Transcript profiling reveals complex auxin signalling pathway and transcription regulation involved in dedifferentiation and redifferentiation during somatic embryogenesis in cotton

Allelic Expression Dynamics of Regulatory Factors During Embryogenic Callus Induction in ABB Banana (Musa spp. cv. Bengal, ABB Group)

The regulatory mechanisms underlying embryogenic callus (EC) formation in polyploid bananas remain unexplored, posing challenges for genetic transformation and biotechnological applications. Here, we conducted transcriptome sequencing on cultured explants, non-embryogenic callus, EC, and browning callus in the ABB cultivar 'MJ' (Musa spp. cv. Bengal). Our analysis of differentially expressed genes (DEGs) revealed significant enrichment in plant hormones, MAPK, and zeatin biosynthesis pathways. Notably, most genes in the MJ variety exhibited balanced expression of the A and B alleles, but A-specific allele expression was dominant in the key signaling pathways, whereas B-specific allele expression was very rare during EC induction. In the auxin signaling pathway, six A-specific MJARF genes were markedly downregulated, underscoring their critical roles in the negative regulation of callus formation. Additionally, six A-specific MJEIN3 alleles were found to play negative regulatory roles in ethylene signaling during EC development. We also identified phenylpropanoids responsible for enzymatic browning. Furthermore, the expression patterns of transcription factors in bananas exhibited specific expression modes, highlighting the unique mechanisms of callus formation. This study enhanced our understanding of the regulatory roles of these alleles in EC induction and offers new insights into the utilization of alleles to improve the efficiency of somatic embryogenesis in bananas.

Molecular insight into auxin signaling and associated network modulating stress responses in rice

Phytohormones are vital regulators of various signaling networks in plants. Among different phytohormones, auxin has been thoroughly studied for its role in regulating plants' growth, development, and stress response. One major function of auxin is modulating the developmental processes in response to environmental cues. Although extensive studies on Arabidopsis have advanced the knowledge of auxin biology, several studies on rice have uncovered key players regulated by auxin that play critical roles in coordinating auxin homeostasis and signaling involved in defense response. The emerging knowledge on auxin biology, auxin-regulated gene expression, and auxin-signaling in rice during various environmental stresses has provided insights into the possible mechanism of rice susceptibility or resistance to different abiotic and biotic stresses. The current review enumerates the possible mechanisms of stress-induced auxin homeostasis in rice. In addition, we provide an overview of the state of knowledge on auxin-mediated defense signaling in rice, highlighting its pivotal role in stress response. In particular, we discuss the auxin pathways and the dynamic regulation in response to biotic and abiotic stress. We highlight the novel findings in the diversity of auxin signaling in the model plant Arabidopsis with an aim to emphasize the need to translate these findings into agronomically and economically important cereals like rice. Addressing the complexity of auxin induction, signaling, and its associated molecular network, an in-depth investigation in rice is required to comprehend auxin-mediated spatial-temporal regulation of developmental processes during stress.

Transcriptome and Gene Expression Analysis Revealed CeNA1: A Potential New Marker for Somatic Embryogenesis in Common Centaury (Centaurium erythraea Rafn.)

Centaurium erythraea Rafn. is a medicinal plant used as a model for studying plant developmental processes due to its developmental plasticity and ease of manipulation in vitro. Identifying the genes involved in its organogenesis and somatic embryogenesis (SE) is the first step toward unraveling the molecular mechanisms underlying its morphogenic plasticity. Although SE is the most common method of centaury regeneration, the genes involved in this have not yet been identified. The aim of this study was to identify the differentially expressed genes (DEGs) during key stages of SE and organogenesis using transcriptome data, with a focus on novel SE-related genes. The transcriptomic analysis revealed a total of 4040 DEGs during SE and 12,708 during organogenesis. Gene Ontology (GO) annotation showed that the highest number of SE-related genes was involved in defense responses. The expression of fifteen selected SE-related candidate genes was assessed by RT-qPCR across nine centaury developmental stages, including embryogenic tissues. Notably, a newly reported transcript, named CeNA1, was specifically activated during embryogenic callus (ec) induction, making it a potential novel marker for early SE. These findings provide, for the first time, insight into SE-related transcriptional patterns, representing a step closer to uncovering the molecular basis of centaury's developmental plasticity.

Protein profile changes during priming explants to embryogenic response in Coffea canephora: identification of the RPN12 proteasome subunit involved in the protein degradation

Plant somatic embryogenesis encompasses somatic cells switch into embryogenic cells that can later produce somatic embryos with the ability to produce plantlets. Previously, we defined in vitro culture settings for the somatic embryogenesis process of Coffea canephora that comprise adequate plantlets with auxin plus cytokinin followed by cut-leaf explant cultivation with cytokinin, producing embryos with the ability to regenerate plantlets. Here, we confirmed that cultivating cut-leaf explants with cytokinin is sufficient to promote somatic embryos proliferation and the high yield of somatic embryos in the protocol requires adequate plantlets with auxin plus cytokinin. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels reveal auxin-plus cytokinin-dependent regulated proteins in plantlets with up and down abundance. Chitinase A class III, proteins involved in the metabolism and folding of proteins, photosynthesis, antioxidant activity, and chromatin organization were identified. The RPN12 protein, which is a subunit of the proteasome 26S, has an abundance that is not associated with transcript changes, suggesting post-translational regulation.

Analysis of the plant hormone expression profile during somatic embryogenesis induction in teak (Tectona grandis)

Plant somatic embryogenesis (SE) is an efficient regeneration system for propagation. It involves the regulation of a complex molecular regulatory network encompassing endogenous hormone synthesis, metabolism, and signal transduction processes, induced through exogenous plant growth regulators. Previous studies have focused primarily on traditional propagation methods for Tectona grandis, but there is limited knowledge on SE and its hormonal regulatory mechanisms. In our study, different SE stages, including the nonembryogenic callus (NEC), embryogenic callus (EC), and globular and heart-shaped embryo (E-SEs) stages, were induced in teak cotyledons incubated on MS medium supplemented with 0.1 mg/L thidiazuron (TDZ). Morphological and histological observations indicated that EC primarily originates from the development of embryogenic cell clusters. During SE induction, the levels of six classes of endogenous hormones, IAA, CTK, ETH, ABA, SA, and JA, changed significantly. Transcriptome analysis revealed that endogenous hormones participate in SE induction in teak through various biological processes, such as biosynthesis, metabolism, and signal transduction pathways. We found that IAA biosynthesis primarily occurs through the IAM pathway during these three stages. The ETH receptor kinase gene SERF1 exhibited the highest expression levels in E-SEs. The ABA-, SA-, and JA-related signal transduction genes ABI3, NPR1, and JAZ exhibited no differential expression during different stages. Moreover, key encoding genes of SE regulators, including WUS, WOX and SERK, were differentially expressed during SE. In conclusion, this study offers insights into the roles of endogenous hormones and their interactions during SE induction.

Proteomic and Phosphoproteomic Analyses during Plant Regeneration Initiation in Cotton (Gossypium hirsutum L.)

Somatic embryogenesis (SE) is a biotechnological tool used to generate new individuals and is the preferred method for rapid plant regeneration. However, the molecular basis underlying somatic cell regeneration through SE is not yet fully understood, particularly regarding interactions between the proteome and post-translational modifications. Here, we performed association analysis of high-throughput proteomics and phosphoproteomics in three representative samples (non-embryogenic calli, NEC; primary embryogenic calli, PEC; globular embryos, GE) during the initiation of plant regeneration in cotton, a pioneer crop for genetic biotechnology applications. Our results showed that protein accumulation is positively regulated by phosphorylation during SE, as revealed by correlation analyses. Of the 1418 proteins that were differentially accumulated in the proteome and the 1106 phosphoproteins that were differentially regulated in the phosphoproteome, 115 proteins with 229 phosphorylation sites overlapped (co-differential). Furthermore, seven dynamic trajectory patterns of differentially accumulated proteins (DAPs) and the correlated differentially regulated phosphoproteins (DRPPs) pairs with enrichment features were observed. During the initiation of plant regeneration, functional enrichment analysis revealed that the overlapping proteins (DAPs-DRPPs) were considerably enriched in cellular nitrogen metabolism, spliceosome formation, and reproductive structure development. Moreover, 198 DRPPs (387 phosphorylation sites) were specifically regulated at the phosphorylation level and showed four patterns of stage-enriched phosphorylation susceptibility. Furthermore, enrichment annotation analysis revealed that these phosphoproteins were significantly enriched in endosomal transport and nucleus organization processes. During embryogenic differentiation, we identified five DAPs-DRPPs with significantly enriched characteristic patterns. These proteins may play essential roles in transcriptional regulation and signaling events that initiate plant regeneration through protein accumulation and/or phosphorylation modification. This study enriched the understanding of key proteins and their correlated phosphorylation patterns during plant regeneration, and also provided a reference for improving plant regeneration efficiency.

Physiological, Metabolic, and Transcriptomic Analyses Reveal Mechanisms of Proliferation and Somatic Embryogenesis of Litchi (Litchi chinensis Sonn.) Embryogenic Callus Promoted by D-Arginine Treatment

D-arginine (D-Arg) can promote embryogenic callus (EC) proliferation and increase the rate of somatic embryo induction of litchi (Litchi chinensis Sonn.), yet the mechanism underlying the processes is incompletely understood. To investigate the mechanism, physiological responses of polyamines (PAs) [putrescine (Put), spermidine (Spd), and spermine (Spm)] were investigated for D-Arg-treated litchi EC and enzyme activity related to polyamine metabolism, plant endogenous hormones, and polyamine- and embryogenic-related genes were explored. Results showed that the exogenous addition of D-Arg reduces the activity of diamine oxidase (DAO) and polyamine oxidase (PAO) in EC, reduces the production of H2O2, promotes EC proliferation, and increases the (Spd + Spm)/Put ratio to promote somatic embryo induction. Exogenous D-Arg application promoted somatic embryogenesis (SE) by increasing indole-3-acetyl glycine (IAA-Gly), kinetin-9-glucoside (K9G), and dihydrozeatin-7-glucoside (DHZ7G) levels and decreasing trans-zeatin riboside (tZR), N-[(-)-jasmonoyl]-(L)-valine (JA-Val), jasmonic acid (JA), and jasmonoyl-L-isoleucine (Ja-ILE) levels on 18 d, as well as promoting cell division and differentiation. The application of exogenous D-Arg regulated EC proliferation and somatic embryo induction by altering gene expression levels of the WRKY family, AP2/ERF family, C3H family, and C2H2 family. These results indicate that exogenous D-Arg could regulate the proliferation of EC and the SE induction of litchi by changing the biosynthesis of PAs through the alteration of gene expression pattern and endogenous hormone metabolism.

Integrated metabolomics and transcriptomics analysis highlight key pathways involved in the somatic embryogenesis of Darjeeling tea

BACKGROUND

Darjeeling tea is a globally renowned beverage, which faces numerous obstacles in sexual reproduction, such as self-incompatibility, poor seed germination, and viability, as well as issues with vegetative propagation. Somatic embryogenesis (SE) is a valuable method for rapid clonal propagation of Darjeeling tea. However, the metabolic regulatory mechanisms underlying SE in Darjeeling tea remain largely unknown. To address this, we conducted an integrated metabolomics and transcriptomics analysis of embryogenic callus (EC), globular embryo (GE), and heart-shaped embryo (HE).

RESULTS

The integrated analyses showed that various genes and metabolites involved in the phenylpropanoid pathway, auxin biosynthesis pathway, gibberellin, brassinosteroid and amino acids biosynthesis pathways were differentially enriched in EC, GE, and HE. Our results revealed that despite highly up-regulated auxin biosynthesis genes YUC1, TAR1 and AAO1 in EC, endogenous indole-3-acetic acid (IAA) was significantly lower in EC than GE and HE. However, bioactive Gibberellin A4 displayed higher accumulation in EC. We also found higher BABY BOOM (BBM) and Leafy cotyledon1 (LEC1) gene expression in GE along with high accumulation of castasterone, a brassinosteroid. Total flavonoids and phenolics levels were elevated in GE and HE compared to EC, especially the phenolic compound chlorogenic acid was highly accumulated in GE.

CONCLUSIONS

Integrated metabolome and transcriptome analysis revealed enriched metabolic pathways, including auxin biosynthesis and signal transduction, brassinosteroid, gibberellin, phenylpropanoid biosynthesis, amino acids metabolism, and transcription factors (TFs) during SE in Darjeeling tea. Notably, EC displayed lower endogenous IAA levels, conducive to maintaining differentiation, while higher IAA concentration in GE and HE was crucial for preserving embryo identity. Additionally, a negative correlation between bioactive gibberellin A4 (GA4) and IAA was observed, impacting callus growth in EC. The high accumulation of chlorogenic acid, a phenolic compound, might contribute to the low success rate in GE and HE formation in Darjeeling tea. TFs such as BBM1, LEC1, FUS3, LEA, WOX3, and WOX11 appeared to regulate gene expression, influencing SE in Darjeeling tea.

Proper doses of brassinolide enhance somatic embryogenesis in different competent Korean pine cell lines during embryogenic callus differentiation

Somatic embryogenesis of Korean pine (Pinus koraiensis Sieb. Et Zucc.), an ecologically and econimically very important conifer species, was hindered by the gradually weakens and fast runaway of the embryogenicity and embryo competence of the embryogenic callus. Brassinolide (BL) has shown the enhancing capability of somatic embryo regeneration. For checking the function of BL in this issue, we applied different concentrations of BL to Korean pine callus materials exhibiting different embryogenic capacities and subsequently monitored the physiological alterations and hormone dynamics of the embryogenic callus. Our study revealed that calli with different embryogenic strengths responded differently to different concentrations of BL, but the effect after the addition of BL was very uniform. The addition of BL during the proliferation phase of embryogenic callus may help to stimulate the biological activity of callus during the proliferation process and improve the level of cell metabolism, which is accompanied by a reduction in storage substances. BL could reduce the level of endogenous auxin IAA in embryogenic callus and increase the level of abscisic acid to regulate cell division and differentiation. In addition, the MDA content in the callus was significantly decreased and the activity of antioxidant enzymes was significantly increased after the addition of BL. During the proliferation of embryogenic callus, BL was added to participate in the metabolism of phenylpropane in the cells and to increase the activity of phenylalanine ammonia-lyase and the content of lignin in the cells. We deduced that the proper doses of BL for Korean pine embryogenic callus culture were as follow: calli with low, high and decreasing embryogenicity were subcultured after the addition of 0.75 mg/L, 0.35 mg/L, 2.00 mg/L BL, respectively, during proliferation culture stage.

Proteomic Approach during the Induction of Somatic Embryogenesis in Coffea canephora

Plant growth regulators (PGR) are essential for somatic embryogenesis (SE) in different species, and Coffea canephora is no exception. In our study model, previously, we have been able to elucidate the participation of various genes involved in SE by using different strategies; however, until now, we have not used a proteomic approach. This research seeks to contribute to understanding the primary cellular pathways involved in developing SE in C. canephora. The process of our model consists of two stages: (1) preconditioning in MS medium with auxin (NAA) and cytokinin (KIN), and (2) induction in Yasuda liquid medium added with cytokinin (BA). Therefore, in this study, we analyzed different days of the SE induction process using shotgun label-free proteomics. An amount of 1630 proteins was found among different sampling days of the process, of which the majority were accumulated during the induction stage. We found that some of the most enriched pathways during this process were the biosynthesis of amino acids and secondary metabolites. Eighteen proteins were found related to auxin homeostasis and two to cytokinin metabolism, such as ABC, BIG, ILR, LOG, and ARR. Ten proteins and transcription factors related to SE were also identified, like SERK1, SKP1, nuclear transcription factor Y, MADS-box, and calreticulin, and 19 related to other processes of plant development, among which the 14-3-3 and PP2A proteins stand out. This is the first report on the proteomic approach to elucidate the mechanisms that operate during the induction of SE in C. canephora. So, our findings provide the groundwork for future, more in-depth research. Data are available via ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD047172.

Regulation of regeneration in Arabidopsis thaliana

We employed several algorithms with high efficacy to analyze the public transcriptomic data, aiming to identify key transcription factors (TFs) that regulate regeneration in Arabidopsis thaliana. Initially, we utilized CollaborativeNet, also known as TF-Cluster, to construct a collaborative network of all TFs, which was subsequently decomposed into many subnetworks using the Triple-Link and Compound Spring Embedder (CoSE) algorithms. Functional analysis of these subnetworks led to the identification of nine subnetworks closely associated with regeneration. We further applied principal component analysis and gene ontology (GO) enrichment analysis to reduce the subnetworks from nine to three, namely subnetworks 1, 12, and 17. Searching for TF-binding sites in the promoters of the co-expressed and co-regulated (CCGs) genes of all TFs in these three subnetworks and Triple-Gene Mutual Interaction analysis of TFs in these three subnetworks with the CCGs involved in regeneration enabled us to rank the TFs in each subnetwork. Finally, six potential candidate TFs-WOX9A, LEC2, PGA37, WIP5, PEI1, and AIL1 from subnetwork 1-were identified, and their roles in somatic embryogenesis (GO:0010262) and regeneration (GO:0031099) were discussed, so were the TFs in Subnetwork 12 and 17 associated with regeneration. The TFs identified were also assessed using the CIS-BP database and Expression Atlas. Our analyses suggest some novel TFs that may have regulatory roles in regeneration and embryogenesis and provide valuable data and insights into the regulatory mechanisms related to regeneration. The tools and the procedures used here are instrumental for analyzing high-throughput transcriptomic data and advancing our understanding of the regulation of various biological processes of interest.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-023-00121-9.

Genome-wide identification and expression analysis of PYL family genes and functional characterization of GhPYL8D2 under drought stress in Gossypium hirsutum

Cotton is a crucial economic crop, serving as a natural fiber source for the textile industry. However, drought stress poses a significant threat to cotton fiber quality and productivity worldwide. Pyrabactin Resistance 1-Like (PYL) proteins, as abscisic acid (ABA) receptors, play a crucial role in adverse stress responses, but knowledge about the PYLs in cotton remains limited. In our study, we identified 40 GhPYL genes in Gossypium hirsutum through a genome-wide analysis of the cotton genome database. Our analysis revealed that the PYL family formed three distinct subfamilies with typical family characteristics in G. hirsutum. Additionally, through quantitative expression analysis, including transcriptome dataset and qRT-PCR, we found that all GhPYLs were expressed in all tissues of G. hirsutum, and all GhPYLs were differentially expressed under drought stress. Among them, GhPYL4A1, GhPY5D1, GhPY8D2, and a member of the type 2C protein phosphatases clade A family in Gossypium hirsutum (GhPP2CA), GhHAI2D, showed significant differences in expression levels within 12 h after stress treatment. Our protein interaction analysis and BiFC demonstrated the complex regulatory network between GhPYL family proteins and GhPP2CA proteins. We also found that there is an interaction between GhPYL8D2 and GhHAI2D, and through drought treatment of transgenic cotton, we found that GhPYL8D2 played a vital role in the response of G. hirsutum to drought through stomatal control via co-regulation with GhHAI2D. Our findings provide useful insights into the regulation of GhPYL family genes that occur in response to abiotic stresses in cotton.

OsCM regulates rice defence system in response to UV light supplemented with drought stress

Studies on plant responses to combined abiotic stresses are very limited, especially in major crop plants. The current study evaluated the response of chorismate mutase overexpressor (OxCM) rice line to combined UV light and drought stress. The experiments were conducted in pots in a growth chamber, and data were assessed for gene expression, antioxidant and hormone regulation, flavonoid accumulation, phenotypic variation, and amino acid accumulation. Wild-type (WT) rice had reduced the growth and vigour, while transgenic rice maintained growth and vigour under combined UV light and drought stress. ROS and lipid peroxidation analysis revealed that chorismate mutase (OsCM) reduced oxidative stress mediated by ROS scavenging and reduced lipid peroxidation. The combined stresses reduced biosynthesis of total flavonoids, kaempferol and quercetin in WT plants, but increased significantly in plants with OxCM. Phytohormone analysis showed that SA was reduced by 50% in WT and 73% in transgenic plants, while ABA was reduced by 22% in WT plants but increased to 129% in transgenic plants. Expression of chorismate mutase regulates phenylalanine biosynthesis, UV light and drought stress-responsive genes, e.g., phenylalanine ammonia lyase (OsPAL), dehydrin (OsDHN), dehydration-responsive element-binding (OsDREB), ras-related protein 7 (OsRab7), ultraviolet-B resistance 8 (OsUVR8), WRKY transcription factor 89 (OsWRKY89) and tryptophan synthase alpha chain (OsTSA). Moreover, OsCM also increases accumulation of free amino acids (aspartic acid, glutamic acid, leucine, tyrosine, phenylalanine and proline) and sodium (Na), potassium (K), and calcium (Ca) ions in response to the combined stresses. Together, these results suggest that chorismate mutase expression induces physiological, biochemical and molecular changes that enhance rice tolerance to combined UV light and drought stresses.

Transcriptome analysis revealed enrichment pathways and regulation of gene expression associated with somatic embryogenesis in Camellia sinensis

The high frequency, stable somatic embryo system of tea has still not been established due to the limitations of its own characteristics and therefore severely restricts the genetic research and breeding process of tea plants. In this study, the transcriptome was used to illustrate the mechanisms of gene expression regulation in the somatic embryogenesis of tea plants. The number of DEGs for the (IS intermediate stage)_PS (preliminary stage), ES (embryoid stage)_IS and ES_PS stages were 109, 2848 and 1697, respectively. The enrichment analysis showed that carbohydrate metabolic processes were considerably enriched at the ES_IS stage and performed a key role in somatic embryogenesis, while enhanced light capture in photosystem I could provide the material basis for carbohydrates. The pathway analysis showed that the enriched pathways in IS_PS process were far less than those in ES_IS or ES_PS, and the photosynthesis and photosynthetic antenna protein pathway of DEGs in ES_IS or ES_PS stage were notably enriched and up-regulated. The key photosynthesis and photosynthesis antenna protein pathways and the Lhcb1 gene were discovered in tea plants somatic embryogenesis. These results were of great significance to clarify the mechanism of somatic embryogenesis and the breeding research of tea plants.

Opposite Auxin Dynamics Determine the Gametophytic and Embryogenic Fates of the Microspore

The microspore can follow two different developmental pathways. In vivo microspores follow the gametophytic program to produce pollen grains. In vitro, isolated microspores can be reprogrammed by stress treatments and follow the embryogenic program, producing doubled-haploid embryos. In the present study, we analyzed the dynamics and role of endogenous auxin in microspore development during these two different scenarios, in Brassica napus. We analyzed auxin concentration, cellular accumulation, the expression of the TAA1 auxin biosynthesis gene, and the PIN1-like efflux carrier gene, as well as the effects of inhibiting auxin biosynthesis by kynurenine on microspore embryogenesis. During the gametophytic pathway, auxin levels and TAA1 and PIN1-like expression were high at early stages, in tetrads and tapetum, while they progressively decreased during gametogenesis in both pollen and tapetum cells. In contrast, in microspore embryogenesis, TAA1 and PIN1-like genes were upregulated, and auxin concentration increased from the first embryogenic divisions. Kynurenine treatment decreased both embryogenesis induction and embryo production, indicating that auxin biosynthesis is required for microspore embryogenesis initiation and progression. The findings indicate that auxin exhibits two opposite profiles during these two microspore developmental pathways, which determine the different cell fates of the microspore.

Genome-wide analysis of the SAUR gene family and function exploration of DlSAUR32 during early longan somatic embryogenesis

The early auxin responsive small auxin up-regulated RNA (SAUR) family is an important gene family in the auxin signal transduction pathway. This study focused on the regulatory mechanism of DlSAUR genes during early somatic embryogenesis (SE) and its response to hormone treatment and abiotic stress. Mining of the available Dimocarpus longan Lour. (D. longan) genome sequence yielded 68 putative SAUR genes. Transcript profiles based on RNA-seq data showed that most of the 24 detected DlSAUR genes were highly expressed in the globular embryos (GE) (10) and most of them responded to heat stress and 2,4-D treatment. The results of qRT-PCR showed that most of DlSAUR genes were up-regulated under auxin inhibitor N-1-naphthylphthalamic acid (NPA) and auxin indole-3-acetic acid (IAA) treatments. Moreover, NPA could promote longan SE. The assay for ATAC-seq data analysis showed that chromatin accessibility of 19 of the 24 DlSAUR genes were open during early SE, and most DlSAUR genes differentially expressed during early SE were not associated with H3K4me1 signal enrichment. The DlSAUR32 was selected for subcellular localization and RNA-seq analysis, which encode a cell nuclear-localized protein. Dual-luciferase assays and transient transformation showed that the transcription factors (TFs) DlWRKY75-1 and DlWRKY75-2 might bind to the DlSAUR32 promoters to inhibition gene transcription. Transient overexpression of DlWRKY75-1 and DlWRKY75-2 decreased IAA content in N. benthamiana leaves. Thus, the regulatory network composed of DlSAUR32 and its related TFs may regulate the early longan SE and be involved in the auxin response regulatory pathway of longan.

Global transcriptome profiling reveals differential regulatory, metabolic and hormonal networks during somatic embryogenesis in Coffea arabica

BACKGROUND

Somatic embryogenesis (SE) is one of the most promising processes for large-scale dissemination of elite varieties. However, for many plant species, optimizing SE protocols still relies on a trial and error approach. We report the first global scale transcriptome profiling performed at all developmental stages of SE in coffee to unravel the mechanisms that regulate cell fate and totipotency.

RESULTS

RNA-seq of 48 samples (12 developmental stages × 4 biological replicates) generated 90 million high quality reads per sample, approximately 74% of which were uniquely mapped to the Arabica genome. First, the statistical analysis of transcript data clearly grouped SE developmental stages into seven important phases (Leaf, Dedifferentiation, Primary callus, Embryogenic callus, Embryogenic cell clusters, Redifferentiation and Embryo) enabling the identification of six key developmental phase switches, which are strategic for the overall biological efficiency of embryo regeneration. Differential gene expression and functional analysis showed that genes encoding transcription factors, stress-related genes, metabolism-related genes and hormone signaling-related genes were significantly enriched. Second, the standard environmental drivers used to control SE, i.e. light, growth regulators and cell density, were clearly perceived at the molecular level at different developmental stages. Third, expression profiles of auxin-related genes, transcription factor-related genes and secondary metabolism-related genes were analyzed during SE. Gene co-expression networks were also inferred. Auxin-related genes were upregulated during dedifferentiation and redifferentiation while transcription factor-related genes were switched on from the embryogenic callus and onward. Secondary metabolism-related genes were switched off during dedifferentiation and switched back on at the onset of redifferentiation. Secondary metabolites and endogenous IAA content were tightly linked with their respective gene expression. Lastly, comparing Arabica embryogenic and non-embryogenic cell transcriptomes enabled the identification of biological processes involved in the acquisition of embryogenic capacity.

CONCLUSIONS

The present analysis showed that transcript fingerprints are discriminating signatures of cell fate and are under the direct influence of environmental drivers. A total of 23 molecular candidates were successfully identified overall the 12 developmental stages and can be tested in many plant species to optimize SE protocols in a rational way.

Genome-wide identification and expression analysis of the SAUR gene family in foxtail millet (Setaria italica L.)

BACKGROUND

Auxin performs important functions in plant growth and development processes, as well as abiotic stress. Small auxin-up RNA (SAUR) is the largest gene family of auxin-responsive factors. However, the knowledge of the SAUR gene family in foxtail millet is largely obscure.

RESULTS

In the current study, 72 SiSAUR genes were identified and renamed according to their chromosomal distribution in the foxtail millet genome. These SiSAUR genes were unevenly distributed on nine chromosomes and were classified into three groups through phylogenetic tree analysis. Most of the SiSAUR members from the same group showed similar gene structure and motif composition characteristics. Analysis of cis-acting elements showed that many hormone and stress response elements were identified in the promoter region of SiSAURs. Gene replication analysis revealed that many SiSAUR genes were derived from gene duplication events. We also found that the expression of 10 SiSAURs was induced by abiotic stress and exogenous hormones, which indicated that SiSAUR genes may participated in complex physiological processes.

CONCLUSIONS

Overall, these results will be valuable for further studies on the biological role of SAUR genes in foxtail development and response to stress conditions and may shed light on the improvement of the genetic breeding of foxtail millet.

Changes and transcriptome regulation of endogenous hormones during somatic embryogenesis in Ormosia henryi Prain

Introduction

Ormosia henryi is a rare and endangered plant growing in southern China. Somatic embryo culture is an effective measure for the rapid propagation of O. henryi. It has not been reported how regulatory genes induce somatic embryogenesis by regulating endogenous hormone changes during the process of somatic embryogenesis in O. henryi.

Methods

In this study, we analysed the endogenous hormone levels and transcriptome data of nonembryogenic callus (NEC), embryogenic callus (EC), globular embryo (GE) and cotyledon embryo (CE) in O. henryi.

Results

The results showed that the indole-3-acetic acid (IAA) content was higher and the cytokinins (CKs) content was lower in EC than in NEC, and the gibberellins (GAs) and abscisic acid (ABA) contents were significantly higher in NEC than in EC. The contents of IAA, CKs, GAs and ABA increased significantly with EC development. The expression patterns of differentially expressed genes (DEGs) involved in the biosynthesis and signal transduction of auxin (AUX) (YUCCA and SAUR), CKs (B-ARR), GAs (GA3ox, GA20ox, GID1 and DELLA) and ABA (ZEP, ABA2, AAO3, CYP97A3, PYL and ABF) were consistent with the levels of endogenous hormones during somatic embryogenesis (SE). In this study, 316 different transcription factors (TFs) regulating phytohormones were detected during SE. AUX/IAA were downregulated in the process of EC formation and GE differentiation into CE, but other TFs were upregulated and downregulated.

Conclusion

Therefore, we believe that relatively high IAA content and low CKs, GAs and ABA contents contribute to EC formation. The differential expression of AUX, CKs, GAs and ABA biosynthesis and signal transduction genes affected the endogenous hormone levels at different stages of SE in O. henryi. The downregulated expression of AUX/IAA inhibited NEC induction, promoted EC formation and GE differentiation into CE.

GhTCE1-GhTCEE1 dimers regulate transcriptional reprogramming during wound-induced callus formation in cotton

Wounded plant cells can form callus to seal the wound site. Alternatively, wounding can cause adventitious organogenesis or somatic embryogenesis. These distinct developmental pathways require specific cell fate decisions. Here, we identify GhTCE1, a basic helix-loop-helix family transcription factor, and its interacting partners as a central regulatory module of early cell fate transition during in vitro dedifferentiation of cotton (Gossypium hirsutum). RNAi- or CRISPR/Cas9-mediated loss of GhTCE1 function resulted in excessive accumulation of reactive oxygen species (ROS), arrested callus cell elongation, and increased adventitious organogenesis. In contrast, GhTCE1-overexpressing tissues underwent callus cell growth, but organogenesis was repressed. Transcriptome analysis revealed that several pathways depend on proper regulation of GhTCE1 expression, including lipid transfer pathway components, ROS homeostasis, and cell expansion. GhTCE1 bound to the promoters of the target genes GhLTP2 and GhLTP3, activating their expression synergistically, and the heterodimer TCE1-TCEE1 enhances this activity. GhLTP2- and GhLTP3-deficient tissues accumulated ROS and had arrested callus cell elongation, which was restored by ROS scavengers. These results reveal a unique regulatory network involving ROS and lipid transfer proteins, which act as potential ROS scavengers. This network acts as a switch between unorganized callus growth and organized development during in vitro dedifferentiation of cotton cells.

A Comparative Transcriptome Analysis Reveals the Molecular Mechanisms That Underlie Somatic Embryogenesis in Peaonia ostii ‘Fengdan’

Low propagation rate is the primary problem that limits industry development of tree peony. In this study, a highly efficient regeneration system for tree peony using somatic embryogenesis (SE) was established. The transcriptomes of zygotic embryo explants (S0), non-embryonic callus (S1), embryonic callus (S2), somatic embryos (S3), and regenerated shoots (S4) were analyzed to determine the regulatory mechanisms that underlie SE in tree peony. The differentially expressed genes (DEGs) were identified in the pairwise comparisons of S1-vs-S2 and S1-vs-S3, respectively. The enriched DEGs were primarily involved in hormone signal transduction, stress response and the nucleus (epigenetic modifications). The results indicated that cell division, particularly asymmetric cell division, was enhanced in S3. Moreover, the genes implicated in cell fate determination played central roles in S3. Hormone signal pathways work in concert with epigenetic modifications and stress responses to regulate SE. SERK, WOX9, BBM, FUS3, CUC, and WUS were characterized as the molecular markers for tree peony SE. To our knowledge, this is the first study of the SE of tree peony using transcriptome sequencing. These results will improve our understanding of the molecular mechanisms that underly SE in tree peony and will benefit the propagation and genetic engineering of this plant.

Dynamic Transcriptome Analysis Reveals Complex Regulatory Pathway Underlying Induction and Dose Effect by Different Exogenous Auxin IAA and 2,4-D During in vitro Embryogenic Redifferentiation in Cotton

Plant somatic cells can reprogram into differentiated embryos through somatic embryogenesis (SE) on the condition of plant growth regulators (PGRs). RNA sequencing analysis was performed to investigate transcriptional profiling on cotton redifferentiated callus that was induced by different auxin types (IAA and 2,4-D), different concentrations (0, 0.025, and 0.05 mg L^-1), and different incubation times (0, 5, and 20 days). Under the 2,4-D induction effect, signal transduction pathways of plant hormones were significantly enriched in the embryogenic response stage (5 days). These results indicated that auxin signal transduction genes were necessary for the initial response of embryogenic differentiation. In the pre-embryonic initial period (20 days), the photosynthetic pathway was significantly enriched. Most differentially expressed genes (DEGs) were downregulated under the induction of 2,4-D. Upon the dose effect of IAA and 2,4-D, respectively, pathways were significantly enriched in phenylpropanoid biosynthesis, fatty acid metabolism, and carbon metabolic pathways. Therefore, primary and secondary metabolism pathways were critical in cotton SE. These results showed that complex synergistic mechanisms involving multiple cellular pathways were the causes of the induction and dose effect of auxin-induced SE. This study reveals a systematic molecular response to auxin signals and reveals the way that regulates embryogenic redifferentiation during cotton SE.

Induction of Somatic Embryogenesis in Tamarillo (Solanum betaceum Cav.) Involves Increases in the Endogenous Auxin Indole-3-Acetic Acid

Somatic embryogenesis (SE) is a complex biological process regulated by several factors, such as the action of plant growth regulators, namely auxins, of which the most physiologically relevant is indole-3-acetic acid (IAA). In tamarillo, an optimized system for induction of SE creates, after an induction process, embryogenic (EC) and non-embryogenic callus (NEC). In this work the endogenous levels of auxin along the induction phase and in the calli samples were investigated using chemical quantifications by colorimetric reactions and HPLC as well as immunohistochemistry approaches. Differential gene expression (IAA 11, IAA 14, IAA 17, TIR 1, and AFB3) analysis during the induction phase was also carried out. The results showed that the endogenous IAA content is considerably higher in embryogenic than in non-embryogenic calli, with a tendency to increase as the dedifferentiation of the original explant (leaf segments) evolves. Furthermore, the degradation rates of IAA seem to be related to these levels, as non-embryogenic tissue presents a higher degradation rate. The immunohistochemical results support the quantifications made, with higher observable labeling on embryogenic tissue that tends to increase along the induction phase. Differential gene expression also suggests a distinct molecular response between EC and NEC.

Uniconazole Augments Abscisic Acid in Promoting Somatic Embryogenesis in Cotton (Gossypium hirsutum L.)

During somatic embryogenesis (SE), somatic cells initiate embryogenic development under appropriate conditions. Uniconazole, a plant growth regulator, was found to inhibit the proliferation of callus but promoted the conversion of callus into an embryogenic callus (EC) in cotton. The supplementation of uniconazole in the culture medium significantly suppressed the endogenous auxin [indole acetic acid (IAA)] level in callus tissues in both the callus initiation and proliferation stage but enhanced the abscisic acid (ABA) level only in the callus proliferation stage. Exogenous ABA and uniconazole showed cooperative effects on promoting the differentiation rate of callus into EC. These findings were verified by RNA-seq analysis, which elucidated that the genes involved in the IAA biosynthesis, metabolism, and signaling, and ABA metabolism pathways were regulated by uniconazole during the callus development and SE. Overall, the results suggest that uniconazole could modulate callus proliferation and callus differentiation rate by regulating the endogenous levels of IAA and ABA.

GhLBDs Promote Callus Initiation and Act as Selectable Markers to Increase Transformation Efficiency

Detached organs or differentiated tissues could form a mass of pluripotent cells termed as callus on an auxin-rich medium, the underlying molecular mechanism of which remains elusive in cotton. LATERAL ORGAN BOUNDARIES DOMAIN (LBD) transcription factor is a key regulator of plant cell totipotency/pluripotency, and a number of cotton GhLBDs with high-level differential expression during the callus induction process have been identified. Their overexpression in cotton calli fostered promotions in and callus induction without exogenous auxin. Expression analysis and histological observation using paraffin sectioning suggested that the first 72 h on culture is a key time point for callus initiation, whereby the GhLBDs showed high transcript abundance and enlarged calli that were rapidly developed from procambium and cambium. GhLBDs' expression level could be precisely modulated by the gradient concentrations of exogenous auxin, whereas auxin transport inhibitor 2,3,5-triiodobenzoic acid could severely inhibit its expression. The LBD-mediated callus formation was also dependent on the expression levels of GhLBDs. Further, a β-estradiol-inducible promoter pER8 was used to drive GhLBD18-1 expression, which led to rapid callus proliferation, suggesting that pER8/GhLBD18-1 could be used as a selectable marker system to replace the existing antibiotic/herbicide-resistance selectable markers in plant transformation. Our study provides new insights for callus initiation regulatory mechanism and strategies for improving transformation efficiency in cotton.

Early stem growth mutation alters metabolic flux changes enhance sesquiterpenoids biosynthesis in Atractylodes lancea (Thunb.) DC

Atractylodes lancea (Thunb.) DC. is a well-known medicinal herb in China, containing abundant active components, including a variety of sesquiterpenoids. Owing to a shortage of wild resources, artificial cultivation has become the main breeding mode, leading to the germplasm degradation. In preliminary research, our research group found that a mutant tissue culture seedling of A. lancea is an excellent germplasm resource, characterized by early stem growth and higher sesquiterpenoid content than that of the wild type. In this study, the physiological and biochemical mechanisms underlying efficient sesquiterpenoids synthesis by this mutant A. lancea were systematically evaluated. The results showed that the photosynthetic efficiency, central carbon metabolism efficiency, and energy metabolism efficiency were significantly improved in mutant A. lancea compared with the wild type, and the content of endogenous hormones, such as gibberellin and jasmonic acid, changed significantly. In addition, levels of key metabolites and the expression level of key genes in the mevalonate and 2-C-methyl-d-erythritol-4-phosphate pathways were significantly higher in mutant type than in wild type, resulting in elevated sesquiterpenoid synthesis in the mutant. These physiological and biochemical properties explain the rapid growth and high sesquiterpenoid content of mutant A. lancea.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11240-022-02240-5.

Small molecule inhibitors of mammalian GSK-3β promote in vitro plant cell reprogramming and somatic embryogenesis in crop and forest species

Plant in vitro regeneration systems, such as somatic embryogenesis, are essential in breeding; they permit propagation of elite genotypes, production of doubled-haploids, and regeneration of whole plants from gene editing or transformation events. However, in many crop and forest species, somatic embryogenesis is highly inefficient. We report a new strategy to improve in vitro embryogenesis using synthetic small molecule inhibitors of mammalian glycogen synthase kinase 3β (GSK-3β), never used in plants. These inhibitors increased in vitro embryo production in three different systems and species, microspore embryogenesis of Brassica napus and Hordeum vulgare, and somatic embryogenesis of Quercus suber. TDZD-8, a representative compound of the molecules tested, inhibited GSK-3 activity in microspore cultures, and increased expression of embryogenesis genes FUS3, LEC2, and AGL15. Plant GSK-3 kinase BIN2 is a master regulator of brassinosteroid (BR) signalling. During microspore embryogenesis, BR biosynthesis and signalling genes CPD, GSK-3-BIN2, BES1, and BZR1 were up-regulated and the BAS1 catabolic gene was repressed, indicating activation of the BR pathway. TDZD-8 increased expression of BR signalling elements, mimicking BR effects. The findings support that the small molecule inhibitors promoted somatic embryogenesis by activating the BR pathway, opening up the way for new strategies using GSK-3β inhibitors that could be extended to other species.

Global Transcriptome and Coexpression Network Analyses Reveal New Insights Into Somatic Embryogenesis in Hybrid Sweetgum (Liquidambar styraciflua × Liquidambar formosana)

Somatic embryogenesis (SE) is a process of somatic cells that dedifferentiate to totipotent embryonic stem cells and generate embryos in vitro. Despite recent scientific headway in deciphering the difficulties of somatic embryogenesis, the overall picture of key genes, pathways, and co-expression networks regulating SE is still fragmented. Therefore, deciphering the molecular basis of somatic embryogenesis of hybrid sweetgum remains pertinent. In the present study, we analyzed the transcriptome profiles and gene expression regulation changes via RNA sequencing from three distinct developmental stages of hybrid sweetgum: non-embryogenic callus (NEC), embryogenic callus (EC), and redifferentiation. Comparative transcriptome analysis showed that 19,957 genes were differentially expressed in ten pairwise comparisons of SE. Among these, plant hormone signaling-related genes, especially the auxin and cytokinin signaling components, were significantly enriched in NEC and EC early. The K-means method was used to identify multiple transcription factors, including HB-WOX, B3-ARF, AP2/ERF, and GRFs (growth regulating factors). These transcription factors showed distinct stage- or tissue-specific expression patterns mirroring each of the 12 superclusters to which they belonged. For example, the WOX transcription factor family was expressed only at NEC and EC stages, ARF transcription factor was expressed in EC early, and GRFs was expressed in late SE. It was noteworthy that the AP2/ERF transcription factor family was expressed during the whole SE process, but almost not in roots, stems and leaves. A weighted gene co-expression network analysis (WGCNA) was used in conjunction with the gene expression profiles to recognize the genes and modules that may associate with specific tissues and stages. We constructed co-expression networks and revealed 22 gene modules. Four of these modules with properties relating to embryonic potential, early somatic embryogenesis, and somatic embryo development, as well as some hub genes, were identified for further functional studied. Through a combination analysis of WGCNA and K-means, SE-related genes including AUX22, ABI3, ARF3, ARF5, AIL1, AIL5, AGL15, WOX11, WOX9, IAA29, BBM1, MYB36, LEA6, SMR4 and others were obtained, indicating that these genes play an important role in the processes underlying the progression from EC to somatic embryos (SEs) morphogenesis. The transcriptome information provided here will form the foundation for future research on genetic transformation and epigenetic control of plant embryogenesis at a molecular level. In follow-up studies, these data could be used to construct a regulatory network for SE; Key genes obtained from coexpression network analysis at each critical stage of somatic embryo can be considered as potential candidate genes to verify these networks.

Multiple analyses of various factors affecting the plantlet regeneration of Picea mongolica (H. Q. Wu) W.D. Xu from somatic embryos

Picea mongolica, a native species with excellent industrial wood quality and strong sand-fixing capacity, may be utilized in construction of urban green spaces in arid areas in China. However, now the sustainability of the ecosystems where this species grows is at serious risk due to a lack of natural regeneration. In this study, we developed an efficient regeneration system and comprehensively analyzed various factors affecting somatic embryogenesis (SE) using zygotic embryos as explants. We identified the optimal plant growth regulators (PGRs) performance and the best donor trees (k81) for the generation of somatic embryos (SEMs). Additionally, we confirmed that the positive developmental window of SEMs initiation was at the end of July to early August, which is when zygotic embryos was at the late embryogeny. In this time period, specific transcripts associated with the regulation of epigenetic modifications, plant hormone-related genes, and embryonic development-related transcription factors play important roles for early SEMs initiation. These results may provide a valuable resource for vegetative propagation of Picea mongolica. Our results may help to establish a reliable protocol for plantlet regeneration, which may facilitate urban greening applications and industrialization in arid areas.

Comparative transcriptome analysis during developmental stages of direct somatic embryogenesis in Tilia amurensis Rupr

Tilia species are valuable woody species due to their beautiful shape and role as honey trees. Somatic embryogenesis can be an alternative method for mass propagation of T. amurensis. However, the molecular mechanisms of T. amurensis somatic embryogenesis are yet to be known. Here, we conducted comparative transcriptional analysis during somatic embryogenesis of T. amurensis. RNA-Seq identified 1505 differentially expressed genes, including developmental regulatory genes. Auxin related genes such as YUC, AUX/IAA and ARF and signal transduction pathway related genes including LEA and SERK were differentially regulated during somatic embryogenesis. Also, B3 domain family (LEC2, FUS3), VAL and PKL, the regulatory transcription factors, were differentially expressed by somatic embryo developmental stages. Our results could provide plausible pathway of signaling somatic embryogenesis of T. amurensis, and serve an important resource for further studies in direct somatic embryogenesis in woody plants.

Selection of reference genes for gene expression analysis in Liriodendron hybrids' somatic embryogenesis and germinative tissues

The differential expression of genes is crucial for plant somatic embryogenesis (SE), and the accurate quantification of gene expression levels relies on choosing appropriate reference genes. To select the most suitable reference genes for SE studies, 10 commonly used reference genes were examined in synchronized somatic embryogenic and subsequent germinative cultures of Liriodendron hybrids by using quantitative real-time reverse transcription PCR. Four popular normalization algorithms: geNorm, NormFinder, Bestkeeper and Delta-Ct were used to select and validate the suitable reference genes. The results showed that elongation factor 1-gamma, histone H1 linker protein, glyceraldehyde-3-phosphate dehydrogenase and α-tubulin were suitable for SE tissues, while elongation factor 1-gamma and actin were best for the germinative organ tissues. Our work will benefit future studies of gene expression and functional analyses of SE in Liriodendron hybrids. It is also serves as a guide of reference gene selection in early embryonic gene expression analyses for other woody plant species.

Reverse Chromatin Immunoprecipitation (R-ChIP) enables investigation of the upstream regulators of plant genes

DNA binding proteins carry out important and diverse functions in the cell, including gene regulation, but identifying these proteins is technically challenging. In the present study, we developed a technique to capture DNA-associated proteins called reverse chromatin immunoprecipitation (R-ChIP). This technology uses a set of specific DNA probes labeled with biotin to isolate chromatin, and the DNA-associated proteins are then identified using mass spectrometry. Using R-ChIP, we identified 439 proteins that potentially bind to the promoter of the Arabidopsis thaliana gene AtCAT3 (AT1G20620). According to functional annotation, we randomly selected 5 transcription factors from these candidates, including bZIP1664, TEM1, bHLH106, BTF3, and HAT1, to verify whether they in fact bind to the AtCAT3 promoter. The binding of these 5 transcription factors was confirmed using chromatin immunoprecipitation quantitative real-time PCR and electrophoretic mobility shift assays. In addition, we improved the R-ChIP method using plants in which the DNA of interest had been transiently introduced, which does not require the T-DNA integration, and showed that this substantially improved the protein capture efficiency. These results together demonstrate that R-ChIP has a wide application to characterize chromatin composition and isolate upstream regulators of a specific gene.

Transcriptomic analysis reveals somatic embryogenesis-associated signaling pathways and gene expression regulation in maize (Zea mays L.)

Transcriptome analysis of maize embryogenic callus and somatic embryos reveals associated genes reprogramming, hormone signaling pathways and transcriptional regulation involved in somatic embryogenesis in maize. Somatic embryos are widely utilized in propagation and genetic engineering of crop plants. In our laboratory, an elite maize inbred line Y423 that could generate intact somatic embryos was obtained and applied to genetic transformation. To enhance our understanding of regulatory mechanisms during maize somatic embryogenesis, we used RNA-based sequencing (RNA-seq) to characterize the transcriptome of immature embryo (IE), embryogenic callus (EC) and somatic embryo (SE) from maize inbred line Y423. The number of differentially expressed genes (DEGs) in three pairwise comparisons (IE-vs-EC, IE-vs-SE and EC-vs-SE) was 5767, 7084 and 1065, respectively. The expression patterns of DEGs were separated into eight major clusters. Somatic embryogenesis associated genes were mainly grouped into cluster A or B with an expression trend toward up-regulation during dedifferentiation. GO annotation and KEGG pathway analysis revealed that DEGs were implicated in plant hormone signal transduction, stress response and metabolic process. Among the differentially expressed transcription factors, the most frequently represented families were associated with the common stress response or related to cell differentiation, embryogenic patterning and embryonic maturation processes. Genes include hormone response/transduction and stress response, as well as several transcription factors were discussed in this study, which may be potential candidates for further analyses regarding their roles in somatic embryogenesis. Furthermore, the temporal expression patterns of candidate genes were analyzed to reveal their roles in somatic embryogenesis. This transcriptomic data provide insights into future functional studies, which will facilitate further dissections of the molecular mechanisms that control maize somatic embryogenesis.

Transcriptome analysis identifies genes involved in the somatic embryogenesis of Eucalyptus

Background

Eucalyptus, a highly diverse genus of the Myrtaceae family, is the most widely planted hardwood in the world due to its increasing importance for fiber and energy. Somatic embryogenesis (SE) is one large-scale method to provide commercial use of the vegetative propagation of Eucalyptus and dedifferentiation is a key step for plant cells to become meristematic. However, little is known about the molecular changes during the Eucalyptus SE.

Results

We compared the transcriptome profiles of the differentiated and dedifferentiated tissues of two Eucalyptus species – E. camaldulensis (high embryogenetic potential) and E. grandis x urophylla (low embryogenetic potential). Initially, we identified 18,777 to 20,240 genes in all samples. Compared to the differentiated tissues, we identified 9229 and 8989 differentially expressed genes (DEGs) in the dedifferentiated tissues of E. camaldulensis and E. grandis x urophylla, respectively, and 2687 up-regulated and 2581 down-regulated genes shared. Next, we identified 2003 up-regulated and 1958 down-regulated genes only in E. camaldulensis, including 6 somatic embryogenesis receptor kinase, 17 ethylene, 12 auxin, 83 ribosomal protein, 28 zinc finger protein, 10 heat shock protein, 9 histone, 122 cell wall related and 98 transcription factor genes. Genes from other families like ABA, arabinogalactan protein and late embryogenesis abundant protein were also found to be specifically dysregulated in the dedifferentiation process of E. camaldulensis. Further, we identified 48,447 variants (SNPs and small indels) specific to E. camaldulensis, including 13,434 exonic variants from 4723 genes (e.g., annexin, GN, ARF and AP2-like ethylene-responsive transcription factor). qRT-PCR was used to confirm the gene expression patterns in both E. camaldulensis and E. grandis x urophylla.

Conclusions

This is the first time to study the somatic embryogenesis of Eucalyptus using transcriptome sequencing. It will improve our understanding of the molecular mechanisms of somatic embryogenesis and dedifferentiation in Eucalyptus. Our results provide a valuable resource for future studies in the field of Eucalyptus and will benefit the Eucalyptus breeding program.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07214-5.

Early nodulin 93 protein gene: essential for induction of somatic embryogenesis in oil palm

Transcript profiling during the early induction phase of oil palm tissue culture and RNAi studies in a model somatic embryogenesis system showed that EgENOD93 expression is essential for somatic embryogenesis. Micropropagation of oil palm through tissue culture is vital for the generation of superior and uniform elite planting materials. Studies were carried out to identify genes to distinguish between leaf explants with the potential to develop into embryogenic or non-embryogenic callus. Oil palm cDNA microarrays were co-hybridized with cDNA probes of reference tissue, separately with embryo forming (media T527) and non-embryo (media T694) forming leaf explants sampled at Day 7, Day 14 and Day 21. Analysis of the normalized datasets has identified 77, 115 and 127 significantly differentially expressed genes at Day 7, Day 14, and Day 21, respectively. An early nodulin 93 protein gene (ENOD93), was highly expressed at Day 7, Day 14, and Day 21 and in callus (media T527), as assessed by RT-qPCR. Validation of EgENOD93 across tissue culture lines of different genetic background and media composition showed the potential of this gene as an embryogenic marker. In situ RNA hybridization and functional characterization in Medicago truncatula provided additional evidence that ENOD93 is essential for somatic embryogenesis. This study supports the suitability of EgENOD93 as a marker to predict the potential of leaf explants to produce embryogenic callus. Crosstalk among stresses, auxin, and Nod-factor like signalling molecules likely induces the expression of EgENOD93 for embryogenic callus formation.

Cell Wall Composition as a Marker of the Reprogramming of the Cell Fate on the Example of a Daucus carota (L.) Hypocotyl in Which Somatic Embryogenesis Was Induced

Changes in the composition of the cell walls are postulated to accompany changes in the cell's fate. We check whether there is a relationship between the presence of selected pectic, arabinogalactan proteins (AGPs), and extensins epitopes and changes in cell reprogramming in order to answer the question of whether they can be markers accompanying changes of cell fate. Selected antibodies were used for spatio-temporal immunolocalization of wall components during the induction of somatic embryogenesis. Based on the obtained results, it can be concluded that (1) the LM6 (pectic), LM2 (AGPs) epitopes are positive markers, but the LM5, LM19 (pectic), JIM8, JIM13 (AGPs) epitopes are negative markers of cells reprogramming to the meristematic/pluripotent state; (2) the LM8 (pectic), JIM8, JIM13, LM2 (AGPs) and JIM11 (extensin) epitopes are positive markers, but LM6 (pectic) epitope is negative marker of cells undergoing detachment; (3) JIM4 (AGPs) is a positive marker, but LM5 (pectic), JIM8, JIM13, LM2 (AGPs) are negative markers for pericycle cells on the xylem pole; (4) LM19, LM20 (pectic), JIM13, LM2 (AGPs) are constitutive wall components, but LM6, LM8 (pectic), JIM4, JIM8, JIM16 (AGPs), JIM11, JIM12 and JIM20 (extensins) are not constitutive wall components; (5) the extensins do not contribute to the cell reprogramming.

Hypermethylation of Auxin-Responsive Motifs in the Promoters of the Transcription Factor Genes Accompanies the Somatic Embryogenesis Induction in Arabidopsis

The auxin-induced embryogenic reprogramming of plant somatic cells is associated with extensive modulation of the gene expression in which epigenetic modifications, including DNA methylation, seem to play a crucial role. However, the function of DNA methylation, including the role of auxin in epigenetic regulation of the SE-controlling genes, remains poorly understood. Hence, in the present study, we analysed the expression and methylation of the TF genes that play a critical regulatory role during SE induction (LEC1, LEC2, BBM, WUS and AGL15) in auxin-treated explants of Arabidopsis. The results showed that auxin treatment substantially affected both the expression and methylation patterns of the SE-involved TF genes in a concentration-dependent manner. The auxin treatment differentially modulated the methylation of the promoter (P) and gene body (GB) sequences of the SE-involved genes. Relevantly, the SE-effective auxin treatment (5.0 µM of 2,4-D) was associated with the stable hypermethylation of the P regions of the SE-involved genes and a significantly higher methylation of the P than the GB fragments was a characteristic feature of the embryogenic culture. The presence of auxin-responsive (AuxRE) motifs in the hypermethylated P regions suggests that auxin might substantially contribute to the DNA methylation-mediated control of the SE-involved genes.

Advances in Plant Regeneration: Shake, Rattle and Roll

Some plant cells are able to rebuild new organs after tissue damage or in response to definite stress treatments and/or exogenous hormone applications. Whole plants can develop through de novo organogenesis or somatic embryogenesis. Recent findings have enlarged our understanding of the molecular and cellular mechanisms required for tissue reprogramming during plant regeneration. Genetic analyses also suggest the key role of epigenetic regulation during de novo plant organogenesis. A deeper understanding of plant regeneration might help us to enhance tissue culture optimization, with multiple applications in plant micropropagation and green biotechnology. In this review, we will provide additional insights into the physiological and molecular framework of plant regeneration, including both direct and indirect de novo organ formation and somatic embryogenesis, and we will discuss the key role of intrinsic and extrinsic constraints for cell reprogramming during plant regeneration.

Dynamic Transcriptome Analysis Reveals Uncharacterized Complex Regulatory Pathway Underlying Genotype-Recalcitrant Somatic Embryogenesis Transdifferentiation in Cotton

As a notable illustration of totipotency and plant regeneration, somatic embryogenesis (SE) is the developmental reprogramming of somatic cells toward the embryogenesis pathway, the key step for genetic engineering. Investigations examining the totipotency process are of great fundamental and practical importance in crop biotechnology. However, high-frequency regeneration of cotton via SE has been limited due to genotype-dependent response. The molecular basis deciphering SE genotype recalcitrance remains largely unexplored in cotton. In the current study, to comprehensively investigate the dynamic transcriptional profiling and gene regulatory patterns involved in SE process, a genome-wide RNA sequencing analysis was performed in two cotton genotypes with distinct embryogenic abilities, the highly embryogenic genotype Yuzao 1 (YZ) and the recalcitrant genotype Lumian 1 (LM). Three typical developmental staged cultures of early SE-hypocotyls (HY), nonembryogenic calli (NEC) and primary embryogenic calli (PEC)-were selected to establish the transcriptional profiles. Our data revealed that a total of 62,562 transcripts were present amongst different developmental stages in the two genotypes. Of these, 18,394 and 26,514 differentially expressed genes (DEGs) were identified during callus dedifferentiation (NEC-VS-HY) and embryogenic transdifferentiation (PEC-VS-NEC), respectively in the recalcitrant genotype, 21,842 and 22,343 DEGs in the highly embryogenic genotype. Furthermore, DEGs were clustered into six expression patterns during cotton SE process in the two genotypes. Moreover, functional enrichment analysis revealed that DEGs were significantly enriched in fatty acid, tryptophan and pyruvate metabolism in the highly embryogenic genotype and in DNA conformation change otherwise in the recalcitrant genotype. In addition, critical SE-associated expressed transcription factors, as well as alternative splicing events, were notably and preferentially activated during embryogenic transdifferentiation in the highly embryogenic genotype compared with the recalcitrant genotype. Taken together, by systematically comparing two genotypes with distinct embryogenic abilities, the findings in our study revealed a comprehensive overview of the dynamic gene regulatory patterns and uncharacterized complex regulatory pathways during cotton SE genotype-dependent response. Our work provides insights into the molecular basis and important gene resources for understanding the underlying genotype recalcitrance during SE process and plant regeneration, thereby holding great promise for accelerating the application of biotechnology to cotton for improving its breeding efficiency.

Current Perspectives on the Auxin-Mediated Genetic Network that Controls the Induction of Somatic Embryogenesis in Plants

Auxin contributes to almost every aspect of plant development and metabolism as well as the transport and signalling of auxin-shaped plant growth and morphogenesis in response to endo- and exogenous signals including stress conditions. Consistently with the common belief that auxin is a central trigger of developmental changes in plants, the auxin treatment of explants was reported to be an indispensable inducer of somatic embryogenesis (SE) in a large number of plant species. Treating in vitro-cultured tissue with auxins (primarily 2,4-dichlorophenoxyacetic acid, which is a synthetic auxin-like plant growth regulator) results in the extensive reprogramming of the somatic cell transcriptome, which involves the modulation of numerous SE-associated transcription factor genes (TFs). A number of SE-modulated TFs that control auxin metabolism and signalling have been identified, and conversely, the regulators of the auxin-signalling pathway seem to control the SE-involved TFs. In turn, the different expression of the genes encoding the core components of the auxin-signalling pathway, the AUXIN/INDOLE-3-ACETIC ACIDs (Aux/IAAs) and AUXIN RESPONSE FACTORs (ARFs), was demonstrated to accompany SE induction. Thus, the extensive crosstalk between the hormones, in particular, auxin and the TFs, was revealed to play a central role in the SE-regulatory network. Accordingly, LEAFY COTYLEDON (LEC1 and LEC2), BABY BOOM (BBM), AGAMOUS-LIKE15 (AGL15) and WUSCHEL (WUS) were found to constitute the central part of the complex regulatory network that directs the somatic plant cell towards embryogenic development in response to auxin. The revealing picture shows a high degree of complexity of the regulatory relationships between the TFs of the SE-regulatory network, which involve direct and indirect interactions and regulatory feedback loops. This review examines the recent advances in studies on the auxin-controlled genetic network, which is involved in the mechanism of SE induction and focuses on the complex regulatory relationships between the down- and up-stream targets of the SE-regulatory TFs. In particular, the outcomes from investigations on Arabidopsis, which became a model plant in research on genetic control of SE, are presented.

Dynamic Transcriptome Analysis Reveals Uncharacterized Complex Regulatory Pathway Underlying Dose IBA-Induced Embryogenic Redifferentiation in Cotton

The somatic embryogenesis (SE) process of plants is regulated by exogenous hormones. During the SE, different genes sensitively respond to hormone signals through complex regulatory networks to exhibit plant totipotency. When cultured in indole-3-butyric acid (IBA) concentration gradient medium supplemented with 0 mg dm^-3, 0.025 mg dm^-3, and 0.05 mg dm^-3 IBA, the callus differentiation rate first increased then decreased in cotton. To characterize the molecular basis of IBA-induced regulating SE, transcriptome analysis was conducted on embryogenic redifferentiation. Upon the examination of the IBA's embryogenic inductive effect, it was revealed that pathways related to plant hormone signal transduction and alcohol degradation were significantly enriched in the embryogenic responsive stage (5 days). The photosynthesis, alcohol metabolism and cell cycle pathways were specifically regulated in the pre-embryonic initial period (20 days). Upon the effect of the IBA dose, in the embryogenic responsive stage (5 days), the metabolism of xenobiotics by the cytochrome P450 pathway and secondary metabolism pathways of steroid, flavonoid, and anthocyanin biosynthesis were significantly enriched. The phenylpropanoid, brassinosteroid, and anthocyanin biosynthesis pathways were specifically associated in the pre-embryonic initial period (20 days). At different developmental stages of embryogenic induction, photosynthesis, flavonoid biosynthesis, phenylpropanoid biosynthesis, mitogen-activated protein kinase (MAPK) signaling, xenobiotics metabolism by cytochrome P450, and brassinosteroid biosynthesis pathways were enriched at low a IBA concentration. Meanwhile, at high IBA concentration, the carbon metabolism, alcohol degradation, circadian rhythm and biosynthesis of amino acids pathways were significantly enriched. The results reveal that complex regulating pathways participate in the process of IBA-induced redifferentiation in cotton somatic embryogenesis. In addition, collections of potential essential signaling and regulatory genes responsible for dose IBA-induced efficient embryogenic redifferentiation were identified. Quantitative real-time PCR (qRT-PCR) was performed on the candidate genes with different expression patterns, and the results are basically consistent with the RNA-seq data. The results suggest that the complicated and concerted IBA-induced mechanisms involving multiple cellular pathways are responsible for dose-dependent plant growth regulator-induced SE. This report represents a systematic study and provides new insight into molecular signaling and regulatory basis underlying the process of dose IBA-induced embryogenic redifferentiation during SE.

Global scale transcriptome analysis reveals differentially expressed genes involve in early somatic embryogenesis in Dimocarpus longan Lour

BACKGROUND

Somatic embryogenesis (SE) is a process of somatic cells that dedifferentiate to totipotent embryonic stem cells and generate embryos in vitro. Longan SE has been established and wildly used as model system for studying embryogenesis in woody plants, SE-related genes had been characterized. In spite of that, a comprehensive overview of SE at a molecular level is still absent. To understand the molecular mechanisms during longan SE, we examined the transcriptome changes by using Illumina HiSeq from the four distinct developmental stages, including non-embryogenic callus (NEC), embryogenic callus (EC), incomplete compact pro-embryogenic cultures (ICpEC), globular embryos (GE).

RESULTS

RNA-seq of the four samples generated a total of 243.78 million high quality reads, approximately 81.5% of the data were mapped to longan genome. The cDNA libraries of NEC, EC, ICpEC and GE, generated 22,743, 19,745, 21,144, 21,102 expressed transcripts, 1935, 1710, 1816, 1732 novel transcripts, 2645, 366, 505, 588 unique genes, respectively. Comparative transcriptome analysis showed that a total of 10,642, 4180, 5846 and 1785 genes were differentially expressed in the pairwise comparisons of NEC_vs_EC, EC_vs_ICpEC, EC_vs_GE, ICpEC_vs_GE, respectively. Among them, plant hormones signalling related genes were significantly enriched, especially the auxin and cytokinin signalling components. The transcripts of flavonoid biosynthesis related genes were mainly expressed in NEC, while fatty acid biosynthesis related genes mainly accumulated in early SE. In addition, the extracelluar protein encoding genes LTP, CHI, GLP, AGP, EP1 were related to longan SE. Combined with the FPKM value of longan nine tissues transcription, 27 SE specific or preferential genes (LEC1, LEC1-like, PDF1.3, GH3.6, AGL80, PIN1, BBM, WOX9, WOX2, ABI3, et al.) and 28 NEC preferential genes (LEA5, CNOT3, DC2.15, PR1-1, NsLTP2, DIR1, PIP1, PIP2.1, TIP2-1, POD-P7 and POD5 et al.) were characterized as molecular markers for longan early SE. qRT-PCR validation of SE-related genes showed a high correlation between RNA-seq and qRT-PCR data.

CONCLUSION

This study provides new insights into the role of the transcriptome during early SE in longan. Differentially expressed genes reveal that plant hormones signalling, flavonoid and fatty acid biosynthesis, and extracelluar protein related genes were involved in longan early SE. It could serve as a valuable platform resource for further functional studies addressing embryogenesis in woody plants.

Hormonal and transcriptional analyses of fruit development and ripening in different varieties of black pepper (Piper nigrum)

Black pepper (Piper nigrum L.) is one of the most popular and oldest spices in the world with culinary uses and various pharmacological properties. In order to satisfy the growing worldwide demand for black pepper, improved productivity of pepper is highly desirable. A primary constraint in black pepper production is the non-synchronous nature of flower development and non-uniform fruit ripening within a spike. The uneven ripening of pepper berries results in a high labour requirement for selective harvesting contributes to low productivity and affects the quality of the pepper products. In Malaysia, there are a few recommended varieties for black pepper planting, each having some limitations in addition to the useful characteristics. Therefore, a comparative study of different black pepper varieties will provide a better understanding of the mechanisms regulates fruit development and ripening. Plant hormones are known to influence the fruit development process and their roles in black pepper flower and fruit development were inferred based on the probe-based gene expression analysis and the quantification of the multiple plant hormones using high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS). In this study, jasmonic acid and salicylic acid were found to play roles in flowering and fruit setting, whereas auxin, gibberellin and cytokinins are important for fruit growth. Abscisic acid has positive role in fruit maturation and ripening in the development process. Distinct pattern of plant hormones related gene expression profiles with the hormones accumulation profiles suggested a complex network of regulation is involved in the signaling process and crosstalk between plant hormones was another layer of regulation in the black pepper fruit development mechanisms. The current study provides clues to help in elucidating the timing of the action of each specific plant hormone during fruit development and ripening which could be applied to enhance our ability to control the ripening process, leading to improving procedures for the production and post-harvest handling of pepper fruits.

Analysis of Soybean Somatic Embryogenesis Using Chromosome Segment Substitution Lines and Transcriptome Sequencing

Soybean is an important cash crop that is widely used as a source of vegetable protein and edible oil. The regeneration ability of soybean directly affects the application of biotechnology. In this study, we used the exogenous hormone 2,4-D to treat immature embryos. Different levels of somatic incidence were selected from the chromosome segment substitution lines (CSSLs) constructed by SN14 and ZYD00006. Transcriptome sequencing of extreme materials was performed, and 2666 differentially expressed genes were obtained. At the same time, a difference table was generated by combining the data on CSSL rearrangement. In the extreme materials, a total of 93 differentially expressed genes were predicted and were then analyzed by cluster analysis and Gene Ontology (GO) annotation. After screening and annotating the target genes, three differentially expressed genes with hormone pathways were identified. The expression patterns of the target genes were verified by real-time quantitative PCR (qRT-PCR). Haplotype polymorphism detection and linkage disequilibrium analysis were performed on the candidate gene Glyma.09g248200. This study provided more information on the regulation network of soybean somatic embryogenesis and regeneration processes, and further identified important genes in the soybean regeneration process and provided a theoretical basis for accelerating the application of biotechnology to soybean for improving its breeding efficiency.

Transcriptome analysis of the induction of somatic embryogenesis in Coffea canephora and the participation of ARF and Aux/IAA genes

Background

Somatic embryogenesis (SE) is a useful biotechnological tool to study the morpho-physiological, biochemical and molecular processes during the development of Coffea canephora. Plant growth regulators (PGR) play a key role during cell differentiation in SE. The Auxin-response-factor (ARF) and Auxin/Indole-3-acetic acid (Aux/IAA) are fundamental components involved in the signaling of the IAA. The IAA signaling pathway activates or represses the expression of genes responsive to auxins during the embryogenic transition of the somatic cells. The growing development of new generation sequencing technologies (NGS), as well as bioinformatics tools, has allowed us to broaden the landscape of SE study of various plant species and identify the genes directly involved.

Methods

Analysis of transcriptome expression profiles of the C. canephora genome and the identification of a particular set of differentially expressed genes (DEG) during SE are described in this study.

Results

A total of eight ARF and seven Aux/IAA differentially expressed genes were identified during the different stages of the SE induction process. The quantitative expression analysis showed that ARF18 and ARF5 genes are highly expressed after 21 days of the SE induction, while Aux/IAA7 and Aux/IAA12 genes are repressed.

Discussion

The results of this study allow a better understanding of the genes involved in the auxin signaling pathway as well as their expression profiles during the SE process.

Genome-Wide Analysis of Cotton Auxin Early Response Gene Families and Their Roles in Somatic Embryogenesis

Auxin is well known to regulate growth and development processes. Auxin early response genes serve as a critical component of auxin signaling and mediate auxin regulation of diverse physiological processes. In the present study, a genome-wide identification and comprehensive analysis of auxin early response genes were conducted in upland cotton. A total of 71 auxin response factor (ARF), 86 Auxin/Indole-3-Acetic Acid (Aux/IAA), 63 Gretchen Hagen3 (GH3), and 194 small auxin upregulated RNA (SAUR) genes were identified in upland cotton, respectively. Phylogenetic analysis revealed that the ARF, GH3, and SAUR families were likely subject to extensive evolutionary divergence between Arabidopsis and upland cotton, while the Aux/IAA family was evolutionary conserved. Expression profiles showed that the ARF, Aux/IAA, GH3, and SAUR family genes were extensively involved in embryogenic competence acquisition of upland cotton callus. The Aux/IAA family genes generally showed a higher expression level in the non-embryogenic callus (NEC) of highly embryogenic cultivar CCRI24 than that of recalcitrant cultivar CCRI12, which may be conducive to initializing the embryogenic transformation. Auxin early response genes were tightly co-expressed with most of the known somatic embryogenesis (SE) related genes, indicating that these genes may regulate upland cotton SE by interacting with auxin early response genes.

Unravelling the Metabolic and Hormonal Machinery During Key Steps of Somatic Embryogenesis: A Case Study in Coffee

Somatic embryogenesis (SE) is one of the most promising processes for large-scale dissemination of elite varieties. However, for many plant species, optimizing SE protocols still relies on a trial-and-error approach. Using coffee as a model plant, we report here the first global analysis of metabolome and hormone dynamics aiming to unravel mechanisms regulating cell fate and totipotency. Sampling from leaf explant dedifferentiation until embryo development covered 15 key stages. An in-depth statistical analysis performed on 104 metabolites revealed that massive re-configuration of metabolic pathways induced SE. During initial dedifferentiation, a sharp decrease in phenolic compounds and caffeine levels was also observed while auxins, cytokinins and ethylene levels were at their highest. Totipotency reached its highest expression during the callus stages when a shut-off in hormonal and metabolic pathways related to sugar and energetic substance hydrolysis was evidenced. Abscisic acid, leucine, maltotriose, myo-inositol, proline, tricarboxylic acid cycle metabolites and zeatin appeared as key metabolic markers of the embryogenic capacity. Combining metabolomics with multiphoton microscopy led to the identification of chlorogenic acids as markers of embryo redifferentiation. The present analysis shows that metabolite fingerprints are signatures of cell fate and represent a starting point for optimizing SE protocols in a rational way.

Screening the Reference Genes for Quantitative Gene Expression by RT-qPCR During SE Initial Dedifferentiation in Four Gossypium hirsutum Cultivars that Have Different SE Capability

RNA sequencing (RNA-Seq)-based gene expression analysis is applicable to a wide range of biological purposes in various species. Reverse transcription quantitative PCR (RT-qPCR) is also used to assess target gene expression utilizing stably expressed reference genes as internal control under a given set of conditions. However, investigations of the reference genes for RT-qPCR normalization in the process of somatic embryogenesis (SE) initial dedifferentiation in Gossypium hirsutum are rarely reported. In this study, on the basis of our previous transcriptome data of three different induction stages during SE initial dedifferentiation process in four G. hirsutum cultivars that have different SE capability, 15 candidate genes were selected during SE initial dedifferentiation process, and their expression stability was evaluated by geNorm, NormFinder, and BestKeeper. The results indicated that the two genes of endonuclease 4 (ENDO4) and 18S ribosomal RNA (18S rRNA) showed stable expression in the four different G. hirsutum cultivars, endowing them to be appropriate reference genes during three induction stages in the four cotton cultivars. In addition, the stability and reliability of the two reference genes of ENDO4 and 18S rRNA were further verified by comparing the expressions of auxin-responsive protein 22 (AUX22) and ethylene-responsive transcription factor 17 (ERF17) between RT-qPCR results and the RNA-seq data, which showed strong positive correlation coefficient (R² = 0.8396–0.9984), validating again the steady expression of ENDO4 and 18S rRNA as the reliable reference genes. Our results provide effective reference genes for RT-qPCR normalization during SE process in different G. hirsutum cultivars.

Metabolome and Transcriptome Association Analysis Reveals Dynamic Regulation of Purine Metabolism and Flavonoid Synthesis in Transdifferentiation during Somatic Embryogenesis in Cotton

Plant regeneration via somatic embryogenesis (SE) is a key step during genetic engineering. In the current study, integrated widely targeted metabolomics and RNA sequencing were performed to investigate the dynamic metabolic and transcriptional profiling of cotton SE. Our data revealed that a total of 581 metabolites were present in nonembryogenic staged calli (NEC), primary embryogenic calli (PEC), and initiation staged globular embryos (GE). Of the differentially accumulated metabolites (DAMs), nucleotides, and lipids were specifically accumulated during embryogenic differentiation, whereas flavones and hydroxycinnamoyl derivatives were accumulated during somatic embryo development. Additionally, metabolites related to purine metabolism were significantly enriched in PEC vs. NEC, whereas in GE vs. PEC, DAMs were remarkably associated with flavonoid biosynthesis. An association analysis of the metabolome and transcriptome data indicated that purine metabolism and flavonoid biosynthesis were co-mapped based on the Kyoto encyclopedia of genes and genomes (KEGG) database. Moreover, purine metabolism-related genes associated with signal recognition, transcription, stress, and lipid binding were significantly upregulated. Moreover, several classic somatic embryogenesis (SE) genes were highly correlated with their corresponding metabolites that were involved in purine metabolism and flavonoid biosynthesis. The current study identified a series of potential metabolites and corresponding genes responsible for SE transdifferentiation, which provides a valuable foundation for a deeper understanding of the regulatory mechanisms underlying cell totipotency at the molecular and biochemical levels.

Crosstalk between cytokinin and ethylene signaling pathways regulates leaf abscission in cotton in response to chemical defoliants

Abscission is a process that allows plants to shed tissues or organs via cell separation, and occurs throughout the life cycle. Removal of leaves through the use of chemical defoliants is very important for mechanical harvesting of cotton (Gossypium hirsutum). However, our knowledge of the molecular mechanisms of the defoliation response involved is limited. In this study, RNA-seq was conducted in order to profile the differentially expressed genes (DEGs) between cultivars X50 (sensitive to chemical defoliants) and X33 (relatively insensitive) at different time points after treatment with thidiazuron and ethephon (TE). A total of 2434 DEGs were identified between the two cultivars across the different time-points. Functional categories according to GO and KEGG analyses revealed that plant hormone signal transduction and zeatin biosynthesis were involved in the response to TE. Cytokinin oxidase/dehydrogenase (CKX) genes and ethylene-related genes were up-regulated following TE treatment, and were associated with increased level of ethylene, especially in cultivar X50. Down-regulation of GhCKX3 resulted in delayed defoliation and a reduced ethylene response. The results show that crosstalk between cytokinin and ethylene regulates cotton defoliation, and provide new insights into the molecular mechanisms underlying the mode of action of defoliants in cotton.