Genome-wide identification and expression analysis of the expansin gene family in tomato

New QTLs involved in the control of stigma position in tomato

BACKGROUND

Tomato mating systems were strongly affected by domestication events. Mutations disrupting self-incompatibility paralleled by changes retracting the stigma position (SP) within the staminal cone conferred strict autogamy and self-fertility to the cultivated forms. Although major genes affecting these changes have been identified, SP control in domesticated forms that retain a constitutive or heat-inducible noninserted SP needs elucidation. To widen the possibility of identifying SP genetic determinants, we analyzed the trait in four populations (two germplasm collections, a multiparental recombinant inbred and a biparental progeny) under different environmental conditions (normal and heat stressed).

RESULTS

Overall, 37 markers significantly associated with the trait were identified. Several colocalizations were found, both among regions first reported in this work and among them and previously reported positions. This finding supported the reliability of the analysis. Three such regions, in the long arms of chromosomes 1, 8 and 11, were validated in an independent segregating population, and candidate genes in confidence intervals were identified among transcription factors and hormone-, stress- and cell wall-related genes.

CONCLUSION

Overall, this work supported the hypothesis that the SP phenotype is controlled by different key genes in tomato, paving the way for the identification of novel players and novel mechanisms involved in the regulation of herkogamy.

Plant Cell Wall Loosening by Expansins

Expansins comprise an ancient group of cell wall proteins ubiquitous in land plants and their algal ancestors. During cell growth, they facilitate passive yielding of the wall's cellulose networks to turgor-generated tensile stresses, without evidence of enzymatic activity. Expansins are also implicated in fruit softening and other developmental processes and in adaptive responses to environmental stresses and pathogens. The major expansin families in plants include α-expansins (EXPAs), which act on cellulose-cellulose junctions, and β-expansins, which can act on xylans. EXPAs mediate acid growth, which contributes to wall enlargement by auxin and other growth agents. The genomes of diverse microbes, including many plant pathogens, also encode expansins designated expansin-like X. Expansins are proposed to disrupt noncovalent bonding between laterally aligned polysaccharides (notably cellulose), facilitating wall loosening for a variety of biological roles.

Alpha-expansins: more than three decades of wall creep and loosening in fruits

Expansins are proteins without catalytic activity, but able to break hydrogen bonds between cell wall polysaccharides hemicellulose and cellulose. This proteins were reported for the first time in 1992, describing cell wall extension in cucumber hypocotyls caused particularly by alpha-expansins. Although these proteins have GH45 and CBM63 domains, characteristic of enzymes related with the cleavage of cell wall polysaccharides, demonstrating in vitro that they extend plant cell wall. Its participation has been associated to molecular processes such as development and growing, fruit ripening and softening, tolerance and resistance to biotic and abiotic stress and seed germination. Structural insights, facilitated by bioinformatics approaches, are highlighted, shedding light on the intricate interactions between alpha-expansins and cell wall polysaccharides. After more than thirty years of its discovery, we want to celebrate the knowledge of alpha-expansins and emphasize their importance to understand the phenomena of disassembly and loosening of the cell wall, specifically in the fruit ripening phenomena, with this state-of-the-art dedicated to them.

Transcriptome- and genome-wide systematic identification of expansin gene family and their expression in tuberous root development and stress responses in sweetpotato (Ipomoea batatas)

Introduction

Expansins (EXPs) are essential components of the plant cell wall that function as relaxation factors to directly promote turgor-driven expansion of the cell wall, thereby controlling plant growth and development and diverse environmental stress responses. EXPs genes have been identified and characterized in numerous plant species, but not in sweetpotato.

Results and methods

In the present study, a total of 59 EXP genes unevenly distributed across 14 of 15 chromosomes were identified in the sweetpotato genome, and segmental and tandem duplications were found to make a dominant contribution to the diversity of functions of the IbEXP family. Phylogenetic analysis showed that IbEXP members could be clustered into four subfamilies based on the EXPs from Arabidopsis and rice, and the regularity of protein motif, domain, and gene structures was consistent with this subfamily classification. Collinearity analysis between IbEXP genes and related homologous sequences in nine plants provided further phylogenetic insights into the EXP gene family. Cis-element analysis further revealed the potential roles of IbEXP genes in sweetpotato development and stress responses. RNA-seq and qRT-PCR analysis of eight selected IbEXPs genes provided evidence of their specificity in different tissues and showed that their transcripts were variously induced or suppressed under different hormone treatments (abscisic acid, salicylic acid, jasmonic acid, and 1-aminocyclopropane-1-carboxylic acid) and abiotic stresses (low and high temperature).

Discussion

These results provide a foundation for further comprehensive investigation of the functions of IbEXP genes and indicate that several members of this family have potential applications as regulators to control plant development and enhance stress resistance in plants.

Identification and expression analysis of expansin gene family in Salvia miltiorrhiza

BACKGROUND

Expansins (EXP) are important enzymes that are involved in the extension of plant cells and regulation of root configurations, which play important roles in resisting various stresses. As a model medicinal plant, Salvia miltiorrhiza is well recognized for treating coronary heart disease, myocardial infection, and other cardiovascular and cerebrovascular diseases; however, the SmEXP gene family has not yet been analyzed.

METHODS

The SmEXP family was systematically analyzed using bioinformatics. Quantitative real-time PCR was employed to analyze the tissue expression patterns of the SmEXP family, as well as its expression under abscisic acid (ABA) treatment and abiotic stress. Subcellular localization assay revealed the localization of SmEXLA1, SmEXLB1, and SmEXPA2.

RESULTS

This study identified 29 SmEXP that belonged to four different subfamilies. SmEXP promoter analysis suggested that it may be involved in the growth, development, and stress adaptation of S. miltiorrhiza. An analysis of the expression patterns of SmEXP revealed that ABA, Cu2+, and NaCl had regulatory effects on its expression. A subcellular localization assay showed that SmEXLA1 and SmEXLB1 were located on the nucleus and cell membrane, while SmEXPA2 was located on the cell wall.

CONCLUSION

For this study, the SmEXP family was systematically analyzed for the first time, which lays a foundation for further elucidating its physiological and biological functionality.

Auxin homeostasis is maintained by sly-miR167-SlARF8A/B-SlGH3.4 feedback module in the development of locular and placental tissues of tomato fruits

The locular gel, produced by the placenta, is important for fruit flavor and seed development in tomato. However, the mechanism underlying locule and placenta development is not fully understood yet. Here, we show that two SlARF transcription factors, SlARF8B and SlARF8A (SlARF8A/B), promote the development of locular and placenta tissues. The expression of both SlARF8A and SlARF8B is repressed by sly-microRNA167 (sly-miR167), allowing for the activation of auxin downstream genes. In slarf8a, slarf8b, and slarf8a/b mutants, the auxin (IAA) levels are decreased, whereas the levels of inactive IAA conjugates including IAA-Ala, IAA-Asp, and IAA-Glu are increased. We further find that SlARF8B directly inhibits the expression of SlGH3.4, an acyl acid amino synthetase that conjugates the amino acids to IAA. Disruption of such auxin balance by the increased expression of SlGH3.4 or SlGH3.2 results in defective locular and placental tissues. Taken together, our findings reveal an important regulatory module constituted by sly-miR167-SlARF8A/B-SlGH3.4 during the development of locular and placenta tissues of tomato fruits.

Genome-wide identification of the expansin gene family in netted melon and their transcriptional responses to fruit peel cracking

Introduction

Fruit cracking not only affects the appearance of netted melons (Cucumis melo L. var. reticulatus Naud.) but also decreases their marketability.

Methods

Herein, to comprehensively understand the role of expansin (EXP) proteins in netted melon, bioinformatics methods were employed to discover the EXP gene family in the melon genome and analyze its characteristic features. Furthermore, transcriptomics analysis was performed to determine the expression patterns of melon EXP (CmEXP) genes in crack-tolerant and crack-susceptible netted melon varieties.

Discussion

Thirty-three CmEXP genes were identified. Chromosomal location analysis revealed that CmEXP gene distribution was uneven on 12 chromosomes. In addition, phylogenetic tree analysis revealed that CmEXP genes could be categorized into four subgroups, among which the EXPA subgroup had the most members. The same subgroup members shared similar protein motifs and gene structures. Thirteen duplicate events were identified in the 33 CmEXP genes. Collinearity analysis revealed that the CmEXP genes had 50, 50, and 44 orthologous genes with EXP genes in cucumber, watermelon, and Arabidopsis, respectively. However, only nine orthologous EXP genes were observed in rice. Promoter cis-acting element analysis demonstrated that numerous cis-acting elements in the upstream promoter region of CmEXP genes participate in plant growth, development, and environmental stress responses. Transcriptomics analysis revealed 14 differentially expressed genes (DEGs) in the non-cracked fruit peels between the crack-tolerant variety 'Xizhoumi 17' (N17) and the crack-susceptible variety 'Xizhoumi 25' (N25). Among the 14 genes, 11 were upregulated, whereas the remaining three were downregulated in N17. In the non-cracked (N25) and cracked (C25) fruit peels of 'Xizhoumi 25', 24 DEGs were identified, and 4 of them were upregulated, whereas the remaining 20 were downregulated in N25. In the two datasets, only CmEXPB1 exhibited consistently upregulated expression, indicating its importance in the fruit peel crack resistance of netted melon. Transcription factor prediction revealed 56 potential transcription factors that regulate CmEXPB1 expression.

Results

Our study findings enrich the understanding of the CmEXP gene family and present candidate genes for the molecular breeding of fruit peel crack resistance of netted melon.

Functional characterization of MaEXPA11 and its roles in response to biotic and abiotic stresses in mulberry

Mulberry is a traditional economic tree with various values in sericulture, ecology, food industry and medicine. Expansins (EXPs) are known as cell wall expansion related proteins and have been characterized to involve in plant development and responses to diverse stresses. In present study, twenty EXP and expansin-like (EXL) genes were identified in mulberry. RNA-seq results indicated that three EXP and EXL genes showed up-regulated expression level under sclerotiniose pathogen infection in three independent RNA-seq datasets. The most significant upregulated EXPA11 was selected as key EXP involving in response to sclerotiniose pathogen infection in mulberry. Furthermore, a comprehensive functional analysis was performed to reveal subcellular location, tissue expression profile of MaEXPA11 in mulberry. Down-regulation of MaEXPA11 using virus induced gene silence (VIGS) was performed to explore the function of MaEXPA11 in Morus alba. Results showed that MaEXPA11 can positively regulate mulberry resistance to Ciboria shiraiana infection and negatively regulate mulberry resistance to cold or drought stress.

Identification of genes associated with abiotic stress tolerance in sweetpotato using weighted gene co-expression network analysis

Sweetpotato, Ipomoea batatas (L.), a key food security crop, is negatively impacted by heat, drought, and salinity stress. The orange-fleshed sweetpotato cultivar "Beauregard" was exposed to heat, salt, and drought treatments for 24 and 48 h to identify genes responding to each stress condition in leaves. Analysis revealed both common (35 up regulated, 259 down regulated genes in the three stress conditions) and unique sets of up regulated (1337 genes by drought, 516 genes by heat, and 97 genes by salt stress) and down regulated (2445 genes by drought, 678 genes by heat, and 204 genes by salt stress) differentially expressed genes (DEGs) suggesting common, yet stress-specific transcriptional responses to these three abiotic stressors. Gene Ontology analysis of down regulated DEGs common to both heat and salt stress revealed enrichment of terms associated with "cell population proliferation" suggestive of an impact on the cell cycle by the two stress conditions. To identify shared and unique gene co-expression networks under multiple abiotic stress conditions, weighted gene co-expression network analysis was performed using gene expression profiles from heat, salt, and drought stress treated 'Beauregard' leaves yielding 18 co-expression modules. One module was enriched for "response to water deprivation," "response to abscisic acid," and "nitrate transport" indicating synergetic crosstalk between nitrogen, water, and phytohormones with genes encoding osmotin, cell expansion, and cell wall modification proteins present as key hub genes in this drought-associated module. This research lays the groundwork for exploring to a further degree, mechanisms for abiotic stress tolerance in sweetpotato.

Investigation of the expansin gene family in sugar beet (Beta vulgaris) by the genome-wide level and their expression responses under abiotic stresses

Sugar beet (Beta vulgaris ssp. vulgaris) is primarily used in sugar production worldwide. Expansins are a gene family of cell wall proteins effective in regulating cell wall structure. They also participate in developmental stages, including cell and leaf growth, root development, and fruit ripening. This study comprehensively characterizes the expansin gene family members found in the sugar beet genome. In addition, in silico expression analysis of sugar beet expansin genes under variable abiotic stress conditions and expression profiles of expansin genes under combined drought and heat stresses by the qRT-PCR method were evaluated in the study. A total of 31 sugar beet expansin genes were identified. BvuEXLA-02 and BvuEXLB-02 genes can have abiotic stress tolerance roles besides their roles in normal development. Determining the properties of sugar beet expansin, family members can help enable the cellulose hydrolysis mechanism and raise plant biomass. Elucidating expression profiles of the sugar beet expansin genes under variable stress conditions can support improving plant productivity. The results of the current study may also contribute to the deep understanding of sugar beet expansin genes in the future.

Genome-Wide Analysis of the Expansin Gene Family in Populus and Characterization of Expression Changes in Response to Phytohormone (Abscisic Acid) and Abiotic (Low-Temperature) Stresses

Expansins are a group of cell wall enzyme proteins that help to loosen cell walls by breaking hydrogen bonds between cellulose microfibrils and hemicellulose. Expansins are essential plant proteins that are involved in several key processes, including seed germination, the growth of pollen tubes and root hairs, fruit ripening and abscission processes. Currently, there is a lack of knowledge concerning the role of expansins in woody plants. In this study, we analyzed expansin genes using Populus genome as the study target. Thirty-six members of the expansin gene family were identified in Populus that were divided into four subfamilies (EXPA, EXPB, EXLA and EXLB). We analyzed the molecular structure, chromosome localization, evolutionary relationships and tissue specificity of these genes and investigated expression changes in responses to phytohormone and abiotic stresses of the expansin genes of Populus tremula L. (PtEXs). Molecular structure analysis revealed that each PtEX protein had several conserved motifs and all of the PtEXs genes had multiple exons. Chromosome structure analysis showed that the expansin gene family is distributed on 14 chromosomes. The PtEXs gene family expansion patterns showed segmental duplication. Transcriptome data of Populus revealed that 36 PtEXs genes were differently expressed in different tissues. Cis-element analysis showed that the PtEXs were closely associated with plant development and responses to phytohormone and abiotic stress. Quantitative real-time PCR showed that abscisic acid (ABA) and low-temperature treatment affected the expression of some PtEXs genes, suggesting that these genes are involved in responses to phytohormone and abiotic stress. This study provides a further understanding of the expansin gene family in Populus and forms a basis for future functional research studies.

Genome-Wide Identification of Expansin Gene Family and Their Response under Hormone Exposure in Ginkgo biloba L

Expansins are pH-dependent enzymatic proteins that irreversibly and continuously facilitate cell-wall loosening and extension. The identification and comprehensive analysis of Ginkgo biloba expansins (GbEXPs) are still lacking. Here, we identified and investigated 46 GbEXPs in Ginkgo biloba. All GbEXPs were grouped into four subgroups based on phylogeny. GbEXPA31 was cloned and subjected to a subcellular localization assay to verify our identification. The conserved motifs, gene organization, cis-elements, and Gene Ontology (GO) annotation were predicted to better understand the functional characteristics of GbEXPs. The collinearity test indicated segmental duplication dominated the expansion of the GbEXPA subgroup, and seven paralogous pairs underwent strong positive selection during expansion. A majority of GbEXPAs were mainly expressed in developing Ginkgo kernels or fruits in transcriptome and real-time quantitative PCR (qRT-PCR). Furthermore, GbEXLA4, GbEXLA5, GbEXPA5, GbEXPA6, GbEXPA8, and GbEXPA24 were inhibited under the exposure of abiotic stresses (UV-B and drought) and plant hormones (ABA, SA, and BR). In general, this study expanded our understanding for expansins in Ginkgo tissues' growth and development and provided a new basis for studying GbEXPs in response to exogenous phytohormones.

Comparative bioinformatics analysis and abiotic stress responses of expansin proteins in Cucurbitaceae members: watermelon and melon

Watermelon and melon are members of the Cucurbitaceae family including economically significant crops in the world. The expansin protein family, which is one of the members of the cell wall, breaks down the non-covalent bonds between cell wall polysaccharides, causing pressure-dependent cell expansion. Comparative bioinformatics and molecular characterization analysis of the expansin protein family were carried out in the watermelon (Citrullus lanatus) and melon (Cucumis melo) plants in the study. Gene expression levels of expansin family members were analyzed in leaf and root tissues of watermelon and melon under ABA, drought, heat, cold, and salt stress conditions by quantitative real-time PCR analysis. After comprehensive searches, 40 expansin proteins (22 ClaEXPA, 14 ClaEXPLA, and 4 ClaEXPB) in watermelon and 43 expansin proteins (19 CmEXPA, 15 CmEXPLA, 3 CmEXPB, and 6 CmEXPLB) in melon were identified. The greatest orthologous genes were identified with soybean expansin genes for watermelon and melon. However, the latest divergence time between orthologous genes was determined with poplar expansin genes for watermelon and melon expansin genes. ClaEXPA-04, ClaEXPA-09, ClaEXPB-01, ClaEXPB-03, and ClaEXPLA-13 genes in watermelon and CmEXPA-12, CmEXPA-10, and CmEXPLA-01 genes in melon can be involved in tissue development and abiotic stress response of the plant. The current study combining bioinformatics and experimental analysis can provide a detailed characterization of the expansin superfamily which has roles in growth and reaction to the stress of the plant. The study ensures detailed data for future studies examining gene functions including the roles in plant growth and stress conditions.

Genome-wide studies of PAL genes in sorghum and their responses to aphid infestation

Phenylalanine ammonia-lyase (PAL, EC 4.3.1.25) plays a crucial role in plant adaptation to biotic and abiotic stresses. However, the current knowledge about PAL proteins in sorghum is essentially lacking. Thus, in this study we aimed to analyze the PAL family genes in sorghum using a genome-wide approach and to explore the role of PAL genes in host plant resistance to aphids via SA-mediated defense signaling. Here, we report gene structural features of 8 PAL (SbPAL) genes in sorghum (Sorghum bicolor), their phylogeny, protein motifs and promoter analysis. Furthermore, we demonstrated that the SbPAL genes were induced by sugarcane aphid (SCA) infestation and SbPAL exhibited differential gene expression in susceptible and resistant genotypes. PAL activity assays further validated upregulated expression of the SbPAL genes in a resistant genotype. In addition, exogenous application of SA reduced plant damage and suppressed aphid population growth and fecundity in susceptible genotype, suggesting that those SbPAL genes act as positive regulator of the SA-mediated defense signaling pathway to combat aphid pests in sorghum. This study provides insights for further examination of the defense role of PAL in sorghum against other pests and pathogens.

Molecular and Genetic Events Determining the Softening of Fleshy Fruits: A Comprehensive Review

Fruit softening that occurs during fruit ripening and postharvest storage determines the fruit quality, shelf life and commercial value and makes fruits more attractive for seed dispersal. In addition, over-softening results in fruit eventual decay, render fruit susceptible to invasion by opportunistic pathogens. Many studies have been conducted to reveal how fruit softens and how to control softening. However, softening is a complex and delicate life process, including physiological, biochemical and metabolic changes, which are closely related to each other and are affected by environmental conditions such as temperature, humidity and light. In this review, the current knowledge regarding fruit softening mechanisms is summarized from cell wall metabolism (cell wall structure changes and cell-wall-degrading enzymes), plant hormones (ETH, ABA, IAA and BR et al.), transcription factors (MADS-Box, AP2/ERF, NAC, MYB and BZR) and epigenetics (DNA methylation, histone demethylation and histone acetylation) and a diagram of the regulatory relationship between these factors is provided. It will provide reference for the cultivation of anti-softening fruits.

Overexpression of AcEXPA23 Promotes Lateral Root Development in Kiwifruit

Kiwifruit is loved by consumers for its unique taste and rich vitamin C content. Kiwifruit are very sensitive to adverse soil environments owing to fleshy and shallow roots, which limits the uptake of water and nutrients into the root system, resulting in low yield and poor fruit quality. Lateral roots are the key organs for plants to absorb water and nutrients. Improving water and fertilizer use efficiency by promoting lateral root development is a feasible method to improve yield and quality. Expansin proteins plays a major role in lateral root growth; hence, it is important to identify expansin protein family members, screen key genes, and explore gene function in root development. In this study, 41 expansin genes were identified based on the genome of kiwifruit (‘Hongyang’, Actinidia chinensis). By clustering with the Arabidopsis thaliana expansin protein family, the 41 AcExpansin proteins were divided into four subfamilies. The AcExpansin protein family was further analysed by bioinformatics methods and was shown to be evolutionarily diverse and conserved at the DNA and protein levels. Based on previous transcriptome data and quantitative real-time PCR assays, we screened the candidate gene AcEXPA23. Overexpression of AcEXPA23 in kiwifruit increased the number of kiwifruit lateral roots.

A YABBY gene CRABS CLAW a (CRCa) negatively regulates flower and fruit sizes in tomato

CRABS CLAW (CRC) is a YABBY transcription factor that plays a pivotal role in carpel development and flower meristem determinacy. Here, we characterized a CRC homolog SlCRCa and elucidated its specific roles in tomato (Solanum lycopersicum). SlCRCa is highly expressed in the petals and stamens, and is responsive to gibberellin (GA) treatment. Overexpression of SlCRCa in tomato reduces the sizes of petals, stamens, and fruits, while the inverse phenotypes are induced by knockdown of SlCRCa. Furthermore, histological investigation suggests that the smaller or larger fruits in SlCRCa-overexpressing or SlCRCa-RNAi plants are mainly determined by the decreases or increases in cell layers and cell sizes in pericarp, respectively. Through transcriptome and qRT-PCR analyses, we speculate that SlCRCa inhibits cell division by regulating the transcription of cell division-related genes, and also suppresses cell expansion by modulating the expansin genes and GA pathway in tomato fruits. Besides, SlCRCa is involved in the feedback regulation of GA biosynthesis. Our findings reveal that SlCRCa negatively regulates fruit size by affecting cell division and cell expansion, and it is also an inhibitor of floral organ sizes in tomato.

Genome-Wide Identification of Expansin Genes in Wild Soybean (Glycine soja) and Functional Characterization of Expansin B1 (GsEXPB1) in Soybean Hair Root

Wild soybean, the progenitor and close relative of cultivated soybean, has an excellent environmental adaptation ability and abundant resistance genes. Expansins, as a class of cell wall relaxation proteins, have important functions in regulating plant growth and stress resistance. In the present study, we identified a total of 75 members of the expansin family on the basis of recent genomic data published for wild soybean. The predicted results of promoter elements structure showed that wild soybean expansin may be associated with plant hormones, stress responses, and growth. Basal transcriptome data of vegetative organs suggest that the transcription of expansin members has some organ specificity. Meanwhile, the transcripts of some members had strong responses to salt, low temperature and drought stress. We screened and obtained an expansin gene, GsEXPB1, which is transcribed specifically in roots and actively responds to salt stress. The results of A. tumefaciens transient transfection showed that this protein was localized in the cell wall of onion epidermal cells. We initially analyzed the function of GsEXPB1 by a soybean hairy root transformation assay and found that overexpression of GsEXPB1 significantly increased the number of hairy roots, root length, root weight, and the tolerance to salt stress. This research provides a foundation for subsequent studies of expansins in wild soybean.

Comparative transcriptome analysis elucidates positive physiological effects of foliar application of pyraclostrobin on tomato (Solanum lycopersicum L.)

Strobilurins, including pyraclostrobin have frequently been reported showing positive physiological effects in various agricultural crops apart from fungicidal activity. Present study elucidates comparative transcriptome analysis of control and pyraclostrobin treated tomato leaf and identifies metabolic pathways and key genes responsible for positive effects of pyraclostrobin on tomato. Pair-end raw reads, generated by Illumina Hi-seq platform were pre-processed and good quality reads were mapped onto tomato reference genome using HISAT2 alignment programme. Transcript assembly and quantification were performed using StringTie assembler. Differential Gene Expression analysis by DESeq2 identified 1,952 upregulated genes including genes encoding pathogenesis related proteins and 835 downregulated genes. RT-PCR study showed increase in expression of RBCs (2.5-fold), GA20o (3-fold), and NR (1.4-fold) genes, which are the key genes of photosynthesis, gibberellic acid synthesis, and nitrogen assimilation pathways respectively identified in KEGG pathway analysis. Pyraclostrobin treated plants showed 1.6-folds increase in plant height, 3.3-folds increase in number of leaves, and 2.8-folds increase in number of flowers. Total protein content increased 1.7, 1.4, 1.2, 1.2, and 1.4 folds at 1 day after application (DAA), 4DAA, 7DAA, 10DAA, and 13DAA respectively in treated plants. Moreover, content of phenol also increased 1.14, 1.5, 2.4, and 1.5 folds in 4DAA, 7DAA, 10DAA, and 13DAA respectively. Nitrate reductase activity increased 2-fold, 1.8-fold, 1.5-fold and 1.15-fold in 1DAA, 7DAA, 10DAA and 13DAA respectively. Carbohydrate decreased in treated plants up to 7DAA. The present study is the first report of transcriptome analysis elucidating positive physiological effects of strobilurin on plant.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01191-7.

Genome-wide identification, characterization of expansin gene family of banana and their expression pattern under various stresses

Expansin, a cell wall-modifying gene family, has been well characterized and its role in biotic and abiotic stress resistance has been proven in many monocots, but not yet studied in banana, a unique model crop. Banana is one of the staple food crops in developing countries and its production is highly influenced by various biotic and abiotic factors. Characterizing the expansin genes of the ancestor genome (M. acuminata and M. balbisiana) of present day cultivated banana will enlighten their role in growth and development, and stress responses. In the present study, 58 (MaEXPs) and 55 (MbaEXPs) putative expansin genes were identified in A and B genome, respectively, and were grouped in four subfamilies based on phylogenetic analysis. Gene structure and its duplications revealed that EXPA genes are highly conserved and are under negative selection whereas the presence of more number of introns in other subfamilies revealed that they are diversifying. Expression profiling of expansin genes showed a distinct expression pattern for biotic and abiotic stress conditions. This study revealed that among the expansin subfamilies, EXPAs contributed significantly towards stress-resistant mechanism. The differential expression of MaEXPA18 and MaEXPA26 under drought stress conditions in the contrasting cultivar suggested their role in drought-tolerant mechanism. Most of the MaEXPA genes are differentially expressed in the root lesion nematode contrasting cultivars which speculated that this expansin subfamily might be the susceptible factor. The downregulation of MaEXPLA6 in resistant cultivar during Sigatoka leaf spot infection suggested that by suppressing this gene, resistance may be enhanced in susceptible cultivar. Further, in-depth studies of these genes will lead to gain insight into their role in various stress conditions in banana.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-021-03106-x.

Drought tolerance improvement in Solanum lycopersicum: an insight into "OMICS" approaches and genome editing

Solanum lycopersicum (tomato) is an internationally acclaimed vegetable crop that is grown worldwide. However, drought stress is one of the most critical challenges for tomato production, and it is a crucial task for agricultural biotechnology to produce drought-resistant cultivars. Although breeders have done a lot of work on the tomato to boost quality and quantity of production and enhance resistance to biotic and abiotic stresses, conventional tomato breeding approaches have been limited to improving drought tolerance because of the intricacy of drought traits. Many efforts have been made to better understand the mechanisms involved in adaptation and tolerance to drought stress in tomatoes throughout the years. "Omics" techniques, such as genomics, transcriptomics, proteomics, and metabolomics in combination with modern sequencing technologies, have tremendously aided the discovery of drought-responsive genes. In addition, the availability of biotechnological tools, such as plant transformation and the recently developed genome editing system for tomatoes, has opened up wider opportunities for validating the function of drought-responsive genes and the generation of drought-tolerant varieties. This review highlighted the recent progresses for tomatoes improvement against drought stress through "omics" and "multi-omics" technologies including genetic engineering. We have also discussed the roles of non-coding RNAs and genome editing techniques for drought stress tolerance improvement in tomatoes.

Auxin-mediated regulation of arbuscular mycorrhizal symbiosis: A role of SlGH3.4 in tomato

Most land plants can establish symbiosis with arbuscular mycorrhizal (AM) fungi to increase fitness to environmental challenges. The development of AM symbiosis is controlled by intricate procedures involving all phytohormones. However, the mechanisms underlying the auxin-mediated regulation of AM symbiosis remains largely unknown. Here, we report that AM colonisation promotes auxin response and indole-3-acetic acid (IAA) accumulation, but downregulates IAA biosynthesis genes in tomato (Solanum lycopersicum). External IAA application modulates the AM symbiosis by promoting arbuscule formation at low concentrations but repressing it at high concentrations. An AM-induced GH3 gene, SlGH3.4, encoding a putative IAA-amido synthetase, negatively regulates mycorrhization via maintaining cellular auxin homoeostasis. Loss of SlGH3.4 function increased free IAA content and arbuscule incidence, while constitutively overexpressing SlGH3.4 in either tomato or rice resulted in decreased IAA content, total colonisation level and arbuscule abundance in mycorrhizal roots. Several auxin-inducible expansin genes involved in AM formation or resistance to pathogen infection were upregulated in slgh3.4 mycorrhizal roots but downregulated in the SlGH3.4-overexpressing plants. Taken together, our results highlight a positive correlation between the endogenous IAA content and mycorrhization level, particularly arbuscule incidence, and suggest that the SlGH3.4-mediated auxin homoeostasis and regulation of expansin genes is involved in finely tuning the AM development.

Comparative genomic analysis of expansin superfamily gene members in zucchini and cucumber and their expression profiles under different abiotic stresses

Zucchini and cucumber belong to the Cucurbitaceae family, a group of economical and nutritious food plants that is consumed worldwide. Expansin superfamily proteins are generally localized in the cell wall of plants and are known to possess an effect on cell wall modification by causing the expansion of this region. Although the whole genome sequences of cucumber and zucchini plants have been resolved, the determination and characterization of expansin superfamily members in these plants using whole genomic data have not been implemented yet. In the current study, a genome-wide analysis of zucchini (Cucurbita pepo) and cucumber (Cucumis sativus) genomes was performed to determine the expansin superfamily genes. In total, 49 and 41 expansin genes were identified in zucchini and cucumber genomes, respectively. All expansin superfamily members were subjected to further bioinformatics analysis including gene and protein structure, ontology of the proteins, phylogenetic relations and conserved motifs, orthologous relations with other plants, targeting miRNAs of those genes and in silico gene expression profiles. In addition, various abiotic stress responses of zucchini and cucumber expansin genes were examined to determine their roles in stress tolerance. CsEXPB-04 and CsEXPA-11 from cucumber and CpEXPA-20 and CpEXPLA-14 from zucchini can be candidate genes for abiotic stress response and tolerance in addition to their roles in the normal developmental processes, which are supported by the gene expression analysis. This work can provide new perspectives for the roles of expansin superfamily genes and offers comprehensive knowledge for future studies investigating the modes of action of expansin proteins.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01108-w.

The evolution of the expansin gene family in Brassica species

Expansin gene (EXP) family plays important roles in plant growth and crop improvement. However, it has not been well studied in the Brassica genus that includes several important agricultural and horticultural crops. To get insight to the evolution and expansion of EXP family in Brassica, Brassica EXPs which are homologues of 35 known AtEXPs of Arabidopsis were comprehensively and systematically analyzed in the present study. In total, 340 Brassica EXPs were clustered into four groups that corresponded multiple alignment to four subfamilies of AtEXPs, with divergent conserved motifs and cis-acting elements among groups. To understand the expansion of EXP family, an integrated genomic block system was constructed among Arabidopsis and Brassica species based on 24 known ancestral karyotype blocks. Obvious gene loss, segmental duplication, tandem duplication and DNA sequence repeat events were found during the expansion of Brassica EXPs, of which the segmental duplication was possibly the major driving force. The divergence time was estimated in 1109 orthologs pairs of EXPs, revealing the divergence of Brassica EXPs from AtEXPs during ~30 MYA, and the divergence of EXPs among Brassica species during 13.50-17.94 MYA. Selective mode analysis revealed that the purifying selection was the major contributor to expansion of Brassica EXPs. This study provides new insights into the evolution and expansion of the EXP family in Brassica genus.

Identification of blossom-end rot loci using joint QTL-seq and linkage-based QTL mapping in tomato

Blossom-End Rot is Quantitatively Inherited and Maps to Four Loci in Tomato. Blossom-end rot (BER) is a devastating physiological disorder that affects tomato and other vegetables, resulting in significant crop losses. To date, most studies on BER have focused on the environmental factors that affect calcium translocation to the fruit; however, the genetic basis of this disorder remains unknown. To investigate the genetic basis of BER, two F₂ and F_3:4 populations along with a BC₁ population that segregated for BER occurrence were evaluated in the greenhouse. Using the QTL-seq approach, quantitative trait loci (QTL) associated with BER Incidence were identified at the bottom of chromosome (ch) 3 and ch11. Additionally, linkage-based QTL mapping detected another QTL, BER3.1, on ch3 and BER4.1 on ch4. To fine map the QTLs identified by QTL-seq, recombinant screening was performed. BER3.2, the major BER QTL on ch3, was narrowed down from 5.68 to 1.58 Mbp with a 1.5-LOD support interval (SI) corresponding to 209 candidate genes. BER3.2 colocalizes with the fruit weight gene FW3.2/SlKLUH, an ortholog of cytochrome P450 KLUH in Arabidopsis. Further, BER11.1, the major BER QTL on ch11, was narrowed down from 3.99 to 1.13 Mbp with a 1.5-LOD SI interval comprising of 141 candidate genes. Taken together, our results identified and fine mapped the first loci for BER resistance in tomato that will facilitate marker-assistant breeding not only in tomato but also in many other vegetables suffering for BER.

The Arabidopsis expansin gene (AtEXPA18) is capable to ameliorate drought stress tolerance in transgenic tobacco plants

BACKGROUND

Expansins are cell wall proteins loosening plant cell in pH-dependent manner. This study aimed to investigate the role of AtEXPA18 in different morphological, physiological, and cellular responses of transgenic tobacco plants to moderate and severe drought stress.

METHODS AND RESULTS

Previously synthesized AtEXPA18 gene construct was successfully transferred to the tobacco plants through an agrobacterium-mediate transformation system. Upon obtaining the second generation, tobacco transgenic plants were confirmed by conventional polymerase chain reaction (PCR) technique alongside reverse transcription PCR (RT-PCR) using specific primers. Under drought stress, the transgenic lines showed remarkable growth and significantly improved based on morphological traits such as height and stem diameter, leaf area, leaf number, root dry weight, and Abscisic acid levels of leaves compared control plants. As a result, the Cytokinin content of transgenic plants has increased under severe stress levels. Notably, the area's expansion for abaxial epidermal cells under the microscope confirmed in transgene cells compared with the -transgene cells.

CONCLUSION

These results, altogether, could support the AtEXPA18 gene implication in cell expansion and improving tolerance capacity of transgenic crops under drought stress.

Evolutionary research on the expansin protein family during the plant transition to land provides new insights into the development of Tartary buckwheat fruit

BACKGROUND

Plant transitions to land require robust cell walls for regulatory adaptations and to resist changing environments. Cell walls provide essential plasticity for plant cell division and defense, which are often conferred by the expansin superfamily with cell wall-loosening functions. However, the evolutionary mechanisms of expansin during plant terrestrialization are unclear.

RESULTS

Here, we identified 323 expansin proteins in 12 genomes from algae to angiosperms. Phylogenetic evolutionary, structural, motif gain and loss and Ka/Ks analyses indicated that highly conserved expansin proteins were already present in algae and expanded and purified after plant terrestrialization. We found that the expansion of the FtEXPA subfamily was caused by duplication events and that the functions of certain duplicated genes may have differentiated. More importantly, we generated space-time expression profiles and finally identified five differentially expressed FtEXPs in both large and small fruit Tartary buckwheat that may regulate fruit size by responding to indoleacetic acid.

CONCLUSIONS

A total of 323 expansin proteins from 12 representative plants were identified in our study during terrestrialization, and the expansin family that originated from algae expanded rapidly after the plants landed. The EXPA subfamily has more members and conservative evolution in angiosperms. FtEXPA1, FtEXPA11, FtEXPA12, FtEXPA19 and FtEXPA24 can respond to indole-3-acetic acid (IAA) signals and regulate fruit development. Our study provides a blueprint for improving the agronomic traits of Tartary buckwheat and a reference for defining the evolutionary history of the expansin family during plant transitions to land.

Identification, genomic organization, and expression profiles of single C2H2 zinc finger transcription factors in tomato (Solanum lycopersicum)

C2H2 zinc finger proteins (ZFPs) play essential roles in leaf morphogenesis and floral development, as well as heat stress response and trichome formation, which activate or inhibit gene transcription mainly through interactions with nucleic acids, such as single-strand DNA, RNA binding or RNA/DNA bidirectional binding, and protein interaction. Single C2H2 ZFPs is the subfamily of ZFPs, but little of single C2H2 ZFP family is known in tomato. In this study, we identified 30 single ZFP genes in tomato using bioinformatics-based methods. Gene structures, phylogeny, conserved motifs, cis-element of promoter, chromosomal localization, gene duplication, and expression patterns of these single C2H2 ZFP genes were analyzed. Sequence analysis showed that most single C2H2 ZFP genes possessed only one exon, except for SlC1-liZFP1 and SlC1-liZFP2. These single C2H2 ZFP genes were asymmetrically distributed on 10 chromosomes, excluding 2 and 12 chromosomes. In addition, 24 of these genes were predicated to have experienced segmental duplication. Cis-element prediction indicated that many important elements were located in the putative promoter regions, like light and gibberellic acid (GA)-responsive elements. The expression profiles of these genes in different tissues and various hormones and stress treatment were further analyzed. Many genes were lowly expressed in all tissues, whereas some were specifically expressed in certain tissues, like SlC1-liZFP2 in young leaves, and SlC1-liZFP15 in fruits. Furthermore, these genes could also be induced by several hormones and stresses, including IAA, ETH, GA, cold, and drought. This study sets a good foundation for further characterizing the biological roles of single C2H2 ZFP genes in tomato.

Genome-wide identification, characterization, and expression analysis of the expansin gene family in watermelon (Citrullus lanatus)

Expansins are plant cell-wall loosening proteins involved in cell enlargement, adaptive responses to environmental stimuli, and various developmental processes. Although expansins have been characterized in many plant species, little is reported on this family in watermelon. In this study, 30 expansin genes in the watermelon genome (ClEXPs) were identified. These genes which were divided into four subfamilies (7 ClEXLAs, 2 ClEXLBs, 18 ClEXPAs, and 3 ClEXPBs) are unevenly distribute on 10 of 11 watermelon chromosomes. Chromosome mapping suggested that tandem duplication events may have played important roles in the expanding of watermelon expansins. Gene structure and motif identification revealed that same subfamily and subgroup have conserved gene structure and motif. Detection of cis-acting elements revealed that ClEXPs gene promoter regions were enriched with light-responsive elements, hormone-responsive, environmental stress-related, and development-related elements. Expression patterns of ClEXPs were investigated by qRT-PCR. The results showed that expression patterns of 15 ClEXP genes differed in three tissues. Through our own and public RNA-seq analysis, we found that ClEXPs had different expression patterns in fruit flesh, fruit rind, and seed at various developmental stages, and most of ClEXPs were highly responsive to abiotic and biotic stresses. Remarkably, 7 ClEXPs (ClEXLA1, ClEXLA6, ClEXLB1, ClEXLB2, ClEXPA5, ClEXPA10, and ClEXPA16) exhibited positive response to at least three kinds of stresses, suggesting that they might play important roles in the crosstalk of stress signal pathways. The results of this study provide useful insights for the functional identification of expansin gene family in watermelon.

Temporal expression patterns of fruit-specific α- EXPANSINS during cell expansion in bell pepper (Capsicum annuum L.)

BACKGROUND

Expansins (EXPs) facilitate non-enzymatic cell wall loosening during several phases of plant growth and development including fruit growth, internode expansion, pollen tube growth, leaf and root development, and during abiotic stress responses. In this study, the spatial and temporal expression patterns of C. annuum α- EXPANSIN (CaEXPA) genes were characterized. Additionally, fruit-specific CaEXPA expression was correlated with the rate of cell expansion during bell pepper fruit development.

RESULTS

Spatial expression patterns revealed that CaEXPA13 was up-regulated in vegetative tissues and flowers, with the most abundant expression in mature leaves. Expression of CaEXPA4 was associated with stems and roots. CaEXPA3 was expressed abundantly in flower at anthesis suggesting a role for CaEXPA3 in flower development. Temporal expression analysis revealed that 9 out of the 21 genes were highly expressed during fruit development. Of these, expression of six genes, CaEXPA5, CaEXPA7, CaEXPA12, CaEXPA14 CaEXPA17 and CaEXPA19 were abundant 7 to 21 days after anthesis (DAA), whereas CaEXPA6 was strongly expressed between 14 and 28 DAA. Further, this study revealed that fruit growth and cell expansion occur throughout bell pepper development until ripening, with highest rates of fruit growth and cell expansion occurring between 7 and 14 DAA. The expression of CaEXPA14 and CaEXPA19 positively correlated with the rate of cell expansion, suggesting their role in post-mitotic cell expansion-mediated growth of the bell pepper fruit. In this study, a ripening specific EXP transcript, CaEXPA9 was identified, suggesting its role in cell wall disassembly during ripening.

CONCLUSIONS

This is the first genome-wide study of CaEXPA expression during fruit growth and development. Identification of fruit-specific EXPAs suggest their importance in facilitating cell expansion during growth and cell wall loosening during ripening in bell pepper. These EXPA genes could be important targets for future manipulation of fruit size and ripening characteristics.

Genome-wide identification of the expansin gene family reveals that expansin genes are involved in fibre cell growth in cotton

BACKGROUND

Expansins (EXPs), a group of proteins that loosen plant cell walls and cellulosic materials, are involved in regulating cell growth and diverse developmental processes in plants. However, the biological functions of this gene family in cotton are still unknown.

RESULTS

In this paper, we identified a total of 93 expansin genes in Gossypium hirsutum. These genes were classified into four subfamilies, including 67 GhEXPAs, 8 GhEXPBs, 6 GhEXLAs, and 12 GhEXLBs, and divided into 15 subgroups. The 93 expansin genes are distributed over 24 chromosomes, excluding Ghir_A02 and Ghir_D06. All GhEXP genes contain multiple exons, and each GhEXP protein has multiple conserved motifs. Transcript profiling and qPCR analysis revealed that the expansin genes have distinct expression patterns among different stages of cotton fibre development. Among them, 3 genes (GhEXPA4o, GhEXPA1A, and GhEXPA8h) were highly expressed in the initiation stage, 9 genes (GhEXPA4a, GhEXPA13a, GhEXPA4f, GhEXPA4q, GhEXPA8f, GhEXPA2, GhEXPA8g, GhEXPA8a, and GhEXPA4n) had high expression during the fast elongation stage, and GhEXLA1c and GhEXLA1f were preferentially expressed in the transition stage of fibre development.

CONCLUSIONS

Our results provide a solid basis for further elucidation of the biological functions of expansin genes in relation to cotton fibre development and valuable genetic resources for future crop improvement.

Complex Molecular Evolution and Expression of Expansin Gene Families in Three Basic Diploid Species of Brassica

Expansins are a kind of structural proteins of the plant cell wall, and they enlarge cells by loosening the cell walls. Therefore, expansins are involved in many growth and development processes. The complete genomic sequences of Brassica rapa, Brassica oleracea and Brassica nigra provide effective platforms for researchers to study expansin genes, and can be compared with analogues in Arabidopsis thaliana. This study identified and characterized expansin families in B. rapa, B. oleracea, and B. nigra. Through the comparative analysis of phylogeny, gene structure, and physicochemical properties, the expansin families were divided into four subfamilies, and then their expansion patterns and evolution details were explored accordingly. Results showed that after the three species underwent independent evolution following their separation from A. thaliana, the expansin families in the three species had increased similarities but fewer divergences. By searching divergences of promoters and coding sequences, significant positive correlations were revealed among orthologs in A. thaliana and the three basic species. Subsequently, differential expressions indicated extensive functional divergences in the expansin families of the three species, especially in reproductive development. Hence, these results support the molecular evolution of basic Brassica species, potential functions of these genes, and genetic improvement of related crops.

Expansin gene loss is a common occurrence during adaptation to an aquatic environment

Expansins comprise a superfamily of plant cell wall loosening proteins that can be divided into four individual families (EXPA, EXPB, EXLA and EXLB). Aside from inferred roles in a variety of plant growth and developmental traits, little is known regarding the function of specific expansin clades, for which there are at least 16 in flowering plants (angiosperms); however, there is evidence to suggest that some expansins have cell-specific functions, in root hair and pollen tube development, for example. Recently, two duckweed genomes have been sequenced (Spirodela polyrhiza strains 7498 and 9509), revealing significantly reduced superfamily sizes. We hypothesized that there would be a correlation between expansin loss and morphological reductions seen among highly adapted aquatic species. In order to provide an answer to this question, we characterized the expansin superfamilies of the greater duckweed Spirodela, the marine eelgrass Zostera marina and the bladderwort Utricularia gibba. We discovered rampant expansin gene and clade loss among the three, including a complete absence of the EXLB family and EXPA-VII. The most convincing correlation between morphological reduction and expansin loss was seen for Utricularia and Spirodela, which both lack root hairs and the root hair expansin clade EXPA-X. Contrary to the pattern observed in other species, four Utricularia expansins failed to branch within any clade, suggesting that they may be the result of neofunctionalization. Last, an expansin clade previously discovered only in eudicots was identified in Spirodela, allowing us to conclude that the last common ancestor of monocots and eudicots contained a minimum of 17 expansins.

Overexpression of paralogues of the wheat expansin gene TaEXPA8 improves low-temperature tolerance in Arabidopsis

Low temperature is one of the important factors limiting wheat yield in cold regions. Expansins are nonenzymatic proteins that loosen cell walls and play important roles in diverse biological processes related to cell wall modification, including development and stress tolerance. Many studies have shown that expansins are involved in resistance to various abiotic stresses, such as heat and drought. However, the role of expansins in response to low-temperature stress remains unclear. Based on our previous transcriptome data of a winter wheat cultivar Dongnongdongmai 2 (DN2), we found that one of the expansin genes, TaEXPA8, was significantly induced by low temperature, indicating a role for TaEXPA8 in cold resistance. In this study, the paralogous TaEXPA8 genes TaEXPA8-A, TaEXPA8-B and TaEXPA8-D were cloned by RT-PCR. These three genes were then transformed into Arabidopsis by the floral dip method. Expression patterns of TaEXPA8 genes in different tissues and in response to several abiotic stresses and hormones were detected by quantitative real-time PCR (qRT-PCR). The results showed that TaEXPA8-A and TaEXPA8-B were expressed mainly in roots, while TaEXPA8-D was expressed predominantly in flowers. TaEXPA8 genes were induced by low-temperature and drought. The overexpression of TaEXPA8-B and TaEXPA8-D enhanced low-temperature resistance and had increased superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activity and soluble protein, MDA and proline content. In summary, our study suggested that the expansins TaEXPA8-B and TaEXPA8-D are involved in the response to low temperature and possibly play a role in cold resistance by activating the protective enzyme system.

Root Transcriptomic Analysis Reveals Global Changes Induced by Systemic Infection of Solanum lycopersicum with Mild and Severe Variants of Potato Spindle Tuber Viroid

Potato spindle tuber viroid (PSTVd) causes systemic infection in plant hosts. There are many studies on viroid-host plant interactions, but they have predominantly focused on the aboveground part of the plant. Here, we investigated transcriptomic profile changes in tomato roots systemically infected with mild or severe PSTVd variants using a combined microarray/RNA-seq approach. Analysis indicated differential expression of genes related to various Gene Ontology categories depending on the stage of infection and PSTVd variant. A majority of cell-wall-related genes were down-regulated at early infection stages, but at the late stage, the number of up-regulated genes increased significantly. Along with observed alterations of many lignin-related genes, performed lignin quantification indicated their disrupted level in PSTVd-infected roots. Altered expression of genes related to biosynthesis and signaling of auxin and cytokinin, which are crucial for lateral root development, was also identified. Comparison of both PSTVd infections showed that transcriptional changes induced by the severe variant were stronger than those caused by the mild variant, especially at the late infection stage. Taken together, we showed that similarly to aboveground plant parts, PSTVd infection in the underground tissues activates the plant immune response.

A comprehensive expression analysis of the expansin gene family in potato (Solanum tuberosum) discloses stress-responsive expansin-like B genes for drought and heat tolerances

Expansin is a type of cell wall elongation and stress relaxation protein involved in various developmental processes and stress resistances in plant. In this study, we identified 36 potato (Solanum tuberosum L.) genes belonging to the expansin (StEXP) gene family from the genome reference. These genes included 24 α-expansins (StEXPAs), five β-expansins (StEXPBs), one expansin-like A (StEXLA) and six expansin-like B (StEXLBs). The RNA-Seq analysis conducted from a variety of tissue types showed 34 expansins differentially expressed among tissues, some of which only expressed in specific tissues. Most of the StEXPAs and StEXPB2 transcripts were more abundant in young tuber compared with other tissues, suggesting they likely play a role in tuber development. There were 31 genes, especially StEXLB6, showed differential expression under the treatments of ABA, IAA and GA3, as well as under the drought and heat stresses, indicating they were likely involved in potato stress resistance. In addition, the gene co-expression analysis indicated the StEXLBs likely contribute to a wider range of stress resistances compared with other genes. We found the StEXLA and six StEXLBs expressed differently under a range of abiotic stresses (salt, alkaline, heavy metals, drought, heat, and cold stresses), which likely participated in the associated signaling pathways. Comparing with the control group, potato growing under the drought or heat stresses exhibited up-regulation of the all six StEXLB genes in leaves, whereas, the StEXLB3, StEXLB4, StEXLB5 and StEXLB6 showed relatively higher expression levels in roots. This suggested these genes likely played a role in the drought and heat tolerance. Overall, this study has shown the potential role of the StEXP genes in potato growth and stress tolerance, and provided fundamental resources for the future studies in potato breeding.

Genome-wide identification, characterization, and expression analysis of the expansin gene family in Chinese jujube (Ziziphus jujuba Mill.)

Main conclusion 30 expansin genes were identified in the jujube genome. Phylogenetic analysis classified expansins into 17 subgroups. Closely related expansins share a conserved gene structure. ZjEXPs had different expression patterns in different tissues. Plant-specific expansins were first discovered as pH-dependent cell-wall-loosening proteins involved in diverse physiological processes. No comprehensive analysis of the expansin gene family has yet been carried out at the whole genome level in Chinese jujube (Ziziphus jujuba Mill.). In this study, 30 expansin genes were identified in the jujube genome. These genes, which were distributed with varying densities across 10 of the 12 jujube chromosomes, could be divided into four subfamilies: 19 ZjEXPAs, 3 ZjEXPBs, 1 ZjEXLA, and 7 ZjEXLBs. Phylogenetic analysis of expansin genes in Arabidopsis, rice, apple, grape, and jujube classified these genes into 17 subgroups. Members of the same subfamily and subgroup shared conserved gene structure and motif compositions. Homology analysis identified 20 homologous gene pairs between jujube and Arabidopsis. Further analysis of ZjEXP gene promoter regions uncovered various growth, development and stress-responsive cis-acting elements. Expression analysis and transcript profiling revealed that ZjEXPs had different expression patterns in different tissues at various developmental stages. ZjEXPA4 and ZjEXPA6 were highly expressed in young fruits, ZjEXPA3 and ZjEXPA5 were significantly expressed in flowers, and ZjEXPA7 was specifically expressed in young leaves. The results of this study, the first systematic analysis of the jujube expansin gene family, can serve as a strong foundation for further elucidation of the physiological functions and biological roles of jujube expansin genes.

Effect of constitutive expression of Arabidopsis CLAVATA3 on cell growth and possible role of cytokinins in leaf size control in transgenic tobacco plants

We generated transgenic tobacco plants (Nicotiana tabacum L.) with overexpression of the Arabidopsis thaliana CLAVATA3 (CLV3) gene which is known to be a negative regulator of cell division. Surprisingly, most of the 35S::CLV3 transgenic plants showed no phenotypic differences with the wild type plants. However, there were considerable changes in the morphological parameters between 35S::CLV3 overexpressors and wild type plants. As expected, the number of meristematic cells in the shoot apical meristem was reduced in 35S::CLV3 plants as compared to the wild type plants. Moreover, overexpression of CLV3 exerted morphological changes not only to shoot apical meristem but also to leaves and flowers. Thus, transgenic plants were characterized by reduced number of epidermal and mesophyll cells as well as stomatal pores in mature leaves. However, there was a compensatory increase in leaf cell size of 35S::CLV3 plants that contributed to maintenance of organ size within the normal range. We observed that expression of cell expansion-promoted genes, expansin NtEXPA4 and endo-xyloglucan transferase NtEXGT, were elevated in mature leaves. In contrast, there was a decrease in the transcript level of the cell division-related AINTEGUMENTA-like (NtANTL) gene in 35S::CLV3 transgenic plants. In addition, we detected an increase in cytokinin level without any changes in the contents of IAA and ABA in 35S::CLV3 overexpressors. Interestingly, cytokinin treatment was shown to stimulate the expression of NtEXPA4 and NtEXGT genes in 35S::CLV3 transgenic plants. We propose that observed compensatory cell expansion in leaves of 35S::CLV3 transgenic plants may be due, at least in part, to a possible link between cytokinin signalling and cell expansion-related genes.

Genome-wide identification of wheat (Triticum aestivum) expansins and expansin expression analysis in cold-tolerant and cold-sensitive wheat cultivars

Plant expansins are proteins involved in cell wall loosening, plant growth, and development, as well as in response to plant diseases and other stresses. In this study, we identified 128 expansin coding sequences from the wheat (Triticum aestivum) genome. These sequences belong to 45 homoeologous copies of TaEXPs, including 26 TaEXPAs, 15 TaEXPBs and four TaEXLAs. No TaEXLB was identified. Gene expression and sub-expression profiles revealed that most of the TaEXPs were expressed either only in root tissues or in multiple organs. Real-time qPCR analysis showed that many TaEXPs were differentially expressed in four different tissues of the two wheat cultivars—the cold-sensitive ‘Chinese Spring (CS)’ and the cold-tolerant ‘Dongnongdongmai 1 (D1)’ cultivars. Our results suggest that the differential expression of TaEXPs could be related to low-temperature tolerance or sensitivity of different wheat cultivars. Our study expands our knowledge on wheat expansins and sheds new light on the functions of expansins in plant development and stress response.

Genome-wide analysis of expansin superfamily in wild Arachis discloses a stress-responsive expansin-like B gene

Expansins are plant cell wall-loosening proteins involved in adaptive responses to environmental stimuli and various developmental processes. The first genome-wide analysis of the expansin superfamily in the Arachis genus identified 40 members in A. duranensis and 44 in A. ipaënsis, the wild progenitors of cultivated peanut (A. hypogaea). These expansins were further characterized regarding their subfamily classification, distribution along the genomes, duplication events, molecular structure, and phylogeny. A RNA-seq expression analysis in different Arachis species showed that the majority of these expansins are modulated in response to diverse stresses such as water deficit, root-knot nematode (RKN) infection, and UV exposure, with an expansin-like B gene (AraEXLB8) displaying a highly distinct stress-responsive expression profile. Further analysis of the AraEXLB8 coding sequences showed high conservation across the Arachis genotypes, with eight haplotypes identified. The modulation of AraEXLB8 expression in response to the aforementioned stresses was confirmed by qRT-PCR analysis in distinct Arachis genotypes, whilst in situ hybridization revealed transcripts in different root tissues according to the stress imposed. The overexpression of AraEXLB8 in soybean (Glycine max) composite plants remarkably decreased the number of galls in transformed hairy roots inoculated with RKN. This study improves the current understanding of the molecular evolution, divergence, and gene expression of expansins in Arachis, and provides molecular and functional insights into the role of expansin-like B, the less-studied plant expansin subfamily.

Expression profiles and hormonal regulation of tobacco expansin genes and their involvement in abiotic stress response

Changes in the expression levels of tobacco expansin genes NtEXPA1, NtEXPA4, NtEXPA5, and NtEXPA6 were studied in different organs of tobacco (Nicotiana tabacum L.) as well as in response to phytohormone and stress treatments. It was shown that NtEXPA1, NtEXPA4 and NtEXPA5 transcripts were predominantly expressed in the shoot apices and young leaves, but almost absent in mature leaves and roots. The NtEXPA6 mRNA was found at high levels in calluses containing a large number of undifferentiated cells, but hardly detectable in the leaves of different ages and roots. In young leaves, expression levels of NtEXPA1, NtEXPA4 and NtEXPA5 genes were induced by cytokinins, auxins and gibberellins. Cytokinins and auxins were also found to increase NtEXPA6 transcripts in young leaves but to the much lower levels than the other expansin mRNAs. Expression analysis demonstrated that brassinosteroid phytohormones were able either to up-regulate or to down-regulate expression of different expansins in leaves of different ages. Furthermore, transcript levels of NtEXPA1, NtEXPA4, and NtEXPA5 genes were increased in response to NaCl, drought, cold, heat, and 10μM abscisic acid (ABA) treatments but reduced in response to more severe stresses, i.e. cadmium, freezing, and 100μM ABA. In contrast, no substantial changes were found in NtEXPA6 transcript level after all stress treatments. In addition, we examined the involvement of tobacco expansins in the regulation of abiotic stress tolerance by transgenic approaches. Transgenic tobacco plants with constitutive expression of NtEXPA1 and NtEXPA5 exhibited improved tolerance to salt stress: these plants showed higher growth indices after NaCl treatment and minimized water loss by reducing stomatal density. In contrast, NtEXPA4-silenced plants were characterized by a considerable growth reduction under salinity and enhanced water loss. Our findings indicate that expression levels of all studied tobacco expansins genes are modulated by plant hormones whereas NtEXPA1, NtEXPA4, and NtEXPA5 expansins may be involved in the regulation of stress tolerance in tobacco plants.

Genome-wide identification of the expansin gene family in tobacco (Nicotiana tabacum)

Expansins are pH-dependent cell wall loosening proteins which form a large family in plants. They have been shown to be involved in various developmental processes and been implicated in enabling plants' ability to absorb nutrients from the soil as well as conferring biotic and abiotic stress resistances. It is therefore clear that they can be potential targets in genetic engineering for crop improvement. Tobacco (Nicotiana tabacum) is a major crop species as well as a model organism. Considering that only a few tobacco expansins have been studied, a genome-wide analysis of the tobacco expansin gene family is necessary. In this study, we identified 52 expansins in tobacco, which were classified into four subfamilies: 36 NtEXPAs, 6 NtEXPBs, 3 NtEXLAs and 7 NtEXLBs. Compared to other species, the NtEXLB subfamily size was relatively larger. Phylogenetic analysis showed that the 52 tobacco expansins were divided into 13 subgroups. Gene structure analysis revealed that genes within subfamilies/subgroups exhibited similar characteristics such as gene structure and protein motif arrangement. Whole-genome duplication and tandem duplication events may have played important roles in the expanding of tobacco expansins. Cis-Acting element analysis revealed that each expansin gene was regulated or several expansin genes were co-regulated by both internal and environmental factors. 35 of these genes were identified as being expressed according to a microarray analysis. In contrast to most NtEXPAs which had higher expression levels in young organs, NtEXLAs and NtEXLBs were preferentially expressed in mature or senescent tissues, suggesting that they might play different roles in different organs or at different developmental stages. As the first step towards genome-wide analysis of the tobacco expansin gene family, our work provides solid background information related to structure, evolution and expression as well as regulatory cis-acting elements of the tobacco expansins. This information will provide a strong foundation for cloning and functional exploration of expansin genes in tobacco.

Comparative Transcriptome Analysis of Primary Roots of Brassica napus Seedlings with Extremely Different Primary Root Lengths Using RNA Sequencing

Primary root (PR) development is a crucial developmental process that is essential for plant survival. The elucidation of the PR transcriptome provides insight into the genetic mechanism controlling PR development in crops. In this study, we performed a comparative transcriptome analysis to investigate the genome-wide gene expression profiles of the seedling PRs of four Brassica napus genotypes that were divided into two groups, short group (D43 and D61), and long group (D69 and D72), according to their extremely different primary root lengths (PRLs). The results generated 55,341,366-64,631,336 clean reads aligned to 62,562 genes (61.9% of the current annotated genes) in the B. napus genome. We provide evidence that at least 44,986 genes are actively expressed in the B. napus PR. The majority of the genes that were expressed during seedling PR development were associated with metabolism, cellular processes, response to stimulus, biological regulation, and signaling. Using a pairwise comparison approach, 509 differentially expressed genes (DEGs; absolute value of log2 fold-change ≥1 and p ≤ 0.05) between the long and short groups were revealed, including phytohormone-related genes, protein kinases and phosphatases, oxygenase, cytochrome P450 proteins, etc. Combining GO functional category, KEGG, and MapMan pathway analyses indicated that the DEGs involved in cell wall metabolism, carbohydrate metabolism, lipid metabolism, secondary metabolism, protein modification and degradation, hormone pathways and signaling pathways were the main causes of the observed PRL differences. We also identified 16 differentially expressed transcription factors (TFs) involved in PR development. Taken together, these transcriptomic datasets may serve as a foundation for the identification of candidate genes and may provide valuable information for understanding the molecular and cellular events related to PR development.