Ethylene inhibits lateral root development, increases IAA transport and expression of PIN3 and PIN7 auxin efflux carriers

Symbiotic plant-bacterial-fungal interaction orchestrates ethylene and auxin signaling for optimized plant growth

The complex and mutual interactions between plants and their associated microbiota are key for plant survival and fitness. From the myriad of microbes that exist in the soil, plants dynamically engineer their surrounding microbiome in response to varying environmental and nutrient conditions. The notion that the rhizosphere bacterial and fungal community acts in harmony with plants is widely acknowledged, yet little is known about how these microorganisms interact with each other and their host plants. Here, we explored the interaction of two well-studied plant beneficial endophytes, Enterobacter sp. SA187 and the fungus Serendipita indica. We show that these microbes show inhibitory growth in vitro but act in a mutually positive manner in the presence of Arabidopsis as a plant host. Although both microbes can promote plant salinity tolerance, plant resilience is enhanced in the ternary interaction, revealing that the host plant has the ability to positively orchestrate the interactions between microbes to everyone's benefit. In conclusion, this study advances our understanding of plant-microbiome interaction beyond individual plant-microbe relationships, unveiling a new layer of complexity in how plants manage microbial communities for optimal growth and stress resistance.

MDF Regulates a Network of Auxin-Dependent and Auxin-Independent Pathways of Adventitious Root Regeneration in Arabidopsis

Plants exhibit strong plasticity in growth and development, seen clearly in lateral and adventitious root development from differentiated tissues in response to environmental stresses. Previous studies have demonstrated the role of both auxin-dependent and auxin-independent signaling pathways in regulating the de novo formation of adventitious roots (ARs) from differentiated tissues, such as leaf petiole in Arabidopsis. One important question is how the auxin-dependent and auxin-independent pathways are coordinated. To investigate this question, we used a combined approach of inducible gene expression, mutant, and signaling reporter gene analysis during AR regeneration in the Arabidopsis petiole to understand regulatory relationships. Auxin signaling components AXR1 and AXR3 are each required for both AR and subsequent lateral root (LR) initiation, as is the ethylene signaling repressor POLARIS, but not EIN2. The PIN trafficking SNARE protein VAMP714 is required for LR rather than AR formation, through effects on auxin-induced gene expression. We identify the RNA splicing regulator MDF and the transcription factor RAP2.7 as new positive regulators of both the auxin-independent and auxin-dependent pathways, and show that MDF regulates RAP2.7, WOX5, and NAC1 while RAP2.7 regulates WOX5 but not NAC1 or YUC1. NAC1 is required for de novo root formation in a pathway independent of YUC1, WOX5, or RAP2.7. We propose a model in which MDF represents a point of molecular crosstalk between auxin-dependent and auxin-independent regeneration processes.

Polyploidization leads to salt stress resilience via ethylene signaling in citrus plants

Polyploidization is a common occurrence in the evolutionary history of flowering plants, significantly contributing to their adaptability and diversity. However, the molecular mechanisms behind these adaptive advantages are not well understood. Through comprehensive phenotyping of diploid and tetraploid clones from Citrus and Poncirus genera, we discovered that genome doubling significantly enhances salt stress resilience. Epigenetic and transcriptomic analyses revealed that increased ethylene production in the roots of tetraploid plants was associated with hypomethylation and enhanced chromatin accessibility of the ACO1 gene. This increased ethylene production activates the transcription of reactive oxygen species scavenging genes and stress-related hormone biosynthesis genes. Consequently, tetraploid plants exhibited superior root functionality under salt stress, maintaining improved cytosolic K+/Na+ homeostasis. To genetically validate the link between salt stress resilience and ACO1 expression, we generated overexpression and knockout lines, confirming the central role of ACO1 expression regulation following genome doubling in salt stress resilience. Our work elucidates the molecular mechanisms underlying the role of genome doubling in stress resilience. We also highlight the importance of chromatin dynamics in fine-tuning ethylene gene expression and activating salt stress resilience pathways, offering valuable insights into plant adaptation and crop genome evolution.

Cell wall dynamic changes and signaling during plant lateral root development

Lateral roots (LRs), are an important component of plant roots, playing a crucial role in anchoring the plant in the soil and facilitating the uptake of water and nutrients. As post-embryonic organs, LRs originate from the pericycle cells of the primary root, and their formation is characterized by precise regulation of cell division and complex intercellular interactions, both of which are closely tied to cell wall regulation. Considering the rapid advances in molecular techniques over the past three decades, we reframe the understanding of the dynamic change in cell wall during LR development by summarizing the factors that precipitate these changes and their effects, as well as the regulated signals involved. Additionally, we discuss current challenges in this field and propose potential solutions.

Ethylene-independent modulation of root development by ACC via downregulation of WOX5 and group I CLE peptide expression

In seed plants, the canonical role of 1-aminocyclopropane-1-carboxylic acid (ACC) is to serve as the precursor in the biosynthesis of the phytohormone ethylene, and indeed, ACC treatment is often used as a proxy for ethylene treatment. Increasing evidence suggests that ACC can also act independently of ethylene to regulate various aspects of plant growth and development. Here, we explore the effects of ACC on Arabidopsis thaliana root growth and the mechanisms by which it acts. ACC inhibits growth of the primary root in Arabidopsis seedlings when ethylene signaling is blocked, which becomes evident after 36 h of treatment with ACC. This reduced root growth is in part the result of suppressed cell proliferation in the root meristem resulting from altered expression of a key regulator of stem cell niche activity, WOX5. ACC also promotes lateral root (LR) development, in contrast to ethylene, which inhibits LR formation. Transcriptomic analysis of roots revealed no significant changes in gene expression after 45 min or 4 h of ACC treatment, but longer treatment times revealed a large number of differentially expressed genes, including the downregulation of the expression of a small group of phylogenetically related CLE peptides. Reduced expression of these group 1 CLEs in response to ACC leads to the activation of a transcription factor, LBD18, which promotes LR development. These results suggest that ACC acts to modulate multiple aspects of Arabidopsis root growth independently of ethylene via distinct transcriptional effects in the root meristem and LR precursor cells.

Auxin-dependent post-translational regulation of MONOPTEROS in the Arabidopsis root

Auxin plays a pivotal role in plant development by activating AUXIN RESPONSE FACTORs (ARFs). Under low auxin levels, ARF activity is inhibited by interacting with Aux/IAAs. Aux/IAAs are degraded when the cellular auxin concentration increases, causing the release of ARF inhibition. Here, we show that levels of the ARF5/MONOPTEROS (MP) protein are regulated in a cell-type-specific and isoform-dependent manner. We find that the stability of MP isoforms is differentially controlled depending on the auxin level. The canonical MP isoform is degraded by the proteasome in root tissues with low auxin levels. While auxin sharpens the MP localization domain in roots, it does not do so in ovules or embryos. Our research highlights a mechanism for providing spatial control of auxin signaling capacity. Together with recent advances in understanding the tissue-specific expression and post-transcriptional modification of auxin signaling components, these results provide insights into understanding how auxin can elicit so many distinct responses.

Actin Isovariant ACT2-Mediated Cellular Auxin Homeostasis Regulates Lateral Root Organogenesis in Arabidopsis thaliana

Lateral root (LR) organogenesis is regulated by cellular flux of auxin within pericycle cells, which depends on the membrane distribution and polar localization of auxin carrier proteins. The correct distribution of auxin carrier proteins relies on the intracellular trafficking of these proteins aided by filamentous actin as a track. However, the precise role of actin in lateral root development is still elusive. Here, using vegetative class actin isovariant mutants, we revealed that loss of actin isovariant ACT8 led to increased lateral root formation. The distribution of auxin within pericycle cells was altered in act8 mutant, primarily due to the altered distribution of AUX1 and PIN7. Interestingly, incorporation of act2 mutant in act8 background (act2act8) effectively nullified the LR phenotype observed in act8 mutant, indicating that ACT2 plays an important role in LR development. To explore further, we investigated the possibility that the act8 mutant's LR phenotype and cellular auxin distribution resulted from ACT2 overexpression. Consistent with the idea, enhanced lateral root formation, altered AUX1, PIN7 expression, and auxin distribution in pericycle cells were observed in ACT2 overexpression lines. Collectively, these results suggest that actin isovariant ACT2 but not ACT8 plays a pivotal role in regulating source-to-sink auxin distribution during lateral root organogenesis.

Shaping root architecture: towards understanding the mechanisms involved in lateral root development

Plants have an amazing ability to adapt to their environment, and this extends beyond biochemical responses and includes developmental changes that help them better exploit resources and survive. The plasticity observed in individual plant morphology is associated with robust developmental pathways that are influenced by environmental factors. However, there is still much to learn about the mechanisms behind the formation of the root system. In Arabidopsis thaliana, the root system displays a hierarchical structure with primary and secondary roots. The process of lateral root (LR) organogenesis involves multiple steps, including LR pre-patterning, LR initiation, LR outgrowth, and LR emergence. The study of root developmental plasticity in Arabidopsis has led to significant progress in understanding the mechanisms governing lateral root formation. The importance of root system architecture lies in its ability to shape the distribution of roots in the soil, which affects the plant's ability to acquire nutrients and water. In Arabidopsis, lateral roots originate from pericycle cells adjacent to the xylem poles known as the xylem-pole-pericycle (XPP). The positioning of LRs along the primary root is underpinned by a repetitive pre-patterning mechanism that establishes primed sites for future lateral root formation. In a subset of primed cells, the memory of a transient priming stimulus leads to the formation of stable pre-branch sites and the establishment of founder cell identity. These founder cells undergo a series of highly organized periclinal and anticlinal cell divisions and expansion to form lateral root primordia. Subsequently, LRP emerges through three overlying cell layers of the primary root, giving rise to fully developed LRs. In addition to LRs Arabidopsis can also develop adventitious lateral roots from the primary root in response to specific stress signals such as wounding or environmental cues. Overall, this review creates an overview of the mechanisms governing root lateral root formation which can be a stepping stone to improved crop yields and a better understanding of plant adaptation to changing environments.

Advances in Plant Auxin Biology: Synthesis, Metabolism, Signaling, Interaction with Other Hormones, and Roles under Abiotic Stress

Auxin is a key hormone that regulates plant growth and development, including plant shape and sensitivity to environmental changes. Auxin is biosynthesized and metabolized via many parallel pathways, and it is sensed and transduced by both normal and atypical pathways. The production, catabolism, and signal transduction pathways of auxin primarily govern its role in plant growth and development, and in the response to stress. Recent research has discovered that auxin not only responds to intrinsic developmental signals, but also mediates various environmental signals (e.g., drought, heavy metals, and temperature stresses) and interacts with hormones such as cytokinin, abscisic acid, gibberellin, and ethylene, all of which are involved in the regulation of plant growth and development, as well as the maintenance of homeostatic equilibrium in plant cells. In this review, we discuss the latest research on auxin types, biosynthesis and metabolism, polar transport, signaling pathways, and interactions with other hormones. We also summarize the important role of auxin in plants under abiotic stresses. These discussions provide new perspectives to understand the molecular mechanisms of auxin's functions in plant development.

Employing Genomic Tools to Explore the Molecular Mechanisms behind the Enhancement of Plant Growth and Stress Resilience Facilitated by a Burkholderia Rhizobacterial Strain

The rhizobacterial strain BJ3 showed 16S rDNA sequence similarity to species within the Burkholderia genus. Its complete genome sequence revealed a 97% match with Burkholderia contaminans and uncovered gene clusters essential for plant-growth-promoting traits (PGPTs). These clusters include genes responsible for producing indole acetic acid (IAA), osmolytes, non-ribosomal peptides (NRPS), volatile organic compounds (VOCs), siderophores, lipopolysaccharides, hydrolytic enzymes, and spermidine. Additionally, the genome contains genes for nitrogen fixation and phosphate solubilization, as well as a gene encoding 1-aminocyclopropane-1-carboxylate (ACC) deaminase. The treatment with BJ3 enhanced root architecture, boosted vegetative growth, and accelerated early flowering in Arabidopsis. Treated seedlings also showed increased lignin production and antioxidant capabilities, as well as notably increased tolerance to water deficit and high salinity. An RNA-seq transcriptome analysis indicated that BJ3 treatment significantly activated genes related to immunity induction, hormone signaling, and vegetative growth. It specifically activated genes involved in the production of auxin, ethylene, and salicylic acid (SA), as well as genes involved in the synthesis of defense compounds like glucosinolates, camalexin, and terpenoids. The expression of AP2/ERF transcription factors was markedly increased. These findings highlight BJ3's potential to produce various bioactive metabolites and its ability to activate auxin, ethylene, and SA signaling in Arabidopsis, positioning it as a new Burkholderia strain that could significantly improve plant growth, stress resilience, and immune function.

The transcription factor WRKY22 modulates ethylene biosynthesis and root development through transactivating the transcription of ACS5 and ACO5 in Arabidopsis

The WRKY transcription factor (TF) genes form a large family in higher plants, with 72 members in Arabidopsis (Arabidopsis thaliana). The gaseous phytohormone ethylene (ET) regulates multiple physiological processes in plants. It is known that 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACSs, EC 4.4.1.14) limit the enzymatic reaction rate of ethylene synthesis. However, whether WRKY TFs regulate the expression of ACSs and/or ACC oxidases (ACOs, EC 1.14.17.4) remains largely elusive. Here, we demonstrated that Arabidopsis WRKY22 positively regulated the expression of a few ACS and ACO genes, thus promoting ethylene production. Inducible overexpression of WRKY22 caused shorter hypocotyls without ACC treatment. A qRT-PCR screening demonstrated that overexpression of WRKY22 activates the expression of several ACS and ACO genes. The promoter regions of ACS5, ACS11, and ACO5 were also activated by WRKY22, which was revealed by a dual luciferase reporter assay. A follow-up chromatin immunoprecipitation coupled with quantitative PCR (ChIP-qPCR) and electrophoretic mobility shift assay (EMSA) showed that the promoter regions of ACS5 and ACO5 could be bound by WRKY22 directly. Moreover, wrky22 mutants had longer primary roots and more lateral roots than wild type, while WRKY22-overexpressing lines showed the opposite phenotype. In conclusion, this study revealed that WRKY22 acts as a novel TF activating, at least, the expression of ACS5 and ACO5 to increase ethylene synthesis and modulate root development.

Control of Plant Height and Lateral Root Development via Stu-miR156 Regulation of SPL9 Transcription Factor in Potato

MicroRNAs (miRNAs) are a class of endogenous, non-coding small-molecule RNAs that usually regulate the expression of target genes at the post-transcriptional level. miR156 is one of a class of evolutionarily highly conserved miRNA families. SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor is one of the target genes that is regulated by miR156. SPL transcription factors are involved in regulating plant growth and development, hormone response, stress response, and photosynthesis. In the present study, transgenic potato plants with overexpressed miR156 were obtained via the Agrobacterium-mediated transformation method. The results showed that the expression levels of the target gene, StSPL9, were all downregulated in the transgenic plants with overexpressed Stu-miR156. Compared with those of the control plants, the plant height and root length of the transgenic plants were significantly decreased, while the number of lateral roots was significantly increased. These results revealed that the miR156/SPLs module was involved in regulating potato plant height and root growth.

Humic acid improves wheat growth by modulating auxin and cytokinin biosynthesis pathways

Humic acids have been widely used for centuries to enhance plant growth and productivity. The beneficial effects of humic acids have been attributed to different functional groups and phytohormone-like compounds enclosed in macrostructure. However, the mechanisms underlying the plant growth-promoting effects of humic acids are only partially understood. We hypothesize that the bio-stimulatory effect of humic acids is mainly due to the modulation of innate pathways of auxin and cytokinin biosynthesis in treated plants. A physiological investigation along with molecular characterization was carried out to understand the mechanism of bio-stimulatory effects of humic acid. A gene expression analysis was performed for the genes involved in auxin and cytokinin biosynthesis pathways in wheat seedlings. Furthermore, Arabidopsis thaliana transgenic lines generated by fusing the auxin-responsive DR5 and cytokinin-responsive ARR5 promoter to ß-glucuronidase (GUS) reporter were used to study the GUS expression analysis in humic acid treated seedlings. This study demonstrates that humic acid treatment improved the shoot and root growth of wheat seedlings. The expression of several genes involved in auxin (Tryptophan Aminotransferase of Arabidopsis and Gretchen Hagen 3.2) and cytokinin (Lonely Guy3) biosynthesis pathways were up-regulated in humic acid-treated seedlings compared to the control. Furthermore, GUS expression analysis showed that bioactive compounds of humic acid stimulate endogenous auxin and cytokinin-like activities. This study is the first report in which using ARR5:GUS lines we demonstrate the biostimulants activity of humic acid.

A GARP transcription factor SlGCC positively regulates lateral root development in tomato via auxin-ethylene interplay

MAIN CONCLUSION

SlGCC, a GARP transcription factor, functions as a root-related transcriptional repressor. SlGCC synchronizes auxin and ethylene signaling involving SlPIN3 and SlIAA3 as intermediate targets sketching a molecular map for lateral root development in tomato. The root system is crucial for growth and development of plants as it performs basic functions such as providing mechanical support, nutrients and water uptake, pathogen resistance and responds to various stresses. SlGCC, a GARP family transcription factor (TF), exhibited predominant expression in age-dependent (initial to mature stages) tomato root. SlGCC is a transcriptional repressor and is regulated at a transcriptional and translational level by auxin and ethylene. Auxin and ethylene mediated SlGCC protein stability is governed via proteasome degradation pathway during lateral root (LR) growth development. SlGCC over-expressor (OE) and under-expressed (UE) tomato transgenic lines demonstrate its role in LR development. This study is an attempt to unravel the vital role of SlGCC in regulating tomato LR architecture.

How plant roots respond to waterlogging

Plant submergence is a major abiotic stress that impairs plant performance. Under water, reduced gas diffusion exposes submerged plant cells to an environment that is enriched in gaseous ethylene and is limited in oxygen (O2) availability (hypoxia). The capacity for plant roots to avoid and/or sustain critical hypoxia damage is essential for plants to survive waterlogging. Plants use spatiotemporal ethylene and O2 dynamics as instrumental flooding signals to modulate potential adaptive root growth and hypoxia stress acclimation responses. However, how non-adapted plant species modulate root growth behaviour during actual waterlogged conditions to overcome flooding stress has hardly been investigated. Here we discuss how changes in the root growth rate, lateral root formation, density, and growth angle of non-flood adapted plant species (mainly Arabidopsis) could contribute to avoiding and enduring critical hypoxic conditions. In addition, we discuss current molecular understanding of how ethylene and hypoxia signalling control these adaptive root growth responses. We propose that future research would benefit from less artificial experimental designs to better understand how plant roots respond to and survive waterlogging. This acquired knowledge would be instrumental to guide targeted breeding of flood-tolerant crops with more resilient root systems.

Characterization of Almond Scion/Rootstock Communication in Cultivar and Rootstock Tissues through an RNA-Seq Approach

The rootstock genotype plays a crucial role in determining various aspects of scion development, including the scion three-dimensional structure, or tree architecture. Consequently, rootstock choice is a pivotal factor in the establishment of new almond (Prunus amygdalus (L.) Batsch, syn P. dulcis (Mill.)) intensive planting systems, demanding cultivars that can adapt to distinct requirements of vigor and shape. Nevertheless, considering the capacity of the rootstock genotype to influence scion development, it is likely that the scion genotype reciprocally affects rootstock performance. In the context of this study, we conducted a transcriptomic analysis of the scion/rootstock interaction in young almond trees, with a specific focus on elucidating the scion impact on the rootstock molecular response. Two commercial almond cultivars were grafted onto two hybrid rootstocks, thereby generating four distinct combinations. Through RNA-Seq analysis, we discerned that indeed, the scion genotype exerts an influence on the rootstock expression profile. This influence manifests through the modulation of genes associated with hormonal regulation, cell division, root development, and light signaling. This intricate interplay between scion and rootstock communication plays a pivotal role in the development of both scion and rootstock, underscoring the critical importance of a correct choice when establishing new almond orchards.

Trehalose metabolism coordinates transcriptional regulatory control and metabolic requirements to trigger the onset of cassava storage root initiation

Cassava storage roots (SR) are an important source of food energy and raw material for a wide range of applications. Understanding SR initiation and the associated regulation is critical to boosting tuber yield in cassava. Decades of transcriptome studies have identified key regulators relevant to SR formation, transcriptional regulation and sugar metabolism. However, there remain uncertainties over the roles of the regulators in modulating the onset of SR development owing to the limitation of the widely applied differential gene expression analysis. Here, we aimed to investigate the regulation underlying the transition from fibrous (FR) to SR based on Dynamic Network Biomarker (DNB) analysis. Gene expression analysis during cassava root initiation showed the transition period to SR happened in FR during 8 weeks after planting (FR8). Ninety-nine DNB genes associated with SR initiation and development were identified. Interestingly, the role of trehalose metabolism, especially trehalase1 (TRE1), in modulating metabolites abundance and coordinating regulatory signaling and carbon substrate availability via the connection of transcriptional regulation and sugar metabolism was highlighted. The results agree with the associated DNB characters of TRE1 reported in other transcriptome studies of cassava SR initiation and Attre1 loss of function in literature. The findings help fill the knowledge gap regarding the regulation underlying cassava SR initiation.

Novel insights into the role of leaf in the cutting process of Camellia sinensis using physiological, biochemical and transcriptome analyses

Cuttage is the preferred approach for rapid propagation of many species including tea plant (Camellia sinensis). Leaf serves as a key part of nodal cutting, but there is a lack of systematic research on its role in the cutting process. In this study, 24 tea cultivars were employed to prove the necessity of leaf and light during cuttage. Further leaf physiological parameters found that lower net photosynthesis rate probably promoted rooting. Phytohormone content detection showed that auxin content and composition pattern were related to rooting ability. Leaf transcriptome analyses of cuttings from a representative easy-to-root cultivar (cv. Echa 10) revealed that genes involved in carbohydrate metabolism, signal transduction, metabolite biosynthesis and transportation were differentially expressed during the rooting process. CsTSA1, CsYUC10, CsAUX1s, CsPIN3 and CsPIN5 were selected as the candidate genes, which possibly regulate the rooting of nodal cuttings. These results illustrate the necessity of the leaf in cuttage and provide molecular evidence that leaf is an important place for signal transduction, metabolite synthesis and transport during the rooting process.

Genetic Interaction between Arabidopsis SUR2/CYP83B1 and GNOM Indicates the Importance of Stabilizing Local Auxin Accumulation in Lateral Root Initiation

Lateral root (LR) formation is an important developmental event for the establishment of the root system in most vascular plants. In Arabidopsis thaliana, the fewer roots (fwr) mutation in the GNOM gene, encoding a guanine nucleotide exchange factor of ADP ribosylation factor that regulates vesicle trafficking, severely inhibits LR formation. Local accumulation of auxin response for LR initiation is severely affected in fwr. To better understand how local accumulation of auxin response for LR initiation is regulated, we identified a mutation, fewer roots suppressor1 (fsp1), that partially restores LR formation in fwr. The gene responsible for fsp1 was identified as SUPERROOT2 (SUR2), encoding CYP83B1 that positions at the metabolic branch point in the biosynthesis of auxin/indole-3-acetic acid (IAA) and indole glucosinolate. The fsp1 mutation increases both endogenous IAA levels and the number of the sites where auxin response locally accumulates prior to LR formation in fwr. SUR2 is expressed in the pericycle of the differentiation zone and in the apical meristem in roots. Time-lapse imaging of the auxin response revealed that local accumulation of auxin response is more stable in fsp1. These results suggest that SUR2/CYP83B1 affects LR founder cell formation at the xylem pole pericycle cells where auxin accumulates. Analysis of the genetic interaction between SUR2 and GNOM indicates the importance of stabilization of local auxin accumulation sites for LR initiation.

Comparative transcriptome analysis of high- and low-embryogenic Hevea brasiliensis genotypes reveals involvement of phytohormones in somatic embryogenesis

BACKGROUND

Rubber plant (Hevea brasiliensis) is one of the major sources of latex. Somatic embryogenesis (SE) is a promising alterative to its propagation by grafting and seed. Phytohormones have been shown to influence SE in different plant species. However, limited knowledge is available on the role of phytohormones in SE in Hevea. The anther cultures of two Hevea genotypes (Yunyan 73477-YT and Reken 628-RT) with contrasting SE rate were established and four stages i.e., anthers (h), anther induced callus (y), callus differentiation state (f), and somatic embryos (p) were studied. UPLC-ESI-MS/MS and transcriptome analyses were used to study phytohormone accumulation and related expression changes in biosynthesis and signaling genes.

RESULTS

YT showed higher callus induction rate than RT. Of the two genotypes, only YT exhibited successful SE. Auxins, cytokinins (CKs), abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), gibberellins (GAs), and ethylene (ETH) were detected in the two genotypes. Indole-3-acetic acid (IAA), CKs, ABA, and ETH had notable differences in the studied stages of the two genotypes. The differentially expressed genes identified in treatment comparisons were majorly enriched in MAPK and phytohormone signaling, biosynthesis of secondary metabolites, and metabolic pathways. The expression changes in IAA, CK, ABA, and ETH biosynthesis and signaling genes confirmed the differential accumulation of respective phytohormones in the two genotypes.

CONCLUSION

These results suggest potential roles of phytohormones in SE in Hevea.

Tapping into the plasticity of plant architecture for increased stress resilience

Plant architecture develops post-embryonically and emerges from a dialogue between the developmental signals and environmental cues. Length and branching of the vegetative and reproductive tissues were the focus of improvement of plant performance from the early days of plant breeding. Current breeding priorities are changing, as we need to prioritize plant productivity under increasingly challenging environmental conditions. While it has been widely recognized that plant architecture changes in response to the environment, its contribution to plant productivity in the changing climate remains to be fully explored. This review will summarize prior discoveries of genetic control of plant architecture traits and their effect on plant performance under environmental stress. We review new tools in phenotyping that will guide future discoveries of genes contributing to plant architecture, its plasticity, and its contributions to stress resilience. Subsequently, we provide a perspective into how integrating the study of new species, modern phenotyping techniques, and modeling can lead to discovering new genetic targets underlying the plasticity of plant architecture and stress resilience. Altogether, this review provides a new perspective on the plasticity of plant architecture and how it can be harnessed for increased performance under environmental stress.

The local and systemic accumulation of ethylene determines the rapid defence responses induced by flg22 in tomato (Solanum lycopersicum L.)

Plant defence responses induced by the bacterial elicitor flg22 are highly dependent on phytohormones, including gaseous ethylene (ET). While the regulatory role of ET in local defence responses to flg22 exposure has been demonstrated, its contribution to the induction of systemic responses is not clearly understood. For this consideration, we examined the effects of different ET modulators on the flg22-induced local and systemic defence progression. In our experiments, ET biosynthesis inhibitor aminoethoxyvinyl glycine (AVG) or ET receptor blocker silver thiosulphate (STS) were applied 1 h before flg22 treatments and 1 h later the rapid local and systemic responses were detected in the leaves of intact tomato plants (Solanum lycopersicum L.). Based on our results, AVG not only diminished the flg22-induced ET accumulation locally, but also in the younger leaves confirming the role of ET in the whole-plant expanding defence progression. This increase in ET emission was accompanied by increased local expression of SlACO1, which was reduced by AVG and STS. Local ET biosynthesis upon flg22 treatment was shown to positively regulate local and systemic superoxide (O2.-) and hydrogen peroxide (H2O2) production, which in turn could contribute to ET accumulation in younger leaves. Confirming the role of ET in flg22-induced rapid defence responses, application of AVG reduced local and systemic ET, O2.- and H2O2 production, whereas STS reduced it primarily in the younger leaves. Interestingly, in addition to flg22, AVG and STS induced stomatal closure alone at whole-plant level, however in the case of combined treatments together with flg22 both ET modulators reduced the rate of stomatal closure in the older- and younger leaves as well. These results demonstrate that both local and systemic ET production in sufficient amounts and active ET signalling are essential for the development of flg22-induced rapid local and systemic defence responses.

SlGH3.15, a member of the GH3 gene family, regulates lateral root development and gravitropism response by modulating auxin homeostasis in tomato

Multiple Gretchen Hagen 3 (GH3) genes have been implicated in a range of processes in plant growth and development through their roles in maintaining hormonal homeostasis. However, there has only been limited study on the functions of GH3 genes in tomato (Solanum lycopersicum). In this work, we investigated the important function of SlGH3.15, a member of the GH3 gene family in tomato. Overexpression of SlGH3.15 led to severe dwarfism in both the above- and below-ground sections of the plant, accompanied by a substantial decrease in free IAA content and reduction in the expression of SlGH3.9, a paralog of SlGH3.15. Exogenous supply of IAA negatively affected the elongation of the primary root and partially restored the gravitropism defects in SlGH3.15-overexpression lines. While no phenotypic change was observed in the SlGH3.15 RNAi lines, double knockout lines of SlGH3.15 and SlGH3.9 were less sensitive to treatments with the auxin polar transport inhibitor. Overall, these findings revealed important roles of SlGH3.15 in IAA homeostasis and as a negative regulator of free IAA accumulation and lateral root formation in tomato.

Ethylene Inhibition Reduces De Novo Shoot Organogenesis and Subsequent Plant Development from Leaf Explants of Solanum betaceum Cav

In de novo shoot organogenesis (DNSO) plant cells develop into new shoots, without the need of an existing meristem. Generally, this process is triggered by wounding and specific growth regulators, such as auxins and cytokinins. Despite the potential significance of the plant hormone ethylene in DNSO, its effect in regeneration processes of woody species has not been thoroughly investigated. To address this gap, Solanum betaceum Cav. was used as an experimental model to explore the role of this hormone on DNSO and potentially extend the findings to other woody species. In this work it was shown that ethylene positively regulates DNSO from tamarillo leaf explants. Ethylene precursors ACC and ethephon stimulated shoot regeneration by increasing the number of buds and shoots regenerated. In contrast, the inhibition of ethylene biosynthesis or perception by AVG and AgNO₃ decreased shoot regeneration. Organogenic callus induced in the presence of ethylene precursors showed an upregulated expression of the auxin efflux carrier gene PIN1, suggesting that ethylene may enhance shoot regeneration by affecting auxin distribution prior to shoot development. Additionally, it was found that the de novo shoot meristems induced in explants in which ethylene biosynthesis and perception was suppressed were unable to further develop into elongated shoots. Overall, these results imply that altering ethylene levels and perception could enhance shoot regeneration efficiency in tamarillo. Moreover, we offer insights into the possible molecular mechanisms involved in ethylene-induced shoot regeneration.

GOLVEN peptides regulate lateral root spacing as part of a negative feedback loop on the establishment of auxin maxima

Lateral root initiation requires the accumulation of auxin in lateral root founder cells, yielding a local auxin maximum. The positioning of auxin maxima along the primary root determines the density and spacing of lateral roots. The GOLVEN6 (GLV6) and GLV10 signaling peptides and their receptors have been established as regulators of lateral root spacing via their inhibitory effect on lateral root initiation in Arabidopsis. However, it remained unclear how these GLV peptides interfere with auxin signaling or homeostasis. Here, we show that GLV6/10 signaling regulates the expression of a subset of auxin response genes, downstream of the canonical auxin signaling pathway, while simultaneously inhibiting the establishment of auxin maxima within xylem-pole pericycle cells that neighbor lateral root initiation sites. We present genetic evidence that this inhibitory effect relies on the activity of the PIN3 and PIN7 auxin export proteins. Furthermore, GLV6/10 peptide signaling was found to enhance PIN7 abundance in the plasma membranes of xylem-pole pericycle cells, which likely stimulates auxin efflux from these cells. Based on these findings, we propose a model in which the GLV6/10 signaling pathway serves as a negative feedback mechanism that contributes to the robust patterning of auxin maxima along the primary root.

Genome-wide identification and characterization of PIN-FORMED (PIN) and PIN-LIKES (PILS) gene family reveals their role in adventitious root development in tea nodal cutting (Camellia Sinensis)

Auxin affects all aspects of plant growth and development, including morphogenesis and adaptive responses. Auxin transmembrane transport is promoted by PIN formation (PIN) and a structurally similar PIN-like (PILS) gene family, which jointly controls the directional transport of the auxin between plant cells, and the accumulation of intracellular auxin. At present, there is no study investigating the roles of CslPIN and CslPILS gene family in root development in the tea plant (Camellia sinensis). In this study, 8 CslPIN and 10 CslPILS genes were identified in the tea plant, and their evolutionary relationships, physical and chemical properties, conserved motifs, cis-acting elements, chromosome location, collinearity, and expression characteristics were analyzed. The mechanism of CslPIN and CslPILS in the formation of tea adventitious roots (ARs) was studied by the AR induction system. Through functional verification, the regulation of CslPIN3 gene on root growth and development of tea plant was studied by over-expression of CslPIN3 in Arabidopsis thaliana and in situ hybridization in Camellia sinensis. The results confirmed CslPIN3 was involved in the regulation of root growth and development as well as auxin accumulation. This study provides a better insight into the regulatory mechanism of CslPIN and CslPILS gene family on the formation of AR in tea plant.

Whole-Genome Comparison Reveals Structural Variations behind Heading Leaf Trait in Brassica oleracea

Brassica oleracea displays remarkable morphological variations. It intrigued researchers to study the underlying cause of the enormous diversification of this organism. However, genomic variations in complex heading traits are less known in B. oleracea. Herein, we performed a comparative population genomics analysis to explore structural variations (SVs) responsible for heading trait formation in B. oleracea. Synteny analysis showed that chromosomes C1 and C2 of B. oleracea (CC) shared strong collinearity with A01 and A02 of B. rapa (AA), respectively. Two historical events, whole genome triplication (WGT) of Brassica species and differentiation time between AA and CC genomes, were observed clearly by phylogenetic and Ks analysis. By comparing heading and non-heading populations of B. oleracea genomes, we found extensive SVs during the diversification of the B. oleracea genome. We identified 1205 SVs that have an impact on 545 genes and might be associated with the heading trait of cabbage. Overlapping the genes affected by SVs and the differentially expressed genes identified by RNA-seq analysis, we identified six vital candidate genes that may be related to heading trait formation in cabbage. Further, qRT-PCR experiments also verified that six genes were differentially expressed between heading leaves and non-heading leaves, respectively. Collectively, we used available genomes to conduct a comparison population genome analysis and identify candidate genes for the heading trait of cabbage, which provides insight into the underlying reason for heading trait formation in B. oleracea.

Calcium-dependent protein kinase CDPK16 phosphorylates serine-856 of glutamate receptor-like GLR3.6 protein leading to salt-responsive root growth in Arabidopsis. FRONTIERS IN PLANT SCIENCE 2023;14:1093472. [PMID: 36818849 PMCID: PMC9935832 DOI: 10.3389/fpls.2023.1093472] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Calcium-permeable channels in the plasma membrane play vital roles in plant growth, development, and response to environmental stimuli. Arabidopsis possesses 20 glutamate receptor-like proteins that share similarities with animal ionotropic glutamate receptors and mediate Ca²⁺ influx in plants. Calcium-dependent protein kinases (CDPKs) phosphorylate serine (Ser)-860 of glutamate receptor-like (GLR)3.7 protein, which interacts with 14-3-3ω and plays an essential role in salt and abscisic acid response in Arabidopsis by modulating Ca²⁺ signaling. However, the significance of CDPK- mediated phosphorylation status of Ser residues of GLR3.6 with regard to the functioning of GLR3.6 remains to be elucidated. In this study, we performed an in vitro kinase assay using CDPK16 and peptides containing the 14-3-3ω interacting domain of GLR3.6. We showed that Ser861/862 of GLR3.6 are required for the interaction with 14-3-3ω and that Ser856 of GLR3.6 is specifically phosphorylated by CDPK16 but not by CDPK3 and CDPK34. In addition, the expression of GLR3.6 was quickly downregulated by salt stress, and plants of glr3.6 mutants and GLR3.6-overexpression lines presented shorter and longer root lengths, respectively, under normal growth conditions than Col. Overexpression of the GLR3.6-Ser856 to Ala mutation resulted in a less sensitive phenotype in response to salt stress similar to glr3.6. Our results indicated that the Ser861/862 residues of GLR3.6 are required for interaction with 14-3-3ω. Additionally, the phosphorylation status of Ser856 residue of GLR3.6, which is mediated specifically by CDPK16, regulates root growth in normal and salt stress and conditions.

Interplay between ethylene and nitrogen nutrition: How ethylene orchestrates nitrogen responses in plants

The stress hormone ethylene plays a key role in plant adaptation to adverse environmental conditions. Nitrogen (N) is the most quantitatively required mineral nutrient for plants, and its availability is a major determinant for crop production. Changes in N availability or N forms can alter ethylene biosynthesis and/or signaling. Ethylene serves as an important cellular signal to mediate root system architecture adaptation, N uptake and translocation, ammonium toxicity, anthocyanin accumulation, and premature senescence, thereby adapting plant growth and development to external N status. Here, we review the ethylene-mediated morphological and physiological responses and highlight how ethylene transduces the N signals to the adaptive responses. We specifically discuss the N-ethylene relations in rice, an important cereal crop in which ethylene is essential for its hypoxia survival.

Molinia caerulea alters forest Quercus petraea seedling growth through reduced mycorrhization

Oak regeneration is jeopardized by purple moor grass, a well-known competitive perennial grass in the temperate forests of Western Europe. Below-ground interactions regarding resource acquisition and interference have been demonstrated and have led to new questions about the negative impact of purple moor grass on ectomycorrhizal colonization. The objective was to examine the effects of moor grass on root system size and ectomycorrhization rate of oak seedlings as well as consequences on nitrogen (N) content in oak and soil. Oak seedlings and moor grass tufts were planted together or separately in pots under semi-controlled conditions (irrigated and natural light) and harvested 1 year after planting. Biomass, N content in shoot and root in oak and moor grass as well as number of lateral roots and ectomycorrhizal rate in oak were measured. Biomass in both oak shoot and root was reduced when planting with moor grass. Concurrently, oak lateral roots number and ectomycorrhization rate decreased, along with a reduction in N content in mixed-grown oak. An interference mechanism of moor grass is affecting oak seedlings performance through reduction in oak lateral roots number and its ectomycorrhization, observed in conjunction with a lower growth and N content in oak. By altering both oak roots and mycorrhizas, moor grass appears to be a species with a high allelopathic potential. More broadly, these results show the complexity of interspecific interactions that involve various ecological processes involving the soil microbial community and need to be explored in situ.

Identification and function of miRNA-mRNA interaction pairs during lateral root development of hemi-parasitic Santalum album L. seedlings

Sandalwood (Santalum album L.) is a hemi-parasitic tree species famous for its santalol and santalene, which are extracted from its heartwood and roots. The ability to understand root functionality within its branched root system would benefit the regulation of sandalwood growth and enhance the commercial value of sandalwood. Phenotypic and anatomical evidence in this study indicated that seed germination stage 4 (SG4) seemed pivotal for lateral root (LR) morphogenesis. Small RNA (sRNA) high-throughput sequencing of root tissues at three sub-stages of SG4 (lateral root primordia initiation (LRPI), lateral root primordia development (LRPD), and lateral root primordia emergence (LRPE)) was performed to identify microRNAs (miRNAs) associated with LR development. A total of 135 miRNAs, including 70 differentially expressed miRNAs (DEMs), were screened. Ten DEMs were selected to investigate transcript abundance in different organs or developmental stages. Among 100 negative DEM-mRNA interaction pairs, four targets (Sa-miR166m_2, 408d, 858a, and novel_Sa-miR8) were selected for studying cleavage sites by 5' RLM-RACE validation. The expression mode of the four miRNA-mRNA pairs was investigated after indole-3-acetic acid (IAA) treatment. IAA enhanced the abundance of homeobox-leucine-zipper protein 32 (HOX32), laccase 12 (LAC12), myeloblastosis86 (MYB86), and pectin methylesterase inhibitor6 (PMEI6) target transcripts by reducing the expression of Sa-miR166m_2, 408d, 858a, and novel_Sa-miR8 in the first 10 min. A schematic model of miRNA-regulated LR development is proposed for this hemi-parasitic species. This novel genetic information for improving sandalwood root growth and development may allow for the cultivation of fast-growing and high-yielding plantations.

24-epibrassinolide improves cadmium tolerance and lateral root growth associated with regulating endogenous auxin and ethylene in Kentucky bluegrass

The application of phytohormones is a viable technique to increase the efficiency of phytoremediation in heavy metal-contaminated soils. The objective of this study was to determine how the application of 24-epibrassinolide (EBR), a brassinosteroid analog, could regulate root growth and tolerance to cadmium (Cd) stress in Kentucky bluegrass. As a result, the number of lateral root primordia and total root length in the Cd-treated seedlings decreased by 33.1 % and 56.5 %, respectively. After the application of EBR, Cd accumulation in roots and leaves, and the negative effect of Cd on root growth were reduced under Cd stress. Additionally, the expression of the brassinosteroid signaling gene PpBRI1 was significantly upregulated by exogenous EBR. Moreover, exogenous EBR upregulated the expression of genes encoding antioxidant enzymes and improved the activity of antioxidant enzymes, thereby reduced oxidative stress in roots. Finally, targeted hormonomics analysis highlighted the utility of the application of EBR to alleviate the effect of Cd on the reduction in auxin (IAA) content and the increase in ethylene (ACC) content. These were known to be associated with the upregulation in the expression of auxin biosynthesis gene PpYUCCA1 and downregulation in the expression of ethylene biosynthesis gene PpACO1 in the roots treated with Cd stress. Overall, the application of EBR alleviated Cd-induced oxidative stress in addition to improving root elongation and lateral root growth crosstalk with auxin and ethylene in Kentucky bluegrass subjected to Cd stress. This study further highlights the potential role of brassinosteroids in improving the efficiency of phytoremediation for Cd-contaminated soils.

WRKY29 transcription factor regulates ethylene biosynthesis and response in arabidopsis

The gaseous phytohormone ethylene participates in a lot of physiological processes in plants. 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS, EC 4.4.1.14) and the ACC oxidase (ACO, EC 1.14.17.4) are key enzymes in ethylene biosynthesis. However, how ACSs and ACOs are regulated at the transcriptional level is largely unknown. In the present study, we showed that an Arabidopsis (Arabidopsis thaliana) WRKY-type transcription factor (TF), WRKY29 positively regulated the expression of ACS5, ACS6, ACS8, ACS11 and ACO5 genes and thus promoted basal ethylene production. WRKY29 protein was localized in nuclei and was a transcriptional activator. Overexpression of WRKY29 caused pleiotropic effect on plant growth, development and showed obvious response even without ACC treatment. Inducible overexpression of WRKY29 also reduced primary root elongation and lateral root growth. A triple response assay of overexpression and mutant seedlings of WRKY29 showed that overexpression seedlings had shorter hypocotyls than the transgenic GFP (Green Fluorescence Protein) control, while mutants had no difference from wild-type. A qRT-PCR assay demonstrated that expression of multiple ACSs and ACO5 was up-regulated in WRKY29 overexpression plants. A transactivation assay through dual luciferase reporter system confirmed the regulation of promoters of ACS5, ACS6, ACS8, ACS11 and ACO5 by WRKY29. Both in vivo chromatin immunoprecipitation (ChIP)- quantitative PCR and in vitro electrophoretic mobility shift assay (EMSA) revealed that WRKY29 directly bound to the promoter regions of its target genes. Taken together, these results suggest that WRKY29 is a novel TF positively regulating ethylene production by modulating the expression of ACS and ACO genes.

Molecular hydrogen positively influences lateral root formation by regulating hydrogen peroxide signaling

Although a previous study discovered that exogenous molecular hydrogen (H₂) supplied with hydrogen-rich water (HRW) can mediate lateral root (LR) development, whether or how endogenous H₂ influences LR formation is still elusive. In this report, mimicking the induction responses in tomato seedlings achieved by HRW or exogenous hydrogen peroxide (H₂O₂; a positive control), transgenic Arabidopsis that overexpressed the hydrogenase1 gene (CrHYD1) from Chlamydomonas reinhardtii not only stimulated endogenous hydrogen peroxide (H₂O₂) production, but also markedly promoted LR formation. Above H₂ and H₂O₂ responses were abolished by the removal of endogenous H₂O₂. Moreover, the changes in transcriptional patterns of representative cell cycle genes and auxin signaling-related genes during LR development in both tomato and transgenic Arabidopsis thaliana matched with above phenotypes. The alternations in the levels of GUS transcripts driven by the CYCB1 promoter and expression of PIN1 protein further indicated that H₂O₂ synthesis was tightly linked to LR formation achieved by endogenous H₂, and cell cycle regulation and auxin-dependent pathway might be their targets. There results might provide a reference for molecular mechanism underlying the regulation of root morphogenesis by H₂.

Ethylene response factor ERF.D7 activates auxin response factor 2 paralogs to regulate tomato fruit ripening

Despite the obligatory role of ethylene in climacteric fruit ripening and the identification of 77 ethylene response factors (ERFs) in the tomato (Solanum lycopersicum) genome, the role of few ERFs has been validated in the ripening process. Here, using a comprehensive morpho-physiological, molecular, and biochemical approach, we demonstrate the regulatory role of ERF D7 (SlERF.D7) in tomato fruit ripening. SlERF.D7 expression positively responded to exogenous ethylene and auxin treatments, most likely in a ripening inhibitor-independent manner. SlERF.D7 overexpression (OE) promoted ripening, and its silencing had the opposite effect. Alterations in its expression modulated ethylene production, pigment accumulation, and fruit firmness. Consistently, genes involved in ethylene biosynthesis and signaling, lycopene biosynthesis, and cell wall loosening were upregulated in the OE lines and downregulated in RNAi lines. These transgenic lines also accumulated altered levels of indole-3-acetic acid at late-breaker stages. A positive association between auxin response factor 2 (ARF2) paralog's transcripts and SlERF.D7 mRNA levels and that SlARF2A and SlARF2B are direct targets of SlERF.D7 underpinned the perturbed auxin-ethylene crosstalk for the altered ripening program observed in the transgenic fruits. Overall, this study uncovers that SlERF.D7 positively regulates SlARF2A/B abundance to amalgamate auxin and ethylene signaling pathways for controlling tomato fruit ripening.

Actin isovariant ACT7 controls root meristem development in Arabidopsis through modulating auxin and ethylene responses

The meristem is the most functionally dynamic part in a plant. The shaping of the meristem requires constant cell division and elongation, which are influenced by hormones and the cytoskeletal component, actin. Although the roles of hormones in modulating meristem development have been extensively studied, the role of actin in this process is still elusive. Using the single and double mutants of the vegetative class actin, we demonstrate that actin isovariant ACT7 plays an important role in root meristem development. In the absence of ACT7, but not ACT8 and ACT2, depolymerization of actin was observed. Consistently, the act7 mutant showed reduced cell division, cell elongation, and meristem length. Intracellular distribution and trafficking of auxin transport proteins in the actin mutants revealed that ACT7 specifically functions in the root meristem to facilitate the trafficking of auxin efflux carriers PIN1 and PIN2, and consequently the transport of auxin. Compared with act7, the act7act8 double mutant exhibited slightly enhanced phenotypic response and altered intracellular trafficking. The altered distribution of auxin in act7 and act7act8 affects the response of the roots to ethylene, but not to cytokinin. Collectively, our results suggest that ACT7-dependent auxin-ethylene response plays a key role in controlling Arabidopsis root meristem development.

Phyto-Friendly Soil Bacteria and Fungi Provide Beneficial Outcomes in the Host Plant by Differently Modulating Its Responses through (In)Direct Mechanisms

Sustainable agricultural systems based on the application of phyto-friendly bacteria and fungi are increasingly needed to preserve soil fertility and microbial biodiversity, as well as to reduce the use of chemical fertilizers and pesticides. Although there is considerable attention on the potential applications of microbial consortia as biofertilizers and biocontrol agents for crop management, knowledge on the molecular responses modulated in host plants because of these beneficial associations is still incomplete. This review provides an up-to-date overview of the different mechanisms of action triggered by plant-growth-promoting microorganisms (PGPMs) to promote host-plant growth and improve its defense system. In addition, we combined available gene-expression profiling data from tomato roots sampled in the early stages of interaction with Pseudomonas or Trichoderma strains to develop an integrated model that describes the common processes activated by both PGPMs and highlights the host's different responses to the two microorganisms. All the information gathered will help define new strategies for the selection of crop varieties with a better ability to benefit from the elicitation of microbial inoculants.

Expression analysis of PIN family genes in Chinese hickory reveals their potential roles during grafting and salt stress

Grafting is an effective way to improve Chinese hickory while salt stress has caused great damage to the Chinese hickory industry. Grafting and salt stress have been regarded as the main abiotic stress types for Chinese hickory. However, how Chinese hickory responds to grafting and salt stress is less studied. Auxin has been proved to play an essential role in the stress response through its re-distribution regulation mediated by polar auxin transporters, including PIN-formed (PIN) proteins. In this study, the PIN gene family in Chinese hickory (CcPINs) was identified and structurally characterized for the first time. The expression profiles of the genes in response to grafting and salt stress were determined. A total of 11 CcPINs with the open reading frames (ORFs) of 1,026-1,983 bp were identified. Transient transformation in tobacco leaves demonstrated that CcPIN1a, CcPIN3, and CcPIN4 were localized in the plasma membrane. There were varying phylogenetic relationships between CcPINs and homologous genes in different species, but the closest relationships were with those in Carya illinoinensis and Juglans regia. Conserved N- and C-terminal transmembrane regions as well as sites controlling the functions of CcPINs were detected in CcPINs. Five types of cis-acting elements, including hormone- and stress-responsive elements, were detected on the promoters of CcPINs. CcPINs exhibited different expression profiles in different tissues, indicating their varied roles during growth and development. The 11 CcPINs responded differently to grafting and salt stress treatment. CcPIN1a might be involved in the regulation of the grafting process, while CcPIN1a and CcPIN8a were related to the regulation of salt stress in Chinese hickory. Our results will lay the foundation for understanding the potential regulatory functions of CcPIN genes during grafting and under salt stress treatment in Chinese hickory.

A Plant Growth Promoting of Rhizobacteria and Endophytic Bacteria in Vegetable Rhizosphere and Root Samples

Rhizobacteria and endophytic bacteria are popular for its abilities in influencing plant growth and development. The strategy employed these bacteria as biofertilizer for planting is believed to bring several benefits such as low cost, eco-friendly, and feasible. One of the remarkable products for plant growth promoting provided by rhizobacteria and endophytic bacteria were the advantageous enzymes such as 1-aminocyclopropane-1-carboxylate deaminase, phosphatase, and cellulase. These biocatalysts then involve in several direct or indirect pathways of nutrient, growth factor, and/or defense factor synthesizes. From five different essential leafy vegetables in Thailand, this study aimed to investigate the plant growth promoting potentials of endophytic bacteria and rhizobacteria isolated from root tissue and rhizosphere, respectively, via IAA quantitative and enzyme activity assays. The selected bacterial strains were further identified using 16S rRNA gene sequencing and observed their interaction with plant root using scanning electron microscope method. Our study, thus far, has isolated two bacterial strains of Bacillus subtilis MSE5 and Bacillus cereus AVR1, respectively, with multifunctional traits of potential on the plant growth. Importantly, these two strains of MSE5 and AVR1 had shown the capacity to advance root colonization. Therefore, MSE5 and AVR1 are recommended for further studies in developing eco-friendly biofertilizer. In addition, some novel cellulose-degrading bacterial strains with significant potential on hydrolysis capacity were also isolated that might be valuable for industrial applications.

The interaction between exogenous IBA with sucrose, light and ventilation alters the expression of ARFs and Aux/IAA genes in Carica papaya plantlets

The interaction between exogenous IBA with sucrose, light and ventilation, alters the expression of ARFs and Aux/IAA genes in in vitro grown Carica papaya plantlets. In vitro papaya plantlets normally show low rooting percentages during their ex vitro establishment that eventually leads to high mortality when transferred to field conditions. Indole-3-butyric acid (IBA) auxin is normally added to culture medium, to achieve adventitious root formation on in vitro papaya plantlets. However, the molecular mechanisms occurring when IBA is added to the medium under varying external conditions of sugar, light and ventilation have not been studied. Auxin response factors (ARF) are auxin-transcription activators, while auxin/indole-3-acetic acid (Aux/IAA) are auxin-transcription repressors, that modulate key components involved in auxin signaling in plants. In the present study, we identified 12 CpARF and 18 CpAux/IAA sequences in the papaya genome. The cis-acting regulatory elements associated to those CpARFs and CpAux/IAA gene families were associated with stress and hormone responses. Furthermore, a comprehensive characterization and expression profiling analysis was performed on 6 genes involved in rhizogenesis formation (CpARF5, 6, 7 and CpAux/IAA11, 13, 14) from in vitro papaya plantlets exposed to different rhizogenesis-inducing treatments. In general, intact in vitro plantlets were not able to produce adventitious roots, when IBA (2 mg L^-1) was added to the culture medium; they became capable to produce roots and increased their ex-vitro survival. However, the best rooting and survival % were obtained when IBA was added in combination with adequate sucrose supply (20 g L^-1), increased light intensity (750 µmol photon m^-2 s^-1) and ventilation systems within the culture vessel. Interestingly, it was precisely under those conditions that promoted high rooting and survival %, where the highest expression of CpARFs, but the lowest expression of CpAux/IAAs occurred. One interesting case occurred when in vitro plantlets were exposed to high levels of light in the absence of added IBA, as high rooting and survival occurred, even though no exogenous auxin was added. In fact, plantlets from this treatment showed the right expression profile between auxin activators/repressors genes, in both stem base and root tissues.

Ethylene Insensitive 3-Like 2 is a Brassicaceae-specific transcriptional regulator involved in fine-tuning ethylene responses in Arabidopsis thaliana

Ethylene signaling directs a pleiotropy of developmental processes in plants. In Arabidopsis, ethylene signaling converges at the master transcription factor Ethylene Insensitive 3 (EIN3), which has five homologs, EIN3-like 1-5 (EIL1-EIL5). EIL1 is most fully characterized and operates similarly to EIN3, while EIL3-5 are not involved in ethylene signaling. EIL2 remains less investigated. Our phylogenetic analysis revealed that EIL2 homologs have only been retrieved in the Brassicaceae family, suggesting that EIL2 diverged to have specific functions in the mustard family. By characterizing eil2 mutants, we found that EIL2 is involved in regulating ethylene-specific developmental processes in Arabidopsis thaliana, albeit in a more subtle way compared with EIN3/EIL1. EIL2 steers ethylene-triggered hypocotyl elongation in light-grown seedlings and is involved in lateral root formation. Furthermore, EIL2 takes part in regulating flowering time as eil2 mutants flower on average 1 d earlier and have fewer leaves. A pEIL2:EIL2:GFP translational reporter line revealed that EIL2 protein abundance is restricted to the stele of young developing roots. EIL2 expression, and not EIL2 protein stability, is regulated by ethylene in an EIN3/EIL1-dependent way. Despite EIL2 taking part in several developmental processes, the precise upstream and downstream regulation of this ethylene- and Brassicaceae-specific transcription factor remains to be elucidated.

Transcriptomic Insights Into Root Development and Overwintering Transcriptional Memory of Brassica rapa L. Grown in the Field

As the only overwintering oil crop in the north area of China, living through winter is the primary feature of winter rapeseed. Roots are the only survival organ during prolonged cold exposure during winter to guarantee flowering in spring. However, little is known about its root development and overwintering memory mechanism. In this study, root collar tissues (including the shoot apical meristem) of three winter rapeseed varieties with different cold resistance, i.e., Longyou-7 (strong cold tolerance), Tianyou-4 (middle cold tolerance), and Lenox (cold-sensitive), were sampled in the pre-winter period (S1), overwintering periods (S2-S5), and re-greening stage (S6), and were used to identify the root development and overwintering memory mechanisms and seek candidate overwintering memory genes by measuring root collar diameter and RNA sequencing. The results showed that the S1-S2 stages were the significant developmental stages of the roots as root collar diameter increased slowly in the S3-S5 stages, and the roots developed fast in the strong cold resistance variety than in the weak cold resistance variety. Subsequently, the RNA-seq analysis revealed that a total of 37,905, 45,102, and 39,276 differentially expressed genes (DEGs), compared to the S1 stage, were identified in Longyou-7, Tianyou-4, and Lenox, respectively. The function enrichment analysis showed that most of the DEGs are significantly involved in phenylpropanoid biosynthesis, plant hormone signal transduction, MAPK signaling pathway, starch and sucrose metabolism, photosynthesis, amino sugar and nucleotide sugar metabolism, and spliceosome, ribosome, proteasome, and protein processing in endoplasmic reticulum pathways. Furthermore, the phenylpropanoid biosynthesis and plant hormone signal transduction pathways were related to the difference in root development of the three varieties, DEGs involved in photosynthesis and carbohydrate metabolism processes may participate in overwintering memory of Longyou-7 and Tianyou-4, and the spliceosome pathway may contribute to the super winter resistance of Longyou-7. The transcription factor enrichment analysis showed that the WRKY family made up the majority in different stages and may play an important regulatory role in root development and overwintering memory. These results provide a comprehensive insight into winter rapeseed's complex overwintering memory mechanisms. The identified candidate overwintering memory genes may also serve as important genetic resources for breeding to further improve the cold resistance of winter rapeseed.

Ethylene signaling increases reactive oxygen species accumulation to drive root hair initiation in Arabidopsis

Root hair initiation is a highly regulated aspect of root development. The plant hormone ethylene and its precursor, 1-amino-cyclopropane-1-carboxylic acid, induce formation and elongation of root hairs. Using confocal microscopy paired with redox biosensors and dyes, we demonstrated that treatments that elevate ethylene levels lead to increased hydrogen peroxide accumulation in hair cells prior to root hair formation. In the ethylene-insensitive receptor mutant, etr1-3, and the signaling double mutant, ein3eil1, the increase in root hair number or reactive oxygen species (ROS) accumulation after ACC and ethylene treatment was lost. Conversely, etr1-7, a constitutive ethylene signaling receptor mutant, has increased root hair formation and ROS accumulation, similar to ethylene-treated Col-0 seedlings. The caprice and werewolf transcription factor mutants have decreased and elevated ROS levels, respectively, which are correlated with levels of root hair initiation. The rhd2-6 mutant, with a defect in the gene encoding the ROS-synthesizing RESPIRATORY BURST OXIDASE HOMOLOG C (RBOHC), and the prx44-2 mutant, which is defective in a class III peroxidase, showed impaired ethylene-dependent ROS synthesis and root hair formation via EIN3EIL1-dependent transcriptional regulation. Together, these results indicate that ethylene increases ROS accumulation through RBOHC and PRX44 to drive root hair formation.

Bradyrhizobium japonicum IRAT FA3 Alters Arabidopsis thaliana Root Architecture via Regulation of Auxin Efflux Transporters PIN2, PIN3, PIN7, and ABCB19

Beneficial rhizobacteria can stimulate changes in plant root development. Although root system growth is mediated by multiple factors, the regulated distribution of the phytohormone auxin within root tissues plays a principal role. Auxin transport facilitators help to generate the auxin gradients and maxima that determine root structure. Here, we show that the plant-growth-promoting rhizobacterial strain Bradyrhizobium japonicum IRAT FA3 influences specific auxin efflux transporters to alter Arabidopsis thaliana root morphology. Gene expression profiling of host transcripts in control and B. japonicum-inoculated roots of the wild-type A. thaliana accession Col-0 confirmed upregulation of PIN2, PIN3, PIN7, and ABCB19 with B. japonicum and identified genes potentially contributing to a diverse array of auxin-related responses. Cocultivation of the bacterium with loss-of-function auxin efflux transport mutants revealed that B. japonicum requires PIN3, PIN7, and ABCB19 to increase lateral root development and utilizes PIN2 to reduce primary root length. Accelerated lateral root primordia production due to B. japonicum was not observed in single pin3, pin7, or abcb19 mutants, suggesting independent roles for PIN3, PIN7, and ABCB19 during the plant-microbe interaction. Our work demonstrates B. japonicum's influence over host transcriptional reprogramming during plant interaction with this beneficial microbe and the subsequent alterations to root system architecture.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Action mechanisms of small microbial volatile compounds in plants

Microorganisms communicate with plants by exchanging chemical signals throughout the phytosphere. Before direct contact with plants occurs, beneficial microorganisms emit a plethora of volatile compounds that promote plant growth and photosynthesis as well as developmental, metabolic, transcriptional, and proteomic changes in plants. These compounds can also induce systemic drought tolerance and improve water and nutrient acquisition. Recent studies have shown that this capacity is not restricted to beneficial microbes; it also extends to phytopathogens. Plant responses to microbial volatile compounds have frequently been associated with volatile organic compounds with molecular masses ranging between ~ 45Da and 300Da. However, microorganisms also release a limited number of volatile compounds with molecular masses of less than ~45Da that react with proteins and/or act as signaling molecules. Some of these compounds promote photosynthesis and growth when exogenously applied in low concentrations. Recently, evidence has shown that small volatile compounds are important determinants of plant responses to microbial volatile emissions. However, the regulatory mechanisms involved in these responses remain poorly understood. This review summarizes current knowledge of biochemical and molecular mechanisms involved in plant growth, development, and metabolic responses to small microbial volatile compounds.

SbbHLH85, a bHLH member, modulates resilience to salt stress by regulating root hair growth in sorghum

bHLH family proteins play an important role in plant stress response. However, the molecular mechanism regulating the salt response of bHLH is largely unknown. This study aimed to investigate the function and regulating mechanism of the sweet sorghum SbbHLH85 during salt stress. The results showed that SbbHLH85 was different from its homologs in other species. Also, it was a new atypical bHLH transcription factor and a key gene for root development in sweet sorghum. The overexpression of SbbHLH85 resulted in significantly increased number and length of root hairs via ABA and auxin signaling pathways, increasing the absorption of Na⁺. Thus, SbbHLH85 plays a negative regulatory role in the salt tolerance of sorghum. We identified a potential interaction partner of SbbHLH85, which was phosphate transporter chaperone PHF1 and modulated the distribution of phosphate, through screening a yeast two-hybrid library. Both yeast two-hybrid and BiFC experiments confirmed the interaction between SbbHLH85 and PHF1. The overexpression of SbbHLH85 led to a decrease in the expression of PHF1 as well as the content of Pi. Based on these results, we suggested that the increase in the Na⁺ content and the decrease in the Pi content resulted in the salt sensitivity of transgenic sorghum.

Nitric oxide, gravity response, and a unified schematic of plant signaling

Plant signaling components are often involved in numerous processes. Calcium, reactive oxygen species, and other signaling molecules are essential to normal biotic and abiotic responses. Yet, the summation of these components is integrated to produce a specific response despite their involvement in a myriad of response cascades. In the response to gravity, the role of many of these individual components has been studied, but a specific sequence of signals has not yet been assembled into a cohesive schematic of gravity response signaling. Herein, we provide a review of existing knowledge of gravity response and differential protein and gene regulation induced by the absence of gravity stimulus aboard the International Space Station and propose an integrated theoretical schematic of gravity response incorporating that information. Recent developments in the role of nitric oxide in gravity signaling provided some of the final contextual pillars for the assembly of the model, where nitric oxide and the role of cysteine S-nitrosation may be central to the gravity response. The proposed schematic accounts for the known responses to reorientation with respect to gravity in roots-the most well studied gravitropic plant tissue-and is supported by the extensive evolutionary conservation of regulatory amino acids within protein components of the signaling schematic. The identification of a role of nitric oxide in regulating the TIR1 auxin receptor is indicative of the broader relevance of the schematic in studying a multitude of environmental and stress responses. Finally, there are several experimental approaches that are highlighted as essential to the further study and validation of this schematic.

PIN3-mediated auxin transport contributes to blue light-induced adventitious root formation in Arabidopsis

Adventitious rooting is a heritable quantitative trait that is influenced by multiple endogenous and exogenous factors in plants, and one important environmental factor required for efficient adventitious root formation is light signaling. However, the physiological significance and molecular mechanism of light underlying adventitious root formation are still largely unexplored. Here, we report that blue light-induced adventitious root formation is regulated by PIN-FORMED3 (PIN3)-mediated auxin transport in Arabidopsis. Adventitious root formation is significantly impaired in the loss-of-function mutants of the blue light receptors, PHOTOROPIN1 (PHOT1) and PHOTOROPIN2 (PHOT2), as well as the phototropic transducer, NON-PHOTOTROPIC HYPOCOTYL3 (NPH3). In addition, blue light enhanced the auxin content in the adventitious root, and the pin3 loss-of-function mutant had a reduced adventitious rooting response under blue light compared to the wild type. The PIN3 protein level was higher in plants treated with blue light than in those in darkness, especially in the hypocotyl pericycle, while PIN3-GFP failed to accumulate in nph3 PIN3::PIN3-GFP. Furthermore, the results showed that PIN3 physically interacted with NPH3, a key transducer in phototropic signaling. Taken together, our study demonstrates that blue light induces adventitious root formation through the phototropic signal transducer, NPH3, which regulates adventitious root formation by affecting PIN3-mediated auxin transport.

Auxin Interactions with Other Hormones in Plant Development

Auxin is a crucial growth regulator that governs plant development and responses to environmental perturbations. It functions at the heart of many developmental processes, from embryogenesis to organ senescence, and is key to plant interactions with the environment, including responses to biotic and abiotic stimuli. As remarkable as auxin is, it does not act alone, but rather solicits the help of, or is solicited by, other endogenous signals, including the plant hormones abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellic acid, jasmonates, salicylic acid, and strigolactones. The interactions between auxin and other hormones occur at multiple levels: hormones regulate one another's synthesis, transport, and/or response; hormone-specific transcriptional regulators for different pathways physically interact and/or converge on common target genes; etc. However, our understanding of this crosstalk is still fragmentary, with only a few pieces of the gigantic puzzle firmly established. In this review, we provide a glimpse into the complexity of hormone interactions that involve auxin, underscoring how patchy our current understanding is.