FAMA is an essential component for the differentiation of two distinct cell types, myrosin cells and guard cells, in Arabidopsis

The dynamic and diverse nature of parenchyma cells in the Arabidopsis root during secondary growth

During secondary growth, the vascular cambium produces conductive xylem and phloem cells, while the phellogen (cork cambium) deposits phellem (cork) as the outermost protective barrier. Although most of the secondary tissues are made up of parenchyma cells, which are also produced by both cambia, their diversity and function are poorly understood. Here we combined single-cell RNA sequencing analysis with lineage tracing to recreate developmental trajectories of the cell types in the Arabidopsis root undergoing secondary growth. By analysing 93 reporter lines, we were able to identify 20 different cell types or cell states, many of which have not been described before. We additionally observed distinct transcriptome signatures of parenchyma cells depending on their maturation state and proximity to the conductive cell types. Our data show that both xylem and phloem parenchyma tissues are required for normal formation of conductive tissue cell types. Furthermore, we show that mature phloem parenchyma gradually obtains periderm identity, and this transformation can be accelerated by jasmonate treatment or wounding. Our study thus reveals the diversity of parenchyma cells and their capacity to undergo considerable identity changes during secondary growth.

The Arabidopsis F-box protein FBS associated with the helix-loop-helix transcription factor FAMA involved in stomatal immunity

Stomatal pores serve as primary entry points for pathogen invasion. Stomatal closure is a crucial strategy that plants employ to counter pathogen attack. Here, we report that F-BOX STRESS-INDUCED (FBS) is essential for modulating stomatal closure, thereby enhancing resistance to bacteria in Arabidopsis thaliana. The fbs2-1 fbs3-1 fbs4-2 triple mutant displayed increased susceptibility to Pseudomonas syringae pv. tomato (PstDC3000) due to impaired stomatal closure. Additionally, FBS4 interacts with and degrades the basic helix-loop-helix (bHLH) transcription factor FAMA. Both the fama-1 single mutant plants and the fama-1 fbs2-1 fbs3-1 fbs4-2 quadruple mutant plants exhibited resistance to PstDC3000 inoculation. Furthermore, the expression levels of abscisic acid (ABA)-responsive genes RD29A, RD29B, ABI2, and CIPK25 were altered in the fbs2-1 fbs3-1 fbs4-2 and fama-1 mutant plants. Collectively, our data demonstrate that FBS, in association with FAMA, plays an important role in pathogen invasion by influencing ABA signaling-related stomatal closure.

Co-option and neofunctionalization of stomatal executors for defence against herbivores in Brassicales

Co-option of gene regulatory networks leads to the acquisition of new cell types and tissues. Stomata, valves formed by guard cells (GCs), are present in most land plants and regulate CO2 exchange. The transcription factor (TF) FAMA globally regulates GC differentiation. In the Brassicales, FAMA also promotes the development of idioblast myrosin cells (MCs), another type of specialized cell along the vasculature essential for Brassicales-specific chemical defences. Here we show that in Arabidopsis thaliana, FAMA directly induces the TF gene WASABI MAKER (WSB), which triggers MC differentiation. WSB and STOMATAL CARPENTER 1 (SCAP1, a stomatal lineage-specific direct FAMA target), synergistically promote GC differentiation. wsb mutants lacked MCs and the wsb scap1 double mutant lacked normal GCs. Evolutionary analyses revealed that WSB is conserved across stomatous angiosperms. We propose that the conserved and reduced transcriptional FAMA-WSB module was co-opted before evolving to induce MC differentiation.

Small molecules and heat treatments reverse vernalization via epigenetic modification in Arabidopsis

Monocarpic plants flower only once and then produce seeds. Many monocarpic plants require a cold treatment known as vernalization before they flower. This requirement delays flowering until the plant senses warm temperatures in the spring. Exposure to high temperatures following vernalization causes devernalization, which cancels the vernalized state, inhibiting flowering and promoting vegetative growth. In this study, we screened over 16,000 chemical compounds and identified five small molecules (devernalizers; DVRs) that induce devernalization in Arabidopsis thaliana at room temperature without requiring a high-temperature treatment. Treatment with DVRs reactivated the expression of FLOWERING LOCUS C (FLC), a master repressor of flowering, by reducing the deposition of repressive histone modifications, thereby delaying flowering time. Three of the DVRs identified shared two structures: a hydantoin-like region and a spiro-like carbon. Treatment with DVR06, which has a simple chemical structure containing these domains, delayed flowering time and reduced the deposition of repressive histone modifications at FLC. RNA-seq and ChIP-seq analyses revealed both shared and specific transcriptomic and epigenetic effects between DVR06- and heat-induced devernalization. Overall, our extensive chemical screening indicated that hydantoin and spiro are key chemical signatures that reduce repressive histone modifications and promote devernalization in plants.

Stem Cells and Differentiation in Vascular Tissues

Plant vascular tissues are crucial for the long-distance transport of water, nutrients, and a multitude of signal molecules throughout the plant body and, therefore, central to plant growth and development. The intricate development of vascular tissues is orchestrated by unique populations of dedicated stem cells integrating endogenous as well as environmental cues. This review summarizes our current understanding of vascular-related stem cell biology and of vascular tissue differentiation. We present an overview of the molecular and cellular mechanisms governing the maintenance and fate determination of vascular stem cells and highlight the interplay between intrinsic and external cues. In this context, we emphasize the role of transcription factors, hormonal signaling, and epigenetic modifications. We also discuss emerging technologies and the large repertoire of cell types associated with vascular tissues, which have the potential to provide unprecedented insights into cellular specialization and anatomical adaptations to distinct ecological niches.

Beyond stomatal development: SMF transcription factors as versatile toolkits for land plant evolution

As master transcription factors of stomatal development, SPEECHLESS, MUTE, and FAMA, collectively termed SMFs, are primary targets of molecular genetic analyses in the model plant Arabidopsis thaliana. Studies in other model systems identified SMF orthologs as key players in evolutionary developmental biology studies on stomata. However, recent studies on the astomatous liverwort Marchantia polymorpha revealed that the functions of these genes are not limited to the stomatal development, but extend to other types of tissues, namely sporophytic setal and gametophytic epidermal tissues. These studies provide insightful examples of gene-regulatory network co-opting, and highlight SMFs and related transcription factors as general toolkits for novel trait evolution in land plant lineages. Here, we critically review recent literature on the SMF-like gene in M. polymorpha and discuss their implications for plant evolutionary biology.

Integration of cell differentiation and initiation of monoterpenoid indole alkaloid metabolism in seed germination of Catharanthus roseus

In Catharanthus roseus, monoterpenoid indole alkaloids (MIAs) are produced through the cooperation of four cell types, with final products accumulating in specialized cells known as idioblasts and laticifers. To explore the relationship between cellular differentiation and cell type-specific MIA metabolism, we analyzed the expression of MIA biosynthesis in germinating seeds. Embryos from immature and mature seeds were observed via stereomicroscopy, fluorescence microscopy, and electron microscopy. Time-series MIA and iridoid quantification, along with transcriptome analysis, were conducted to determine the initiation of MIA biosynthesis. In addition, the localization of MIAs was examined using alkaloid staining and imaging mass spectrometry (IMS). Laticifers were present in embryos before seed maturation. MIA biosynthesis commenced 12 h after germination. MIAs accumulated in laticifers of embryos following seed germination, and MIA metabolism is induced after germination in a tissue-specific manner. These findings suggest that cellular morphological differentiation precedes metabolic differentiation. Considering the well-known toxicity and defense role of MIAs in matured plants, MIAs may be an important defense strategy already in the delicate developmental phase of seed germination, and biosynthesis and accumulation of MIAs may require the tissue and cellular differentiation.

Bna.EPF2 Enhances Drought Tolerance by Regulating Stomatal Development and Stomatal Size in Brassica napus

Drought stress severely affects global plant growth and production. The enhancement of plant water-use efficiency (WUE) and drought tolerance by the manipulation of the stomata is an effective strategy to deal with water shortage. However, increasing the WUE and drought tolerance by manipulation on the stomata has rarely been tested in Brassica napus. Here, we isolated Bna.EPF2, an epidermal patterning factor (EPF) in Brassica napus (ecotype Westar), and identified its role in drought performance. Bna.EPF2 overexpression lines had a reduction average of 19.02% in abaxial stomatal density and smaller stomatal pore size, leading to approximately 25% lower transpiration, which finally resulted in greater instantaneous WUE and enhanced drought tolerance. Interestingly, the reduction in stomatal density did not affect the CO₂ assimilation or yield-related agronomic traits in Bna.EPF2 overexpression plants. Together with the complementation of Bna.EPF2 significantly decreasing the stomatal density of Arabidopsis epf2, and Bna.EPF2 being expressed in mature guard cells, these results suggest that Bna.EPF2 not only functions in stomatal density development, but also in stomatal dimension in Brassicas. Taken together, our results suggest that Bna.EPF2 improves WUE and drought tolerance by the regulation of stomatal density and stomatal size in Brassica without growth and yield penalty, and provide insight into the manipulation of this gene in the breeding of drought tolerant plants with increased production under water deficit conditions.

Single-Cell RNA Sequencing of Arabidopsis Leaf Tissues Identifies Multiple Specialized Cell Types: Idioblast Myrosin Cells and Potential Glucosinolate-Producing Cells

The glucosinolate-myrosinase defense system (GMDS), characteristic of Brassicales, is involved in plant defense. Previous single-cell transcriptomic analyses have reported the expression profiles of multiple GMDS-related cell types (i.e. myrosinase-rich myrosin idioblasts and multiple types of potential glucosinolate synthetic cells as well as a candidate S-cell for glucosinolate accumulation). However, differences in plant stages and cell-type annotation methods have hindered comparisons among studies. Here, we used the single-cell transcriptome profiles of extended Arabidopsis leaves and verified the distribution of previously used markers to refine the expression profiles of GMDS-associated cell types. Moreover, we performed beta-glucuronidase promoter assays to confirm the histological expression patterns of newly obtained markers for GMDS-associated candidates. As a result, we found a set of new specific reporters for myrosin cells and potential glucosinolate-producing cells.

Targeted expression of bgl23-D, a dominant-negative allele of ATCSLD5, affects cytokinesis of guard mother cells and exine formation of pollen in Arabidopsis thaliana

Targeted expression of bgl23-D, a dominant-negative allele of ATCSLD5, is a useful genetic approach for functional analysis of ATCSLDs in specific cells and tissues in plants. Stomata are key cellular structures for gas and water exchange in plants and their development is influenced by several genes. We found the A. thaliana bagel23-D (bgl23-D) mutant showing abnormal bagel-shaped single guard cells. The bgl23-D was a novel dominant mutation in the A. thaliana cellulose synthase-like D5 (ATCSLD5) gene that was reported to function in the division of guard mother cells. The dominant character of bgl23-D was used to inhibit ATCSLD5 function in specific cells and tissues. Transgenic A. thaliana expressing bgl23-D cDNA with the promoter of stomata lineage genes, SDD1, MUTE, and FAMA, showed bagel-shaped stomata as observed in the bgl23-D mutant. Especially, the FAMA promoter exhibited a higher frequency of bagel-shaped stomata with severe cytokinesis defects. Expression of bgl23-D cDNA in the tapetum with SP11 promoter or in the anther with ATSP146 promoter induced defects in exine pattern and pollen shape, novel phenotypes that were not shown in the bgl23-D mutant. These results indicated that bgl23-D inhibited unknown ATCSLD(s) that exert the function of exine formation in the tapetum. Furthermore, transgenic A. thaliana expressing bgl23-D cDNA with SDD1, MUTE, and FAMA promoters showed enhanced rosette diameter and increased leaf growth. Taken together, these findings suggest that the bgl23-D mutation could be a helpful genetic tool for functional analysis of ATCSLDs and manipulating plant growth.

Expanded roles and divergent regulation of FAMA in Brachypodium and Arabidopsis stomatal development

Stomata, cellular valves found on the surfaces of aerial plant tissues, present a paradigm for studying cell fate and patterning in plants. A highly conserved core set of related basic helix-loop-helix (bHLH) transcription factors regulates stomatal development across diverse species. We characterized BdFAMA in the temperate grass Brachypodium distachyon and found this late-acting transcription factor was necessary and sufficient for specifying stomatal guard cell fate, and unexpectedly, could also induce the recruitment of subsidiary cells in the absence of its paralogue, BdMUTE. The overlap in function is paralleled by an overlap in expression pattern and by unique regulatory relationships between BdMUTE and BdFAMA. To better appreciate the relationships among the Brachypodium stomatal bHLHs, we used in vivo proteomics in developing leaves and found evidence for multiple shared interaction partners. We reexamined the roles of these genes in Arabidopsis thaliana by testing genetic sufficiency within and across species, and found that while BdFAMA and AtFAMA can rescue stomatal production in Arabidopsis fama and mute mutants, only AtFAMA can specify Brassica-specific myrosin idioblasts. Taken together, our findings refine the current models of stomatal bHLH function and regulatory feedback among paralogues within grasses as well as across the monocot/dicot divide.

Stomatal regulators are co-opted for seta development in the astomatous liverwort Marchantia polymorpha

The evolution of special types of cells requires the acquisition of new gene regulatory networks controlled by transcription factors (TFs). In stomatous plants, a TF module formed by subfamilies Ia and IIIb basic helix-loop-helix TFs (Ia-IIIb bHLH) regulates stomatal formation; however, how this module evolved during land plant diversification remains unclear. Here we show that, in the astomatous liverwort Marchantia polymorpha, a Ia-IIIb bHLH module regulates the development of a unique sporophyte tissue, the seta, which is found in mosses and liverworts. The sole Ia bHLH gene, MpSETA, and a IIIb bHLH gene, MpICE2, regulate the cell division and/or differentiation of seta lineage cells. MpSETA can partially replace the stomatal function of Ia bHLH TFs in Arabidopsis thaliana, suggesting that a common regulatory mechanism underlies setal and stomatal formation. Our findings reveal the co-option of a Ia-IIIb bHLH TF module for regulating cell fate determination and/or cell division of distinct types of cells during land plant evolution.

An optimized FACS-free single-nucleus RNA sequencing (snRNA-seq) method for plant science research

Recently, single-cell RNA sequencing (scRNA-seq) provides unprecedented power for accurately understanding gene expression regulatory mechanisms. However, scRNA-seq studies have limitations in plants, due to difficulty in protoplast isolation that requires enzymatic digestion of the cell walls from various plant tissues. Therefore, to overcome this problem, we developed a nuclei isolation approach that does not rely on Fluorescence Activated Cell Sorting (FACS). We validated the robustness of the FACS-free single-nucleus RNA sequencing (snRNA-seq) methodology in mature Arabidopsis plant tissue by comparing it to scRNA-seq results based on protoplasts extracted from the same batch of leaf materials. Sequencing results demonstrated the high quality of snRNA-seq data, as well as its utility in cell type classification and marker gene identification. This approach also showed several advantages, including the ability to use frozen samples, taking less suspension preparation time, and reducing biased cellular coverage and dissociation-induced transcriptional artifacts. Surprisingly, snRNA-seq detected two epidermal pavement cell clusters, while scRNA-seq only had one. Furthermore, we hypothesized that these two epidermal cells represent the top and lower epidermis based on differences in expression patterns of cluster-specific expressed genes. In summary, this study has advanced the application of snRNA-seq in Arabidopsis leaves and confirmed the advantages of snRNA-seq in plant research.

Research strategies for single-cell transcriptome analysis in plant leaves

The recent and continuous improvement in single-cell RNA sequencing (scRNA-seq) technology has led to its emergence as an efficient experimental approach in plant research. However, compared with single-cell research in animals and humans, the application of scRNA-seq in plant research is limited by several challenges, including cell separation, cell type annotation, cellular function analysis, and cell-cell communication networks. In addition, the unavailability of corresponding reliable and stable analysis methods and standards has resulted in the relative decentralization of plant single-cell research. Considering these shortcomings, this review summarizes the research progress in plant leaf using scRNA-seq. In addition, it describes the corresponding feasible analytical methods and associated difficulties and problems encountered in the current research. In the end, we provide a speculative overview of the development of plant single-cell transcriptome research in the future.

Leaf cell-specific and single-cell transcriptional profiling reveals a role for the palisade layer in UV light protection

Like other complex multicellular organisms, plants are composed of different cell types with specialized shapes and functions. For example, most laminar leaves consist of multiple photosynthetic cell types. These cell types include the palisade mesophyll, which typically forms one or more cell layers on the adaxial side of the leaf. Despite their importance for photosynthesis, we know little about how palisade cells differ at the molecular level from other photosynthetic cell types. To this end, we have used a combination of cell-specific profiling using fluorescence-activated cell sorting and single-cell RNA-sequencing methods to generate a transcriptional blueprint of the palisade mesophyll in Arabidopsis thaliana leaves. We find that despite their unique morphology, palisade cells are otherwise transcriptionally similar to other photosynthetic cell types. Nevertheless, we show that some genes in the phenylpropanoid biosynthesis pathway have both palisade-enriched expression and are light-regulated. Phenylpropanoid gene activity in the palisade was required for production of the ultraviolet (UV)-B protectant sinapoylmalate, which may protect the palisade and/or other leaf cells against damaging UV light. These findings improve our understanding of how different photosynthetic cell types in the leaf can function uniquely to optimize leaf performance, despite their transcriptional similarities.

Histone Demethylases JMJ30 and JMJ32 Modulate the Speed of Vernalization Through the Activation of FLOWERING LOCUS C in Arabidopsis thaliana

Vernalization is the promotion of flowering after prolonged exposure to cold. In Arabidopsis thaliana, vernalization induces epigenetic silencing of the floral repressor gene FLOWERING LOCUS C (FLC). Among the repressive epigenetic marks, the trimethylation of lysine 27 on histone H3 proteins (H3K27me3) is a critical contributor to the epigenetic silencing of FLC. The deposition of H3K27me3 is mediated by Polycomb Repressive Complex 2 (PRC2). Conversely, the elimination of H3K27me3 is mediated by histone demethylases, Jumonji-C domain-containing protein JMJ30 and its homolog JMJ32. However, the role of JMJ30 and JMJ32 in vernalization is largely unknown. In this study, we found that cold treatment dramatically reduced the expression levels of JMJ30 and did not reduce those of JMJ32. Next, by using the genetic approach, we found that the flowering of jmj30 jmj32 was accelerated under moderate vernalized conditions. Under moderate vernalized conditions, the silencing of FLC occurred more quickly in jmj30 jmj32 than in the wild type. These results suggested that the histone demethylases JMJ30 and JMJ32 brake vernalization through the activation of FLC. Our study suggested that PRC2 and Jumonji histone demethylases act in an opposing manner to regulate flowering time via epigenetic modifications.

Two independent cis-acting elements are required for the guard cell-specific expression of SCAP1, which is essential for late stomatal development

Regulating the stomatal aperture to adapt to environmental changes is critical for plants as stomatal guard cells are responsible for gas exchange between plants and the atmosphere. We previously showed that a plant-specific DNA-binding with one finger (Dof)-type transcription factor, SCAP1, functions as a key regulator in the final stages of guard cell differentiation. In the present study, we performed deletion and gain-of-function analyses with the 5' flanking region of SCAP1 to identify the regulatory region controlling the guard cell-specific expression of SCAP1. The results revealed that two cis-acting elements, 5'-CACGAGA-3' and 5'-CACATGTTTCCC-3', are crucial for the guard cell-specific expression of SCAP1. Consistently, when an 80-bp promoter region including these two cis-elements was fused to a gene promoter that is not active in guard cells, it functioned as a promoter that directed gene expression in guard cells. Furthermore, the promoter region of HT1 encoding the central regulator of stomatal CO₂ signaling was also found to contain a 5'-CACGAGA-3' sequence, which was confirmed to function as a cis-element necessary for guard cell-specific expression of HT1. These findings suggest the existence of a novel transcriptional regulatory mechanism that synchronously promotes the expression of multiple genes required for the stomatal maturation and function.

Dynamic Changes in Reactive Oxygen Species in the Shoot Apex Contribute to Stem Cell Death in Arabidopsis thaliana

In monocarpic plants, stem cells are fated to die. However, the potential mechanism of stem cell death has remained elusive. Here, we reveal that the levels of two forms of reactive oxygen species (ROS), superoxide anion free radical (O2·−) and hydrogen peroxide (H₂O₂), show dynamic changes in the shoot apex during the plant life cycle of Arabidopsis thaliana. We found that the level of O2·− decreased and disappeared at four weeks after bolting (WAB), while H₂O₂ appeared at 3 WAB and showed a burst at 5 WAB. The timing of dynamic changes in O2·− and H₂O₂ was delayed for approximately three weeks in clv3-2, which has a longer lifespan. Moreover, exogenous application of H₂O₂ inhibited the expression of the stem cell determinant WUSCHEL (WUS) and promoted the expression of the developmentally programmed cell death (dPCD) marker gene ORESARA 1 (ORE1). These results indicate that H₂O₂ triggers an important signal inducing dPCD in stem cells. Given that O2·− plays roles in maintaining WUS expression and stem cell activity, we speculate that the dynamic shift from O2·− to H₂O₂ in the shoot apex results in stem cell death. Our findings provide novel insights for understanding ROS-mediated regulation during plant stem cell death.

The Cellular and Subcellular Organization of the Glucosinolate–Myrosinase System against Herbivores and Pathogens

Glucosinolates are an important class of secondary metabolites in Brassicales plants with a critical role in chemical defense. Glucosinolates are chemically inactive but can be hydrolyzed by myrosinases to produce a range of chemically active compounds toxic to herbivores and pathogens, thereby constituting the glucosinolate–myrosinase defense system or the mustard oil bomb. During the evolution, Brassicales plants have developed not only complex biosynthetic pathways for production of a large number of glucosinolate structures but also different classes of myrosinases that differ in catalytic mechanisms and substrate specificity. Studies over the past several decades have made important progress in the understanding of the cellular and subcellular organization of the glucosinolate–myrosinase system for rapid and timely detonation of the mustard oil bomb upon tissue damage after herbivore feeding and pathogen infection. Progress has also been made in understanding the mechanisms that herbivores and pathogens have evolved to counter the mustard oil bomb. In this review, we summarize our current understanding of the function and organization of the glucosinolate–myrosinase system in Brassicales plants and discuss both the progresses and future challenges in addressing this complex defense system as an excellent model for analyzing plant chemical defense.

The Cell Differentiation of Idioblast Myrosin Cells: Similarities With Vascular and Guard Cells

Idioblasts are defined by abnormal shapes, sizes, and contents that are different from neighboring cells. Myrosin cells are Brassicales-specific idioblasts and accumulate a large amount of thioglucoside glucohydrolases (TGGs, also known as myrosinases) in their vacuoles. Myrosinases convert their substrates, glucosinolates, into toxic compounds when herbivories and pests attack plants. In this review, we highlight the similarities and differences between myrosin cells and vascular cells/guard cells (GCs) because myrosin cells are distributed along vascular cells, especially the phloem parenchyma, and myrosin cells share the master transcription factor FAMA with GCs for their cell differentiation. In addition, we analyzed the overlap of cell type-specific genes between myrosin cells and GCs by using published single-cell transcriptomics (scRNA-seq) data, suggesting significant similarities in the gene expression patterns of these two specialized cells.

Jasmonate regulates the FAMA/mediator complex subunit 8-THIOGLUCOSIDE GLUCOHYDROLASE 1 cascade and myrosinase activity

Myrosinases are β-thioglucoside glucosidases that are unique to the Brassicales order. These enzymes hydrolyze glucosinolates to produce compounds that have direct antibiotic effects or that function as signaling molecules in the plant immune system, protecting plants from pathogens and insect pests. However, the effects of jasmonic acid (JA), a plant hormone that is crucial for plant disease resistance, on myrosinase activity remain unclear. Here, we systematically studied the effects of JA on myrosinase activity and explored the associated internal transcriptional regulation mechanisms. Exogenous application of JA significantly increased myrosinase activity, while the inhibition of endogenous JA biosynthesis and signaling reduced myrosinase activity. In addition, some myrosinase genes in Arabidopsis (Arabidopsis thaliana) were upregulated by JA. Further genetic and biochemical evidence showed that transcription factor FAMA interacted with a series of JASMONATE ZIM-DOMAIN proteins and affected JA-mediated myrosinase activity. However, among the JA-upregulated myrosinase genes, only THIOGLUCOSIDE GLUCOHYDROLASE 1 (TGG1) was positively regulated by FAMA. Further biochemical analysis showed that FAMA bound to the TGG1 promoter to directly mediate TGG1 expression in conjunction with Mediator complex subunit 8 (MED8). Together, our results provide evidence that JA acts as an important signal upstream of the FAMA/MED8-TGG1 pathway to positively regulate myrosinase activity in Arabidopsis.

Multiple indole glucosinolates and myrosinases defend Arabidopsis against Tetranychus urticae herbivory

Arabidopsis (Arabidopsis thaliana) defenses against herbivores are regulated by the jasmonate (JA) hormonal signaling pathway, which leads to the production of a plethora of defense compounds. Arabidopsis defense compounds include tryptophan-derived metabolites, which limit Arabidopsis infestation by the generalist herbivore two-spotted spider mite, Tetranychus urticae. However, the phytochemicals responsible for Arabidopsis protection against T. urticae are unknown. Here, we used Arabidopsis mutants disrupted in the synthesis of tryptophan-derived secondary metabolites to identify phytochemicals involved in the defense against T. urticae. We show that of the three tryptophan-dependent pathways found in Arabidopsis, the indole glucosinolate (IG) pathway is necessary and sufficient to assure tryptophan-mediated defense against T. urticae. We demonstrate that all three IGs can limit T. urticae herbivory, but that they must be processed by myrosinases to hinder T. urticae oviposition. Putative IG breakdown products were detected in mite-infested leaves, suggesting in planta processing by myrosinases. Finally, we demonstrate that besides IGs, there are additional JA-regulated defenses that control T. urticae herbivory. Together, our results reveal the complexity of Arabidopsis defenses against T. urticae that rely on multiple IGs, specific myrosinases, and additional JA-dependent defenses.

LEAFY, a Pioneer Transcription Factor in Plants: A Mini-Review

A subset of eukaryotic transcription factors (TFs) possess the ability to reprogram one cell type into another. Genes important for cellular reprograming are typically located in closed chromatin, which is covered by nucleosomes. Pioneer factors are a special class of TFs that can initially engage their target sites in closed chromatin prior to the engagement with, opening of, or modification of the sites by other factors. Although many pioneer factors are known in animals, a few have been characterized in plants. The TF LEAFY (LFY) acts as a pioneer factor specifying floral fate in Arabidopsis. In response to endogenous and environmental cues, plants produce appropriate floral inducers (florigens). During the vegetative phase, LFY is repressed by the TERMINAL FLOWER 1 (TFL1)-FD complex, which functions as a floral inhibitor, or anti-florigen. The florigen FLOWERING LOCUS T (FT) competes with TFL1 to prevent the binding of the FD TF to the LFY locus. The resulting FT-FD complex functions as a transient stimulus to activate its targets. Once LFY has been transcribed in the appropriate spatiotemporal manner, LFY binds to nucleosomes in closed chromatin regions. Subsequently, LFY opens the chromatin by displacing H1 linker histones and recruiting the SWI/SNF chromatin-remodeling complex. Such local changes permit the binding of other TFs, leading to the expression of the floral meristem identity gene APETALA1. This mini-review describes the latest advances in our understanding of the pioneer TF LFY, providing insight into the establishment of gene expression competence through the shaping of the plant epigenetic landscape.

Plant evolution driven by interactions with symbiotic and pathogenic microbes

During 450 million years of diversification on land, plants and microbes have evolved together. This is reflected in today's continuum of associations, ranging from parasitism to mutualism. Through phylogenetics, cell biology, and reverse genetics extending beyond flowering plants into bryophytes, scientists have started to unravel the genetic basis and evolutionary trajectories of plant-microbe associations. Protection against pathogens and support of beneficial, symbiotic, microorganisms are sustained by a blend of conserved and clade-specific plant mechanisms evolving at different speeds. We propose that symbiosis consistently emerges from the co-option of protection mechanisms and general cell biology principles. Exploring and harnessing the diversity of molecular mechanisms used in nonflowering plant-microbe interactions may extend the possibilities for engineering symbiosis-competent and pathogen-resilient crops.

Identification of a Devernalization Inducer by Chemical Screening Approaches in Arabidopsis thaliana

Vernalization is the promotion of flowering after prolonged exposure to cold. In Arabidopsis thaliana, vernalization induces epigenetic silencing of the floral repressor gene FLOWERING LOCUS C (FLC). The repressive epigenetic mark trimethylation of lysine 27 on histone H3 proteins (H3K27me3) is a critical contributor to the epigenetic silencing of FLC. Interestingly, the deposited H3K27me3 in the FLC locus can be erased by short-term high-temperature treatment. This is referred to as devernalization. In this study, we identified a novel chemical compound, 4-Isoxazolecarboxylic acid, 3,5-dimethyl-2-(4-fluorophenyl)-4-isoxazole carboxylic acid 1-methyl-2-oxoethyl ester named as DEVERNALIZER01 (DVR01), which induces devernalization in Arabidopsis seedlings, by an FLC-luciferase reporter-based high-throughput screening assay. DVR01 decreased the amount of H3K27me3 in the FLC locus in vernalized plants, resulting in the upregulation of FLC in the whole plant, including the vasculature and meristem, where FLC represses floral induction genes. We also showed that a 2-week treatment with DVR01 reverted plants with a vernalized status back to a fully non-vernalized status. Collectively, this study provides a novel structure of DVR01, which modulates devernalization via demethylation of H3K27me3 in the FLC locus.

Transcriptional networks regulating root vascular development

Vascular development involves multiple processes, including the establishment of vascular stem cells (e.g. procambium/cambium cells), stem cell divisions, and cell specification. A number of key transcription factors regulating vascular development have been identified, and the molecular mechanisms underlying these regulators have been keenly investigated. These studies uncovered that transcriptional regulation and phytohormone signaling have central roles in proceeding vascular developmental processes. Recent research approaches contributed to identify key transcription factors and their downstream genes, which enhanced our understanding of vascular development. This review discusses some research approaches and emerging molecular mechanisms that mediate the activation of transcriptional networks regulating root vascular development.

Developing and validating protocols for mechanical isolation of guard-cell enriched epidermal peels for omics studies

Stomata, which are microscopic valves on the leaf surface formed by two guard cells (GC), play a critical role in the regulation of leaf water and gas exchange and, hence, determine plant adaptive potential. However, little data is available on GC biochemistry, protein abundance and gene expression, mainly due to technical difficulties and challenges in isolating sufficient amounts of high-quality pure GC. In the present study we applied some modifications to the mechanical isolation of guard-cell to generalise this method for diverse growth conditions as well as plant species. Epidermal peel fragments enriched in guard cells were mechanically isolated from quinoa, spinach and sugar beet leaves grown at two conditions (normal and salt stress). Multiple analysis was performed to confirm the suitability and superiority of the modified technique to the original method. At the first step, the viability and purity of GC-enriched epidermal fragments were assessed under the microscope. Then, the RNA integrity, gene expression, and 1D SDS-PAGE tests were performed to validate the suitability of this technique for omics studies. The data revealed a wide range of proteins as well as a high integrity of RNA extracted from guard cell samples. The expression level of several GC-specific genes and mesophyll-dominant genes were investigated using a comparative analysis of transcriptome datasets of GC and whole-leaf samples. We found that Rubisco and photosynthesis-related proteins such as chlorophyll a/b binding protein were substantially higher in the whole leaf compared with the GCs. More importantly, GC-specific genes such as OST1, SLAC1, MYB60, FAMA and HT1 were highly expressed in the GCs, confirming that our guard cell preparation was highly enriched in GC gene transcripts. Real-time quantitative reverse transcription PCR further confirmed the efficacy of the GC isolation technique for exploring responses of GC to diverse types of stress at the molecular level.

Morphological and Physiological Framework Underlying Plant Longevity in Arabidopsis thaliana

Monocarpic plants have a single reproductive phase, in which their longevity is developmentally programmed by molecular networks. In the reproductive phase of Arabidopsis thaliana, the inflorescence meristem (IM) maintains a central pool of stem cells and produces a limited number of flower primordia, which result in seed formation and the death of the whole plant. In this study, we observed morphological changes in the IM at cellular and intracellular resolutions until the end of the plant life cycle. We observed four biological events during the periods from 1 week after bolting (WAB) till the death of stem cells: (1) the gradual reduction in the size of the IM, (2) the dynamic vacuolation of IM cells, (3) the loss of the expression of the stem cell determinant WUSCHEL (WUS), and (4) the upregulation of the programmed cell death marker BIFUNCTIONAL NUCLEASE1 (BFN1) in association with the death of stem cells. These results indicate that the stem cell population gradually decreases in IM during plant aging and eventually is fully terminated. We further show that the expression of WUS became undetectable in IM at 3 WAB prior to the loss of CLAVATA3 (CLV3) expression at 5 WAB; CLV3 is a negative regulator of WUS. Moreover, clv3 plants showed delayed loss of WUS and lived 6 weeks longer compared with wild-type plants. These results indicated that the prolonged expression of CLV3 at 4-5 WAB may be a safeguard that inhibits the reactivation of WUS and promotes plant death. Finally, through transcriptome analysis, we determined that reactive oxygen species (ROS) are involved in the control of plant longevity. Our work presents a morphological and physiological framework for the regulation of plant longevity in Arabidopsis.

A Splice Variant of BrrWSD1 in Turnip (Brassica rapa var. rapa) and Its Possible Role in Wax Ester Synthesis under Drought Stress

Cuticular wax accumulation in plants contributes to drought tolerance. Here, we compared the drought levels on two varieties with different genotypes in turnip (Brassica rapa var. rapa) and found that the drought tolerance was higher in the waxy KTRG-B48a than in the wax-free KTRG-B48b. A combination of PacBio and Illumina sequencing analyses revealed that differential transcripts were mainly enriched in the wax synthesis pathway, and a splice variant (BrrWSD1-X2) was identified in the waxy KTRG-B48a. BrrWSD1-X2 had a stronger ability to synthesize wax esters than BrrWSD1-X1 using heterologous expression in yeast (Saccharomyces cerevisiae) mutant H1246a. Then, we speculated that the T to C transversion of the third intron and the higher number of TA repeats in the third intron of BrrWSD1 DNA in the waxy KTRG-B48a may result in a lower efficiency of splicing recognition of the third intron, resulting in the emergence of BrrWSD1-X2 in waxy varieties.

Myrosinase: insights on structural, catalytic, regulatory, and environmental interactions

Glucosinolate-myrosinase is a substrate-enzyme defense mechanism present in Brassica crops. This binary system provides the plant with an efficient system against herbivores and pathogens. For humans, it is well known for its anti-carcinogenic, anti-inflammatory, immunomodulatory, anti-bacterial, cardio-protective, and central nervous system protective activities. Glucosinolate and myrosinase are spatially present in different cells that upon tissue disruption come together and result in the formation of a variety of hydrolysis products with diverse physicochemical and biological properties. The myrosinase-catalyzed reaction starts with cleavage of the thioglucosidic linkage resulting in release of a D-glucose and an unstable thiohydroximate-O-sulfate. The outcome of this thiohydroximate-O-sulfate has been shown to depend on the structure of the glucosinolate side chain, the presence of supplementary proteins known as specifier proteins and/or on the physiochemical condition. Myrosinase was first reported in mustard seed during 1939 as a protein responsible for release of essential oil. Until this date, myrosinases have been characterized from more than 20 species of Brassica, cabbage aphid, and many bacteria residing in the human intestine. All the plant myrosinases are reported to be activated by ascorbic acid while aphid and bacterial myrosinases are found to be either neutral or inhibited. Myrosinase catalyzes hydrolysis of the S-glycosyl bond, O-β glycosyl bond, and O-glycosyl bond. This review summarizes information on myrosinase, an essential component of this binary system, including its structural and molecular properties, mechanism of action, and its regulation and will be beneficial for the research going on the understanding and betterment of the glucosinolate-myrosinase system from an ecological and nutraceutical perspective.

Specialized Vacuoles of Myrosin Cells: Chemical Defense Strategy in Brassicales Plants

Plant vacuoles display many versatile functions. Vacuoles in vegetative tissues are generally involved in protein degradation, and are called lytic vacuoles. However, vegetative vacuoles in specialized cells can accumulate large concentrations of proteins, such as those in idioblast myrosin cells along veins in the order Brassicales, which store large amounts of myrosinases (thioglucoside glucohydrolase and thioglucoside glucohydrolase). Myrosinases cleave the bond between sulfur and glucose in sulfur-rich compounds (glucosinolates) to produce toxic compounds (isothiocyanates) when plants are damaged by pests. This defense strategy is called the myrosinase-glucosinolate system. Recent studies identified atypical myrosinases, PENETRATION 2 (PEN2) and PYK10, along with key components for development of myrosin cells. In this review, we discuss three topics in the myrosinase-glucosinolate system. First, we summarize the complexity and importance of the myrosinase-glucosinolate system, including classical myrosinases, atypical myrosinases and the system that counteracts the myrosinase-glucosinolate system. Secondly, we describe molecular machineries underlying myrosin cell development, including specific reporters, cell lineage, cell differentiation and cell fate determination. The master regulators for myrosin cell differentiation, FAMA and SCREAM, are key transcription factors involved in guard cell differentiation. This indicates that myrosin cells and guard cells share similar transcriptional networks. Finally, we hypothesize that the myrosinase-glucosinolate system may have originated in stomata of ancestral Brassicales plants and, after that, plants co-opted this defense strategy into idioblasts near veins at inner tissue layers.

Localization of the glucosinolate biosynthetic enzymes reveals distinct spatial patterns for the biosynthesis of indole and aliphatic glucosinolates

Glucosinolates constitute the primary defense metabolites in Arabidopsis thaliana (Arabidopsis). Indole and aliphatic glucosinolates, biosynthesized from tryptophan and methionine, respectively, are known to serve distinct biological functions. Although all genes in the biosynthetic pathways are identified, and it is known where glucosinolates are stored, it has remained elusive where glucosinolates are produced at the cellular and tissue level. To understand how the spatial organization of the different glucosinolate biosynthetic pathways contributes to their distinct biological functions, we investigated the localization of enzymes of the pathways under constitutive conditions and, for indole glucosinolates, also under induced conditions, by analyzing the spatial distribution of several fluorophore-tagged enzymes at the whole plant and the cellular level. We show that key steps in the biosynthesis of the different types of glucosinolates are localized in distinct cells in separate as well as overlapping vascular tissues. The presence of glucosinolate biosynthetic enzymes in parenchyma cells of the vasculature may assign new defense-related functions to these cell types. The knowledge gained in this study is an important prerequisite for understanding the orchestration of chemical defenses from site of synthesis to site of storage and potential (re)mobilization upon attack.

Plant Vacuoles

Plant vacuoles are multifunctional organelles. On the one hand, most vegetative tissues develop lytic vacuoles that have a role in degradation. On the other hand, seed cells have two types of storage vacuoles: protein storage vacuoles (PSVs) in endosperm and embryonic cells and metabolite storage vacuoles in seed coats. Vacuolar proteins and metabolites are synthesized on the endoplasmic reticulum and then transported to the vacuoles via Golgi-dependent and Golgi-independent pathways. Proprotein precursors delivered to the vacuoles are converted into their respective mature forms by vacuolar processing enzyme, which also regulates various kinds of programmed cell death in plants. We summarize two types of vacuolar membrane dynamics that occur during defense responses: vacuolar membrane collapse to attack viral pathogens and fusion of vacuolar and plasma membranes to attack bacterial pathogens. We also describe the chemical defense against herbivores brought about by the presence of PSVs in the idioblast myrosin cell.

Combinatorial Transcriptional Control of Plant Specialized Metabolism

Plants produce countless specialized compounds of diverse chemical nature and biological activities. Their biosynthesis often exclusively occurs either in response to environmental stresses or is limited to dedicated anatomical structures. In both scenarios, regulation of biosynthesis appears to be mainly controlled at the transcriptional level, which is generally dependent on a combined interplay of DNA-related mechanisms and the activity of transcription factors that may act in a combinatorial manner. How environmental and developmental cues are integrated into a coordinated cell type-specific stress response has only partially been unraveled so far. Building on the available examples from (metabolic) gene expression, here we propose theoretical models of how this integration of signals may occur at the level of transcriptional control.

The Arabidopsis thaliana Mediator subunit MED8 regulates plant immunity to Botrytis Cinerea through interacting with the basic helix-loop-helix (bHLH) transcription factor FAMA

The Mediator complex is at the core of transcriptional regulation and plays a central role in plant immunity. The MEDIATOR25 (MED25) subunit of Arabidopsis thaliana regulates jasmonate-dependent resistance to Botrytis cinerea through interacting with the basic helix-loop-helix (bHLH) transcription factor of jasmonate signaling, MYC2. Another Mediator subunit, MED8, acts independently or together with MED25 in plant immunity. However, unlike MED25, the underlying action mechanisms of MED8 in regulating B. cinerea resistance are still unknown. Here, we demonstrated that MED8 regulated plant immunity to B. cinerea through interacting with another bHLH transcription factor, FAMA, which was previously shown to control the final proliferation/differentiation switch during stomatal development. Our research demonstrates that FAMA is also an essential component of B. cinerea resistance. The fama loss-of-function mutants (fama-1 and fama-2) increased susceptibility to B. cinerea infection and reduced defense-gene expression. On the contrary, transgenic lines constitutively overexpressing FAMA showed opposite B. cinerea responses compared with the fama loss-of-function mutants. FAMA-overexpressed plants displayed enhanced resistance to B. cinerea infection and increased expression levels of defensin genes following B. cinerea treatment. Genetic analysis of MED8 and FAMA suggested that FAMA-regulated pathogen resistance was dependent on MED8. In addition, MED8 and FAMA were both associated with the G-box region in the promoter of ORA59. Our findings indicate that the MED8 subunit of the A. thaliana Mediator regulates plant immunity to B. cinerea through interacting with the transcription factor FAMA, which was discovered to be a key component in B. cinerea resistance.

Genome-wide analysis of basic helix-loop-helix (bHLH) transcription factors in Brachypodium distachyon

BACKGROUND

As a superfamily of transcription factors (TFs), the basic helix-loop-helix (bHLH) proteins have been characterized functionally in many plants with a vital role in the regulation of diverse biological processes including growth, development, response to various stresses, and so on. However, no systemic analysis of the bHLH TFs has been reported in Brachypodium distachyon, an emerging model plant in Poaceae.

RESULTS

A total of 146 bHLH TFs were identified in the Brachypodium distachyon genome and classified into 24 subfamilies. BdbHLHs in the same subfamily share similar protein motifs and gene structures. Gene duplication events showed a close relationship to rice, maize and sorghum, and segment duplications might play a key role in the expansion of this gene family. The amino acid sequence of the bHLH domains were quite conservative, especially Leu-27 and Leu-54. Based on the predicted binding activities, the BdbHLHs were divided into DNA binding and non-DNA binding types. According to the gene ontology (GO) analysis, BdbHLHs were speculated to function in homodimer or heterodimer manner. By integrating the available high throughput data in public database and results of quantitative RT-PCR, we found the expression profiles of BdbHLHs were different, implying their differentiated functions.

CONCLUSION

One hundred fourty-six BdbHLHs were identified and their conserved domains, sequence features, phylogenetic relationship, chromosomal distribution, GO annotations, gene structures, gene duplication and expression profiles were investigated. Our findings lay a foundation for further evolutionary and functional elucidation of BdbHLH genes.

Inhibition of cell polarity establishment in stomatal asymmetric cell division using the chemical compound bubblin

Stem-cell polarization is a crucial step in asymmetric cell division, which is a universal system for generating cellular diversity in multicellular organisms. Several conventional genetics studies have attempted to elucidate the mechanisms underlying cell polarization in plants, but it remains largely unknown. In plants, stomata, which are valves for gas exchange, are generated through several rounds of asymmetric divisions. In this study, we identified and characterized a chemical compound that affects stomatal stem-cell polarity. High-throughput screening for bioactive molecules identified a pyridine-thiazole derivative, named bubblin, which induced stomatal clustering in Arabidopsis epidermis. Bubblin perturbed stomatal asymmetric division, resulting in the generation of two identical daughter cells. Both cells continued to express the stomatal-fate determinant SPEECHLESS, and then differentiated into mispatterned stomata. Bubblin-treated cells had a defect in the polarized localization of BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL), which is required for asymmetric cell fate determination. Our results suggest that bubblin induces stomatal lineage cells to divide without BASL-dependent pre-mitotic establishment of polarity. Bubblin is a potentially valuable tool for investigating cell polarity establishment in stomatal asymmetric division.

PYK10 myrosinase reveals a functional coordination between endoplasmic reticulum bodies and glucosinolates in Arabidopsis thaliana

The endoplasmic reticulum body (ER body) is an organelle derived from the ER that occurs in only three families of the order Brassicales and is suggested to be involved in plant defense. ER bodies in Arabidopsis thaliana contain large amounts of β-glucosidases, but the physiological functions of ER bodies and these enzymes remain largely unclear. Here we show that PYK10, the most abundant β-glucosidase in A. thaliana root ER bodies, hydrolyzes indole glucosinolates (IGs) in addition to the previously reported in vitro substrate scopolin. We found a striking co-expression between ER body-related genes (including PYK10), glucosinolate biosynthetic genes and the genes for so-called specifier proteins affecting the terminal products of myrosinase-mediated glucosinolate metabolism, indicating that these systems have been integrated into a common transcriptional network. Consistent with this, comparative metabolite profiling utilizing a number of A. thaliana relatives within Brassicaceae identified a clear phylogenetic co-occurrence between ER bodies and IGs, but not between ER bodies and scopolin. Collectively, our findings suggest a functional link between ER bodies and glucosinolate metabolism in planta. In addition, in silico three-dimensional modeling, combined with phylogenomic analysis, suggests that PYK10 represents a clade of 16 myrosinases that arose independently from the other well-documented class of six thioglucoside glucohydrolases. These findings provide deeper insights into how glucosinolates are metabolized in cruciferous plants and reveal variation of the myrosinase-glucosinolate system within individual plants.

Regulation of plant secondary metabolism and associated specialized cell development by MYBs and bHLHs

Plants are unrivaled in the natural world in both the number and complexity of secondary metabolites they produce, and the ubiquitous phenylpropanoids and the lineage-specific glucosinolates represent two such large and chemically diverse groups. Advances in genome-enabled biochemistry and metabolomic technologies have greatly increased the understanding of their metabolic networks in diverse plant species. There also has been some progress in elucidating the gene regulatory networks that are key to their synthesis, accumulation and function. This review highlights what is currently known about the gene regulatory networks and the stable sub-networks of transcription factors at their cores that regulate the production of these plant secondary metabolites and the differentiation of specialized cell types that are equally important to their defensive function. Remarkably, some of these core components are evolutionarily conserved between secondary metabolism and specialized cell development and across distantly related plant species. These findings suggest that the more ancient gene regulatory networks for the differentiation of fundamental cell types may have been recruited and remodeled for the generation of the vast majority of plant secondary metabolites and their specialized tissues.

FAMA: A Molecular Link between Stomata and Myrosin Cells

Plants use sophisticated defense strategies against herbivores, including the myrosinase-glucosinolate system in Brassicales plants. This system sequesters myrosinase in myrosin cells, which are idioblasts in inner leaf tissues, and produces a toxic compound when cells are damaged by herbivores. Although the molecular mechanisms underlying myrosin cell development are largely unknown, recent studies have revealed that two key components, a basic helix-loop-helix (bHLH) transcription factor (FAMA) and vesicle trafficking factors (such as SYNTAXIN OF PLANTS 22), regulate the differentiation and fate determination of myrosin cells. FAMA also functions as a master regulator of guard cell (GC) differentiation. In this review, we discuss how FAMA operates two distinct genetic programs: the generation of myrosin cells in inner plant tissue and GCs in the epidermis.

Quantitative proteomics reveals the importance of nitrogen source to control glucosinolate metabolism in Arabidopsis thaliana and Brassica oleracea

Accessing different nitrogen (N) sources involves a profound adaptation of plant metabolism. In this study, a quantitative proteomic approach was used to further understand how the model plant Arabidopsis thaliana adjusts to different N sources when grown exclusively under nitrate or ammonium nutrition. Proteome data evidenced that glucosinolate metabolism was differentially regulated by the N source and that both TGG1 and TGG2 myrosinases were more abundant under ammonium nutrition, which is generally considered to be a stressful situation. Moreover, Arabidopsis plants displayed glucosinolate accumulation and induced myrosinase activity under ammonium nutrition. Interestingly, these results were also confirmed in the economically important crop broccoli (Brassica oleracea var. italica). Moreover, these metabolic changes were correlated in Arabidopsis with the differential expression of genes from the aliphatic glucosinolate metabolic pathway. This study underlines the importance of nitrogen nutrition and the potential of using ammonium as the N source in order to stimulate glucosinolate metabolism, which may have important applications not only in terms of reducing pesticide use, but also for increasing plants' nutritional value.

Transcriptomic Analysis of Soil-Grown Arabidopsis thaliana Roots and Shoots in Response to a Drought Stress

Drought stress has a negative impact on crop yield. Thus, understanding the molecular mechanisms responsible for plant drought stress tolerance is essential for improving this beneficial trait in crops. In the current study, a transcriptional analysis was conducted of gene regulatory networks in roots of soil-grown Arabidopsis plants in response to a drought stress treatment. A microarray analysis of drought-stressed roots and shoots was performed at 0, 1, 3, 5, 7, and 9 days. Results indicated that the expression of many drought stress-responsive genes and abscisic acid biosynthesis-related genes was differentially regulated in roots and shoots from days 3 to 9. The expression of cellular and metabolic process-related genes was up-regulated at an earlier time-point in roots than in shoots. In this regard, the expression of genes involved in oxidative signaling, chromatin structure, and cell wall modification also increased significantly in roots compared to shoots. Moreover, the increased expression of genes involved in the transport of amino acids and other solutes; including malate, iron, and sulfur, was observed in roots during the early time points following the initiation of the drought stress. These data suggest that plants may utilize these signaling channels and metabolic adjustments as adaptive responses in the early stages of a drought stress. Collectively, the results of the present study increases our understanding of the differences pertaining to the molecular mechanisms occurring in roots vs. shoots in response to a drought stress. Furthermore, these findings also aid in the selection of novel genes and promoters that can be used to potentially produce crop plants with increased drought tolerance.

Myrosin cells are differentiated directly from ground meristem cells and are developmentally independent of the vasculature in Arabidopsis leaves

Myrosin cells accumulate myrosinases in their vacuoles to catalyze the production of toxic compounds when tissues are damaged by herbivores. Myrosin cells are positioned adjacent to the abaxial side of the vasculature but their origin is unclear. To determine whether the myrosin cells are differentiated from vascular precursor cells, we generated a transgenic Arabidopsis line that expressed a myrosin cell reporter together with one of 3 vascular precursor cell reporters. The myrosin-positive cells were discontinuously distributed while the vascular precursor-positive cells were continuously distributed. The fluorescent signals of the myosin and vascular reporters did not overlap. Furthermore, the shape of myrosin-positive cells was different from the shape of vascular precursor-positive cells. These results indicate that myosin cells develop independently of the vasculature.

Organ-level analysis of idioblast patterning in Egeria densa Planch. leaves

Leaf tissues of plants usually contain several types of idioblasts, defined as specialized cells whose shape and contents differ from the surrounding homogeneous cells. The spatial patterning of idioblasts, particularly of trichomes and guard cells, across the leaf epidermis has received considerable attention as it offers a useful biological model for studying the intercellular regulation of cell fate and patterning. Excretory idioblasts in the leaves of the aquatic monocotyledonous plant Egeria densa produced light blue autofluorescence when irradiated with ultraviolet light. The use of epifluorescence microscopy to detect this autofluorescence provided a simple and convenient method for detecting excretory idioblasts and allowed tracking of those cells across the leaf surfaces, enabling quantitative measurement of the clustering and spacing patterns of idioblasts at the whole leaf level. Occurrence of idioblasts was coordinated along the proximal-distal, medial-lateral, and adaxial-abaxial axes, producing a recognizable consensus spatial pattern of idioblast formation among fully expanded leaves. Idioblast clusters, which comprised up to nine cells aligned along the proximal-distal axis, showed no positional bias or regularity in idioblast-forming areas when compared with singlet idioblasts. Up to 75% of idioblasts existed as clusters on every leaf side examined. The idioblast-forming areas varied between leaves, implying phenotypic plasticity. Furthermore, in young expanding leaves, autofluorescence was occasionally detected in a single giant vesicle or else in one or more small vesicles, which eventually grew to occupy a large portion of the idioblast volume as a central vacuole. Differentiation of vacuoles by accumulating the fluorescence substance might be an integral part of idioblast differentiation. Red autofluorescence from chloroplasts was not detected in idioblasts of young expanding leaves, suggesting idioblast differentiation involves an arrest in chloroplast development at a very early stage, rather than transdifferentiation of chloroplast-containing epidermal cells.

Myrosin cell development is regulated by endocytosis machinery and PIN1 polarity in leaf primordia of Arabidopsis thaliana

Myrosin cells, which accumulate myrosinase to produce toxic compounds when they are ruptured by herbivores, form specifically along leaf veins in Arabidopsis thaliana. However, the mechanism underlying this pattern formation is unknown. Here, we show that myrosin cell development requires the endocytosis-mediated polar localization of the auxin-efflux carrier PIN1 in leaf primordia. Defects in the endocytic/vacuolar SNAREs (syp22 and syp22 vti11) enhanced myrosin cell development. The syp22 phenotype was rescued by expressing SYP22 under the control of the PIN1 promoter. Additionally, myrosin cell development was enhanced either by lacking the activator of endocytic/vacuolar RAB5 GTPase (VPS9A) or by PIN1 promoter-driven expression of a dominant-negative form of RAB5 GTPase (ARA7). By contrast, myrosin cell development was not affected by deficiencies of vacuolar trafficking factors, including the vacuolar sorting receptor VSR1 and the retromer components VPS29 and VPS35, suggesting that endocytic pathway rather than vacuolar trafficking pathway is important for myrosin cell development. The phosphomimic PIN1 variant (PIN1-Asp), which is unable to be polarized, caused myrosin cells to form not only along leaf vein but also in the intervein leaf area. We propose that Brassicales plants might arrange myrosin cells near vascular cells in order to protect the flux of nutrients and water via polar PIN1 localization.