Guard cells: transcription factors regulate stomatal movements

RtNAC055 promotes drought tolerance via a stomatal closure pathway linked to methyl jasmonate/hydrogen peroxide signaling in Reaumuria trigyna

The stomata regulate CO2 uptake and efficient water usage, thereby promoting drought stress tolerance. NAC proteins (NAM, ATAF1/2, and CUC2) participate in plant reactions following drought stress, but the molecular mechanisms underlying NAC-mediated regulation of stomatal movement are unclear. In this study, a novel NAC gene from Reaumuria trigyna, RtNAC055, was found to enhance drought tolerance via a stomatal closure pathway. It was regulated by RtMYC2 and integrated with jasmonic acid signaling and was predominantly expressed in stomata and root. The suppression of RtNAC055 could improve jasmonic acid and H2O2 production and increase the drought tolerance of transgenic R. trigyna callus. Ectopic expression of RtNAC055 in the Arabidopsis atnac055 mutant rescued its drought-sensitive phenotype by decreasing stomatal aperture. Under drought stress, overexpression of RtNAC055 in poplar promoted ROS (H2O2) accumulation in stomata, which accelerated stomatal closure and maintained a high photosynthetic rate. Drought upregulated the expression of PtRbohD/F, PtP5CS2, and PtDREB1.1, as well as antioxidant enzyme activities in heterologous expression poplars. RtNAC055 promoted H2O2 production in guard cells by directly binding to the promoter of RtRbohE, thus regulating stomatal closure. The stress-related genes RtDREB1.1/P5CS1 were directly regulated by RtNAC055. These results indicate that RtNAC055 regulates stomatal closure by maintaining the balance between the antioxidant system and H2O2 level, reducing the transpiration rate and water loss, and improving photosynthetic efficiency and drought resistance.

Diel dynamics of multi-omics in elkhorn fern provide new insights into weak CAM photosynthesis

Crassulacean acid metabolism (CAM) has high water-use efficiency (WUE) and is widely recognized to have evolved from C3 photosynthesis. Different plant lineages have convergently evolved CAM, but the molecular mechanism that underlies C3-to-CAM evolution remains to be clarified. Platycerium bifurcatum (elkhorn fern) provides an opportunity to study the molecular changes underlying the transition from C3 to CAM photosynthesis because both modes of photosynthesis occur in this species, with sporotrophophyll leaves (SLs) and cover leaves (CLs) performing C3 and weak CAM photosynthesis, respectively. Here, we report that the physiological and biochemical attributes of CAM in weak CAM-performing CLs differed from those in strong CAM species. We investigated the diel dynamics of the metabolome, proteome, and transcriptome in these dimorphic leaves within the same genetic background and under identical environmental conditions. We found that multi-omic diel dynamics in P. bifurcatum exhibit both tissue and diel effects. Our analysis revealed temporal rewiring of biochemistry relevant to the energy-producing pathway (TCA cycle), CAM pathway, and stomatal movement in CLs compared with SLs. We also confirmed that PHOSPHOENOLPYRUVATE CARBOXYLASE KINASE (PPCK) exhibits convergence in gene expression among highly divergent CAM lineages. Gene regulatory network analysis identified candidate transcription factors regulating the CAM pathway and stomatal movement. Taken together, our results provide new insights into weak CAM photosynthesis and new avenues for CAM bioengineering.

Meta-Analysis Reveals Challenges and Gaps for Genome-to-Phenome Research Underpinning Plant Drought Response

Severe drought conditions and extreme weather events are increasing worldwide with climate change, threatening the persistence of native plant communities and ecosystems. Many studies have investigated the genomic basis of plant responses to drought. However, the extent of this research throughout the plant kingdom is unclear, particularly among species critical for the sustainability of natural ecosystems. This study aimed to broaden our understanding of genome-to-phenome (G2P) connections in drought-stressed plants and identify focal taxa for future research. Bioinformatics pipelines were developed to mine and link information from databases and abstracts from 7730 publications. This approach identified 1634 genes involved in drought responses among 497 plant taxa. Most (83.30%) of these species have been classified for human use, and most G2P interactions have been described within model organisms or crop species. Our analysis identifies several gaps in G2P research literature and database connectivity, with 21% of abstracts being linked to gene and taxonomy data in NCBI. Abstract text mining was more successful at identifying potential G2P pathways, with 34% of abstracts containing gene, taxa, and phenotype information. Expanding G2P studies to include non-model plants, especially those that are adapted to drought stress, will help advance our understanding of drought responsive G2P pathways.

MITOGEN-ACTIVATED PROTEIN KINASE 4 impacts leaf development, temperature, and stomatal movement in hybrid aspen

Stomatal movement and density influence plant water use efficiency and thus biomass production. Studies in model plants within controlled environments suggest MITOGEN-ACTIVATED PROTEIN KINASE 4 (MPK4) may be crucial for stomatal regulation. We present functional analysis of MPK4 for hybrid aspen (Populus tremula × tremuloides) grown under natural field conditions for several seasons. We provide evidence of the role of MPK4 in the genetic and environmental regulation of stomatal formation, differentiation, signaling, and function; control of the photosynthetic and thermal status of leaves; and growth and acclimation responses. The long-term acclimation manifested as variations in stomatal density and distribution. Short-term acclimation responses were derived from changes in the stomatal aperture. MPK4 localized in the cytoplasm of guard cells (GCs) was a positive regulator of abscisic acid (ABA)-dependent stomatal closure and nitric oxide metabolism in the ABA-dependent pathways, while to a lesser extent, it was involved in ABA-induced hydrogen peroxide accumulation. MPK4 also affected the stomatal aperture through deregulation of microtubule patterns and cell wall structure and composition, including via pectin methyl-esterification, and extensin levels in the GC wall. Deregulation of leaf anatomy (cell compaction) and stomatal movement, together with increased light energy absorption, resulted in altered leaf temperature, photosynthesis, cell death, and biomass accumulation in mpk4 transgenic plants. Divergence between absorbed energy and assimilated energy is a bottleneck, and MPK4 can participate in the control of energy dissipation (thermal effects). Furthermore, MPK4 can participate in balancing the photosynthetic energy distribution via its effective use in growth or redirection to acclimation/defense responses.

Global Dynamic Molecular Profiling of Stomatal Lineage Cell Development by Single-Cell RNA Sequencing

The regulation of stomatal lineage cell development has been extensively investigated. However, a comprehensive characterization of this biological process based on single-cell transcriptome analysis has not yet been reported. In this study, we performed RNA sequencing on 12 844 individual cells from the cotyledons of 5-day-old Arabidopsis seedlings. We identified 11 cell clusters corresponding mostly to cells at specific stomatal developmental stages using a series of marker genes. Comparative analysis of genes with the highest variable expression among these cell clusters revealed transcriptional networks that regulate development from meristemoid mother cells to guard mother cells. Examination of the developmental dynamics of marker genes via pseudo-time analysis revealed potential interactions between these genes. Collectively, our study opens the door for understanding how the identified novel marker genes participate in the regulation of stomatal lineage cell development.

WD40-REPEAT 5a functions in drought stress tolerance by regulating nitric oxide accumulation in Arabidopsis

Nitric oxide (NO) generation by NO synthase (NOS) in guard cells plays a vital role in stomatal closure for adaptive plant response to drought stress. However, the mechanism underlying the regulation of NOS activity in plants is unclear. Here, by screening yeast deletion mutants with decreased NO accumulation and NOS-like activity when subjected to H₂ O₂ stress, we identified TUP1 as a novel regulator of NOS-like activity in yeast. Arabidopsis WD40-REPEAT 5a (WDR5a), a homolog of yeast TUP1, complemented H₂ O₂ -induced NO accumulation of a yeast mutant Δtup1, suggesting the conserved role of WDR5a in regulating NO accumulation and NOS-like activity. This note was further confirmed by using an Arabidopsis RNAi line wdr5a-1 and two T-DNA insertion mutants of WDR5a with reduced WDR5a expression, in which both H₂ O₂ -induced NO accumulation and stomatal closure were repressed. This was because H₂ O₂ -induced NOS-like activity was inhibited in the mutants compared with that of the wild type. Furthermore, these wdr5a mutants were more sensitive to drought stress as they had reduced stomatal closure and decreased expression of drought-related genes. Together, our results revealed that WDR5a functions as a novel factor to modulate NOS-like activity for changes of NO accumulation and stomatal closure in drought stress tolerance.

COP1 jointly modulates cytoskeletal processes and electrophysiological responses required for stomatal closure

Reorganization of the cortical microtubule cytoskeleton is critical for guard cell function. Here, we investigate how environmental and hormonal signals cause these rearrangements and find that COP1, a RING-finger-type ubiquitin E3 ligase, is required for degradation of tubulin, likely by the 26S proteasome. This degradation is required for stomatal closing. In addition to regulating the cytoskeleton, we show that cop1 mutation impaired the activity of S-type anion channels, which are critical for stomatal closure. Thus, COP1 is revealed as a potential coordinator of cytoskeletal and electrophysiological activities required for guard cell function.

A comparative study of the Arabidopsis thaliana guard-cell transcriptome and its modulation by sucrose

Microarray analysis was performed on RNA isolated from guard cells that were manually dissected from leaves of Arabidopsis. By pooling our data with those of two earlier studies on Arabidopsis guard cell protoplasts, we provide a robust view of the guard-cell transcriptome, which is rich in transcripts for transcription factors, signaling proteins, transporters, and carbohydrate-modifying enzymes. To test the hypothesis that photosynthesis-derived sugar signals guard cells to adjust stomatal opening, we determined the profile of genes expressed in guard cells from leaves that had been treated with sucrose. The results revealed that expression of 440 genes changed in guard cells in response to sucrose. Consistent with this hypothesis, these genes encoded cellular functions for photosynthesis and transport of sugars, water, amino acids, and ions. Plants of T-DNA insertion lines for 50 genes highly responsive to sucrose were examined for defects in guard cell function. Twelve genes not previously known to function in guard cells were shown to be important in leaf conductance, water-use efficiency, and/or stomate development. Of these, three are of particular interest, having shown effects in nearly every test of stomatal function without a change in stomatal density: TPS5 (At4g17770), a TRAF domain-containing protein (At1g65370), and a WD repeat–containing protein (At1g15440).

Control of stomatal aperture: a renaissance of the old guard

Stomata, functionally specialized small pores on the surfaces of leaves, regulate the flow of gases in and out of plants. The pore is opened by an increase in osmotic pressure in the guard cells, resulting in the uptake of water. The subsequent increase in cell volume inflates the guard cell and culminates with the opening of the pore. Although guard cells can be regarded as one of the most thoroughly investigated cell types, our knowledge of the signaling pathways which regulate guard cell function remains fragmented. Recent research in guard cells has led to several new hypotheses, however, it is still a matter of debate as to whether guard cells function autonomously or are subject to regulation by their neighboring mesophyll cells.This review synthesizes what is known about the mechanisms and genes critical for modulating stomatal movement. Recent progress on the regulation of guard cell function is reviewed here including the involvement of environmental signals such as light, the concentration of atmospheric CO2 and endogenous plant hormones. In addition we re-evaluate the important role of organic acids such as malate and fumarate play in guard cell metabolism in this process.

Epidermis: the formation and functions of a fundamental plant tissue

Epidermis differentiation and maintenance are essential for plant survival. Constant cross-talk between epidermal cells and their immediate environment is at the heart of epidermal cell fate, and regulates epidermis-specific transcription factors. These factors in turn direct epidermal differentiation involving a whole array of epidermis-specific pathways including specialized lipid metabolism necessary to build the protective cuticle layer. An intact epidermis is crucial for certain key processes in plant development, shoot growth and plant defence. Here, we discuss the control of epidermal cell fate and the function of the epidermal cell layer in the light of recent advances in the field.

Phytochrome B is involved in mediating red light-induced stomatal opening in Arabidopsis thaliana

The stomatal pores of higher plants enable gaseous exchange into and out of leaves for photosynthesis and evaporation. Stomatal opening is induced by both blue and red lights. It is shown that blue light-induced stomatal opening is mediated by the blue light receptor phototropins (PHOT1 and PHOT2) and cryptochromes (CRY1 and CRY2). However, whether phytochrome B (phyB) is involved in red light regulation of stomatal opening remains largely unclear. Here, we report a positive role for Arabidopsis (Arabidopsis thaliana) phyB in the regulation of red light-induced stomatal opening. The phyB mutant stomata displayed a reduced red light response, whereas stomata of the phyB-overexpressing plants displayed a hypersensitive response to red light. In addition, stomata of the cry1 cry2 phyB, phot1 phot2 phyB, and cry1 phyA phyB triple mutant plants showed more reduced light response than those of the single or double mutant plants under white light, implying that phyB acts in concert with phyA, CRY, and PHOT in light regulation of stomatal opening. Stomata of phyB cop1 mutant opened less wide than those of the cop1 mutant, and stomata of the pif3 pif4 mutant opened wider than those of the wild-type, indicating that COP1, together with the PIFs (phytochrome interacting factors), may act downstream of PHYB in regulating stomatal opening. Furthermore, quantitative RT-PCR analysis showed that the expression of MYB60 was reduced in the cry1 cry2 and phyA phyB mutants under blue and red lights, respectively, but induced in the CRY1- and phyB-overexpressing plants. These results demonstrate that phyB and CRY might regulate stomatal opening, at least in part, by regulating MYB60 expression.

The guard cell as a single-cell model towards understanding drought tolerance and abscisic acid action

Stomatal guard cells are functionally specialized epidermal cells usually arranged in pairs surrounding a pore. Changes in ion fluxes, and more specifically osmolytes, within the guard cells drive opening/closing of the pore, allowing gas exchange while limiting water loss through evapo-transpiration. Adjustments of the pore aperture to optimize these conflicting needs are thus centrally important for land plants to survive, especially with the rise in CO(2) associated with global warming and increasing water scarcity this century. The basic biophysical events in modulating membrane transport have been gradually delineated over two decades. Genetics and molecular approaches in recent years have complemented and extended these earlier studies to identify major regulatory nodes. In Arabidopsis, the reference for guard cell genetics, stomatal opening driven by K(+) entry is mainly through KAT1 and KAT2, two voltage-gated K(+) inward-rectifying channels that are activated on hyperpolarization of the plasma membrane principally by the OST2 H(+)-ATPase (proton pump coupled to ATP hydrolysis). By contrast, stomatal closing is caused by K(+) efflux mainly through GORK, the outward-rectifying channel activated by membrane depolarization. The depolarization is most likely initiated by SLAC1, an anion channel distantly related to the dicarboxylate/malic acid transport protein found in fungi and bacteria. Beyond this established framework, there is also burgeoning evidence for the involvement of additional transporters, such as homologues to the multi-drug resistance proteins (or ABC transporters) as intimated by several pharmacological and reverse genetics studies. General inhibitors of protein kinases and protein phosphatases have been shown to profoundly affect guard cell membrane transport properties. Indeed, the first regulatory enzymes underpinning these transport processes revealed genetically were several protein phosphatases of the 2C class and the OST1 kinase, a member of the SnRK2 family. Taken together, these results are providing the first glimpses of an emerging signalling complex critical for modulating the stomatal aperture in response to environmental stimuli.

Guard cell photosynthesis and stomatal function

Chloroplasts are a key feature of most guard cells; however, the function of these organelles in stomatal responses has been a subject of debate. This review examines evidence for and against a role of guard cell chloroplasts in stimulating stomatal opening. Controversy remains over the extent to which guard cell Calvin cycle activity contributes to stomatal regulation. However, this is only one of four possible functions of guard cell chloroplasts; other roles include supply of ATP, blue-light signalling and starch storage. Evidence exists for all these mechanisms, but is highly dependent upon species and growth/measurement conditions, with inconsistencies between different laboratories reported. Significant plasticity and extreme flexibility in guard cell osmoregulatory, signalling and sensory pathways may be one explanation. The use of chlorophyll a fluorescence analysis of individual guard cells is discussed in assessing guard and mesophyll cell physiology in relation to stomatal function. Developments in transgenic and molecular techniques have recently provided interesting, albeit contrasting, data regarding the role of these highly conserved organelles in stomatal function. Recent studies examining the link between mesophyll photosynthesis and stomatal conductance are discussed. An enhanced understanding of these processes may be fundamental in generating crop plants with greater water use efficiencies, capable of combating future climatic changes.

A systems biology approach identifies a R2R3 MYB gene subfamily with distinct and overlapping functions in regulation of aliphatic glucosinolates

BACKGROUND

Glucosinolates are natural metabolites in the order Brassicales that defend plants against both herbivores and pathogens and can attract specialized insects. Knowledge about the genes controlling glucosinolate regulation is limited. Here, we identify three R2R3 MYB transcription factors regulating aliphatic glucosinolate biosynthesis in Arabidopsis by combining several systems biology tools.

METHODOLOGY/PRINCIPAL FINDINGS

MYB28 was identified as a candidate regulator of aliphatic glucosinolates based on its co-localization within a genomic region controlling variation both in aliphatic glucosinolate content (metabolite QTL) and in transcript level for genes involved in the biosynthesis of aliphatic glucosinolates (expression QTL), as well as its co-expression with genes in aliphatic glucosinolate biosynthesis. A phylogenetic analysis with the R2R3 motif of MYB28 showed that it and two homologues, MYB29 and MYB76, were members of an Arabidopsis-specific clade that included three characterized regulators of indole glucosinolates. Over-expression of the individual MYB genes showed that they all had the capacity to increase the production of aliphatic glucosinolates in leaves and seeds and induce gene expression of aliphatic biosynthetic genes within leaves. Analysis of leaves and seeds of single knockout mutants showed that mutants of MYB29 and MYB76 have reductions in only short-chained aliphatic glucosinolates whereas a mutant in MYB28 has reductions in both short- and long-chained aliphatic glucosinolates. Furthermore, analysis of a double knockout in MYB28 and MYB29 identified an emergent property of the system since the absence of aliphatic glucosinolates in these plants could not be predicted by the chemotype of the single knockouts.

CONCLUSIONS/SIGNIFICANCE

It seems that these cruciferous-specific MYB regulatory genes have evolved both overlapping and specific regulatory capacities. This provides a unique system within which to study the evolution of MYB regulatory factors and their downstream targets.

The response of photosynthesis and stomatal conductance to rising [CO2]: mechanisms and environmental interactions

This review summarizes current understanding of the mechanisms that underlie the response of photosynthesis and stomatal conductance to elevated carbon dioxide concentration ([CO2]), and examines how downstream processes and environmental constraints modulate these two fundamental responses. The results from free-air CO2 enrichment (FACE) experiments were summarized via meta-analysis to quantify the mean responses of stomatal and photosynthetic parameters to elevated [CO2]. Elevation of [CO2] in FACE experiments reduced stomatal conductance by 22%, yet, this reduction was not associated with a similar change in stomatal density. Elevated [CO2] stimulated light-saturated photosynthesis (Asat) in C3 plants grown in FACE by an average of 31%. However, the magnitude of the increase in Asat varied with functional group and environment. Functional groups with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis at elevated [CO2] had greater potential for increases in Asat than those where photosynthesis became ribulose-1,5-bisphosphate (RubP)-limited at elevated [CO2]. Both nitrogen supply and sink capacity modulated the response of photosynthesis to elevated [CO2] through their impact on the acclimation of carboxylation capacity. Increased understanding of the molecular and biochemical mechanisms by which plants respond to elevated [CO2], and the feedback of environmental factors upon them, will improve our ability to predict ecosystem responses to rising [CO2] and increase our potential to adapt crops and managed ecosystems to future atmospheric [CO2].