A flowering integrator, SOC1, affects stomatal opening in Arabidopsis thaliana

Stomatal movements are regulated by multiple environmental signals. Recent investigations indicate that photoperiodic flowering components, such as CRY, GI, CO, FT and TSF, are expressed in guard cells and positively affect stomatal opening in Arabidopsis thaliana. Here we show that SOC1, which encodes a MADS box transcription factor and integrates multiple flowering signals, also exerts a positive effect on stomatal opening. FLC encodes a potent repressor of FT and SOC1, and FRI acts as an activator of FLC. Thus, we examined stomatal phenotypes in FRI-Col, which contains an active FRI allele of accession Sf-2 by introgression. We found higher expression of FLC and lower expression of FT, SOC1 and TSF in guard cells from FRI-Col than in those from Col. Light-induced stomatal opening was significantly suppressed in FRI-Col. Interestingly, vernalization of FRI-Col partially restored light-induced stomatal opening, concomitant with a decrease of FLC and increase of FT, SOC1 and TSF. Furthermore, we observed the constitutive open-stomata phenotype in transgenic plants overexpressing SOC1-GFP (green fluorescent protein) in guard cells (SOC1-GFP overexpressor), and found that light-induced stomatal opening was significantly suppressed in a soc1 knockout mutant. RNA sequencing using epidermis from the SOC1-GFP overexpressor revealed that the expression levels of several genes involved in stomatal opening, such as BLUS1 and the plasma membrane H(+)-ATPases, were higher than those in background plants. From these results, we conclude that SOC1 is involved in the regulation of stomatal opening via transcriptional regulation in guard cells.

The Arabidopsis leucine-rich repeat receptor-like kinase MUSTACHES enforces stomatal bilateral symmetry in Arabidopsis

Stomata display a mirror-like symmetry that is adaptive for shoot/atmosphere gas exchange. This symmetry includes the facing guard cells around a lens-shaped and bilaterally symmetric pore, as well as radially arranged microtubule arrays that primarily originate at the pore and then grow outwards. Mutations in MUSTACHES (MUS), which encodes a leucine-rich repeat receptor-like kinase, disrupt this symmetry, resulting in defects ranging from skewed pores and abnormally focused and depolarized radial microtubule arrays, to paired guard cells that face away from each other, or a severe loss of stomatal shape. Translational MUSproMUS:tripleGFP fusions are expressed in cell plates in most cells types in roots and shoots, and cytokinesis and cell plates are mostly normal in mus mutants. However, in guard mother cells, which divide and then form stomata, MUS expression is notably absent from new cell plates, and instead is peripherally located. These results are consistent with a role for MUS in enforcing wall building and cytoskeletal polarity at the centre of the developing stoma via signalling from the vicinity of the guard cell membrane.

Decreased photosynthesis in the erect panicle 3 (ep3) mutant of rice is associated with reduced stomatal conductance and attenuated guard cell development

The ERECT PANICLE 3 gene of rice encodes a peptide that exhibits more than 50% sequence identity with the Arabidopsis F-box protein HAWAIIAN SKIRT (HWS). Ectopic expression of the Os02g15950 coding sequence, driven by the HWS (At3g61950) promoter, rescued the hws-1 flower phenotype in Arabidopsis confirming that EP3 is a functional orthologue of HWS. In addition to displaying an erect inflorescence phenotype, loss-of-function mutants of Os02g15950 exhibited a decrease in leaf photosynthetic capacity and stomatal conductance. Analysis of a range of physiological and anatomical features related to leaf photosynthesis revealed no alteration in Rubisco content and no notable changes in mesophyll size or arrangement. However, both ep3 mutant plants and transgenic lines that have a T-DNA insertion within the Os02g15950 (EP3) gene exhibit smaller stomatal guard cells compared with their wild-type controls. This anatomical characteristic may account for the observed decrease in leaf photosynthesis and provides evidence that EP3 plays a role in regulating stomatal guard cell development.

The barley anion channel, HvALMT1, has multiple roles in guard cell physiology and grain metabolism

The barley (Hordeum vulgare) gene HvALMT1 encodes an anion channel in guard cells and in certain root tissues indicating that it may perform multiple roles. The protein localizes to the plasma membrane and facilitates malate efflux from cells when constitutively expressed in barley plants and Xenopus oocytes. This study investigated the function of HvALMT1 further by identifying its tissue-specific expression and by generating and characterizing RNAi lines with reduced HvALMT1 expression. We show that transgenic plants with 18-30% of wild-type HvALMT1 expression had impaired guard cell function. They maintained higher stomatal conductance in low light intensity and lost water more rapidly from excised leaves than the null segregant control plants. Tissue-specific expression of HvALMT1 was investigated in developing grain and during germination using transgenic barley lines expressing the green fluorescent protein (GFP) with the HvALMT1 promoter. We found that HvALMT1 is expressed in the nucellar projection, the aleurone layer and the scutellum of developing barley grain. Malate release measured from isolated aleurone layers prepared from imbibed grain was significantly lower in the RNAi barley plants compared with control plants. These data provide molecular and physiological evidence that HvALMT1 functions in guard cells, in grain development and during germination. We propose that HvALMT1 releases malate and perhaps other anions from guard cells to promote stomatal closure. The likely roles of HvALMT1 during seed development and grain germination are also discussed.

Spatial and temporal dynamics of primary and secondary metabolism in Phaseolus vulgaris challenged by Pseudomonas syringae

Many defense mechanisms contribute to the plant immune system against pathogens, involving the regulation of different processes of the primary and secondary metabolism. At the same time, pathogens have evolved mechanisms to hijack the plant defense in order to establish the infection and proliferate. Localization and timing of the host response are essential to understand defense mechanisms and resistance to pathogens (Rico et al. 2011). Imaging techniques, such as fluorescence imaging and thermography, are a very valuable tool providing spatial and temporal information about a series of plant processes. In this study, bean plants challenged with two pathovars of Pseudomonas syringae have been investigated. Pseudomonas syringae pv. phaseolicola 1448A and P. syringae pv. tomato DC3000 elicit a compatible and incompatible interaction in bean, respectively. Both types of host-pathogen interaction triggered different changes in the activity of photosynthesis and the secondary metabolism. We conclude that the combined analysis of leaf temperature, chlorophyll fluorescence and green fluorescence emitted by phenolics allows to discriminate compatible from incompatible P. syringae-Phaseolus vulgaris interactions in very early times of the infection, prior to the development of symptoms. These can constitute disease signatures that would allow an early identification of emerging plagues in crops.

Elevation-related variation in leaf stomatal traits as a function of plant functional type: evidence from Changbai Mountain, China

Understanding the variation in stomatal characteristics in relation to climatic gradients can reveal the adaptation strategies of plants, and help us to predict their responses to future climate changes. In this study, we investigated stomatal density (SD) and stomatal length (SL) in 150 plant species along an elevation gradient (540-2357 m) in Changbai Mountain, China, and explored the patterns and drivers of stomatal characteristics across species and plant functional types (PFTs: trees, shrubs, and herbs). The average values of SD and SL for all species combined were 156 mm(-2) and 35 µm, respectively. SD was higher in trees (224 mm(-2)) than in shrubs (156 mm(-2)) or herbs (124 mm(-2)), and SL was largest in herbs (37 µm). SD was negatively correlated with SL in all species and PFTs (P < 0.01). The relationship between stomatal characteristics and elevation differed among PFTs. In trees, SD decreased and SL increased with elevation; in shrubs and herbs, SD initially increased and then decreased. Elevation-related differences in SL were not significant. PFT explained 7.20-17.6% of the total variation in SD and SL; the contributions of CO2 partial pressure (P CO2), precipitation, and soil water content (SWC) were weak (0.02-2.28%). Our findings suggest that elevation-related patterns of stomatal characteristics in leaves are primarily a function of PFT, and highlight the importance of differences among PFTs in modeling gas exchange in terrestrial ecosystems under global climate change.

Auxin--a novel regulator of stomata differentiation

Three recent publications identify the phytohormone auxin as a novel regulator of stomata development and distribution. We discuss how auxin signaling and polar auxin transport blend in with the established signaling network that controls cell division and differentiation within the stomatal cell lineage.

Cytosolic calcium signals elicited by the pathogen-associated molecular pattern flg22 in stomatal guard cells are of an oscillatory nature

Changes in cytosolic free calcium ([Ca(2+)]cyt) are an early and essential element of signalling networks activated by the perception of pathogen-associated molecular patterns (PAMPs), such as flg22. The flg22-induced calcium signal has been described on whole-plant, but not on single-cell scale so far. Also, the Ca(2+) sources and channels contributing to its generation are still obscure. Ratiometric fluorescence imaging employing the calcium reporter Yellow Cameleon 3.6 was performed to analyse the flg22-induced calcium signature in single guard cells of Arabidopsis thaliana. Calcium stores and channel types involved in its generation were determined by a pharmacological approach. In contrast to the calcium signal determined on whole-plant level, the signature on single-cell level is not characterized by one sustained response, but by oscillations in [Ca(2+)]cyt. These oscillations were abolished by EGTA and lanthanum, as well as by U73122, neomycin and TMB-8, but only partially or not at all affected by inhibitors of glutamate receptor-like channels and cyclic nucleotide-gated channels. Our analyses suggest that the response observed on whole-plant level is the summary of oscillations occurring in single cells. Parallel to external calcium, influx via channels located at internal stores contributes to the signal.

Myrosin idioblast cell fate and development are regulated by the Arabidopsis transcription factor FAMA, the auxin pathway, and vesicular trafficking

Crucifer shoots harbor a glucosinolate-myrosinase system that defends against insect predation. Arabidopsis thaliana myrosinase (thioglucoside glucohydrolase [TGG]) accumulates in stomata and in myrosin idioblasts (MIs). This work reports that the basic helix-loop-helix transcription factor FAMA that is key to stomatal development is also expressed in MIs. The loss of FAMA function abolishes MI fate as well as the expression of the myrosinase genes TGG1 and TGG2. MI cells have previously been reported to be located in the phloem. Instead, we found that MIs arise from the ground meristem rather than provascular tissues and thus are not homologous with phloem. Moreover, MI patterning and morphogenesis are abnormal when the function of the ARF-GEF gene GNOM is lost as well as when auxin efflux and vesicular trafficking are chemically disrupted. Stomata and MI cells constitute part of a wider system that reduces plant predation, the so-called "mustard oil bomb," in which vacuole breakage in cells harboring myrosinase and glucosinolate yields a brew toxic to many animals, especially insects. This identification of the gene that confers the fate of MIs, as well as stomata, might facilitate the development of strategies for engineering crops to mitigate predation.

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

Brassicales plants, including Arabidopsis thaliana, have an ingenious two-compartment defense system, which sequesters myrosinase from the substrate glucosinolate and produces a toxic compound when cells are damaged by herbivores. Myrosinase is stored in vacuoles of idioblast myrosin cells. The molecular mechanism that regulates myrosin cell development remains elusive. Here, we identify the basic helix-loop-helix transcription factor FAMA as an essential component for myrosin cell development along Arabidopsis leaf veins. FAMA is known as a regulator of stomatal development. We detected FAMA expression in myrosin cell precursors in leaf primordia in addition to stomatal lineage cells. FAMA deficiency caused defects in myrosin cell development and in the biosynthesis of myrosinases THIOGLUCOSIDE GLUCOHYDROLASE1 (TGG1) and TGG2. Conversely, ectopic FAMA expression conferred myrosin cell characteristics to hypocotyl and root cells, both of which normally lack myrosin cells. The FAMA interactors ICE1/SCREAM and its closest paralog SCREAM2/ICE2 were essential for myrosin cell development. DNA microarray analysis identified 32 candidate genes involved in myrosin cell development under the control of FAMA. This study provides a common regulatory pathway that determines two distinct cell types in leaves: epidermal guard cells and inner-tissue myrosin cells.

NRGA1, a putative mitochondrial pyruvate carrier, mediates ABA regulation of guard cell ion channels and drought stress responses in Arabidopsis

Abscisic acid (ABA) regulates ion channel activity and stomatal movements in response to drought and other stresses. Here, we show that the Arabidopsis thaliana gene NRGA1 is a putative mitochondrial pyruvate carrier which negatively regulates ABA-induced guard cell signaling. NRGA1 transcript was abundant in the A. thaliana leaf and particularly in the guard cells, and its product was directed to the mitochondria. The heterologous co-expression of NRGA1 and AtMPC1 in yeast complemented a loss-of-function mitochondrial pyruvate carrier (MPC) mutant. The nrga1 loss-of-function mutant was very sensitive to the presence of ABA in the context of stomatal movements, and exhibited a heightened tolerance to drought stress. Disruption of NRGA1 gene resulted in increased ABA inhibition of inward K(+) currents and ABA activation of slow anion currents in guard cells. The nrga1/NRGA1 functional complementation lines restored the mutant's phenotypes. Furthermore, transgenic lines of constitutively overexpressing NRGA1 showed opposite stomatal responses, reduced drought tolerance, and ABA sensitivity of guard cell inward K(+) channel inhibition and anion channel activation. Our findings highlight a putative role for the mitochondrial pyruvate carrier in guard cell ABA signaling in response to drought.

Gate control: guard cell regulation by microbial stress

Terrestrial plants rely on stomata, small pores in the leaf surface, for photosynthetic gas exchange and transpiration of water. The stomata, formed by a pair of guard cells, dynamically increase and decrease their volume to control the pore size in response to environmental cues. Stresses can trigger similar or opposing movements: for example, drought induces closure of stomata, whereas many pathogens exploit stomata and cause them to open to facilitate entry into plant tissues. The latter is an active process as stomatal closure is part of the plant's immune response. Stomatal research has contributed much to clarify the signalling pathways of abiotic stress, but guard cell signalling in response to microbes is a relatively new area of research. In this article, we discuss present knowledge of stomatal regulation in response to microbes and highlight common points of convergence, and differences, compared to stomatal regulation by abiotic stresses. We also expand on the mechanisms by which pathogens manipulate these processes to promote disease, for example by delivering effectors to inhibit closure or trigger opening of stomata. The study of pathogen effectors in stomatal manipulation will aid our understanding of guard cell signalling.

Lacking chloroplasts in guard cells of crumpled leaf attenuates stomatal opening: both guard cell chloroplasts and mesophyll contribute to guard cell ATP levels

Controversies regarding the function of guard cell chloroplasts and the contribution of mesophyll in stomatal movements have persisted for several decades. Here, by comparing the stomatal opening of guard cells with (crl-ch) or without chloroplasts (crl-no ch) in one epidermis of crl (crumpled leaf) mutant in Arabidopsis, we showed that stomatal apertures of crl-no ch were approximately 65-70% those of crl-ch and approximately 50-60% those of wild type. The weakened stomatal opening in crl-no ch could be partially restored by imposing lower extracellular pH. Correspondingly, the external pH changes and K(+) accumulations following fusicoccin (FC) treatment were greatly reduced in the guard cells of crl-no ch compared with crl-ch and wild type. Determination of the relative ATP levels in individual cells showed that crl-no ch guard cells contained considerably lower levels of ATP than did crl-ch and wild type after 2 h of white light illumination. In addition, guard cell ATP levels were lower in the epidermis than in leaves, which is consistent with the observed weaker stomatal opening response to white light in the epidermis than in leaves. These results provide evidence that both guard cell chloroplasts and mesophyll contribute to the ATP source for H(+) extrusion by guard cells.

Role of H2O2 dynamics in brassinosteroid-induced stomatal closure and opening in Solanum lycopersicum

Brassinosteroids (BRs) are essential for plant growth and development; however, their roles in the regulation of stomatal opening or closure remain obscure. Here, the mechanism underlying BR-induced stomatal movements is studied. The effects of 24-epibrassinolide (EBR) on the stomatal apertures of tomato (Solanum lycopersicum) were measured by light microscopy using epidermal strips of wild type (WT), the abscisic acid (ABA)-deficient notabilis (not) mutant, and plants silenced for SlBRI1, SlRBOH1 and SlGSH1. EBR induced stomatal opening within an appropriate range of concentrations, whereas high concentrations of EBR induced stomatal closure. EBR-induced stomatal movements were closely related to dynamic changes in H(2)O(2) and redox status in guard cells. The stomata of SlRBOH1-silenced plants showed a significant loss of sensitivity to EBR. However, ABA deficiency abolished EBR-induced stomatal closure but did not affect EBR-induced stomatal opening. Silencing of SlGSH1, the critical gene involved in glutathione biosynthesis, disrupted glutathione redox homeostasis and abolished EBR-induced stomatal opening. The results suggest that transient H(2)O(2) production is essential for poising the cellular redox status of glutathione, which plays an important role in BR-induced stomatal opening. However, a prolonged increase in H(2)O(2) facilitated ABA signalling and stomatal closure.

Mesophyll photosynthesis and guard cell metabolism impacts on stomatal behaviour

Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere. Guard cells determine stomatal aperture and must operate to ensure an appropriate balance between CO2 uptake for photosynthesis (A) and water loss, and ultimately plant water use efficiency (WUE). A strong correlation between A and stomatal conductance (gs ) is well documented and often observed, but the underlying mechanisms, possible signals and metabolites that promote this relationship are currently unknown. In this review we evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behaviour with mesophyll carbon assimilation. We explore a possible role of various metabolites including sucrose and malate (from several potential sources; including guard cell photosynthesis) and new evidence that improvements in WUE have been made by manipulating sucrose metabolism within the guard cells. Finally we discuss the new tools and techniques available for potentially manipulating cell-specific metabolism, including guard and mesophyll cells, in order to elucidate mesophyll-derived signals that coordinate mesophyll CO2 demands with stomatal behaviour, in order to provide a mechanistic understanding of these processes as this may identify potential targets for manipulations in order to improve plant WUE and crop yield.

Stomatal and pavement cell density linked to leaf internal CO2 concentration

BACKGROUND AND AIMS

Stomatal density (SD) generally decreases with rising atmospheric CO2 concentration, Ca. However, SD is also affected by light, air humidity and drought, all under systemic signalling from older leaves. This makes our understanding of how Ca controls SD incomplete. This study tested the hypotheses that SD is affected by the internal CO2 concentration of the leaf, Ci, rather than Ca, and that cotyledons, as the first plant assimilation organs, lack the systemic signal.

METHODS

Sunflower (Helianthus annuus), beech (Fagus sylvatica), arabidopsis (Arabidopsis thaliana) and garden cress (Lepidium sativum) were grown under contrasting environmental conditions that affected Ci while Ca was kept constant. The SD, pavement cell density (PCD) and stomatal index (SI) responses to Ci in cotyledons and the first leaves of garden cress were compared. (13)C abundance (δ(13)C) in leaf dry matter was used to estimate the effective Ci during leaf development. The SD was estimated from leaf imprints.

KEY RESULTS

SD correlated negatively with Ci in leaves of all four species and under three different treatments (irradiance, abscisic acid and osmotic stress). PCD in arabidopsis and garden cress responded similarly, so that SI was largely unaffected. However, SD and PCD of cotyledons were insensitive to Ci, indicating an essential role for systemic signalling.

CONCLUSIONS

It is proposed that Ci or a Ci-linked factor plays an important role in modulating SD and PCD during epidermis development and leaf expansion. The absence of a Ci-SD relationship in the cotyledons of garden cress indicates the key role of lower-insertion CO2 assimilation organs in signal perception and its long-distance transport.

Manipulation of mitogen-activated protein kinase kinase signaling in the Arabidopsis stomatal lineage reveals motifs that contribute to protein localization and signaling specificity

When multiple mitogen-activated protein kinase (MAPK) components are recruited recurrently to transduce signals of different origins, and often opposing outcomes, mechanisms to enforce signaling specificity are of utmost importance. These mechanisms are largely uncharacterized in plant MAPK signaling networks. The Arabidopsis thaliana stomatal lineage was previously used to show that when rendered constitutively active, four MAPK kinases (MKKs), MKK4/5/7/9, are capable of perturbing stomatal development and that these kinases comprise two pairs, MKK4/5 and MKK7/9, with both overlapping and divergent functions. We characterized the contributions of specific structural domains of these four "stomatal" MKKs to MAPK signaling output and specificity both in vitro and in vivo within the three discrete cell types of the stomatal lineage. These results verify the influence of functional docking (D) domains of MKKs on MAPK signal output and identify novel regulatory functions for previously uncharacterized structures within the N termini of MKK4/5. Beyond this, we present a novel function of the D-domains of MKK7/9 in regulating the subcellular localization of these kinases. These results provide tools to broadly assess the extent to which these and additional motifs within MKKs function to regulate MAPK signal output throughout the plant.

Arabidopsis SNAREs SYP61 and SYP121 coordinate the trafficking of plasma membrane aquaporin PIP2;7 to modulate the cell membrane water permeability

Plant plasma membrane intrinsic proteins (PIPs) are aquaporins that facilitate the passive movement of water and small neutral solutes through biological membranes. Here, we report that post-Golgi trafficking of PIP2;7 in Arabidopsis thaliana involves specific interactions with two syntaxin proteins, namely, the Qc-SNARE SYP61 and the Qa-SNARE SYP121, that the proper delivery of PIP2;7 to the plasma membrane depends on the activity of the two SNAREs, and that the SNAREs colocalize and physically interact. These findings are indicative of an important role for SYP61 and SYP121, possibly forming a SNARE complex. Our data support a model in which direct interactions between specific SNARE proteins and PIP aquaporins modulate their post-Golgi trafficking and thus contribute to the fine-tuning of the water permeability of the plasma membrane.

Closing gaps: linking elements that control stomatal movement

Stomata are an attractive experimental system in plant biology, because the responses of guard cells to environmental signals can be directly linked to changes in the aperture of stomatal pores. In this review, the mechanics of stomatal movement are discussed in relation to ion transport in guard cells. Emphasis is placed on the ion pumps, transporters, and channels in the plasma membrane, as well as in the vacuolar membrane. The biophysical properties of transport proteins for H(+), K(+), Ca(2+), and anions are discussed and related to their function in guard cells during stomatal movements. Guard cell signaling pathways for ABA, CO2, ozone, microbe-associated molecular patterns (MAMPs) and blue light are presented. Special attention is given to the regulation of the slow anion channel (SLAC) and SLAC homolog (SLAH)-type anion channels by the ABA signalosome. Over the last decade, several knowledge gaps in the regulation of ion transport in guard cells have been closed. The current state of knowledge is an excellent starting point for tackling important open questions concerning stress tolerance in plants.

ARP2/3 complex-mediated actin dynamics is required for hydrogen peroxide-induced stomatal closure in Arabidopsis

Multiple cellular events like dynamic actin reorganization and hydrogen peroxide (H(2)O(2)) production were demonstrated to be involved in abscisic acid (ABA)-induced stomatal closure. However, the relationship between them as well as the underlying mechanisms remains poorly understood. Here, we showed that H(2)O(2) generation is indispensable for ABA induction of actin reorganization in guard cells of Arabidopsis that requires the presence of ARP2/3 complex. H(2)O(2) -induced stomatal closure was delayed in the mutants of arpc4 and arpc5, and the rate of actin reorganization was slowed down in arpc4 and arpc5 in response to H(2)O(2), suggesting that ARP2/3-mediated actin nucleation is required for H(2)O(2) -induced actin cytoskeleton remodelling. Furthermore, the expression of H(2)O(2) biosynthetic related gene AtrbohD and the accumulation of H(2)O(2) was delayed in response to ABA in arpc4 and arpc5, demonstrating that misregulated actin dynamics affects H(2)O(2) production upon ABA treatment. These results support a possible causal relation between the production of H(2)O(2) and actin dynamics in ABA-mediated guard cell signalling: ABA triggers H(2)O(2) generation that causes the reorganization of the actin cytoskeleton partially mediated by ARP2/3 complex, and ARP2/3 complex-mediated actin dynamics may feedback regulate H(2)O(2) production.

Transcription factor WRKY46 regulates osmotic stress responses and stomatal movement independently in Arabidopsis

Drought and salt stress severely inhibit plant growth and development; however, the regulatory mechanisms of plants in response to these stresses are not fully understood. Here we report that the expression of a WRKY transcription factor WRKY46 is rapidly induced by drought, salt and oxidative stresses. T-DNA insertion of WRKY46 leads to more sensitivity to drought and salt stress, whereas overexpression of WRKY46 (OV46) results in hypersensitivity in soil-grown plants, with a higher water loss rate, but with increased tolerance on the sealed agar plates. Stomatal closing in the OV46 line is insensitive to ABA because of a reduced accumulation of reactive oxygen species (ROS) in the guard cells. We further find that WRKY46 is expressed in guard cells, where its expression is not affected by dehydration, and is involved in light-dependent stomatal opening. Microarray analysis reveals that WRKY46 regulates a set of genes involved in cellular osmoprotection and redox homeostasis under dehydration stress, which is confirmed by ROS and malondialdehyde (MDA) levels in stressed seedlings. Moreover, WRKY46 modulates light-dependent starch metabolism in guard cells via regulating QUA-QUINE STARCH (QQS) gene expression. Taken together, we demonstrate that WRKY46 plays dual roles in regulating plant responses to drought and salt stress and light-dependent stomatal opening in guard cells.

ALY proteins participate in multifaceted Nep1Mo-triggered responses in Nicotiana benthamiana and Arabidopsis thaliana

Previously, it was found that Nep1Mo (a Nep1-like protein from Magnaporthe oryzae) could trigger a variety of plant responses, including stomatal closure, hypersensitive cell death (HCD), and defence-related gene expression, in Nicotiana benthamiana. In this study, it was found that Nep1Mo-induced cell death could be inhibited by the virus-induced gene silencing of NbALY916 in N. benthamiana. NbALY916-silenced plants showed impaired Nep1Mo-induced stomatal closure, decreased Nep1Mo-induced production of hydrogen peroxide (H2O2) and nitric oxide (NO) in guard cells, and reduced Nep1Mo-induced resistance against Phytophthora nicotianae. It also found that the deletion of AtALY4, an orthologue of NbALY916 in Arabidopsis thaliana, impaired Nep1Mo-triggered stomatal closure, HCD, and defence-related gene expression. The compromised stomatal closure observed in the NbALY916-silenced plants and AtALY4 mutants was inhibited by the application of H2O2 and sodium nitroprusside (an NO donor), and both Nep1Mo and H2O2 stimulated guard cell NO synthesis. Conversely, NO-induced stomatal closure was found not to require H2O2 synthesis; and NO treatment did not induce H2O2 production in guard cells. Taken together, these results demonstrate that the NbAlY916/AtAlY4-H2O2-NO pathway mediates multiple Nep1Mo-triggered responses, including stomatal closure, HCD, and defence-related gene expression.

A new loss-of-function allele 28y reveals a role of ARGONAUTE1 in limiting asymmetric division of stomatal lineage ground cell

In Arabidopsis thaliana L., stomata are produced through a series of divisions including asymmetric and symmetric divisions. Asymmetric entry division of meristemoid mother cell produces two daughter cells, the smaller meristemoid and the larger sister cell, a stomatal lineage ground cell (SLGC). Stomatal lineage ground cells can differentiate into epidermal pavement cells but have the potential to divide asymmetrically, spacing divisions, to create satellite meristemoids. Peptide ligands and TOO MANY MOUTHS (TMM) and ERECTA family receptors regulate the initiation of stomatal lineages, activity, and orientation of spacing divisions. Here, we reported that a natural mutant 28y displayed an increased stomatal density and index. Using map-based cloning, we identified mutation in ARGONAUTE1 (AGO1) as the cause of 28y phenotypes. Time-lapse tracing of stomatal lineage cells reveals that stomatal overproduction in 28y is caused by the excessive asymmetric spacing division of SLGCs. Further genetic results demonstrated that AGO1 acts downstream of TMM and negatively regulates the SPCH transcripts, but in a brassinosteroid-independent manner. Upregulation of AGAMOUS-LIKE16 (AGL16) in 28y mutants suggests that AGO1 is required to restrict AGL16-mediated stomatal spacing divisions, an miRNA pathway in addition to ligand-receptor signaling modules.

Requirement for A-type cyclin-dependent kinase and cyclins for the terminal division in the stomatal lineage of Arabidopsis

The Arabidopsis stoma is a specialized epidermal valve made up of a pair of guard cells around a pore whose aperture controls gas exchange between the shoot and atmosphere. Guard cells (GCs) are produced by a symmetric division of guard mother cells (GMCs). The R2R3-MYB transcription factor FOUR LIPS (FLP) and its paralogue MYB88 restrict the division of a GMC to one. Previously, the upstream regions of several core cell cycle genes were identified as the direct targets of FLP/MYB88, including the B-type cyclin-dependent kinase CDKB1;1 and A2-type cyclin CYCA2;3. Here we show that CDKA;1 is also an immediate direct target of FLP/MYB88 through the binding to cis-regulatory elements in the CDKA;1 promoter region. CDKA;1 activity is required not only for normal GMC divisions but also for the excessive cell overproliferation in flp myb88 mutant GMCs. The impaired defects of GMC division in cdkb1;1 1;2 mutants could be partially rescued by a stage-specific expression of CDKA;1. Although targeted overexpression of CDKA;1 does not affect stomatal development, ectopic expression of the D3-type cyclin CYCD3;2 induces GC subdivision, resulting in a stoma with 3-4 GCs instead of the normal two. Co-overexpression of CDKA;1 with CYCD3;2, but not with CYCA2;3, confers a synergistic effect with respect to GC subdivision. Thus, in addition to a role in stomatal formative asymmetric divisions at early developmental stages, CDKA;1 is needed in triggering GMC symmetric divisions at the late stage of stomatal development. However, timely down-regulation of CDKA;1-CYCD3 activity is required for restriction of GC proliferation.