The Plasma Membrane H+-ATPase AHA1 Plays a Major Role in Stomatal Opening in Response to Blue Light

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.

Tomato Atypical Receptor Kinase1 Is Involved in the Regulation of Preinvasion Defense

Tomato Atypical Receptor Kinase 1 (TARK1) is a pseudokinase required for postinvasion immunity. TARK1 was originally identified as a target of the Xanthomonas euvesicatoria effector protein Xanthomonas outer protein N (XopN), a suppressor of early defense signaling. How TARK1 participates in immune signal transduction is not well understood. To gain insight into TARK1's role in tomato (Solanum lycopersicum) immunity, we used a proteomics approach to isolate and identify TARK1-associated immune complexes formed during infection. We found that TARK1 interacts with proteins predicted to be associated with stomatal movement. TARK1 CRISPR mutants and overexpression (OE) lines did not display differences in light-induced stomatal opening or abscisic acid-induced stomatal closure; however, they did show altered stomatal movement responses to bacteria and biotic elicitors. Notably, we found that TARK1 CRISPR plants were resistant to Pseudomonas syringae pathovar tomato strain DC3000-induced stomatal reopening, and TARK1 OE plants were insensitive to P syringae pathovar tomato strain DC3118 (coronatine deficit)-induced stomatal closure. We also found that TARK1 OE in leaves resulted in increased susceptibility to bacterial invasion. Collectively, our results indicate that TARK1 functions in stomatal movement only in response to biotic elicitors and support a model in which TARK1 regulates stomatal opening postelicitation.

Guard Cell Starch Degradation Yields Glucose for Rapid Stomatal Opening in Arabidopsis

Starch in Arabidopsis (Arabidopsis thaliana) guard cells is rapidly degraded at the start of the day by the glucan hydrolases α-AMYLASE3 (AMY3) and β-AMYLASE1 (BAM1) to promote stomatal opening. This process is activated via phototropin-mediated blue light signaling downstream of the plasma membrane H+-ATPase. It remains unknown how guard cell starch degradation integrates with light-regulated membrane transport processes in the fine control of stomatal opening kinetics. We report that H+, K+, and Cl- transport across the guard cell plasma membrane is unaltered in the amy3 bam1 mutant, suggesting that starch degradation products do not directly affect the capacity to transport ions. Enzymatic quantification revealed that after 30 min of blue light illumination, amy3 bam1 guard cells had similar malate levels as the wild type, but had dramatically altered sugar homeostasis, with almost undetectable amounts of Glc. Thus, Glc, not malate, is the major starch-derived metabolite in Arabidopsis guard cells. We further show that impaired starch degradation in the amy3 bam1 mutant resulted in an increase in the time constant for opening of 40 min. We conclude that rapid starch degradation at dawn is required to maintain the cytoplasmic sugar pool, clearly needed for fast stomatal opening. The conversion and exchange of metabolites between subcellular compartments therefore coordinates the energetic and metabolic status of the cell with membrane ion transport.

Predicting the unexpected in stomatal gas exchange: not just an open-and-shut case

Plant membrane transport, like transport across all eukaryotic membranes, is highly non-linear and leads to interactions with characteristics so complex that they defy intuitive understanding. The physiological behaviour of stomatal guard cells is a case in point in which, for example, mutations expected to influence stomatal closing have profound effects on stomatal opening and manipulating transport across the vacuolar membrane affects the plasma membrane. Quantitative mathematical modelling is an essential tool in these circumstances, both to integrate the knowledge of each transport process and to understand the consequences of their manipulation in vivo. Here, we outline the OnGuard modelling environment and its use as a guide to predicting the emergent properties arising from the interactions between non-linear transport processes. We summarise some of the recent insights arising from OnGuard, demonstrate its utility in interpreting stomatal behaviour, and suggest ways in which the OnGuard environment may facilitate 'reverse-engineering' of stomata to improve water use efficiency and carbon assimilation.

NO is involved in H2-induced adventitious rooting in cucumber by regulating the expression and interaction of plasma membrane H+-ATPase and 14-3-3

NO was involved in H₂-induced adventitious rooting by regulating the protein and gene expressions of PM H⁺-ATPase and 14-3-3. Simultaneously, the interaction of PM H⁺-ATPase and 14-3-3 protein was also involved in this process. Hydrogen gas (H₂) and nitric oxide (NO) have been shown to be involved in plant growth and development. The results in this study revealed that NO was involved in H₂-induced adventitious root formation. Western blot (WB) analysis showed that the protein abundances of plasma membrane H⁺-ATPase (PM H⁺-ATPase) and 14-3-3 protein were increased after H₂, NO, H₂ plus NO treatments, whereas their protein abundances were down regulated when NO scavenger carboxy-2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTI O) was added. Moreover, the mRNA abundances of the HA3 and 14-3-3(7) gene as well as the activities of PM H⁺-ATPase (EC 3.6.1.35) and H⁺ pump were in full agreement with the changes of protein abundance. Phosphorylation of PM H⁺-ATPase and the interaction of PM H⁺-ATPase and 14-3-3 protein were detected by co-immunoprecipitation analysis. H₂ and NO significantly up regulated the phosphorylation of PM H⁺-ATPase and the interaction of PM H⁺-ATPase and 14-3-3 protein. Conversely, the stimulation of PM H⁺-ATPase phosphorylation and protein interaction were significantly diminished by cPTIO. Protein interaction activator fusicoccin (FC) and inhibitor adenosine monophosphate (AMP) of PM H⁺-ATPase and 14-3-3 were used in this study, and the results showed that FC significantly increased the abundances of PM H⁺-ATPase and 14-3-3, while AMP showed opposite trends. We further proved the critical roles of PM H⁺-ATPase and 14-3-3 protein interaction in NO-H₂-induced adventitious root formation. Taken together, our results suggested that NO might be involved in H₂-induced adventitious rooting by regulating the expression and the interaction of PM H⁺-ATPase and 14-3-3 protein.

Role of blue and red light in stomatal dynamic behaviour

Plants experience changes in light intensity and quality due to variations in solar angle and shading from clouds and overlapping leaves. Stomatal opening to increasing irradiance is often an order of magnitude slower than photosynthetic responses, which can result in CO2 diffusional limitations on leaf photosynthesis, as well as unnecessary water loss when stomata continue to open after photosynthesis has reached saturation. Stomatal opening to light is driven by two distinct pathways; the 'red' or photosynthetic response that occurs at high fluence rates and saturates with photosynthesis, and is thought to be the main mechanism that coordinates stomatal behaviour with photosynthesis; and the guard cell-specific 'blue' light response that saturates at low fluence rates, and is often considered independent of photosynthesis, and important for early morning stomatal opening. Here we review the literature on these complicated signal transduction pathways and osmoregulatory processes in guard cells that are influenced by the light environment. We discuss the possibility of tuning the sensitivity and magnitude of stomatal response to blue light which potentially represents a novel target to develop ideotypes with the 'ideal' balance between carbon gain, evaporative cooling, and maintenance of hydraulic status that is crucial for maximizing crop performance and productivity.

Raf-like kinases CBC1 and CBC2 negatively regulate stomatal opening by negatively regulating plasma membrane H+-ATPase phosphorylation in Arabidopsis

Stomatal pores, which are surrounded by pairs of guard cells in the plant epidermis, regulate gas exchange between plants and the atmosphere, thereby controlling photosynthesis and transpiration. Blue light works as a signal to guard cells, to induce intracellular signaling and open stomata. Blue light receptor phototropins (phots) are activated by blue light; phot-mediated signals promote plasma membrane (PM) H+-ATPase activity via C-terminal Thr phosphorylation, serving as the driving force for stomatal opening in guard cells. However, the details of this signaling process are not fully understood. In this study, through an in vitro screening of phot-interacting protein kinases, we obtained the CBC1 and CBC2 that had been reported as signal transducers in stomatal opening. Promoter activities of CBC1 and CBC2 indicated that both genes were expressed in guard cells. Single and double knockout mutants of CBC1 and CBC2 showed no lesions in the context of phot-mediated phototropism, chloroplast movement, or leaf flattening. In contrast, the cbc1cbc2 double mutant showed larger stomatal opening under both dark and blue light conditions. Interestingly, the level of phosphorylation of C-terminal Thr of PM H+-ATPase was higher in double mutant guard cells. The larger stomatal openings of the double mutant were effectively suppressed by the phytohormone abscisic acid (ABA). CBC1 and CBC2 interacted with BLUS1 and PM H+-ATPase in vitro. From these results, we conclude that CBC1 and CBC2 act as negative regulators of stomatal opening, probably via inhibition of PM H+-ATPase activity.

Coordinated Transport of Nitrate, Potassium, and Sodium

Potassium (K+) and nitrogen (N) are essential nutrients, and their absorption and distribution within the plant must be coordinated for optimal growth and development. Potassium is involved in charge balance of inorganic and organic anions and macromolecules, control of membrane electrical potential, pH homeostasis and the regulation of cell osmotic pressure, whereas nitrogen is an essential component of amino acids, proteins, and nucleic acids. Nitrate (NO3 -) is often the primary nitrogen source, but it also serves as a signaling molecule to the plant. Nitrate regulates root architecture, stimulates shoot growth, delays flowering, regulates abscisic acid-independent stomata opening, and relieves seed dormancy. Plants can sense K+/NO3 - levels in soils and adjust accordingly the uptake and root-to-shoot transport to balance the distribution of these ions between organs. On the other hand, in small amounts sodium (Na+) is categorized as a "beneficial element" for plants, mainly as a "cheap" osmolyte. However, at high concentrations in the soil, Na+ can inhibit various physiological processes impairing plant growth. Hence, plants have developed specific mechanisms to transport, sense, and respond to a variety of Na+ conditions. Sodium is taken up by many K+ transporters, and a large proportion of Na+ ions accumulated in shoots appear to be loaded into the xylem by systems that show nitrate dependence. Thus, an adequate supply of mineral nutrients is paramount to reduce the noxious effects of salts and to sustain crop productivity under salt stress. In this review, we will focus on recent research unraveling the mechanisms that coordinate the K+-NO3 -; Na+-NO3 -, and K+-Na+ transports, and the regulators controlling their uptake and allocation.

Guard Cells Integrate Light and Temperature Signals to Control Stomatal Aperture

High temperature promotes guard cell expansion, which opens stomatal pores to facilitate leaf cooling. How the high-temperature signal is perceived and transmitted to regulate stomatal aperture is, however, unknown. Here, we used a reverse-genetics approach to understand high temperature-mediated stomatal opening in Arabidopsis (Arabidopsis thaliana). Our findings reveal that high temperature-induced guard cell movement requires components involved in blue light-mediated stomatal opening, suggesting cross talk between light and temperature signaling pathways. The molecular players involved include phototropin photoreceptors, plasma membrane H+-ATPases, and multiple members of the 14-3-3 protein family. We further show that phototropin-deficient mutants display impaired rosette evapotranspiration and leaf cooling at high temperatures. Blocking the interaction of 14-3-3 proteins with their client proteins severely impairs high temperature-induced stomatal opening but has no effect on the induction of heat-sensitive guard cell transcripts, supporting the existence of an additional intracellular high-temperature response pathway in plants.

A Tandem Amino Acid Residue Motif in Guard Cell SLAC1 Anion Channel of Grasses Allows for the Control of Stomatal Aperture by Nitrate

The latest major group of plants to evolve were the grasses. These became important in the mid-Paleogene about 40 million years ago. During evolution, leaf CO₂ uptake and transpirational water loss were optimized by the acquisition of grass-specific stomatal complexes. In contrast to the kidney-shaped guard cells (GCs) typical of the dicots such as Arabidopsis, in the grasses and agronomically important cereals, the GCs are dumbbell shaped and are associated with morphologically distinct subsidiary cells (SCs). We studied the molecular basis of GC action in the major cereal crop barley. Upon feeding ABA to xylem sap of an intact barley leaf, stomata closed in a nitrate-dependent manner. This process was initiated by activation of GC SLAC-type anion channel currents. HvSLAC1 expressed in Xenopus oocytes gave rise to S-type anion currents that increased several-fold upon stimulation with >3 mM nitrate. We identified a tandem amino acid residue motif that within the SLAC1 channels differs fundamentally between monocots and dicots. When the motif of nitrate-insensitive dicot Arabidopsis SLAC1 was replaced by the monocot signature, AtSLAC1 converted into a grass-type like nitrate-sensitive channel. Our work reveals a fundamental difference between monocot and dicot GCs and prompts questions into the selective pressures during evolution that resulted in fundamental changes in the regulation of SLAC1 function.

Ionic selectivity and coordinated transport of Na+ and K+ in flag leaves render differential salt tolerance in rice at the reproductive stage

The present study shows that salt tolerance in the reproductive stage of rice is primarily governed by the selective Na⁺ and K⁺ transport from the root to upper plant parts. Ionic discrimination at the flag leaf, governed by differential expression of Na⁺- and K⁺-specific transporters/ion pumps, is associated with reduced spikelet sterility and reproductive stage salt tolerance. Reproductive stage salt tolerance is crucial in rice to guarantee yield under saline condition. In the present study, differential ionic selectivity and the coordinated transport (from root to flag leaf) of Na⁺ and K⁺ were investigated to assess their impact on reproductive stage salt tolerance. Four rice genotypes having differential salt sensitivity were subjected to reproductive stage salinity stress in pots. The selective Na⁺ and K⁺ transport from the root to upper plant parts was observed in tolerant genotypes. We noticed that prolonged salt exposure did not alter flag leaf greenness even up to 6 weeks; however, it had a detrimental effect on panicle development especially in the salt-susceptible genotype Sabita. But more precise chlorophyll fluorescence imaging analysis revealed salinity-induced damages in Sabita. The salt-tolerant genotype Pokkali (AC41585), a potential Na⁺ excluder, managed to sequester higher Na⁺ load in the roots with little upward transport as evident from greater expression of HKT1 and HKT2 transporters. In contrast, the moderately salt-tolerant Lunidhan was less selective in Na⁺ transport, but possessed a higher capacity to Na⁺ sequestration in leaves. Higher K⁺ uptake and tissue-specific redistribution mediated by HAK and AKT transporters showed robust control in selective K⁺ movement from the root to flag leaf and developing panicles. On the contrary, expressions of Na⁺-specific transporters in developing panicles were either down-regulated or unaffected in tolerant and moderately tolerant genotypes. Yet, in the panicles of the susceptible genotype Sabita, some of the Na⁺-specific transporter genes (SOS1, HKT1;5, HKT2;4) were upregulated. Apart from the ionic regulation strategy, cellular energy balance mediated by different plasma-membrane and tonoplastic H⁺-pumps were also associated with the reproductive stage salt tolerance in rice.

High-yielding rice Takanari has superior photosynthetic response to a commercial rice Koshihikari under fluctuating light

Leaves within crop canopies experience variable light over the course of a day, which greatly affects photosynthesis and crop productivity. Little is known about the mechanisms of the photosynthetic response to fluctuating light and their genetic control. Here, we examined gas exchange, metabolite levels, and chlorophyll fluorescence during the photosynthetic induction response in an Oryza sativa indica cultivar with high yield (Takanari) and a japonica cultivar with lower yield (Koshihikari). Takanari had a faster induction response to sudden increases in light intensity than Koshihikari, as demonstrated by faster increases in net CO2 assimilation rate, stomatal conductance, and electron transport rate. In a simulated light regime that mimicked a typical summer day, the faster induction response in Takanari increased daily CO2 assimilation by 10%. The faster response of Takanari was explained in part by its maintenance of a larger pool of Calvin-Benson cycle metabolites. Together, the rapid responses of electron transport rate, metabolic flux, and stomatal conductance in Takanari contributed to the greater daily carbon gain under fluctuating light typical of natural environments.

Molecular Evolution and Interaction of Membrane Transport and Photoreception in Plants

Light is a vital regulator that controls physiological and cellular responses to regulate plant growth, development, yield, and quality. Light is the driving force for electron and ion transport in the thylakoid membrane and other membranes of plant cells. In different plant species and cell types, light activates photoreceptors, thereby modulating plasma membrane transport. Plants maximize their growth and photosynthesis by facilitating the coordinated regulation of ion channels, pumps, and co-transporters across membranes to fine-tune nutrient uptake. The signal-transducing functions associated with membrane transporters, pumps, and channels impart a complex array of mechanisms to regulate plant responses to light. The identification of light responsive membrane transport components and understanding of their potential interaction with photoreceptors will elucidate how light-activated signaling pathways optimize plant growth, production, and nutrition to the prevailing environmental changes. This review summarizes the mechanisms underlying the physiological and molecular regulations of light-induced membrane transport and their potential interaction with photoreceptors in a plant evolutionary and nutrition context. It will shed new light on plant ecological conservation as well as agricultural production and crop quality, bringing potential nutrition and health benefits to humans and animals.

Autophagy controls reactive oxygen species homeostasis in guard cells that is essential for stomatal opening

Reactive oxygen species (ROS) function as key signaling molecules to inhibit stomatal opening and promote stomatal closure in response to diverse environmental stresses. However, how guard cells maintain basal intracellular ROS levels is not yet known. This study aimed to determine the role of autophagy in the maintenance of basal ROS levels in guard cells. We isolated the Arabidopsis autophagy-related 2 (atg2) mutant, which is impaired in stomatal opening in response to light and low CO₂ concentrations. Disruption of other autophagy genes, including ATG5, ATG7, ATG10, and ATG12, also caused similar stomatal defects. The atg mutants constitutively accumulated high levels of ROS in guard cells, and antioxidants such as ascorbate and glutathione rescued ROS accumulation and stomatal opening. Furthermore, the atg mutations increased the number and aggregation of peroxisomes in guard cells, and these peroxisomes exhibited reduced activity of the ROS scavenger catalase and elevated hydrogen peroxide (H₂O₂) as visualized using the peroxisome-targeted H₂O₂ sensor HyPer. Moreover, such ROS accumulation decreased by the application of 2-hydroxy-3-butynoate, an inhibitor of peroxisomal H₂O₂-producing glycolate oxidase. Our results showed that autophagy controls guard cell ROS homeostasis by eliminating oxidized peroxisomes, thereby allowing stomatal opening.

A role for SPEECHLESS in the integration of leaf stomatal patterning with the growth vs disease trade-off in poplar

Occurrence of stomata on both leaf surfaces (amphistomaty) promotes higher stomatal conductance and photosynthesis while simultaneously increasing exposure to potential disease agents in black cottonwood (Populus trichocarpa). A genome-wide association study (GWAS) with 2.2M single nucleotide polymorphisms generated through whole-genome sequencing found 280 loci associated with variation in adaxial stomatal traits, implicating genes regulating stomatal development and behavior. Strikingly, numerous loci regulating plant growth and response to biotic and abiotic stresses were also identified. The most significant locus was a poplar homologue of SPEECHLESS (PtSPCH1). Individuals possessing PtSPCH1 alleles associated with greater adaxial stomatal density originated primarily from environments with shorter growing seasons (e.g. northern latitudes, high elevations) or with less precipitation. PtSPCH1 was expressed in developing leaves but not developing stem xylem. In developing leaves, RNA sequencing showed patterns of coordinated expression between PtSPCH1 and other GWAS-identified genes. The breadth of our GWAS results suggests that the evolution of amphistomaty is part of a larger, complex response in plants. Suites of genes underpin this response, retrieved through genetic association to adaxial stomata, and show coordinated expression during development. We propose that the occurrence of amphistomaty in P. trichocarpa involves PtSPCH1 and reflects selection for supporting rapid growth over investment in immunity.

Regulation of stomatal opening and histone modification by photoperiod in Arabidopsis thaliana

Stomatal movements are regulated by many environmental signals, such as light, CO₂, temperature, humidity, and drought. Recently, we showed that photoperiodic flowering components have positive effects on light-induced stomatal opening in Arabidopsis thaliana. In this study, we determined that light-induced stomatal opening and increased stomatal conductance were larger in plants grown under long-day (LD) conditions than in those grown under short-day (SD) conditions. Gene expression analyses using purified guard cell protoplasts revealed that FT and SOC1 expression levels were significantly increased under LD conditions. Interestingly, the enhancement of light-induced stomatal opening and increased SOC1 expression in guard cells due to LD conditions persisted for at least 1 week after plants were transferred to SD conditions. We then investigated histone modification using chromatin immunoprecipitation–PCR, and observed increased trimethylation of lysine 4 on histone 3 (H3K4) around SOC1. We also found that LD-dependent enhancement of light-induced stomatal opening and H3K4 trimethylation in SOC1 were suppressed in the ft-2 mutant. These results indicate that photoperiod is an important environmental cue regulating stomatal opening, and that LD conditions enhance light-induced stomatal opening and epigenetic modification (H3K4 trimethylation) around SOC1, a positive regulator of stomatal opening, in an FT-dependent manner. Thus, this study provides novel insights into stomatal responses to photoperiod.

Unusual Roles of Secretory SNARE SYP132 in Plasma Membrane H+-ATPase Traffic and Vegetative Plant Growth

The plasma membrane proton (H⁺)-ATPases of plants generate steep electrochemical gradients and activate osmotic solute uptake. H⁺-ATPase-mediated proton pumping orchestrates cellular homeostasis and is a prerequisite for plastic cell expansion and plant growth. All evidence suggests that the population of H⁺-ATPase proteins at the plasma membrane reflects a balance of their roles in exocytosis, endocytosis, and recycling. Auxin governs both traffic and activation of the plasma membrane H⁺-ATPase proteins already present at the membrane. As in other eukaryotes, in plants, SNARE-mediated membrane traffic influences the density of several proteins at the plasma membrane. Even so, H⁺-ATPase traffic, its relationship with SNAREs, and its regulation by auxin have remained enigmatic. Here, we identify the Arabidopsis (Arabidopsis thaliana) Qa-SNARE SYP132 (Syntaxin of Plants132) as a key factor in H⁺-ATPase traffic and demonstrate its association with endocytosis. SYP132 is a low-abundant, secretory SNARE that primarily localizes to the plasma membrane. We find that SYP132 expression is tightly regulated by auxin and that augmented SYP132 expression reduces the amount of H⁺-ATPase proteins at the plasma membrane. The physiological consequences of SYP132 overexpression include reduced apoplast acidification and suppressed vegetative growth. Thus, SYP132 plays unexpected and vital roles in auxin-regulated H⁺-ATPase traffic and associated functions at the plasma membrane.

Acquiring Control: The Evolution of Stomatal Signalling Pathways

In vascular plants, stomata balance two opposing functions: they open to facilitate CO₂ uptake and close to prevent excessive water loss. Here, we discuss the evolution of three major signalling pathways that are known to control stomatal movements in angiosperms in response to light, CO₂, and abscisic acid (ABA). We examine the evolutionary origins of key signalling genes involved in these pathways, and compare their expression patterns between an angiosperm and moss. We propose that variation in stomatal sensitivity to stimuli between plant groups are rooted in differences in: (i) gene presence/absence, (ii) specificity of gene spatial expression pattern, and (iii) protein characteristics and functional interactions.

Reactive Oxygen Species, Photosynthesis, and Environment in the Regulation of Stomata

SIGNIFICANCE

Stomata sense the intercellular carbon dioxide (CO₂) concentration (C_i) and water availability under changing environmental conditions and adjust their apertures to maintain optimal cellular conditions for photosynthesis. Stomatal movements are regulated by a complex network of signaling cascades where reactive oxygen species (ROS) play a key role as signaling molecules. Recent Advances: Recent research has uncovered several new signaling components involved in CO₂- and abscisic acid-triggered guard cell signaling pathways. In addition, we are beginning to understand the complex interactions between different signaling pathways.

CRITICAL ISSUES

Plants close their stomata in reaction to stress conditions, such as drought, and the subsequent decrease in C_i leads to ROS production through photorespiration and over-reduction of the chloroplast electron transport chain. This reduces plant growth and thus drought may cause severe yield losses for agriculture especially in arid areas.

FUTURE DIRECTIONS

The focus of future research should be drawn toward understanding the interplay between various signaling pathways and how ROS, redox, and hormonal balance changes in space and time. Translating this knowledge from model species to crop plants will help in the development of new drought-resistant crop species with high yields.

Quantitative phosphoproteomic analysis reveals common regulatory mechanisms between effector- and PAMP-triggered immunity in plants

Plant immunity consists of two arms: pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), induced by surface-localized receptors, and effector-triggered immunity (ETI), induced by intracellular receptors. Despite the little structural similarity, both receptor types activate similar responses with different dynamics. To better understand phosphorylation events during ETI, we employed a phosphoproteomic screen using an inducible expression system of the bacterial effector avrRpt2 in Arabidopsis thaliana, and identified 109 differentially phosphorylated residues of membrane-associated proteins on activation of the intracellular RPS2 receptor. Interestingly, several RPS2-regulated phosphosites overlap with sites that are regulated during PTI, suggesting that these phosphosites may be convergent points of both signaling arms. Moreover, some of these sites are residues of important defense components, including the NADPH oxidase RBOHD, ABC-transporter PEN3, calcium-ATPase ACA8, noncanonical Gα protein XLG2 and H⁺ -ATPases. In particular, we found that S343 and S347 of RBOHD are common phosphorylation targets during PTI and ETI. Our mutational analyses showed that these sites are required for the production of reactive oxygen species during both PTI and ETI, and immunity against avirulent bacteria and a virulent necrotrophic fungus. We provide, for the first time, large-scale phosphoproteomic data of ETI, thereby suggesting crucial roles of common phosphosites in plant immunity.

Fluence rate dependence of red light-induced phosphorylation of plasma membrane H+-ATPase in stomatal guard cells

Stomatal opening is induced by red light as well as blue light. Recently, we established an immunohistochemical technique using whole leaves to study plasma membrane (PM) H⁺-ATPase in guard cells, which is an important enzyme driving stomatal opening. Our technique revealed that red light illuminated to whole leaves induces photosynthesis-dependent phosphorylation of C-terminal penultimate residue of PM H⁺-ATPase, threonine, in guard cells, which has been considered to be important for activation of PM H⁺-ATPase, and we proposed that red light promotes stomatal opening via activation of PM H⁺-ATPase in guard cells in whole leaves. Here, using our new immunohistochemical technique, we investigated fluence rate dependence of red light-induced phosphorylation of PM H⁺-ATPase. We found that illumination of red light at 50 µmol m^-2 s^-1, which was suggested to initiate photosynthesis, saturates phosphorylation of PM H⁺-ATPase. Furthermore, we immunohistochemically confirmed decrease in the amount of PM H⁺-ATPase protein in a knock-out mutant of AHA1, an isogene encoding the major isoform of PM H⁺-ATPase in guard cells, implying the importance of AHA1 as the major PM H⁺-ATPase protein in guard cells for light-induced stomatal opening.

Boolean Calcium Signalling Model Predicts Calcium Role in Acceleration and Stability of Abscisic Acid-Mediated Stomatal Closure

Inconsistent hypotheses have proposed Ca²⁺ as either being essential or irrelevant and redundant in ABA induced stomatal closure. This study integrates all available information from literature to define ABA signalling pathway and presents it in a systems view for clearer understanding of the role of Ca²⁺ in stomatal closure. Importantly, it incorporates into an Asynchronous Boolean model time delays sourced from an extensive literature search. The model predicted the timing of ABA events and mutant behaviour close to biology. It revealed biologically reported timing for Ca²⁺ activation and Ca²⁺ dynamics consistent with biology. It also predicts that Ca²⁺ elevation is not essential in stomatal closure but it can accelerate closure, consistent with previous findings, but our model further explains that acting as a mediator, Ca²⁺ accelerates stomatal closure by enhancing plasma membrane slowly activating anion channel SLAC1 and actin rearrangement. It shows statistical significance of Ca²⁺ induced acceleration of closure and that of Ca²⁺ induced acceleration of SLAC1 activation. Further, the model demonstrates that Ca²⁺ enhances resilience of closure to perturbation of important elements; especially, ROS pathway, as did previous ABA model, and even to the ABA signal disruption. It goes further to elucidate the mechanisms by which Ca²⁺ engenders stomatal closure in these perturbations.

Dawn regulates guard cell proteins in Arabidopsis thaliana that function in ATP production from fatty acid beta-oxidation

Based on the nature of the proteins that are altered in abundance, we conclude that guard cells switch their energy source from fatty acid metabolism to chloroplast activity, at the onset of dawn. During stomatal opening at dawn, evidence was recently presented for a breakdown and liquidation of stored triacylglycerols in guard cells to supply ATP for use in stomatal opening. However, proteome changes that happen in the guard cells during dawn were until now poorly understood. Bad accessibility to pure and intact guard cell samples can be considered as the primary reason behind this lack of knowledge. To overcome these technical constraints, epidermal guard cell samples with ruptured pavement cells were isolated at 1 h pre-dawn, 15 min post-dawn and 1 h post-dawn from Arabidopsis thaliana. Proteomic changes were analysed by ultra-performance-liquid-chromatography-mass-spectrometry. With 994 confidently identified proteins, we present the first analysis of the A. thaliana guard cell proteome that is not influenced by side effects of guard cell protoplasting. Data are available via ProteomeXchange with identifier PXD009918. By elucidating the identities of enzymes that change in abundance by the transition from dark to light, we corroborate the hypothesis that respiratory ATP production for stomatal opening results from fatty acid beta-oxidation. Moreover, we identified many proteins that were never reported in the context of guard cell biology. Among them are proteins that might play a role in signalling or circadian rhythm.

Stomata in a saline world

Salt stress results in a dramatic increase in ABA biosynthesis, H₂O₂ accumulation, and reduced K⁺ availability in the shoot. Each of these factors leads to stomata closure, so reducing CO₂ assimilation and imposing yield penalties. However, halophytes, naturally salt tolerant plant species, flourish under saline conditions that would cause massive yield penalties in glycophytic crops. Is there anything special about the stomata of halophytes, why is guard cell function in these salt tolerant species not affected by the above factors? This opinion paper addresses these questions by providing a comprehensive assessment of the molecular identity and operational modes of major plasma membrane transporters that mediate stomata movements.

VAMP711 Is Required for Abscisic Acid-Mediated Inhibition of Plasma Membrane H+-ATPase Activity

Drought stress is a limiting environmental factor that affects plant growth and development. The plant hormone abscisic acid (ABA) plays an important role in plant drought responses. Previous studies have indicated that ABA inhibits plasma membrane H⁺-ATPase (PM H⁺-ATPase) activity, and the decrease in PM H⁺-ATPase activity promotes stomatal closure under drought stress, thereby reducing water loss. However, the underlying molecular mechanisms are not well understood. In this study, we found that in Arabidopsis (Arabidopsis thaliana), ABA induces an N-ethylmaleimide-sensitive factor attachment protein receptor protein, namely, VESICLE-ASSOCIATED MEMBRANE PROTEIN 711 (VAMP711), to interact with the Arabidopsis PM H⁺-ATPases AHA1 and AHA2. The interaction occurs at their C-termini and inhibits PM H⁺-ATPase activity. Deletion of VAMP711 in Arabidopsis results in a higher PM H⁺-ATPase activity and slower stomatal closure in response to ABA and drought treatments. In addition, overexpression of VAMP711 partially rescues the drought-sensitive phenotype of ost2-2D, a mutation in AHA1 resulting in a constitutive activated PM H⁺-ATPase. Our results demonstrate that VAMP711 is involved in regulating ABA-mediated inhibition of PM H⁺-ATPase activity and stomatal closure in response to drought stress.

Red Light-Induced Phosphorylation of Plasma Membrane H+-ATPase in Stomatal Guard Cells

Stomatal opening is stimulated by red and blue light. Blue light activates plasma membrane (PM) H+-ATPase by phosphorylating its penultimate residue, threonine, via a blue light photoreceptor phototropin-mediated signaling pathway in guard cells. Blue light-activated PM H+-ATPase promotes the accumulation of osmolytes and, thus, the osmotic influx of water into guard cells, driving stomatal opening. Red light-induced stomatal opening is thought to be dependent on photosynthesis in both guard cell chloroplasts and mesophyll cells; however, how red light induces stomatal opening and whether PM H+-ATPase is involved in this process have remained unclear. In this study, we established an immunohistochemical technique to detect the phosphorylation level of PM H+-ATPase in guard cells using whole leaves of Arabidopsis (Arabidopsis thaliana) and unexpectedly found that red light induces PM H+-ATPase phosphorylation in whole leaves. Red light-induced PM H+-ATPase phosphorylation in whole leaves was correlated with stomatal opening under red light and was inhibited by the plant hormone abscisic acid. In aha1-9, a knockout mutant of one of the major isoforms of PM H+-ATPase in guard cells, red light-dependent stomatal opening was delayed in whole leaves. Furthermore, the photosynthetic electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibited red light-induced PM H+-ATPase phosphorylation as well as red light-induced stomatal opening in whole leaves. Our results indicate that red light-induced PM H+-ATPase phosphorylation in guard cells promotes stomatal opening in whole leaves, providing insight into the photosynthetic regulation of stomatal opening.

Identification and Characterization of Compounds that Affect Stomatal Movements

Regulation of stomatal aperture is essential for plant growth and survival in response to environmental stimuli. Opening of stomata induces uptake of CO2 for photosynthesis and transpiration, which enhances uptake of nutrients from roots. Light is the most important stimulus for stomatal opening. Under drought stress, the plant hormone ABA induces stomatal closure to prevent water loss. However, the molecular mechanisms of stomatal movements are not fully understood. In this study, we screened chemical libraries to identify compounds that affect stomatal movements in Commelina benghalensis and characterize the underlying molecular mechanisms. We identified nine stomatal closing compounds (SCL1-SCL9) that suppress light-induced stomatal opening by >50%, and two compounds (temsirolimus and CP-100356) that induce stomatal opening in the dark. Further investigations revealed that SCL1 and SCL2 had no effect on autophosphorylation of phototropin or the activity of the inward-rectifying plasma membrane (PM) K+ channel, KAT1, but suppressed blue light-induced phosphorylation of the penultimate residue, threonine, in PM H+-ATPase, which is a key enzyme for stomatal opening. SCL1 and SCL2 had no effect on ABA-dependent responses, including seed germination and expression of ABA-induced genes. These results suggest that SCL1 and SCL2 suppress light-induced stomatal opening at least in part by inhibiting blue light-induced activation of PM H+-ATPase, but not by the ABA signaling pathway. Interestingly, spraying leaves onto dicot and monocot plants with SCL1 suppressed wilting of leaves, indicating that inhibition of stomatal opening by these compounds confers tolerance to drought stress in plants.

Spatio-temporal Aspects of Ca2+ Signalling: Lessons from Guard Cells and Pollen Tubes

Changes in cytosolic Ca2+ concentration ([Ca2+]cyt) serve to transmit information in eukaryotic cells. The involvement of this second messenger in plant cell growth as well as osmotic- and water relations is well established. After almost 40 years of intense research on the coding and decoding of plant Ca2+ signals, numerous proteins involved in Ca2+ action have been identified. However, we are still far from understanding the complexity of Ca2+ networks. New in vivo Ca2+ imaging techniques combined with molecular genetics allow visualisation of spatio-temporal aspects of Ca2+ signalling. In parallel, cell biology together with protein biochemistry and electrophysiology are able to dissect information processing by this second messenger in space and time. Here we focus on the time-resolved changes in cellular events upon Ca2+ signals, concentrating on the two best-studied cell types, pollen tubes and guard cells. We put their signalling networks side by side, compare them with those of other cell types and discuss rapid signalling in the context of Ca2+ transients and oscillations to regulate ion homeostasis.

Constitutive BiP protein accumulation in Arabidopsis mutants defective in a gene encoding chloroplast-resident stearoyl-acyl carrier protein desaturase

The unfolded protein response (UPR) occurs when protein folding and maturation are disturbed in the endoplasmic reticulum (ER). During the UPR, a number of genes including those encoding ER-resident molecular chaperones are induced. In Arabidopsis, BiP3 has been used as a UPR marker gene whose expression is strongly induced in response to ER stress. In this study, we mutagenized Arabidopsis plants expressing β-glucuronidase (GUS) gene under the control of BiP3 promoter and isolated a mutant that exhibits strong GUS activity without treatment with ER stress inducers. By whole genome sequencing, we identified a causal gene in the mutant as SUPPRESSOR OF SALICYLIC ACID INSENSITIVITY2 (SSI2), which encodes stearoyl-acyl carrier protein desaturase that converts stearic acids to oleic acids in the chloroplasts. In addition to GUS proteins, the ssi2 mutant accumulates endogenous BiP3 proteins without treatment by any stress reagents. Interestingly, although the degree of endogenous BiP3 protein accumulation in the ssi2 mutant was comparable to that in wild-type plants treated with the ER stress inducer tunicamycin, much less BiP3 transcripts were detected in the ssi2 mutant compared to tunicamycin-treated wild-type plants. Our finding suggests a genetic link between fatty acid metabolism in the chloroplasts and ER functions.

Mapping of Plasma Membrane Proteins Interacting With Arabidopsis thaliana Flotillin 2

Arabidopsis flotillin 2 (At5g25260) belongs to the group of plant flotillins, which are not well characterized. In contrast, metazoan flotillins are well known as plasma membrane proteins associated with membrane microdomains that act as a signaling hub. The similarity of plant and metazoan flotillins, whose functions most likely consist of affecting other proteins via protein-protein interactions, determines the necessity of detecting their interacting partners in plants. Nevertheless, identifying the proteins that form complexes on the plasma membrane is a challenging task due to their low abundance and hydrophobic character. Here we present an approach for mapping Arabidopsis thaliana flotillin 2 plasma membrane interactors, based on the immunoaffinity purification of crosslinked and enriched plasma membrane proteins with mass spectrometry detection. Using this approach, 61 proteins were enriched in the AtFlot-GFP plasma membrane fraction, and 19 of them were proposed to be flotillin 2 interaction partners. Among our proposed partners of Flot2, proteins playing a role in the plant response to various biotic and abiotic stresses were detected. Additionally, the use of the split-ubiquitin yeast system helped us to confirm that plasma-membrane ATPase 1, early-responsive to dehydration stress protein 4, syntaxin-71, harpin-induced protein-like 3, hypersensitive-induced response protein 2 and two aquaporin isoforms interact with flotillin 2 directly. Based on the results of our study and the reported properties of Flot2 interactors, we propose that Flot2 complexes may be involved in plant-pathogen interactions, water transport and intracellular trafficking.

Determination of H+-ATPase Activity in Arabidopsis Guard Cell Protoplasts through H+-pumping Measurement and H+-ATPase Quantification

The opening of stomata in plants in response to blue light is driven by the plasma membrane H⁺-ATPase in guard cells. To evaluate the activation of the H⁺-ATPase in vivo, we can use H⁺-pumping by guard cells in response to blue light and fusicoccin. To do this, it is required to prepare a large amount of guard cell protoplasts and measure H⁺-pumping in the protoplasts. It is also necessary to determine the protein amount of H⁺-ATPase. In this protocol, we describe the procedures required for these preparations and measurements.

The desert plant Phoenix dactylifera closes stomata via nitrate-regulated SLAC1 anion channel

Date palm Phoenix dactylifera is a desert crop well adapted to survive and produce fruits under extreme drought and heat. How are palms under such harsh environmental conditions able to limit transpirational water loss? Here, we analysed the cuticular waxes, stomata structure and function, and molecular biology of guard cells from P. dactylifera. To understand the stomatal response to the water stress phytohormone of the desert plant, we cloned the major elements necessary for guard cell fast abscisic acid (ABA) signalling and reconstituted this ABA signalosome in Xenopus oocytes. The PhoenixSLAC1-type anion channel is regulated by ABA kinase PdOST1. Energy-dispersive X-ray analysis (EDXA) demonstrated that date palm guard cells release chloride during stomatal closure. However, in Cl^- medium, PdOST1 did not activate the desert plant anion channel PdSLAC1 per se. Only when nitrate was present at the extracellular face of the anion channel did the OST1-gated PdSLAC1 open, thus enabling chloride release. In the presence of nitrate, ABA enhanced and accelerated stomatal closure. Our findings indicate that, in date palm, the guard cell osmotic motor driving stomatal closure uses nitrate as the signal to open the major anion channel SLAC1. This initiates guard cell depolarization and the release of anions together with potassium.

Biology of SLAC1-type anion channels - from nutrient uptake to stomatal closure

Contents 46 I. 46 II. 47 III. 50 IV. 53 V. 56 VI. 57 58 58 References 58 SUMMARY: Stomatal guard cells control leaf CO₂ intake and concomitant water loss to the atmosphere. When photosynthetic CO₂ assimilation is limited and the ratio of CO₂ intake to transpiration becomes suboptimal, guard cells, sensing the rise in CO₂ concentration in the substomatal cavity, deflate and the stomata close. Screens for mutants that do not close in response to experimentally imposed high CO₂ atmospheres identified the guard cell-expressed Slowly activating anion channel, SLAC1, as the key player in the regulation of stomatal closure. SLAC1 evolved, though, before the emergence of guard cells. In Arabidopsis, SLAC1 is the founder member of a family of anion channels, which comprises four homologues. SLAC1 and SLAH3 mediate chloride and nitrate transport in guard cells, while SLAH1, SLAH2 and SLAH3 are engaged in root nitrate and chloride acquisition, and anion translocation to the shoot. The signal transduction pathways involved in CO₂ , water stress and nutrient-sensing activate SLAC/SLAH via distinct protein kinase/phosphatase pairs. In this review, we discuss the role that SLAC/SLAH channels play in guard cell closure, on the one hand, and in the root-shoot continuum on the other, along with the molecular basis of the channels' anion selectivity and gating.

Blue Light Regulation of Stomatal Opening and the Plasma Membrane H+-ATPase

Recent progress of the blue light signaling pathway in guard cells highlights its regulation of H⁺-ATPase activity.

The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics

Stomatal guard cells are widely recognized as the premier plant cell model for membrane transport, signaling, and homeostasis. This recognition is rooted in half a century of research into ion transport across the plasma and vacuolar membranes of guard cells that drive stomatal movements and the signaling mechanisms that regulate them. Stomatal guard cells surround pores in the epidermis of plant leaves, controlling the aperture of the pore to balance CO₂ entry into the leaf for photosynthesis with water loss via transpiration. The position of guard cells in the epidermis is ideally suited for cellular and subcellular research, and their sensitivity to endogenous signals and environmental stimuli makes them a primary target for physiological studies. Stomata underpin the challenges of water availability and crop production that are expected to unfold over the next 20 to 30 years. A quantitative understanding of how ion transport is integrated and controlled is key to meeting these challenges and to engineering guard cells for improved water use efficiency and agricultural yields.

Global Sensitivity Analysis of OnGuard Models Identifies Key Hubs for Transport Interaction in Stomatal Dynamics

The physical requirement for charge to balance across biological membranes means that the transmembrane transport of each ionic species is interrelated, and manipulating solute flux through any one transporter will affect other transporters at the same membrane, often with unforeseen consequences. The OnGuard systems modeling platform has helped to resolve the mechanics of stomatal movements, uncovering previously unexpected behaviors of stomata. To date, however, the manual approach to exploring model parameter space has captured little formal information about the emergent connections between parameters that define the most interesting properties of the system as a whole. Here, we introduce global sensitivity analysis to identify interacting parameters affecting a number of outputs commonly accessed in experiments in Arabidopsis (Arabidopsis thaliana). The analysis highlights synergies between transporters affecting the balance between Ca2+ sequestration and Ca2+ release pathways, notably those associated with internal Ca2+ stores and their turnover. Other, unexpected synergies appear, including with the plasma membrane anion channels and H+-ATPase and with the tonoplast TPK K+ channel. These emergent synergies, and the core hubs of interaction that they define, identify subsets of transporters associated with free cytosolic Ca2+ concentration that represent key targets to enhance plant performance in the future. They also highlight the importance of interactions between the voltage regulation of the plasma membrane and tonoplast in coordinating transport between the different cellular compartments.

Proteome Profiling of Paulownia Seedlings Infected with Phytoplasma

Phytoplasma is an insect-transmitted pathogen that causes witches' broom disease in many plants. Paulownia witches' broom is one of the most destructive diseases threatening Paulownia production. The molecular mechanisms associated with this disease have been investigated by transcriptome sequencing, but changes in protein abundance have not been investigated with isobaric tags for relative and absolute quantitation. Previous results have shown that methyl methane sulfonate (MMS) can help Paulownia seedlings recover from the symptoms of witches' broom and reinstate a healthy morphology. In this study, a transcriptomic-assisted proteomic technique was used to analyze the protein changes in phytoplasma-infected Paulownia tomentosa seedlings, phytoplasma-infected seedlings treated with 20 and 60 mg·L^-1 MMS, and healthy seedlings. A total of 2,051 proteins were obtained, 879 of which were found to be differentially abundant in pairwise comparisons between the sample groups. Among the differentially abundant proteins, 43 were related to Paulownia witches' broom disease and many of them were annotated to be involved in photosynthesis, expression of dwarf symptom, energy production, and cell signal pathways.