Metabolomics of red-light-induced stomatal opening in Arabidopsis thaliana: Coupling with abscisic acid and jasmonic acid metabolism

Abscisic acid-mediated guard cell metabolism regulation

Abscisic acid (ABA) is crucial for plant water deficit (WD) acclimation, but how the interplay between ABA and guard cell (GC) metabolism aids plant WD acclimation remains unclear. Here, we investigated how ABA regulates GC metabolism and how this contributes to plant WD acclimation using tomato wild type (WT) and the ABA-deficient sitiens mutant. These genotypes were characterized at physiological, metabolic, and transcriptional levels under recurring WD periods and were used to perform a13C-glucose labelling experiment using isolated guard cells following exogenously applied ABA. ABA deficiency altered the level of sugars and organic acids in GCs in both irrigated and WD plants and the dynamic of accumulation/degradation of these compounds in GCs during the dark-to-light transition. WD-induced metabolic changes were more pronounced in sitiens than WT GCs. Results from the 13C-labelling experiment indicate that ABA is required for the glycolytic fluxes toward malate and acts as a negative regulator of a putative sucrose substrate cycle. The expression of key ABA-biosynthetic genes was higher in WT than in sitiens GCs after two cycles of WD. Additionally, the intrinsic leaf water use efficiency increased only in WT after the second WD cycle, compared to sitiens. Our results highlight that ABA deficiency disrupts the homeostasis of GC primary metabolism and the WD memory, negatively affecting plant WD acclimation. Our study demonstrates which metabolic pathways are activated by WD and/or regulated by ABA in GCs, which improves our understanding of plant WD acclimation, with clear consequences for plant metabolic engineering in the future.

Screening of environmental stimuli for the positive regulation of stomatal aperture in centipedegrass

Grasslands, the largest carbon pool in China, possess enormous potential for carbon sequestration. Increasing the stomatal aperture to increase the CO2 absorption capacity is a potential method to improve plant photosynthetic efficiency and ultimately enhance the carbon sequestration capacity of grass plants. Research on stomatal aperture regulation has focused mostly on Arabidopsis or crops, while research on grass plants in these areas is scarce, which seriously restricts the implementation of this grassland carbon sequestration strategy. Here, a widely used ecological grass, centipedegrass, was used as the experimental material. First, a convenient method for observing the stomatal aperture was developed. The leaves were floated in a potassium ion-containing open solution (67 mM KCl, pH 6.0) with the adaxial surface rather than the abaxial surface in contact with the solution and were cultivated under light for 1.5 h. Then, nail polish was applied on the adaxial surface, and a large number of open stomata were imprinted. Second, with the help of this improved method, the concentration‒response characteristics of the stomatal aperture to eleven environmental stimuli were tested. The stomatal aperture is dependent on these environmental stimuli in a concentration-dependent manner. The addition of 100 μM brassinolide led to the maximal stomatal aperture. This study provided a technical basis for manipulating stomatal opening to increase the carbon sequestration capacity of centipedegrass.

PIF transcriptional regulators are required for rhythmic stomatal movements

Stomata govern the gaseous exchange between the leaf and the external atmosphere, and their function is essential for photosynthesis and the global carbon and oxygen cycles. Rhythmic stomata movements in daily dark/light cycles prevent water loss at night and allow CO2 uptake during the day. How the actors involved are transcriptionally regulated and how this might contribute to rhythmicity is largely unknown. Here, we show that morning stomata opening depends on the previous night period. The transcription factors PHYTOCHROME-INTERACTING FACTORS (PIFs) accumulate at the end of the night and directly induce the guard cell-specific K+ channel KAT1. Remarkably, PIFs and KAT1 are required for blue light-induced stomata opening. Together, our data establish a molecular framework for daily rhythmic stomatal movements under well-watered conditions, whereby PIFs are required for accumulation of KAT1 at night, which upon activation by blue light in the morning leads to the K+ intake driving stomata opening.

Combined metabolome and transcriptome analyses reveal that growing under Red shade affects secondary metabolite content in Huangjinya green tea

Shading treatments impact the tea (Camellia sinensis L.) quality. The sunlight sensitive varieties can be grown under shading nets for better growth and secondary metabolite content. Here, we studied the responses of a sunlight sensitive green tea variety "Huangjinya" by growing under colored shading nets (red, yellow, blue, and black (75% and 95%) shading rates) to find out the most suitable color of the shading net. Red shading was the most promising treatment as it positively affected the weight and length of 100 one-bud-three leaves and reduced the degree and rate of new shoots burn compared to control (natural sunlight). We then explored the comparative metabolomic changes in response to red shading by using UPLC-ESI-MS/MS system. The amino acids and derivatives, flavonoids, and alkaloids were downaccumulated whereas lipids, organic acids, and lignans were upaccumulated in Red shade grown tea samples. The red shading nets caused a decreased catechin, epicatechin, dopamine, and L-tyramine contents but increased caffeine content. We then employed transcriptome sequencing to find key changes in expressions of related genes and pathways. Notably, key genes associated with the phenylpropanoid and flavonoid biosynthesis pathways exhibited complex regulation. These expression changes suggested a potential trend of polymerization or condensation of simple molecules like catechin or pelargonidin into larger molecules like glucoside or proanthocyanidins. Here, Red shading net triggered higher expression of genes enriched in lipid biosynthesis and jasmonic acid biosynthesis, suggesting an interplay of fatty acids and JA in improving tea performance. These findings contribute to the metabolic responses of Huangjinya tea to red shading nets which might have implications for flavor and health benefits. Our data provide a foundation for further exploration and optimization of cultivation practices for this unique tea variety.

PhyB-dependent phosphorylation of mitogen-activated protein kinase cascade MKK2-MPK2 positively regulates red light-induced stomatal opening

Red light induces stomatal opening by affecting photosynthesis, metabolism and triggering signal transductions in guard cells. Phytochrome B (phyB) plays a positive role in mediating red light-induced stomatal opening. However, phyB-mediated red light guard cell signalling is poorly understood. Here, we found that phyB-mediated sequential phosphorylation of mitogen-activated protein kinase kinase 2 (MAPKK2, MKK2) and MPK2 in guard cells is essential for red light-induced stomatal opening. Mutations in MKK2 and MPK2 led to reduced stomatal opening in response to white light, and these phenotypes could be observed under red light, not blue light. MKK2 interacted with MPK2 in vitro and in plants. MPK2 was directly phosphorylated by MKK2 in vitro. Red light triggered the phosphorylation of MKK2 in guard cells, and MKK2 phosphorylation was greatly reduced in phyB mutant. Simultaneously, red light-stimulated MPK2 phosphorylation in guard cells was inhibited in mkk2 mutant. Furthermore, mkk2 and mpk2 mutants exhibit significantly smaller stomatal apertures than that of wild type during the stomatal opening stage in the diurnal stomatal movements. Our results indicate that red light-promoted phyB-dependent phosphorylation of MKK2-MPK2 cascade in guard cells is essential for stomatal opening, which contributes to the fine-tuning of stomatal opening apertures under light.

A comparison of stomatal conductance responses to blue and red light between C3 and C4 photosynthetic species in three phylogenetically-controlled experiments

Introduction

C4 photosynthesis is an adaptation that has independently evolved at least 66 times in angiosperms. C4 plants, unlike their C3 ancestral, have a carbon concentrating mechanism which suppresses photorespiration, often resulting in faster photosynthetic rates, higher yields, and enhanced water use efficiency. Moreover, the presence of C4 photosynthesis greatly alters the relation between CO2 assimilation and stomatal conductance. Previous papers have suggested that the adjustment involves a decrease in stomatal density. Here, we tested if C4 species also have differing stomatal responses to environmental cues, to accommodate the modified CO2 assimilation patterns compared to C3 species.

Methods

To test this hypothesis, stomatal responses to blue and red-light were analysed in three phylogenetically linked pairs of C3 and C4 species from the Cleomaceae (Gynandropsis and Tarenaya), Flaveria, and Alloteropsis, that use either C3 or C4 photosynthesis.

Results

The results showed strongly decreased stomatal sensitivity to blue light in C4 dicots, compared to their C3 counterparts, which exhibited significant blue light responses. In contrast, in C3 and C4 subspecies of the monocot A. semialata, the blue light response was observed regardless of photosynthetic type. Further, the quantitative red-light response varied across species, but the presence or absence of a significant stomatal red-light response was not directly associated with differences in photosynthetic pathway. Interestingly, stomatal density and morphology patterns observed across the three comparisons were also not consistent with patterns commonly asserted for C3 and C4 species.

Discussion

The strongly diminished blue-light sensitivity of stomatal responses in C4 species across two of the comparisons suggests a common C4 feature that may have functional implications. Altogether, the strong prevalence of species-specific effects clearly emphasizes the importance of phylogenetic controls in comparisons between C3 and C4 photosynthetic pathways.

Cell-type-specific metabolism in plants

Every plant organ contains tens of different cell types, each with a specialized function. These functions are intrinsically associated with specific metabolic flux distributions that permit the synthesis of the ATP, reducing equivalents and biosynthetic precursors demanded by the cell. Investigating such cell-type-specific metabolism is complicated by the mosaic of different cells within each tissue combined with the relative scarcity of certain types. However, techniques for the isolation of specific cells, their analysis in situ by microscopy, or modeling of their function in silico have permitted insight into cell-type-specific metabolism. In this review we present some of the methods used in the analysis of cell-type-specific metabolism before describing what we know about metabolism in several cell types that have been studied in depth; (i) leaf source and sink cells; (ii) glandular trichomes that are capable of rapid synthesis of specialized metabolites; (iii) guard cells that must accumulate large quantities of the osmolytes needed for stomatal opening; (iv) cells of seeds involved in storage of reserves; and (v) the mesophyll and bundle sheath cells of C4 plants that participate in a CO₂ concentrating cycle. Metabolism is discussed in terms of its principal features, connection to cell function and what factors affect the flux distribution. Demand for precursors and energy, availability of substrates and suppression of deleterious processes are identified as key factors in shaping cell-type-specific metabolism.

Red light-upregulated MPK11 negatively regulates red light-induced stomatal opening in Arabidopsis

Stomatal movements allow the uptake of CO₂ for photosynthesis and water loss through transpiration, therefore play a crucial role in determining water use efficiency. Both red and blue lights induce stomatal opening, and the stomatal apertures under light are finetuned by both positive and negative regulators in guard cells. However, the molecular mechanisms for precisely adjusting stomatal apertures under light have not been completely understood. Here, we provided evidence supporting that Arabidopsis thaliana mitogen-activated protein kinase 11 (MPK11) plays a negative role in red light-induced stomatal opening. First, MPK11 was found to be highly expressed in guard cells, and MPK11-GFP signals were detected in both nuclear and cytoplasm of guard cells. The transcript levels of MPK11 in guard cells were upregulated by white light, and the stomata of mpk11 opened wider than that of wild type under white light. Consistent with the larger stomatal aperture, mpk11 mutant exhibited higher stomatal conductance and CO₂ assimilation rate under white light. The transcript levels of the genes responsible for osmolytes increases were higher in guard cells of mpk11 than that of wild type, which may contribute to the larger stomatal aperture of mpk11 under white light. Furthermore, MPK11 transcript levels in guard cells were upregulated by red light, and mpk11 mutant showed a larger stomatal aperture under red light. Taken together, these results demonstrate that red light-upregulated MPK11 plays a negative role in stomatal opening, which finetuning the stomatal opening apertures and preventing excessive water loss by transpiration under light.

GlPS1 overexpression accumulates coumarin secondary metabolites in transgenic Arabidopsis

The dried root of Glehnia littoralis is a traditional Chinese herbal medicine mainly used to treat lung diseases and plays an important role in fighting coronavirus disease 2019 pneumonia in China. This study focused on the key enzyme gene GlPS1 for furanocoumarin synthesis in G. littoralis. In the 35S:GlPS1 transgenic Arabidopsis study, the Arabidopsis thaliana-overexpressing GlPS1 gene was more salt-tolerant than Arabidopsis in the blank group. Metabolomics analysis showed 30 differential metabolites in Arabidopsis, which overexpressed the GlPS1 gene. Twelve coumarin compounds were significantly upregulated, and six of these coumarin compounds were not detected in the blank group. Among these differential coumarin metabolites, isopimpinellin and aesculetin have been annotated by the Kyoto Encyclopedia of Genes and Genomes and isopimpinellin was not detected in the blank group. Through structural comparison, imperatorin was formed by dehydration and condensation of zanthotoxol and a molecule of isoprenol, and the difference between them was only one isoprene. Results showed that the GlPS1 gene positively regulated the synthesis of coumarin metabolites in A. thaliana and at the same time improved the salt tolerance of A. thaliana.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11240-022-02427-w.

Chickpea and Lupin Sprouts, Stimulated by Different LED Lights, As Novel Examples of Isoflavones-Rich Functional Food, and Their Impact on Breast and Prostate Cells

Among all legumes sprouts' active compounds, isoflavones seem to be the most important; nevertheless, their high content is not always associated with beneficial effects. These compounds may prevent or stimulate hormone-dependent cancers due to their estrogen-like activity. Different LED light quality can change the synthesis of active compounds and significantly influence the biological activity of the sprouts. This study aimed to evaluate the effects of LED light (red, blue, green, yellow), as well as total darkness, and natural light conditions (as reference), on isoflavones content, determined by HPLC-UV-VIS, during 10 days of harvesting of chickpea and lupin sprouts. Due to the ambiguous estrogenic potential of isoflavones, the impact of these sprouts on normal and cancer prostate and breast cells was evaluated. Yellow LED light resulted in the highest sum of isoflavones in chickpea sprouts (up to 1 g/100 g dw), while for green LED light, the isoflavones sum was the lowest. The exact opposite effect was noted for lupin sprouts, with the predominance of green over the yellow LED light. The examined sprouts were of high safety to non-neoplastic breast and prostate cells, with interesting cytotoxic effects on breast MCF7 and prostate DU145 cancer cells. No clear relationship was observed between the activity and isoflavones content.

CC-type glutaredoxin, MeGRXC3, associates with catalases and negatively regulates drought tolerance in cassava (Manihot esculenta Crantz)

Glutaredoxins (GRXs) are essential for reactive oxygen species (ROS) homeostasis in responses of plants to environment changes. We previously identified several drought-responsive CC-type GRXs in cassava, an important tropical crop. However, how CC-type GRX regulates ROS homeostasis of cassava under drought stress remained largely unknown. Here, we report that a drought-responsive CC-type GRX, namely MeGRXC3, was associated with activity of catalase in the leaves of 100 cultivars (or unique unnamed genotypes) of cassava under drought stress. MeGRXC3 negatively regulated drought tolerance by modulating drought- and abscisic acid-induced stomatal closure in transgenic cassava. It antagonistically regulated hydrogen peroxide (H₂ O₂ ) accumulation in epidermal cells and guard cells. Moreover, MeGRXC3 interacted with two catalases of cassava, MeCAT1 and MeCAT2, and regulated their activity in vivo. Additionally, MeGRXC3 interacts with a cassava TGA transcription factor, MeTGA2, in the nucleus, and regulates the expression of MeCAT7 through a MeTGA2-MeMYB63 pathway. Overall, we demonstrated the roles of MeGRXC3 in regulating activity of catalase at both transcriptional and post-translational levels, therefore involving in ROS homeostasis and stomatal movement in responses of cassava to drought stress. Our study provides the first insights into how MeGRXC3 may be used in molecular breeding of cassava crops.

Phytochrome interacting factor regulates stomatal aperture by coordinating red light and abscisic acid

Stomata are crucial valves coordinating the fixation of carbon dioxide by photosynthesis and water loss through leaf transpiration. Phytochrome interacting factors (PIFs) are negative regulators of red light responses that belong to the basic helix-loop-helix family of transcription factors. Here, we show that the rice (Oryza sativa) PIF family gene OsPIL15 acts as a negative regulator of stomatal aperture to control transpiration in rice. OsPIL15 reduces stomatal aperture by activating rice ABSCISIC ACID INSENSITIVE 5 (OsABI5), which encodes a critical positive regulator of ABSCISIC ACID (ABA) signaling in rice. Moreover, OsPIL15 interacts with the NIGT1/HRS1/HHO family transcription factor rice HRS1 HOMOLOG 3 (OsHHO3) to possibly enhance the regulation of stomatal aperture. Notably, we discovered that the maize (Zea mays) PIF family genes ZmPIF1 and ZmPIF3, which are homologous to OsPIL15, are also involved in the regulation of stomatal aperture in maize, indicating that PIF-mediated regulation of stomatal aperture may be conserved in the plant lineage. Our findings explain the molecular mechanism by which PIFs play a role in red-light-mediated stomatal opening, and demonstrate that PIFs regulate stomatal aperture by coordinating the red light and ABA signaling pathways.

Light Intensity- and Spectrum-Dependent Redox Regulation of Plant Metabolism

Both light intensity and spectrum (280–800 nm) affect photosynthesis and, consequently, the formation of reactive oxygen species (ROS) during photosynthetic electron transport. ROS, together with antioxidants, determine the redox environment in tissues and cells, which in turn has a major role in the adjustment of metabolism to changes in environmental conditions. This process is very important since there are great spatial (latitude, altitude) and temporal (daily, seasonal) changes in light conditions which are accompanied by fluctuations in temperature, water supply, and biotic stresses. The blue and red spectral regimens are decisive in the regulation of metabolism because of the absorption maximums of chlorophylls and the sensitivity of photoreceptors. Based on recent publications, photoreceptor-controlled transcription factors such as ELONGATED HYPOCOTYL5 (HY5) and changes in the cellular redox environment may have a major role in the coordinated fine-tuning of metabolic processes during changes in light conditions. This review gives an overview of the current knowledge of the light-associated redox control of basic metabolic pathways (carbon, nitrogen, amino acid, sulphur, lipid, and nucleic acid metabolism), secondary metabolism (terpenoids, flavonoids, and alkaloids), and related molecular mechanisms. Light condition-related reprogramming of metabolism is the basis for proper growth and development of plants; therefore, its better understanding can contribute to more efficient crop production in the future.

Advances in Plant Metabolomics and Its Applications in Stress and Single-Cell Biology

In the past two decades, the post-genomic era envisaged high-throughput technologies, resulting in more species with available genome sequences. In-depth multi-omics approaches have evolved to integrate cellular processes at various levels into a systems biology knowledge base. Metabolomics plays a crucial role in molecular networking to bridge the gaps between genotypes and phenotypes. However, the greater complexity of metabolites with diverse chemical and physical properties has limited the advances in plant metabolomics. For several years, applications of liquid/gas chromatography (LC/GC)-mass spectrometry (MS) and nuclear magnetic resonance (NMR) have been constantly developed. Recently, ion mobility spectrometry (IMS)-MS has shown utility in resolving isomeric and isobaric metabolites. Both MS and NMR combined metabolomics significantly increased the identification and quantification of metabolites in an untargeted and targeted manner. Thus, hyphenated metabolomics tools will narrow the gap between the number of metabolite features and the identified metabolites. Metabolites change in response to environmental conditions, including biotic and abiotic stress factors. The spatial distribution of metabolites across different organs, tissues, cells and cellular compartments is a trending research area in metabolomics. Herein, we review recent technological advancements in metabolomics and their applications in understanding plant stress biology and different levels of spatial organization. In addition, we discuss the opportunities and challenges in multiple stress interactions, multi-omics, and single-cell metabolomics.

Past accomplishments and future challenges of the multi-omics characterization of leaf growth

The advent of omics technologies has revolutionized biology and advanced our understanding of all biological processes, including major developmental transitions in plants and animals. Here, we review the vast knowledge accumulated concerning leaf growth in terms of transcriptional regulation before turning our attention to the historically less well-characterized alterations at the protein and metabolite level. We will then discuss how the advent of biochemical methods coupled with metabolomics and proteomics can provide insight into the protein-protein and protein-metabolite interactome of the growing leaves. We finally highlight the substantial challenges in detection, spatial resolution, integration, and functional validation of the omics results, focusing on metabolomics as a prerequisite for a comprehensive understanding of small-molecule regulation of plant growth.

MADS-box transcription factors determine the duration of temporary winter dormancy in closely related evergreen and deciduous Iris spp

Winter dormancy (WD) is a crucial strategy for plants coping with potentially deadly environments. In recent decades, this process has been extensively studied in economically important perennial eudicots due to changing climate. However, in evergreen monocots with no chilling requirements, dormancy processes are so far a mystery. In this study, we compared the WD process in closely related evergreen (Iris japonica) and deciduous (I. tectorum) iris species across crucial developmental time points. Both iris species exhibit a 'temporary' WD process with distinct durations, and could easily resume growth under warm conditions. To decipher transcriptional changes, full-length sequencing for evergreen iris and short read RNA sequencing for deciduous iris were applied to generate respective reference transcriptomes. Combining results from a multipronged approach, SHORT VEGETATIVE PHASE and FRUITFULL (FUL) from MADS-box was associated with a dormancy- and a growth-related module, respectively. They were co-expressed with genes involved in phytohormone signaling, carbohydrate metabolism, and environmental adaptation. Also, gene expression patterns and physiological changes in the above pathways highlighted potential abscisic acid and jasmonic acid antagonism in coordinating growth and stress responses, whereas differences in carbohydrate metabolism and reactive oxygen species scavenging might lead to species-specific WD durations. Moreover, a detailed analysis of MIKCCMADS-box in irises revealed common features described in eudicots as well as possible new roles for monocots during temporary WD, such as FLOWERING LOCUS C and FUL. In essence, our results not only provide a portrait of temporary WD in perennial monocots but also offer new insights into the regulatory mechanism underlying WD in plants.

Guard-cell phytochromes impact seedling photomorphogenesis and rosette leaf morphology

Using a previously established transgenic approach to inactivate phytochrome chromophore synthesis in specific organs or tissues, we used a guard cell-specific promoter to induce phytochrome deficiencies in guard cells of Arabidopsis thaliana. Analyses of multiple homozygous lines depleted of phytochromes in stomatal guard cells indicated elongated hypocotyls specifically in red and far-red growth conditions. Furthermore, rosette leaves of adult plants with guard cell-specific phytochrome deficiencies showed enhanced serration compared to the wild-type Col-0 parent. Thus, we demonstrate that guard cell-localized phytochromes impact the inhibition of hypocotyl elongation, as well as leaf margin morphology of adult rosette leaves in A. thaliana.

Mesophyll-derived sugars are positive regulators of light-driven stomatal opening

Guard cell membrane ion transport and metabolism are deeply interconnected, and their coordinated regulation is integral to stomatal opening. Whereas ion transport is exceptionally well understood, how guard cell metabolism influences stomatal movements is less well known. Organic metabolites, such as malate and sugars, fulfill several functions in guard cells during stomatal opening as allosteric activators, counter-ions, energy source and osmolytes. However, their origin and exact fate remain debated. Recent work revealed that the guard cell carbon pool regulating stomatal function and plant growth is mostly of mesophyll origin, highlighting a tight correlation between mesophyll and guard cell metabolism. This review discusses latest research into guard cell carbon metabolism and its impact on stomatal function and whole plant physiology.

Advances and perspectives in the metabolomics of stomatal movement and the disease triangle

Crops are continuously exposed to microbial pathogens that cause tremendous yield losses worldwide. Stomatal pores formed by pairs of specialized guard cells in the leaf epidermis represent a major route of pathogen entry. Guard cells have an essential role as a first line of defense against pathogens. Metabolomics is an indispensable systems biology tool that has facilitated discovery and functional studies of metabolites that regulate stomatal movement in response to pathogens and other environmental factors. Guard cells, pathogens and environmental factors constitute the "stomatal disease triangle". The aim of this review is to highlight recent advances toward understanding the stomatal disease triangle in the context of newly discovered signaling molecules, hormone crosstalk, and consequent molecular changes that integrate pathogens and environmental sensing into stomatal immune responses. Future perspectives on emerging single-cell studies, multiomics and molecular imaging in the context of stomatal defense are discussed. Advances in this important area of plant biology will inform rational crop engineering and breeding for enhanced stomatal defense without disruption of other pathways that impact crop yield.

Light-Mediated Signaling and Metabolic Changes Coordinate Stomatal Opening and Closure

Stomata are valves on the leaf surface controlling carbon dioxide (CO2) influx for photosynthesis and water loss by transpiration. Thus, plants have to evolve elaborate mechanisms controlling stomatal aperture to allow efficient photosynthesis while avoid excessive water loss. Light is not only the energy source for photosynthesis but also an important signal regulating stomatal movement during dark-to-light transition. Our knowledge concerning blue and red light signaling and light-induced metabolite changes that contribute to stomatal opening are accumulating. This review summarizes recent advances on the signaling components that lie between the perception of blue/red light and activation of the PM H+-ATPases, and on the negative regulation of stomatal opening by red light-activated phyB signaling and ultraviolet (UV-B and UV-A) irradiation. Besides, light-regulated guard cell (GC)-specific metabolic levels, mesophyll-derived sucrose, and CO2 concentration within GCs also play dual roles in stomatal opening. Thus, light-induced stomatal opening is tightly accompanied by brake mechanisms, allowing plants to coordinate carbon gain and water loss. Knowledge on the mechanisms regulating the trade-off between stomatal opening and closure may have potential applications toward generating superior crops with improved water use efficiency (CO2 gain vs. water loss).

FRET kinase sensor development reveals SnRK2/OST1 activation by ABA but not by MeJA and high CO2 during stomatal closure

Sucrose-non-fermenting-1-related protein kinase-2s (SnRK2s) are critical for plant abiotic stress responses, including abscisic acid (ABA) signaling. Here, we develop a genetically encoded reporter for SnRK2 kinase activity. This sensor, named SNACS, shows an increase in the ratio of yellow to cyan fluorescence emission by OST1/SnRK2.6-mediated phosphorylation of a defined serine residue in SNACS. ABA rapidly increases FRET efficiency in N. benthamiana leaf cells and Arabidopsis guard cells. Interestingly, protein kinase inhibition decreases FRET efficiency in guard cells, providing direct experimental evidence that basal SnRK2 activity prevails in guard cells. Moreover, in contrast to ABA, the stomatal closing stimuli, elevated CO2 and MeJA, did not increase SNACS FRET ratios. These findings and gas exchange analyses of quintuple/sextuple ABA receptor mutants show that stomatal CO2 signaling requires basal ABA and SnRK2 signaling, but not SnRK2 activation. A recent model that CO2 signaling is mediated by PYL4/PYL5 ABA-receptors could not be supported here in two independent labs. We report a potent approach for real-time live-cell investigations of stress signaling.