Cytosolic alkalization and cytosolic calcium oscillation in Arabidopsis guard cells response to ABA and MeJA

Characterization of the heat shock factor RcHsfA6 in Rosa chinensis and function in the thermotolerance of Arabidopsis

BACKGROUND

Environmental stresses, especially high temperatures, severely limit the growth and development of many horticultural plants. As a woody ornamental flower with rich flower colors and flower types, rose (R. chinensis) leaves wilt and shriveled petals at high temperatures, which severely affects its growth and ornamental value. The defense mechanism of rose plants against high-temperature stress has not been fully elucidated.

RESULTS

In the present study, the transcriptomes of rose petals at normal (25 °C) and high (35 °C) temperature were compared. A total of 2519 differentially expressed genes (DEGs) were identified, including 1491 upregulated DEGs and 1028 downregulated DEGs. The plant hormone signal transduction pathway, especially the abscisic acid (ABA) signaling pathway, was the most enriched signaling pathway for DEGs in rose at high temperature. Heat shock factors (Hsfs), especially class A Hsfs, have been confirmed to be involved in thermotolerance mechanisms. Among the DEGs, eight genes were annotated as Hsfs, including 5 upregulated Hsfs at high temperature. RcHsfA6 is rapidly induced by high temperatures and is a candidate regulatory factor in the plant ABA signaling pathway. Therefore, we focused on RcHsfA6. RcHsfA6 encodes a protein containing 308 amino acids and contains typical Hsf domains, such as the DNA-binding domain (DBD), the N-terminal oligomerization domain (OD), the nuclear localization signal (NLS) and AHA motifs at the C-terminal activator domain (CTAD). The heterologous overexpression of RcHsfA6 in Arabidopsis increased the thermotolerance of Arabidopsis seeds. In addition, RcHsfA6 overexpression increased the ABA content and the expression of ABA biosynthetic gene AtABI5 and signal transduction gene AtPYL12, thereby inhibiting the germination of Arabidopsis seeds under exogenous ABA conditions.

CONCLUSIONS

Taken together, our results suggest that RcHsfA6 is involved in the high-temperature response of rose and its heterologous overexpression in Arabidopsis increased the thermotolerance of Arabidopsis at high temperatures via the ABA signaling pathway.

TGG1 and TGG2 mutations impair allyl isothiocyanate-mediated stomatal closure in Arabidopsis thaliana

Myrosinase, referred to as thioglucoside glucohydrolase (TGG), plays a crucial role in plant physiology through catalyzing the hydrolysis of glucosinolates into bioactive isothiocyanates. In Arabidopsis thaliana, the myrosinases TGG1 and TGG2 are essential for abscisic acid- and methyl jasmonate-induced stomata closure. Allyl isothiocyanate (AITC), one of myrosinase products, triggers stomatal closure in A. thaliana. We investigated stomatal responses to AITC to clarify the role of TGG1 and TGG2 in Arabidopsis guard-cell signaling. Allyl isothiocyanate at 50 μM and 100 μM induced stomatal closure in the tgg1 and tgg2 single mutants but not in the tgg1 tgg2 double mutant. Furthermore, AITC at 50 μM induced the production of reactive oxygen species and nitric oxide, cytosolic alkalization, and oscillations in cytosolic free calcium concentration in guard cells of both wild-type and mutant plants. These findings suggest that TGG1 and TGG2 are involved in AITC signaling pathway through interaction with signal component(s) downstream of these signaling events, which is not accompanied by hydrolysis of glucosinolates because of the difference in subcellular localization between enzymes (myrosinases) and substrates (glucosinolates).

Cytosolic alkalinization in guard cells: an intriguing but interesting event during stomatal closure that merits further validation of its importance

Stomatal closure is essential to conserve water and prevent microbial entry into leaves. Alkalinization of guard cells is common during closure by factors such as abscisic acid, methyl jasmonate, and even darkness. Despite reports pointing at the role of cytosolic pH, there have been doubts about whether the guard cell pH change is a cause for stomatal closure or an associated event, as changes in membrane potential or ion flux can modulate the pH. However, the importance of cytosolic alkalinization is strongly supported by the ability of externally added weak acids to restrict stomatal closure. Using genetically encoded pH sensors has confirmed the rise in pH to precede the elevation of Ca2+ levels. Yet some reports claim that the rise in pH follows the increase in ROS or Ca2+. We propose a feedback interaction among the rise in pH or ROS or Ca2+ to explain the contrasting opinions on the positioning of pH rise. Stomatal closure and guard cell pH changes are compromised in mutants deficient in vacuolar H+-ATPase (V-ATPase), indicating the importance of V-ATPase in promoting stomatal closure. Thus, cytosolic pH change in guard cells can be related to the rise in ROS and Ca2+, leading to stomatal closure. We emphasize that cytosolic pH in stomatal guard cells deserves further attention and evaluation.

Negative regulation of chitosan-induced stomatal closure by glutathione in Arabidopsis thaliana

Chitosan (CHT) is a deacylated derivative of chitin and improves growth and yield performance, activates defensive genes, and also induces stomatal closure in plants. Glutathione (GSH) has significant functions in the growth, development, defense systems, signaling, and gene expression. GSH negatively regulates abscisic acid-, methyl jasmonate-, and salicylic acid-induced stomatal closure. However, the negative regulation by GSH of CHT-induced stomatal closure is still unknown. Regulation of CHT-induced stomatal closure by GSH in guard cells was investigated using two GSH-deficient mutants, cad2-1 and chlorina 1-1 (ch1-1), and a GSH-decreasing chemical, 1-chloro-2,4-dinitrobenzene (CDNB). The cad2-1 and ch1-1 mutations and CDNB treatment enhanced CHT-induced stomatal closure. Treatment with glutathione monoethyl ester restored the GSH level in the guard cells of cad2-1 and ch1-1 and complemented the stomatal phenotype of the mutants. These results indicate that GSH negatively regulates CHT-induced stomatal closure in Arabidopsis thaliana.

Ca2+-independent ZmCPK2 is inhibited by Ca2+-dependent ZmCPK17 during drought response in maize

Calcium oscillations are induced by different stresses. Calcium-dependent protein kinases (CDPKs/CPKs) are one major group of the plant calcium decoders that are involved in various processes including drought response. Some CPKs are calcium-independent. Here, we identified ZmCPK2 as a negative regulator of drought resistance by screening an overexpression transgenic maize pool. We found that ZmCPK2 does not bind calcium, and its activity is mainly inhibited during short term abscisic acid (ABA) treatment, and dynamically changed in prolonged treatment. Interestingly, ZmCPK2 interacts with and is inhibited by calcium-dependent ZmCPK17, a positive regulator of drought resistance, which is activated by ABA. ZmCPK17 could prevent the nuclear localization of ZmCPK2 through phosphorylation of ZmCPK2T60. ZmCPK2 interacts with and phosphorylates and activates ZmYAB15, a negative transcriptional factor for drought resistance. Our results suggest that drought stress-induced Ca2+ can be decoded directly by ZmCPK17 that inhibits ZmCPK2, thereby promoting plant adaptation to water deficit.

OPEN STOMATA 1 phosphorylates CYCLIC NUCLEOTIDE-GATED CHANNELs to trigger Ca2+ signaling for abscisic acid-induced stomatal closure in Arabidopsis

Multiple cyclic nucleotide-gated channels (CNGCs) are abscisic acid (ABA)-activated Ca2+ channels in Arabidopsis (Arabidopsis thaliana) guard cells. In particular, CNGC5, CNGC6, CNGC9, and CNGC12 are essential for ABA-specific cytosolic Ca2+ signaling and stomatal movements. However, the mechanisms underlying ABA-mediated regulation of CNGCs and Ca2+ signaling are still unknown. In this study, we identified the Ca2+-independent protein kinase OPEN STOMATA 1 (OST1) as a CNGC activator in Arabidopsis. OST1-targeted phosphorylation sites were identified in CNGC5, CNGC6, CNGC9, and CNGC12. These CNGCs were strongly inhibited by Ser-to-Ala mutations and fully activated by Ser-to-Asp mutations at the OST1-targeted sites. The overexpression of individual inactive CNGCs (iCNGCs) under the UBIQUITIN10 promoter in wild-type Arabidopsis conferred a strong dominant-negative-like ABA-insensitive stomatal closure phenotype. In contrast, expressing active CNGCs (aCNGCs) under their respective native promoters in the cngc5-1 cngc6-2 cngc9-1 cngc12-1 quadruple mutant fully restored ABA-activated cytosolic Ca2+ oscillations and Ca2+ currents in guard cells, and rescued the ABA-insensitive stomatal movement mutant phenotypes. Thus, we uncovered that ABA elicits cytosolic Ca2+ signaling via an OST1-CNGC module, in which OST1 functions as a convergence point of the Ca2+-dependent and -independent pathways in Arabidopsis guard cells.

External jasmonic acid isoleucine mediates amplification of plant elicitor peptide receptor (PEPR) and jasmonate-based immune signalling

Jasmonic acid-isoleucine (JA-Ile) is a plant defence hormone whose cellular levels are elevated upon herbivory and regulate defence signalling. Despite their pivotal role, our understanding of the rapid cellular perception of bioactive JA-Ile is limited. This study identifies cell type-specific JA-Ile-induced Ca2+ signal and its role in self-amplification and plant elicitor peptide receptor (PEPR)-mediated signalling. Using the Ca2+ reporter, R-GECO1 in Arabidopsis, we have characterized a monophasic and sustained JA-Ile-dependent Ca2+ signature in leaf epidermal cells. The rapid Ca2+ signal is independent of positive feedback by the JA-Ile receptor, COI1 and the transporter, JAT1. Microarray analysis identified up-regulation of receptors, PEPR1 and PEPR2 upon JA-Ile treatment. The pepr1 pepr2 double mutant in R-GECO1 background exhibits impaired external JA-Ile induced Ca2+ cyt elevation and impacts the canonical JA-Ile responsive genes. JA responsive transcription factor, MYC2 binds to the G-Box motif of PEPR1 and PEPR2 promoter and activates their expression upon JA-Ile treatment and in myc2 mutant, this is reduced. External JA-Ile amplifies AtPep-PEPR pathway by increasing the AtPep precursor, PROPEP expression. Our work shows a previously unknown non-canonical PEPR-JA-Ile-Ca2+ -MYC2 signalling module through which plants sense JA-Ile rapidly to amplify both AtPep-PEPR and jasmonate signalling in undamaged cells.

Integrative regulatory mechanisms of stomatal movements under changing climate

Global climate change-caused drought stress, high temperatures and other extreme weather profoundly impact plant growth and development, restricting sustainable crop production. To cope with various environmental stimuli, plants can optimize the opening and closing of stomata to balance CO2 uptake for photosynthesis and water loss from leaves. Guard cells perceive and integrate various signals to adjust stomatal pores through turgor pressure regulation. Molecular mechanisms and signaling networks underlying the stomatal movements in response to environmental stresses have been extensively studied and elucidated. This review focuses on the molecular mechanisms of stomatal movements mediated by abscisic acid, light, CO2 , reactive oxygen species, pathogens, temperature, and other phytohormones. We discussed the significance of elucidating the integrative mechanisms that regulate stomatal movements in helping design smart crops with enhanced water use efficiency and resilience in a climate-changing world.

K+ and pH homeostasis in plant cells is controlled by a synchronized K+ /H+ antiport at the plasma and vacuolar membrane

Stomatal movement involves ion transport across the plasma membrane (PM) and vacuolar membrane (VM) of guard cells. However, the coupling mechanisms of ion transporters in both membranes and their interplay with Ca2+ and pH changes are largely unclear. Here, we investigated transporter networks in tobacco guard cells and mesophyll cells using multiparametric live-cell ion imaging and computational simulations. K+ and anion fluxes at both, PM and VM, affected H+ and Ca2+ , as changes in extracellular KCl or KNO3 concentrations were accompanied by cytosolic and vacuolar pH shifts and changes in [Ca2+ ]cyt and the membrane potential. At both membranes, the K+ transporter networks mediated an antiport of K+ and H+ . By contrast, net transport of anions was accompanied by parallel H+ transport, with differences in transport capacity for chloride and nitrate. Guard and mesophyll cells exhibited similarities in K+ /H+ transport but cell type-specific differences in [H+ ]cyt and pH-dependent [Ca2+ ]cyt signals. Computational cell biology models explained mechanistically the properties of transporter networks and the coupling of transport across the PM and VM. Our integrated approach indicates fundamental principles of coupled ion transport at membrane sandwiches to control H+ /K+ homeostasis and points to transceptor-like Ca2+ /H+ -based ion signaling in plant cells.

Light-gated channelrhodopsin sparks proton-induced calcium release in guard cells

Although there has been long-standing recognition that stimuli-induced cytosolic pH alterations coincide with changes in calcium ion (Ca2+) levels, the interdependence between protons (H+) and Ca2+ remains poorly understood. We addressed this topic using the light-gated channelrhodopsin HcKCR2 from the pseudofungus Hyphochytrium catenoides, which operates as a H+ conductive, Ca2+ impermeable ion channel on the plasma membrane of plant cells. Light activation of HcKCR2 in Arabidopsis guard cells evokes a transient cytoplasmic acidification that sparks Ca2+ release from the endoplasmic reticulum. A H+-induced cytosolic Ca2+ signal results in membrane depolarization through the activation of Ca2+-dependent SLAC1/SLAH3 anion channels, which enabled us to remotely control stomatal movement. Our study suggests a H+-induced Ca2+ release mechanism in plant cells and establishes HcKCR2 as a tool to dissect the molecular basis of plant intracellular pH and Ca2+ signaling.

Potassium deficiency stress reduces Rubisco activity in Brassica napus leaves by subcellular acidification decreasing photosynthetic rate

Under potassium (K) deficiency photosynthetic carboxylation capacities are limited, affecting the photosynthetic rate of plants. However, it is not clear how ionic K within plants regulates carboxylation capacities. Therefore, the photosynthetic rate (A), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) characteristics, and cytoplasmic pH of Brassica napus leaves with different K levels were measured to evaluate the effects of K on the carboxylation capacity by regulating subcellular pH. The results showed that biochemical limitation dominates the decrease of A. There was a close positive correlation between A and the Rubisco maximum carboxylation rate (Vcmax), which was closer than that between A and the maximum electron transport rate. The thresholds of leaf K concentrations causing decreased A, Vcmax, and Rubisco initial activity were consistent and close to 1.0% in the hydroponic experiments and 1.2% in the field experiments. K deficiency resulted in decreased Rubisco activity, which reduced carboxylation capacity. Moreover, the Rubisco initial activities in vitro with sufficient K supply or under K deficiency all were significantly reduced when the pH was decreased. The cytoplasmic pH was kept neutral at 7.5 under sufficient K supply, and decreased as the leaf K concentration declined below the threshold. Acidified cytoplasmic environment caused by K deficiency could not maintain the pH balance of the chloroplasts, leading to decreased Rubisco initial activity and photosynthetic capacity.

Negative regulation of salicylic acid-induced stomatal closure by glutathione in Arabidopsis thaliana

Salicylic acid (SA) is a ubiquitous phenolic phytohormone that induces stomatal closure. Glutathione (GSH) negatively regulates stomatal closure induced by other plant hormones such as abscisic acid (ABA) and methyl jasmonate (MeJA). However, the involvement of GSH in SA-induced stomatal closure is still unknown. We investigated the regulation of SA signaling by GSH in guard cells using an Arabidopsis thaliana mutant, cad2-1, which is deficient in the first GSH biosynthesis enzyme, γ-glutamylcysteine synthetase. Application of SA decreased stomatal apertures with decreasing intracellular GSH level in guard cells. Decreasing GSH by the cad2-1 mutation and by a GSH-decreasing chemical, 1-chloro-2,4-dinitrobenzene, enhanced the SA-induced stomatal closure. A treatment with glutathione monoethyl ester restored the GSH level in the cad2-1 guard cells and complemented the stomatal phenotype of the mutant. These results indicate that GSH negatively modulates SA-induced stomatal closure in A. thaliana.

Phosphorylation of the plasma membrane H+-ATPase AHA2 by BAK1 is required for ABA-induced stomatal closure in Arabidopsis

Stomatal opening is largely promoted by light-activated plasma membrane-localized proton ATPases (PM H+-ATPases), while their closure is mainly modulated by abscisic acid (ABA) signaling during drought stress. It is unknown whether PM H+-ATPases participate in ABA-induced stomatal closure. We established that BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) interacts with, phosphorylates and activates the major PM Arabidopsis H+-ATPase isoform 2 (AHA2). Detached leaves from aha2-6 single mutant Arabidopsis thaliana plants lost as much water as bak1-4 single and aha2-6 bak1-4 double mutants, with all three mutants losing more water than the wild-type (Columbia-0 [Col-0]). In agreement with these observations, aha2-6, bak1-4, and aha2-6 bak1-4 mutants were less sensitive to ABA-induced stomatal closure than Col-0, whereas the aha2-6 mutation did not affect ABA-inhibited stomatal opening under light conditions. ABA-activated BAK1 phosphorylated AHA2 at Ser-944 in its C-terminus and activated AHA2, leading to rapid H+ efflux, cytoplasmic alkalinization, and reactive oxygen species (ROS) accumulation, to initiate ABA signal transduction and stomatal closure. The phosphorylation-mimicking mutation AHA2S944D driven by its own promoter could largely compensate for the defective phenotypes of water loss, cytoplasmic alkalinization, and ROS accumulation in both aha2-6 and bak1-4 mutants. Our results uncover a crucial role of AHA2 in cytoplasmic alkalinization and ABA-induced stomatal closure during the plant's response to drought stress.

Cytosolic and Nucleosolic Calcium-Regulated Molecular Networks in Response to Long-Term Treatment with Abscisic Acid and Methyl Jasmonate in Arabidopsis thaliana

Calcium acts as a universal secondary messenger that transfers developmental cues and stress signals for gene expression and adaptive growth. A prior study showed that abiotic stresses induce mutually independent cytosolic Ca²⁺ ([Ca²⁺]_cyt) and nucleosolic Ca²⁺ ([Ca²⁺]_nuc) increases in Arabidopsis thaliana root cells. However, gene expression networks deciphering [Ca²⁺]_cyt and [Ca²⁺]_nuc signalling pathways remain elusive. Here, using transgenic A. thaliana to selectively impair abscisic acid (ABA)- or methyl jasmonate (MeJA)-induced [Ca²⁺]_cyt and [Ca²⁺]_nuc increases, we identified [Ca²⁺]_cyt- and [Ca²⁺]_nuc-regulated ABA- or MeJA-responsive genes with a genome oligo-array. Gene co-expression network analysis revealed four Ca²⁺ signal-decoding genes, CAM1, CIPK8, GAD1, and CPN20, as hub genes co-expressed with Ca²⁺-regulated hormone-responsive genes and hormone signalling genes. Luciferase complementation imaging assays showed interactions among CAM1, CIPK8, and GAD1; they also showed interactions with several proteins encoded by Ca²⁺-regulated hormone-responsive genes. Furthermore, CAM1 and CIPK8 were required for MeJA-induced stomatal closure; they were associated with ABA-inhibited seed germination. Quantitative reverse transcription polymerase chain reaction analysis showed the unique expression pattern of [Ca²⁺]-regulated hormone-responsive genes in cam1, cipk8, and gad1. This comprehensive understanding of distinct Ca²⁺ and hormonal signalling will allow the application of approaches to uncover novel molecular foundations for responses to developmental and stress signals in plants.

Methyl jasmonate promotes salinity adaptation responses in two grapevine (Vitis vinifera L.) cultivars differing in salt tolerance

The effects of foliar spray of methyl jasmonate (MJ; 0, 50 and 100 µM) on phytochemical changes and nutritional balance of two grapevine (Vitis vinifera L.) cultivars ['Sultana' (as moderately salt-tolerant) and 'Rishbaba' (as a least salt-tolerant)] were evaluated under different saline conditions (NaCl; 0 and 75 mM). To explore optimum concentration of MJ treatment, grapevine plants were sprayed with MJ twice a day at 12-h intervals during the second week of salinity stress. The results showed that foliar spray of MJ especially at 100 µM was effective in alleviating deleterious effects of NaCl-induced salinity stress on both cultivars. The MJ at 50 and 100 µM significantly decreased ion leakage and production of malondialdehyde and hydrogen peroxide, but increased photosynthetic pigments, soluble carbohydrate, proline, and total phenol and flavonoid contents, especially in 'Sultana' compared with 'Rishbaba'. Moreover, application of 100 µM MJ was associated with enhanced the activities of catalase, guaiacol peroxidase, and ascorbate peroxidase, resulting in greater leaf cell biomembranes stability in vines under saline condition compared with control plants (no treatment). The MJ was also associated with decreased accumulation of sodium and chlorine in leaves, but increased leaf minerals and trace elements compared with control vines. These findings suggest that foliar application of MJ, especially at 100 µM, might be useful to reduce negative effects of salinity stress in vineyards and nurseries.

Protein Carbonylation: Emerging Roles in Plant Redox Biology and Future Prospects

Plants are sessile in nature and they perceive and react to environmental stresses such as abiotic and biotic factors. These induce a change in the cellular homeostasis of reactive oxygen species (ROS). ROS are known to react with cellular components, including DNA, lipids, and proteins, and to interfere with hormone signaling via several post-translational modifications (PTMs). Protein carbonylation (PC) is a non-enzymatic and irreversible PTM induced by ROS. The non-enzymatic feature of the carbonylation reaction has slowed the efforts to identify functions regulated by PC in plants. Yet, in prokaryotic and animal cells, studies have shown the relevance of protein carbonylation as a signal transduction mechanism in physiological processes including hydrogen peroxide sensing, cell proliferation and survival, ferroptosis, and antioxidant response. In this review, we provide a detailed update on the most recent findings pertaining to the role of PC and its implications in various physiological processes in plants. By leveraging the progress made in bacteria and animals, we highlight the main challenges in studying the impacts of carbonylation on protein functions in vivo and the knowledge gap in plants. Inspired by the success stories in animal sciences, we then suggest a few approaches that could be undertaken to overcome these challenges in plant research. Overall, this review describes the state of protein carbonylation research in plants and proposes new research avenues on the link between protein carbonylation and plant redox biology.

Dose-dependent methyl jasmonate effects on photosynthetic traits and volatile emissions: biphasic kinetics and stomatal regulation

Exogenous application of methyl jasmonate (MeJA) has been extensively used to study jasmonate-dependent signaling events triggered by biotic stresses. MeJA application leads to complex jasmonate-dependent physiological responses, including changes in stomatal openness and induction of emissions of a multitude of volatile compounds. Whether the alterations in stomatal conductance and emissions of MeJA-induced volatiles are quantitatively associated with MeJA dose, and whether the induced volatile emissions are regulated by modifications in stomatal conductance had been poorly known until recently. Our latest studies highlighted a biphasic kinetics of jasmonate-dependent volatile emissions induced by MeJA treatment in the model species cucumber (Cucumis sativus), indicating induction of an immediate stress response and subsequent gene-expression level response. Both the immediate and delayed responses were MeJA dose-dependent. The studies further demonstrated that stomata modulated the kinetics of emissions of water-soluble volatiles in a MeJA dose-dependent manner. These studies contribute to understanding of plant short- and long-term responses to different biotic stress severities as simulated by treatments with a range of MeJA doses corresponding to mild to acute stress.

Potassium modulates central carbon metabolism to participate in regulating CO2 transport and assimilation in Brassica napus leaves

Potassium (K) regulates plant metabolism and enhances plant's ability to adapt to adversity. However, under different K deficiency stress, the net photosynthetic rate (A_n) was reduced, influenced by CO₂ conductance or biochemical capacities. The interplay between metabolome and photosynthetic characteristics under K deficiency stress was analyzed to explore the mechanisms by which K regulates photosynthetic capacity. With increasing K deficiency stress, dominations limiting A_n varied from CO₂ conductance to biochemical limitations. Multivariate analyses indicated that organic acids, amino acids and sedoheptulose-7-bisphosphate were significantly related to A_n, CO₂ conductance and carboxylation rate. Under moderate K deficiency, organic acids were up-regulated. Acidification of subcellular compartments reduced sedoheptulose-1,7-bisphosphatase activity, inducing downregulation of sedoheptulose-7-bisphosphate and hindrance of ribulose bisphosphate regeneration. Moreover, increased CO₂ shortage with increasing K deficiency induced a shift of increased citric acid to amino acid synthesis, causing excessive accumulation of amino acids. In addition, the reduced serine level indicated impaired photorespiration. These two changes triggered more serious reduction in photosynthetic capacity. The intimate, changes in photosynthetic capacities were tightly coupled with shifts in central C metabolism, which provides insights into the methods used to enhance A_n and plant's adaptability to abiotic stresses, through the regulation of C metabolites using molecular technology.

An optimized genetically encoded dual reporter for simultaneous ratio imaging of Ca2+ and H+ reveals new insights into ion signaling in plants

Whereas the role of calcium ions (Ca2+ ) in plant signaling is well studied, the physiological significance of pH-changes remains largely undefined. Here we developed CapHensor, an optimized dual-reporter for simultaneous Ca2+ and pH ratio-imaging and studied signaling events in pollen tubes (PTs), guard cells (GCs), and mesophyll cells (MCs). Monitoring spatio-temporal relationships between membrane voltage, Ca2+ - and pH-dynamics revealed interconnections previously not described. In tobacco PTs, we demonstrated Ca2+ -dynamics lag behind pH-dynamics during oscillatory growth, and pH correlates more with growth than Ca2+ . In GCs, we demonstrated abscisic acid (ABA) to initiate stomatal closure via rapid cytosolic alkalization followed by Ca2+ elevation. Preventing the alkalization blocked GC ABA-responses and even opened stomata in the presence of ABA, disclosing an important pH-dependent GC signaling node. In MCs, a flg22-induced membrane depolarization preceded Ca2+ -increases and cytosolic acidification by c. 2 min, suggesting a Ca2+ /pH-independent early pathogen signaling step. Imaging Ca2+ and pH resolved similar cytosol and nuclear signals and demonstrated flg22, but not ABA and hydrogen peroxide to initiate rapid membrane voltage-, Ca2+ - and pH-responses. We propose close interrelation in Ca2+ - and pH-signaling that is cell type- and stimulus-specific and the pH having crucial roles in regulating PT growth and stomata movement.

Stomatal response to isothiocyanates in Arabidopsis thaliana

Allyl isothiocyanate (AITC) induces stomatal closure accompanied by reactive oxygen species (ROS) production and glutathione (GSH) depletion in Arabidopsis thaliana. In this study, stomatal responses to three other isothiocyanates (ITCs), benzyl isothiocyanate (BITC), sulforaphane (SFN), and phenethyl isothiocyanate (PEITC), were investigated in A. thaliana. All these ITCs significantly induced stomatal closure, where PEITC and BITC were most effective. The selected ITCs also induced ROS accumulation, cytosolic alkalization, and GSH depletion in guard cells. Moreover, all ITCs increased the frequency of cytosolic free calcium ([Ca2+]cyt) spikes (transient elevation), while PEITC and BITC showed the highest frequency. There was a strong positive correlation between the number of [Ca2+]cyt spikes per guard cell and the decrease in stomatal aperture. Both cytosolic alkalization and GSH content have a positive correlation with the decrease in stomatal aperture, but ROS production did not have a significant correlation with the decrease in stomatal apertures. These results indicate that the molecules with a functional ITC group induce stomatal closure that is accompanied by GSH depletion, cytosolic alkalization, [Ca2+]cyt spikes, and ROS production, and that the former three cellular events, rather than ROS production, are highly correlated with the decrease in stomatal aperture.

Reactive Carbonyl Species Mediate Methyl Jasmonate-Induced Stomatal Closure

Production of reactive oxygen species (ROS) is a key signal event for methyl jasmonate (MeJA)- and abscisic acid (ABA)-induced stomatal closure. We recently showed that reactive carbonyl species (RCS) stimulates stomatal closure as an intermediate downstream of hydrogen peroxide (H2O2) production in the ABA signaling pathway in guard cells of Nicotiana tabacum and Arabidopsis thaliana. In this study, we examined whether RCS functions as an intermediate downstream of H2O2 production in MeJA signaling in guard cells using transgenic tobacco plants overexpressing A. thaliana 2-alkenal reductase (n-alkanal + NAD(P)+ ⇌ 2-alkenal + NAD(P)H + H+) (AER-OE tobacco) and Arabidopsis plants. The stomatal closure induced by MeJA was impaired in the AER-OE tobacco and was inhibited by RCS scavengers, carnosine and pyridoxamine, in the wild-type (WT) tobacco plants and Arabidopsis plants. Application of MeJA significantly induced the accumulation of RCS, including acrolein and 4-hydroxy-(E)-2-nonenal, in the WT tobacco but not in the AER-OE plants. Application of MeJA induced H2O2 production in the WT tobacco and the AER-OE plants and the H2O2 production was not inhibited by the RCS scavengers. These results suggest that RCS functions as an intermediate downstream of ROS production in MeJA signaling and in ABA signaling in guard cells.

The Myrosinases TGG1 and TGG2 Function Redundantly in Reactive Carbonyl Species Signaling in Arabidopsis Guard Cells

Myrosinase (β-thioglucoside glucohydrolase, enzyme nomenclature, EC 3.2.1.147, TGG) is a highly abundant protein in Arabidopsis guard cells, of which TGG1 and TGG2 function redundantly in abscisic acid (ABA)- and methyl jasmonate-induced stomatal closure. Reactive carbonyl species (RCS) are α,β-unsaturated aldehydes and ketones, which function downstream of reactive oxygen species (ROS) production in the ABA signalling pathway in guard cells. Among the RCS, acrolein is the most highly reactive, which is significantly produced in ABA-treated guard cells. To clarify the ABA signal pathway downstream of ROS production, we investigated the responses of tgg mutants (tgg1-3, tgg2-1 and tgg1-3 tgg2-1) to acrolein. Acrolein induced stomatal closure and triggered cytosolic alkalization in wild type (WT), tgg1-3 single mutants and in tgg2-1 single mutants, but not in tgg1-3 tgg2-1 double mutants. Exogenous Ca2+ induced stomatal closure and cytosolic alkalization not only in WT but also in all of the mutants. Acrolein- and Ca2+-induced stomatal closures were inhibited by an intracellular acidifying agent, butyrate, a Ca2+ chelator, ethylene glycol tetraacetic acid (EGTA) and a Ca2+ channel blocker, LaCl3. Acrolein induced cytosolic free calcium concentration ([Ca2+]cyt) elevation in guard cells of WT plants but not in the tgg1-3 tgg2-1 double mutants. Exogenous Ca2+ elicited [Ca2+]cyt elevation in guard cells of WT and tgg1-3 tgg2-1. Our results suggest that TGG1 and TGG2 function redundantly, not between ROS production and RCS production, but downstream of RCS production in the ABA signal pathway in Arabidopsis guard cells.

Intraorganellar calcium imaging in Arabidopsis seedling roots using the GCaMP variants GCaMP6m and R-CEPIA1er

Ca²⁺ acts as a universal second messenger in eukaryotes. In animals, a wide variety of environmental and developmental stimuli trigger Ca²⁺ dynamics in organelles, such as the cytoplasm, nucleus, and endoplasmic reticulum (ER). However, ER Ca²⁺ ([Ca²⁺]_er) homeostasis and its contributions in cytosolic and/or nucleosolic Ca²⁺ dynamics in plants remain elusive. GCaMPs are comprised of a circularly permutated form of enhanced green fluorescent protein fused to calmodulin and myosin light-chain kinase M13 and used for monitoring Ca²⁺ dynamics in mammalian cells. Here, we targeted a high-affinity variant of GCaMP with nuclear export signal in the cytoplasm (NES-GCaMP6m), with a nuclear-localised signal in the nucleus (NLS-GCaMP6m), and a low-affinity variant of GCaMP, also known as calcium-measuring organelle-entrapped protein indicators (CEPIA), with a signal peptide sequence of the ER-localised protein Calreticulin 1a in the ER lumen (CRT1a-R-CEPIA1er) for intraorganellar Ca²⁺ imaging in Arabidopsis. We found that cytosolic Ca²⁺ ([Ca²⁺]_cyt) increases induced by 250 mM sorbitol as an osmotic stress stimulus, 50 μM abscisic acid (ABA), or 1 mM carbachol (CCh) were mainly due to extracellular Ca²⁺ influx, whereas nucleosolic Ca²⁺ ([Ca²⁺]_nuc) increases triggered by osmotic stress, ABA, or CCh were contributed by [Ca²⁺]_er release. In addition, [Ca²⁺]_er dynamics presented specific patterns in response to different stimuli such as osmotic stress, ABA, or CCh, indicating that Ca²⁺ signalling occurs in the ER in plants. These results provide valuable insights into subcellular Ca²⁺ dynamics in response to different stresses in Arabidopsis root cells and prove that GCaMP imaging is a useful tool for furthering our understanding of plant organelle functions.

Genetically Encoded Biosensors in Plants: Pathways to Discovery

Genetically encoded biosensors that directly interact with a molecule of interest were first introduced more than 20 years ago with fusion proteins that served as fluorescent indicators for calcium ions. Since then, the technology has matured into a diverse array of biosensors that have been deployed to improve our spatiotemporal understanding of molecules whose dynamics have profound influence on plant physiology and development. In this review, we address several types of biosensors with a focus on genetically encoded calcium indicators, which are now the most diverse and advanced group of biosensors. We then consider the discoveries in plant biology made by using biosensors for calcium, pH, reactive oxygen species, redox conditions, primary metabolites, phytohormones, and nutrients. These discoveries were dependent on the engineering, characterization, and optimization required to develop a successful biosensor; they were also dependent on the methodological developments required to express, detect, and analyze the readout of such biosensors.

Advances and current challenges in calcium signaling

Content Summary 414 I. Introduction 415 II. Ca²⁺ importer and exporter in plants 415 III. The Ca²⁺ decoding toolkit in plants 415 IV. Mechanisms of Ca²⁺ signal decoding 417 V. Immediate Ca²⁺ signaling in the regulation of ion transport 418 VI. Ca²⁺ signal integration into long-term ABA responses 419 VII Integration of Ca²⁺ and hormone signaling through dynamic complex modulation of the CCaMK/CYCLOPS complex 420 VIII Ca²⁺ signaling in mitochondria and chloroplasts 422 IX A view beyond recent advances in Ca²⁺ imaging 423 X Modeling approaches in Ca²⁺ signaling 424 XI Conclusions: Ca²⁺ signaling a still young blooming field of plant research 424 Acknowledgements 425 ORCID 425 References 425 SUMMARY: Temporally and spatially defined changes in Ca²⁺ concentration in distinct compartments of cells represent a universal information code in plants. Recently, it has become evident that Ca²⁺ signals not only govern intracellular regulation but also appear to contribute to long distance or even organismic signal propagation and physiological response regulation. Ca²⁺ signals are shaped by an intimate interplay of channels and transporters, and during past years important contributing individual components have been identified and characterized. Ca²⁺ signals are translated by an elaborate toolkit of Ca²⁺ -binding proteins, many of which function as Ca²⁺ sensors, into defined downstream responses. Intriguing progress has been achieved in identifying specific modules that interconnect Ca²⁺ decoding proteins and protein kinases with downstream target effectors, and in characterizing molecular details of these processes. In this review, we reflect on recent major advances in our understanding of Ca²⁺ signaling and cover emerging concepts and existing open questions that should be informative also for scientists that are currently entering this field of ever-increasing breath and impact.

Plant ion channels and transporters in herbivory-induced signalling

In contrast to many biotic stresses that plants face, feeding by herbivores produces unique mechanical and chemical signatures. Plants have evolved effective systems to recognise these mechanical stimuli and chemical elicitors at the plasma membrane (PM), where this recognition generates ion fluxes, including an influx of Ca2+ that elicits cellular Ca2+ signalling, production of reactive oxygen species (ROS), and variation in transmembrane potential. These signalling events also function in propagation of long-distance signals (Ca2+ waves, ROS waves, and electrical signals), which contribute to rapid, systemic induction of defence responses. Recent studies have identified several candidate channels or transporters that likely produce these ion fluxes at the PM. Here, we describe the important roles of these channels/transporters in transduction or transmission of herbivory-induced early signalling events, long-distance signals, and jasmonic acid and green leaf volatile signalling in plants.

Multiparameter imaging of calcium and abscisic acid and high-resolution quantitative calcium measurements using R-GECO1-mTurquoise in Arabidopsis

Calcium signals occur in specific spatio-temporal patterns in response to various stimuli and are coordinated with, for example, hormonal signals, for physiological and developmental adaptations. Quantification of calcium together with other signalling molecules is required for correlative analyses and to decipher downstream calcium-decoding mechanisms. Simultaneous in vivo imaging of calcium and abscisic acid has been performed here to investigate the interdependence of the respective signalling processes in Arabidopsis thaliana roots. Advanced ratiometric genetically encoded calcium indicators have been generated and in vivo calcium calibration protocols were established to determine absolute calcium concentration changes in response to auxin and ATP. In roots, abscisic acid induced long-term basal calcium concentration increases, while auxin triggered rapid signals in the elongation zone. The advanced ratiometric calcium indicator R-GECO1-mTurquoise exhibited an increased calcium signal resolution compared to commonly used Förster resonance energy transfer-based indicators. Quantitative calcium measurements in Arabidopsis root tips using R-GECO1-mTurquoise revealed detailed maps of absolute calcium concentration changes in response to auxin and ATP. Calcium calibration protocols using R-GECO1-mTurquoise enabled high-resolution quantitative imaging of resting cytosolic calcium concentrations and their dynamic changes that revealed distinct hormonal and ATP responses in roots.

A new discrete dynamic model of ABA-induced stomatal closure predicts key feedback loops

Stomata, microscopic pores in leaf surfaces through which water loss and carbon dioxide uptake occur, are closed in response to drought by the phytohormone abscisic acid (ABA). This process is vital for drought tolerance and has been the topic of extensive experimental investigation in the last decades. Although a core signaling chain has been elucidated consisting of ABA binding to receptors, which alleviates negative regulation by protein phosphatases 2C (PP2Cs) of the protein kinase OPEN STOMATA 1 (OST1) and ultimately results in activation of anion channels, osmotic water loss, and stomatal closure, over 70 additional components have been identified, yet their relationships with each other and the core components are poorly elucidated. We integrated and processed hundreds of disparate observations regarding ABA signal transduction responses underlying stomatal closure into a network of 84 nodes and 156 edges and, as a result, established those relationships, including identification of a 36-node, strongly connected (feedback-rich) component as well as its in- and out-components. The network's domination by a feedback-rich component may reflect a general feature of rapid signaling events. We developed a discrete dynamic model of this network and elucidated the effects of ABA plus knockout or constitutive activity of 79 nodes on both the outcome of the system (closure) and the status of all internal nodes. The model, with more than 1024 system states, is far from fully determined by the available data, yet model results agree with existing experiments in 82 cases and disagree in only 17 cases, a validation rate of 75%. Our results reveal nodes that could be engineered to impact stomatal closure in a controlled fashion and also provide over 140 novel predictions for which experimental data are currently lacking. Noting the paucity of wet-bench data regarding combinatorial effects of ABA and internal node activation, we experimentally confirmed several predictions of the model with regard to reactive oxygen species, cytosolic Ca2+ (Ca2+c), and heterotrimeric G-protein signaling. We analyzed dynamics-determining positive and negative feedback loops, thereby elucidating the attractor (dynamic behavior) repertoire of the system and the groups of nodes that determine each attractor. Based on this analysis, we predict the likely presence of a previously unrecognized feedback mechanism dependent on Ca2+c. This mechanism would provide model agreement with 10 additional experimental observations, for a validation rate of 85%. Our research underscores the importance of feedback regulation in generating robust and adaptable biological responses. The high validation rate of our model illustrates the advantages of discrete dynamic modeling for complex, nonlinear systems common in biology.

Chitosan signaling in guard cells requires endogenous salicylic acid

An elicitor chitosan (CHT) induces stomatal closure but the mechanism remains to be clarified. A phytohormone salicylic acid (SA) is crucial for elicitor-induced defense signaling in plants. Here we investigated whether endogenous SA is required for CHT signaling in guard cells. In the SA-deficient nahG mutant, treatment of CHT did not induce either apoplastic reactive oxygen species (ROS) production or stomatal closure but co-treatment of CHT and SA induced both apoplastic ROS production and stomatal closure, indicating the involvement of endogenous SA in CHT-induced apoplastic ROS production and CHT-induced stomatal closure. Furthermore, CHT induced transient cytosolic free calcium concentration increments in the nahG mutant in the presence of exogenous SA but not in the absence of exogenous SA. These results provide evidence that endogenous SA is a crucial element in CHT-induced stomatal closure.

Stomatal closure induced by phytosphingosine-1-phosphate and sphingosine-1-phosphate depends on nitric oxide and pH of guard cells in Pisum sativum

MAIN CONCLUSION

Phyto-S1P and S1P induced stomatal closure in epidermis of pea ( Pisum sativum ) by raising the levels of NO and pH in guard cells. Phosphosphingolipids, such as phytosphingosine-1-phosphate (phyto-S1P) and sphingosine-1-phosphate (S1P), are important signaling components during drought stress. The biosynthesis of phyto-S1P or S1P is mediated by sphingosine kinases (SPHKs). Although phyto-S1P and S1P are known to be signaling components in higher plants, their ability to induce stomatal closure has been ambiguous. We evaluated in detail the effects of phyto-S1P, S1P and SPHK inhibitors on signaling events leading to stomatal closure in the epidermis of Pisum sativum. Phyto-S1P or S1P induced stomatal closure, along with a marked rise in nitric oxide (NO) and cytoplasmic pH of guard cells, as in case of ABA. Two SPHK inhibitors, DL-threo dihydrosphingosine and N',N'-dimethylsphingosine, restricted ABA-induced stomatal closure and prevented the increase of NO or pH by ABA. Modulators of NO or pH impaired both stomatal closure and increase in NO or pH by phyto-S1P/S1P. The stomatal closure by phyto-S1P/S1P was mediated by phospholipase D and phosphatidic acid (PA). When present, PA elevated the levels of pH, but not NO of guard cells. Our results demonstrate that stomatal closure induced by phyto-S1P and S1P depends on rise in pH as well as NO of guard cells. A scheme of signaling events initiated by phyto-S1P/S1P, and converging to cause stomatal closure, is proposed.

Involvement of OST1 Protein Kinase and PYR/PYL/RCAR Receptors in Methyl Jasmonate-Induced Stomatal Closure in Arabidopsis Guard Cells

Methyl jasmonate (MeJA) induces stomatal closure. It has been shown that stomata of many ABA-insensitive mutants are also insensitive to MeJA, and a low amount of ABA is a prerequisite for the MeJA response. However, the molecular mechanisms of the interaction between ABA and MeJA signaling remain to be elucidated. Here we studied the interplay of signaling of the two hormones in guard cells using the quadruple ABA receptor mutant pyr1 pyl1 pyl2 pyl4 and ABA-activated protein kinase mutants ost1-2 and srk2e. In the quadruple mutant, MeJA-induced stomatal closure, H2O2 production, nitric oxide (NO) production, cytosolic alkalization and plasma membrane Ca(2+)-permeable current (ICa) activation were not impaired. At the same time, the inactivation of the inward-rectifying K(+) current was impaired. In contrast to the quadruple mutant, MeJA-induced stomatal closure, H2O2 production, NO production and cytosolic alkalization were impaired in ost1-2 and srk2e as well as in aba2-2, the ABA-deficient mutant. The activation of ICa was also impaired in srk2e. Collectively, these results indicated that OST1 was essential for MeJA-induced stomatal closure, while PYR1, PYL1, PYL2 and PYL4 ABA receptors were not sufficient factors. MeJA did not appear to activate OST1 kinase activity. This implies that OST1 mediates MeJA signaling through an undetectable level of activity or a non-enzymatic action. MeJA induced the expression of an ABA synthesis gene, NCED3, and increased ABA contents only modestly. Taken together with previous reports, this study suggests that MeJA signaling in guard cells is primed by ABA and is not brought about through the pathway mediated by PYR1, PYL1 PYL2 and PYL4.

Convergence and Divergence of Signaling Events in Guard Cells during Stomatal Closure by Plant Hormones or Microbial Elicitors

Dynamic regulation of stomatal aperture is essential for plants to optimize water use and CO2 uptake. Stomatal opening or closure is accompanied by the modulation of guard cell turgor. Among the events leading to stomatal closure by plant hormones or microbial elicitors, three signaling components stand out as the major converging points. These are reactive oxygen species (ROS), cytosolic free Ca(2+), and ion channels. Once formed, the ROS and free Ca(2+) of guard cells regulate both downstream and upstream events. A major influence of ROS is to increase the levels of NO and cytosolic free Ca(2+) in guard cells. Although the rise in NO is an important event during stomatal closure, the available evidences do not support the description of NO as the point of convergence. The rise in ROS and NO would cause an increase of free Ca(2+) and modulate ion channels, through a network of events, in such a way that the guard cells lose K(+)/Cl(-)/anions. The efflux of these ions decreases the turgor of guard cells and leads to stomatal closure. Thus, ROS, NO, and cytosolic free Ca(2+) act as points of divergence. The other guard cell components, which are modulated during stomatal closure are G-proteins, cytosolic pH, phospholipids, and sphingolipids. However, the current information on the role of these components is not convincing so as to assign them as the points of convergence or divergence. The interrelationships and interactions of ROS, NO, cytosolic pH, and free Ca(2+) are quite complex and need further detailed examination. Our review is an attempt to critically assess the current status of information on guard cells, while emphasizing the convergence and divergence of signaling components during stomatal closure. The existing gaps in our knowledge are identified to stimulate further research.

Two guard cell mitogen-activated protein kinases, MPK9 and MPK12, function in methyl jasmonate-induced stomatal closure in Arabidopsis thaliana

Methyl jasmonate (MeJA) and abscisic acid (ABA) signalling cascades share several signalling components in guard cells. We previously showed that two guard cell-preferential mitogen-activated protein kinases (MAPKs), MPK9 and MPK12, positively regulate ABA signalling in Arabidopsis thaliana. In this study, we examined whether these two MAP kinases function in MeJA signalling using genetic mutants for MPK9 and MPK12 combined with a pharmacological approach. MeJA induced stomatal closure in mpk9-1 and mpk12-1 single mutants as well as wild-type plants, but not in mpk9-1 mpk12-1 double mutants. Consistently, the MAPKK inhibitor PD98059 inhibited the MeJA-induced stomatal closure in wild-type plants. MeJA elicited reactive oxygen species (ROS) production and cytosolic alkalisation in guard cells of the mpk9-1, mpk12-1 and mpk9-1 mpk12-1 mutants, as well in wild-type plants. Furthermore, MeJA triggered elevation of cytosolic Ca(2+) concentration ([Ca(2+)]cyt ) in the mpk9-1 mpk12-1 double mutant as well as wild-type plants. Activation of S-type anion channels by MeJA was impaired in mpk9-1 mpk12-1. Together, these results indicate that MPK9 and MPK12 function upstream of S-type anion channel activation and downstream of ROS production, cytosolic alkalisation and [Ca(2+)]cyt elevation in guard cell MeJA signalling, suggesting that MPK9 and MPK12 are key regulators mediating both ABA and MeJA signalling in guard cells.

The role of plasma membrane H(+) -ATPase in jasmonate-induced ion fluxes and stomatal closure in Arabidopsis thaliana

Methyl jasmonate (MeJA) elicits stomatal closure in many plant species. Stomatal closure is accompanied by large ion fluxes across the plasma membrane (PM). Here, we recorded the transmembrane ion fluxes of H(+) , Ca(2+) and K(+) in guard cells of wild-type (Col-0) Arabidopsis, the CORONATINE INSENSITIVE1 (COI1) mutant coi1-1 and the PM H(+) -ATPase mutants aha1-6 and aha1-7, using a non-invasive micro-test technique. We showed that MeJA induced transmembrane H(+) efflux, Ca(2+) influx and K(+) efflux across the PM of Col-0 guard cells. However, this ion transport was abolished in coi1-1 guard cells, suggesting that MeJA-induced transmembrane ion flux requires COI1. Furthermore, the H(+) efflux and Ca(2+) influx in Col-0 guard cells was impaired by vanadate pre-treatment or PM H(+) -ATPase mutation, suggesting that the rapid H(+) efflux mediated by PM H(+) -ATPases could function upstream of the Ca(2+) flux. After the rapid H(+) efflux, the Col-0 guard cells had a longer oscillation period than before MeJA treatment, indicating that the activity of the PM H(+) -ATPase was reduced. Finally, the elevation of cytosolic Ca(2+) concentration and the depolarized PM drive the efflux of K(+) from the cell, resulting in loss of turgor and closure of the stomata.

Plasma membrane H(+)-ATPase is involved in methyl jasmonate-induced root hair formation in lettuce (Lactuca sativa L.) seedlings

KEY MESSAGE

Our results show that methyl jasmonate induces plasma membrane H (+) -ATPase activity and subsequently influences the apoplastic pH of trichoblasts to maintain a cell wall pH environment appropriate for root hair development. Root hairs, which arise from root epidermal cells, are tubular structures that increase the efficiency of water absorption and nutrient uptake. Plant hormones are critical regulators of root hair development. In this study, we investigated the regulatory role of the plasma membrane (PM) H(+)-ATPase in methyl jasmonate (MeJA)-induced root hair formation. We found that MeJA had a pronounced effect on the promotion of root hair formation in lettuce seedlings, but that this effect was blocked by the PM H(+)-ATPase inhibitor vanadate. Furthermore, MeJA treatment increased PM H(+)-ATPase activity in parallel with H(+) efflux from the root tips of lettuce seedlings and rhizosphere acidification. Our results also showed that MeJA-induced root hair formation was accompanied by hydrogen peroxide accumulation. The apoplastic acidification acted in concert with reactive oxygen species to modulate root hair formation. Our results suggest that the effect of MeJA on root hair formation is mediated by modulation of PM H(+)-ATPase activity.

Allyl isothiocyanate induces stomatal closure in Vicia faba

Isothiocyanates are enzymatically produced from glucosinolates in plants, and allyl isothiocyanate (AITC) induces stomatal closure in Arabidopsis thaliana. In this study, we investigated stomatal responses to AITC in Vicia faba. AITC-induced stomatal closure accompanied by reactive oxygen species (ROS) and NO production, cytosolic alkalization and glutathione (GSH) depletion in V. faba. GSH monoethyl ester induced stomatal reopening and suppressed AITC-induced GSH depletion in guard cells. Exogenous catalase and a peroxidase inhibitor, salicylhydroxamic acid, inhibited AITC-induced stomatal closure, unlike an NAD(P)H oxidase inhibitor, diphenylene iodonium chloride. The peroxidase inhibitor also abolished the AITC-induced ROS production, NO production, and cytosolic alkalization. AITC-induced stomatal closure was suppressed by an NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, and an agent to acidify cytosol, butyrate. These results indicate that AITC-induced stomatal closure in V. faba as well as in A. thaliana and suggest that AITC signaling in guard cells is conserved in both plants.

Heterotrimeric G-proteins involved in the MeJA regulated ion flux and stomatal closure in Arabidopsis thaliana

Heterotrimeric G-proteins play an important role in plant signalling pathways. The plant hormone methyl jasmonate (MeJA) can induce stomatal closure in many plant species. The signal cascade in MeJA-induced stomatal closure has been studied previously. However, the function of G proteins in this process has not yet been evaluated. In this study, the stomatal movement induced by MeJA in the wild-type Arabidopsis thaliana (L. Heynh.) (WS), Gα subunit loss-of-function mutant gpa1-1 and gpa1-2 guard cells were measured. Further, the transmembrane ion flux (H+, Ca2+ and K+) and reactive oxygen species (ROS) experiments were performed in guard cells from WS, GDP-β-S pre-treated WS, gpa1-1 and gpa1-2 using non-invasive micro-test technique (NMT) and confocal technique. It was observed that the MeJA-induced stomatal closure was abolished in guard cells of gpa1 mutants. GDP-β-S pre-treatment and gpa1 mutants impaired the MeJA-activated H+ efflux, Ca2+ influx and K+ efflux. The accumulation of ROS in gpa1-1 and gpa1-2 guard cells was also lower than that in WS guard cells under MeJA treatment. These results suggested that Gα subunits are involved in regulating the signal events in JA signal pathway and stomatal closure.

Diverse stomatal signaling and the signal integration mechanism

Guard cells perceive a variety of chemicals produced metabolically in response to abiotic and biotic stresses, integrate the signals into reactive oxygen species and calcium signatures, and convert these signatures into stomatal movements by regulating turgor pressure. Guard cell behaviors in response to such complex signals are critical for plant growth and sustenance in stressful, ever-changing environments. The key open question is how guard cells achieve the signal integration to optimize stomatal aperture. Abscisic acid is responsible for stomatal closure in plants in response to drought, and its signal transduction has been well studied. Other plant hormones and low-molecular-weight compounds function as inducers of stomatal closure and mediators of signaling in guard cells. In this review, we summarize recent advances in research on the diverse stomatal signaling pathways, with specific emphasis on signal integration and signal interaction in guard cell movement.

Inhibition of ABA-induced stomatal closure by fusicoccin is associated with cytosolic acidification-mediated hydrogen peroxide removal

BACKGROUND

Fusicoccin (FC), a fungal phytotoxin produced by Fusicoccum amygdale, causes the inhibition of ABA-induced stomatal closure. The mechanism of inhibition is remaining unclear. We analyzed the role of hydrogen peroxide (H2O2) and relationship between H2O2 removal and cytosolic pH changes during inhibition of ABA-induced stomatal closure by FC.

RESULTS

According to the results, ABA treatment induced H2O2 production and stomatal closure, but FC inhibited the effects of ABA on these two parameters. Treatment with catalase (CAT) and NADPH oxidase inhibitor diphenylene iodonium (DPI) mimicked the effect of FC. These data suggest that inhibition of ABA effect by FC is related to the decrease of H2O2 levels in guard cells. Furthermore, similar to CAT, FC not only suppressed stomatal closure and H2O2 levels in guard cells treated with exogenous H2O2, but also reopened the stomata which had been closed by ABA and reduced the level of H2O2 that had been produced by ABA, indicating that FC causes H2O2 removal in guard cells. The butyric acid treatment simulated the effects of FC on the stomatal aperture and H2O2 levels in guard cells treated with exogenous H2O2 and had been closed by ABA, and both FC and butyric acid reduced cytosolic pH in guard cells of stomata treated with H2O2 and had been closed by ABA, which demonstrate that cytosolic acidification mediates FC-induced H2O2 removal.

CONCLUSION

These results suggest that FC causes cytosolic acidification in guard cells, then induces H2O2 removal and reduces H2O2 levels in guard cells, finally inhibits stomatal closure induced by ABA.

CPK13, a noncanonical Ca2+-dependent protein kinase, specifically inhibits KAT2 and KAT1 shaker K+ channels and reduces stomatal opening

Ca(2) (+)-dependent protein kinases (CPKs) form a large family of 34 genes in Arabidopsis (Arabidopsis thaliana). Based on their dependence on Ca(2+), CPKs can be sorted into three types: strictly Ca(2+)-dependent CPKs, Ca(2+)-stimulated CPKs (with a significant basal activity in the absence of Ca(2+)), and essentially calcium-insensitive CPKs. Here, we report on the third type of CPK, CPK13, which is expressed in guard cells but whose role is still unknown. We confirm the expression of CPK13 in Arabidopsis guard cells, and we show that its overexpression inhibits light-induced stomatal opening. We combine several approaches to identify a guard cell-expressed target. We provide evidence that CPK13 (1) specifically phosphorylates peptide arrays featuring Arabidopsis K(+) Channel KAT2 and KAT1 polypeptides, (2) inhibits KAT2 and/or KAT1 when expressed in Xenopus laevis oocytes, and (3) closely interacts in plant cells with KAT2 channels (Förster resonance energy transfer-fluorescence lifetime imaging microscopy). We propose that CPK13 reduces stomatal aperture through its inhibition of the guard cell-expressed KAT2 and KAT1 channels.

Cytosolic alkalisation and nitric oxide production in UVB-induced stomatal closure in Arabidopsis thaliana

The role and the interrelationship of cytosolic alkalisation and nitric oxide (NO) in UVB-induced stomatal closure were investigated in Arabidopsis thaliana (L.) Heynh. by stomatal bioassay and laser-scanning confocal microscopy. In response to 0.5Wm-2 UVB radiation, the rise of NO levels in guard cells occurred after cytosolic alkalisation but preceded stomatal closure. UVB-induced NO production and stomatal closure were both inhibited by NO scavengers, nitrate reductase (NR) inhibitors and a Nia2-5/Nia1-2 mutation, and also by butyrate. Methylamine induced NO generation and stomatal closure in the wild-type but not in the Nia2-5/Nia1-2 mutant or wild-type plants pretreated with NO scavengers or NR inhibitors while enhancing the cytosolic pH in guard cells under light. NO generation in wild-type guard cells was largely induced after 60min of UVB radiation. The defect in UVB-induced NO generation in Nia2-5/Nia1-2 guard cells did not affect the changes of guard cell pH before 60min of UVB radiation, but prevented the UVB-induced cytosolic alkalisation after 60min of radiation. Meanwhile, exogenous NO caused a marked rise of cytosolic pH in guard cells. Together, our results show that cytosolic alkalisation and NR-dependent NO production coordinately function in UVB signalling in A. thaliana guard cells.

FRET-based reporters for the direct visualization of abscisic acid concentration changes and distribution in Arabidopsis

Abscisic acid (ABA) is a plant hormone that regulates plant growth and development and mediates abiotic stress responses. Direct cellular monitoring of dynamic ABA concentration changes in response to environmental cues is essential for understanding ABA action. We have developed ABAleons: ABA-specific optogenetic reporters that instantaneously convert the phytohormone-triggered interaction of ABA receptors with PP2C-type phosphatases to send a fluorescence resonance energy transfer (FRET) signal in response to ABA. We report the design, engineering and use of ABAleons with ABA affinities in the range of 100–600 nM to map ABA concentration changes in plant tissues with spatial and temporal resolution. High ABAleon expression can partially repress Arabidopsis ABA responses. ABAleons report ABA concentration differences in distinct cell types, ABA concentration increases in response to low humidity and NaCl in guard cells and to NaCl and osmotic stress in roots and ABA transport from the hypocotyl to the shoot and root.

DOI:http://dx.doi.org/10.7554/eLife.01739.001

Plants are able to respond to detrimental changes in their environment—when, for example, water becomes scarce or the soil becomes too salty—in ways that minimize stress and damage caused by these changes. Hormones are chemicals that trigger the plant’s response under these circumstances.

Abscisic acid is the hormone that regulates how plants respond to drought and salt stress and that controls the plant growth in these conditions. In the past, it was possible to measure the average level of this hormone in a given tissue, but not the level in individual cells in a living plant. Moreover, it was difficult to follow directly how abscisic acid moved between the plant cells, tissues or organs.

Now, Waadt et al. (and independently Jones et al.) have developed tools that can measure the levels of abscisic acid within individual cells in living plants and in real time. The plants were genetically engineered to produce sensor proteins with two properties: they can bind to abscisic acid in a reversible manner, and they contain two ‘tags’ that fluoresce at different wavelengths. Shining light onto the plant at a specific wavelength that is only absorbed by one of the tags actually causes both of the tags on the sensor proteins to fluoresce. However, the sensors fluoresce more at one wavelength when they are bound to abscisic acid, and more at the other wavelength when they are not bound to abscisic acid. Hence, measuring the ratio of these two wavelengths in the light that is given off by the sensor proteins can be used as a measure of the concentration of abscisic acid in a plant cell.

Waadt et al. developed sensor proteins called ‘ABAleons’, and used one of these to analyze the uptake, distribution and movement of abscisic acid in different tissues in the model plant Arabidopsis thaliana. Changes in the level of abscisic acid could be detected at the level of an individual plant cell, and over time scales of fractions of seconds to hours. ABAleons also revealed that the concentration of abscisic acid in guard cells—specialized cells that help stop the loss of water vapor from a leaf—increases when humidity levels are low, or when salt levels are high. Low water levels, or high salt levels, also slowly increased the concentration of abscisic acid in the roots of the plant. Furthermore, Waadt et al. saw that abscisic acid moved long distances from the base of the stem up into the shoot, and down to the root.

Waadt et al. also report that the ABAleons made plants less responsive to abscisic acid, possibly because binding to the ABAleons reduced the amount of abscisic acid that was available to perform its role as a hormone. The next challenge is to engineer ABAleons that minimize this unwanted side effect, and then go on to use ABAleons to study environmental conditions and proteins involved in plant hormone responses.

DOI:http://dx.doi.org/10.7554/eLife.01739.002

Enhancement of root hydraulic conductivity by methyl jasmonate and the role of calcium and abscisic acid in this process

The role of jasmonic acid in the induction of stomatal closure is well known. However, its role in regulating root hydraulic conductivity (L) has not yet been explored. The objectives of the present research were to evaluate how JA regulates L and how calcium and abscisic acid (ABA) could be involved in such regulation. We found that exogenous methyl jasmonate (MeJA) increased L of Phaseolus vulgaris, Solanum lycopersicum and Arabidopsis thaliana roots. Tomato plants defective in JA biosynthesis had lower values of L than wild-type plants, and that L was restored by addition of MeJA. The increase of L by MeJA was accompanied by an increase of the phosphorylation state of the aquaporin PIP2. We observed that MeJA addition increased the concentration of cytosolic calcium and that calcium channel blockers inhibited the rise of L caused by MeJA. Treatment with fluoridone, an inhibitor of ABA biosynthesis, partially inhibited the increase of L caused by MeJA, and tomato plants defective in ABA biosynthesis increased their L after application of MeJA. It is concluded that JA enhances L and that this enhancement is linked to calcium and ABA dependent and independent signalling pathways.

Behind the scenes: the roles of reactive oxygen species in guard cells

Guard cells regulate stomatal pore size through integration of both endogenous and environmental signals; they are widely recognized as providing a key switching mechanism that maximizes both the efficient use of water and rates of CO₂ exchange for photosynthesis; this is essential for the adaptation of plants to water stress. Reactive oxygen species (ROS) are widely considered to be an important player in guard cell signalling. In this review, we focus on recent progress concerning the role of ROS as signal molecules in controlling stomatal movement, the interaction between ROS and intrinsic and environmental response pathways, the specificity of ROS signalling, and how ROS signals are sensed and relayed. However, the picture of ROS-mediated signalling is still fragmented and the issues of ROS sensing and the specificity of ROS signalling remain unclear. Here, we review some recent advances in our understanding of ROS signalling in guard cells, with an emphasis on the main players known to interact with abscisic acid signalling.

The role and interactions of cytosolic alkalization and hydrogen peroxide in ultraviolet B-induced stomatal closure in Arabidopsis

Cytosolic alkalization has been shown to function as a key player in multiple stimuli-induced stomatal closure, but its role and relationship with hydrogen peroxide (H2O2) in ultraviolet B (UV-B)-induced stomatal closure remains unknown. In this paper, by stomatal bioassay and laser-scanning confocal microscopy, we observed that 0.5 W m(-2) UV-B induced cytosolic alkalization and H2O2 production in guard cells while inducing stomatal closure in Arabidopsis (Arabidopsis thaliana). Butyrate (a weak acid) reduced the cytosolic pH/H2O2 production and prevented stomatal closure by UV-B. Methylamine (a weak base) induced H2O2 production and stomatal closure while enhancing the cytosolic alkalization in guard cells under light alone. The rise in cytosolic pH of wild-type guard cells on exposure to UV-B was evident at 15 min and substantial at 45 min while H2O2 production started to largely increase after 60 min. The failure of UV-B-induced H2O2 production in AtrbohD/F guard cells did not affect the changes of guard cell pH during the first 60 min of UV-B radiation, but largely suppressed cytosolic alkalization after 60 min of UV-B radiation. These results indicate that cytosolic alkalization mediates UV-B-induced stomatal closure via activating H2O2 production and that H2O2 production can feedback-enhance cytosolic alkalization in Arabidopsis guard cells.

Difference in abscisic acid perception mechanisms between closure induction and opening inhibition of stomata

Abscisic acid (ABA) induces stomatal closure and inhibits light-induced stomatal opening. The mechanisms in these two processes are not necessarily the same. It has been postulated that the ABA receptors involved in opening inhibition are different from those involved in closure induction. Here, we provide evidence that four recently identified ABA receptors (PYRABACTIN RESISTANCE1 [PYR1], PYRABACTIN RESISTANCE-LIKE1 [PYL1], PYL2, and PYL4) are not sufficient for opening inhibition in Arabidopsis (Arabidopsis thaliana). ABA-induced stomatal closure was impaired in the pyr1/pyl1/pyl2/pyl4 quadruple ABA receptor mutant. ABA inhibition of the opening of the mutant's stomata remained intact. ABA did not induce either the production of reactive oxygen species and nitric oxide or the alkalization of the cytosol in the quadruple mutant, in accordance with the closure phenotype. Whole cell patch-clamp analysis of inward-rectifying K(+) current in guard cells showed a partial inhibition by ABA, indicating that the ABA sensitivity of the mutant was not fully impaired. ABA substantially inhibited blue light-induced phosphorylation of H(+)-ATPase in guard cells in both the mutant and the wild type. On the other hand, in a knockout mutant of the SNF1-related protein kinase, srk2e, stomatal opening and closure, reactive oxygen species and nitric oxide production, cytosolic alkalization, inward-rectifying K(+) current inactivation, and H(+)-ATPase phosphorylation were not sensitive to ABA.

Endogenous abscisic acid is involved in methyl jasmonate-induced reactive oxygen species and nitric oxide production but not in cytosolic alkalization in Arabidopsis guard cells

We recently demonstrated that endogenous abscisic acid (ABA) is involved in methyl jasmonate (MeJA)-induced stomatal closure in Arabidopsis thaliana. In this study, we investigated whether endogenous ABA is involved in MeJA-induced reactive oxygen species (ROS) and nitric oxide (NO) production and cytosolic alkalization in guard cells using an ABA-deficient Arabidopsis mutant, aba2-2, and an inhibitor of ABA biosynthesis, fluridon (FLU). The aba2-2 mutation impaired MeJA-induced ROS and NO production. FLU inhibited MeJA-induced ROS production in wild-type guard cells. Pretreatment with 0.1 μM ABA, which does not induce stomatal closure in the wild type, complemented the insensitivity to MeJA of the aba2-2 mutant. However, MeJA induced cytosolic alkalization in both wild-type and aba2-2 guard cells. These results suggest that endogenous ABA is involved in MeJA-induced ROS and NO production but not in MeJA-induced cytosolic alkalization in Arabidopsis guard cells.

Rapid structural changes and acidification of guard cell vacuoles during stomatal closure require phosphatidylinositol 3,5-bisphosphate

Rapid stomatal closure is essential for water conservation in plants and is thus critical for survival under water deficiency. To close stomata rapidly, guard cells reduce their volume by converting a large central vacuole into a highly convoluted structure. However, the molecular mechanisms underlying this change are poorly understood. In this study, we used pH-indicator dyes to demonstrate that vacuolar convolution is accompanied by acidification of the vacuole in fava bean (Vicia faba) guard cells during abscisic acid (ABA)-induced stomatal closure. Vacuolar acidification is necessary for the rapid stomatal closure induced by ABA, since a double mutant of the vacuolar H(+)-ATPase vha-a2 vha-a3 and vacuolar H(+)-PPase mutant vhp1 showed delayed stomatal closure. Furthermore, we provide evidence for the critical role of phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P2] in changes in pH and morphology of the vacuole. Single and double Arabidopsis thaliana null mutants of phosphatidylinositol 3-phosphate 5-kinases (PI3P5Ks) exhibited slow stomatal closure upon ABA treatment compared with the wild type. Moreover, an inhibitor of PI3P5K reduced vacuolar acidification and convolution and delayed stomatal closure in response to ABA. Taken together, these results suggest that rapid ABA-induced stomatal closure requires PtdIns(3,5)P2, which is essential for vacuolar acidification and convolution.

Osmotic stress responses and plant growth controlled by potassium transporters in Arabidopsis

Osmotic adjustment plays a fundamental role in water stress responses and growth in plants; however, the molecular mechanisms governing this process are not fully understood. Here, we demonstrated that the KUP potassium transporter family plays important roles in this process, under the control of abscisic acid (ABA) and auxin. We generated Arabidopsis thaliana multiple mutants for K(+) uptake transporter 6 (KUP6), KUP8, KUP2/SHORT HYPOCOTYL3, and an ABA-responsive potassium efflux channel, guard cell outward rectifying K(+) channel (GORK). The triple mutants, kup268 and kup68 gork, exhibited enhanced cell expansion, suggesting that these KUPs negatively regulate turgor-dependent growth. Potassium uptake experiments using (86)radioactive rubidium ion ((86)Rb(+)) in the mutants indicated that these KUPs might be involved in potassium efflux in Arabidopsis roots. The mutants showed increased auxin responses and decreased sensitivity to an auxin inhibitor (1-N-naphthylphthalamic acid) and ABA in lateral root growth. During water deficit stress, kup68 gork impaired ABA-mediated stomatal closing, and kup268 and kup68 gork decreased survival of drought stress. The protein kinase SNF1-related protein kinases 2E (SRK2E), a key component of ABA signaling, interacted with and phosphorylated KUP6, suggesting that KUP functions are regulated directly via an ABA signaling complex. We propose that the KUP6 subfamily transporters act as key factors in osmotic adjustment by balancing potassium homeostasis in cell growth and drought stress responses.