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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. [ABSTRACT FROM AUTHOR]

Summary: Drought is a major environmental stress that threatens crop production. Therefore, identification of genes involved in drought stress response is of vital importance to decipher the molecular mechanism of stress signal transduction and breed drought tolerance crops, especially for maize.Clade A PP2C phosphatases are core abscisic acid (ABA) signaling components, regulating ABA signal transduction and drought response. However, the roles of other clade PP2Cs in drought resistance remain largely unknown. Here, we discovered a clade F PP2C, ZmPP84, that negatively regulates drought tolerance by screening a transgenic overexpression maize library.Quantitative RT‐PCR indicates that the transcription of ZmPP84 is suppressed by drought stress. We identified that ZmMEK1, a member of the MAPKK family, interacts with ZmPP84 by immunoprecipitation and mass spectrometry analysis. Additionally, we found that ZmPP84 can dephosphorylate ZmMEK1 and repress its kinase activity on the downstream substrate kinase ZmSIMK1, while ZmSIMK1 is able to phosphorylate S‐type anion channel ZmSLAC1 at S146 and T520 in vitro. Mutations of S146 and T520 to phosphomimetic aspartate could activate ZmSLAC1 currents in Xenopus oocytes.Taken together, our study suggests that ZmPP84 is a negative regulator of drought stress response that inhibits stomatal closure through dephosphorylating ZmMEK1, thereby repressing ZmMEK1–ZmSIMK1 signaling pathway. [ABSTRACT FROM AUTHOR]

Summary: Intracellular malate–starch interconversion plays an important role in stomatal movements. We investigated whether malate or oxaloacetate from the cytosolic membrane side regulate anion channels in the plasma membrane of Arabidopsis thaliana guard cells. Physiological concentrations of cytosolic malate have been reported in the range of 0.4–3 mM in leaf cells. Guard cell patch clamp and two‐electrode oocyte voltage‐clamp experiments were pursued. We show that a concentration of 1 mM cytosolic malate greatly activates S‐type anion channels in Arabidopsis thaliana guard cells. Interestingly, 1 mM cytosolic oxaloacetate also activates S‐type anion channels. Malate activation was abrogated at 10 mM malate and in SLAC1 anion channel mutant alleles. Interestingly, malate activation of S‐type anion currents was disrupted at below resting cytosolic‐free calcium concentrations ([Ca2+]cyt), suggesting a key role for basal [Ca2+]cyt signaling. Cytosolic malate was not able to directly activate or enhance SLAC1‐mediated anion currents in Xenopus oocytes, whereas in positive controls, cytosolic NaHCO3 enhanced SLAC1 activity, suggesting that malate may not directly modulate SLAC1. Cytosolic malate activation of S‐type anion currents was impaired in ost1 and in cpk5/6/11/23 quadruple mutant guard cells. Together these findings show that these cytosolic organic anions function in guard cell ‘plasma membrane’ ion channel regulation. [ABSTRACT FROM AUTHOR]

Summary: We conducted an infrared thermal imaging‐based genetic screen to identify Arabidopsis mutants displaying aberrant stomatal behavior in response to elevated concentrations of CO2. This approach resulted in the isolation of a novel allele of the Arabidopsis BIG locus (At3g02260) that we have called CO2insensitive 1 (cis1). BIG mutants are compromised in elevated CO2‐induced stomatal closure and bicarbonate activation of S‐type anion channel currents. In contrast with the wild‐type, they fail to exhibit reductions in stomatal density and index when grown in elevated CO2. However, like the wild‐type, BIG mutants display inhibition of stomatal opening when exposed to elevated CO2. BIG mutants also display wild‐type stomatal aperture responses to the closure‐inducing stimulus abscisic acid (ABA). Our results indicate that BIG is a signaling component involved in the elevated CO2‐mediated control of stomatal development. In the control of stomatal aperture by CO2, BIG is only required in elevated CO2‐induced closure and not in the inhibition of stomatal opening by this environmental signal. These data show that, at the molecular level, the CO2‐mediated inhibition of opening and promotion of stomatal closure signaling pathways are separable and BIG represents a distinguishing element in these two CO2‐mediated responses. [ABSTRACT FROM AUTHOR]

Summary: We conducted an infrared thermal imaging‐based genetic screen to identify Arabidopsis mutants displaying aberrant stomatal behavior in response to elevated concentrations of CO2. This approach resulted in the isolation of a novel allele of the Arabidopsis BIG locus (At3g02260) that we have called CO2insensitive 1 (cis1). BIG mutants are compromised in elevated CO2‐induced stomatal closure and bicarbonate activation of S‐type anion channel currents. In contrast with the wild‐type, they fail to exhibit reductions in stomatal density and index when grown in elevated CO2. However, like the wild‐type, BIG mutants display inhibition of stomatal opening when exposed to elevated CO2. BIG mutants also display wild‐type stomatal aperture responses to the closure‐inducing stimulus abscisic acid (ABA). Our results indicate that BIG is a signaling component involved in the elevated CO2‐mediated control of stomatal development. In the control of stomatal aperture by CO2, BIG is only required in elevated CO2‐induced closure and not in the inhibition of stomatal opening by this environmental signal. These data show that, at the molecular level, the CO2‐mediated inhibition of opening and promotion of stomatal closure signaling pathways are separable and BIG represents a distinguishing element in these two CO2‐mediated responses. [ABSTRACT FROM AUTHOR]

Stomatal pores are formed between a pair of guard cells and allow plant uptake of CO2 and water evaporation. Their aperture depends on changes in osmolyte concentration of guard cell vacuoles, specifically of K+ and Mal2−. Efflux of Mal2− from the vacuole is required for stomatal closure; however, it is not clear how the anion is released. Here, we report the identification of ALMT4 (ALUMINUM ACTIVATED MALATE TRANSPORTER4) as an Arabidopsis thaliana ion channel that can mediate Mal2− release from the vacuole and is required for stomatal closure in response to abscisic acid (ABA). Knockout mutants showed impaired stomatal closure in response to the drought stress hormone ABA and increased whole-plant wilting in response to drought and ABA. Electrophysiological data show that ALMT4 can mediate Mal2− efflux and that the channel activity is dependent on a phosphorylatable C-terminal serine. Dephosphomimetic mutants of ALMT4 S382 showed increased channel activity and Mal2− efflux. Reconstituting the active channel in almt4 mutants impaired growth and stomatal opening. Phosphomimetic mutants were electrically inactive and phenocopied the almt4 mutants. Surprisingly, S382 can be phosphorylated by mitogen-activated protein kinases in vitro. In brief, ALMT4 likely mediates Mal2− efflux during ABA-induced stomatal closure and its activity depends on phosphorylation. [ABSTRACT FROM AUTHOR]