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 CO_(2)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,CO_(2),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.

Reactive oxygen species signaling and stomatal movement in plant responses to drought stress and pathogen attack

Stomata, the pores formed by a pair of guard cells, are the main gateways for water transpiration and photosynthetic CO2 exchange, as well as pathogen invasion in land plants. Guard cell movement is regulated by a combination of environmental factors, including water status, light, CO2 levels and pathogen attack, as well as endogenous signals, such as abscisic acid and apoplastic reactive oxygen species （ROS）. Under abiotic and biotic stress conditions, extracellular ROS are mainly produced by plasma membrane-localized NADPH oxidases, whereas intracellular ROS are produced in multiple organelles. These ROS form a sophisticated cellular signaling network, with the accumulation of apoplastic ROS an early hallmark of stomatal movement. Here, we review recent progress in understanding the molecular mechanisms of the ROS signaling network, primarily during drought stress and pathogen attack. We summarize the roles of apoplastic ROS in regulating stomatal movement, ABA and CO2 signaling, and immunity responses. Finally, we discuss ROS accumulation and communication between organelles and cells. This information provides a conceptual framework for understanding how ROS signaling is integrated with various signaling pathways during plant responses to abiotic and biotic stress stimuli.

SCAB1 coordinates sequential Ca^(2+) and ABA signals during osmotic stress induced stomatal closure in Arabidopsis

Hyperosmotic stress caused by drought is a detrimental threat to plant growth and agricultural productivity due to limited water availability.Stomata are gateways of transpiration and gas exchange,the swift adjustment of stomatal aperture has a strong influence on plant drought resistance.Despite intensive investigations of stomatal closure during drought stress in past decades,little is known about how sequential signals are integrated during complete processes.Here,we discovered that the rapid Ca^(2+) signaling and subsequent abscisic acid(ABA)signaling contribute to the kinetics of both F-actin reorganizations and stomatal closure in Arabidopsis thaliana,while STOMATAL CLOSURE-RELATED ACTIN BINDING PROTEIN1(SCAB1)is the molecular switch for this entire process.During the early stage of osmotic shock responses,swift elevated calcium signaling promotes SCAB1 phosphorylation through calcium sensors CALCIUM DEPENDENT PROTEIN KINASE3(CPK3)and CPK6.The phosphorylation restrained the microfilament binding affinity of SCAB1,which bring about the Factin disassembly and stomatal closure initiation.As the osmotic stress signal continued,both the kinase activity of CPK3 and the phosphorylation level of SCAB1 attenuated significantly.We further found that ABA signaling is indispensable for these attenuations,which presumably contributed to the actin filament reassembly process as well as completion of stomatal closure.Notably,the dynamic changes of SCAB1 phosphorylation status are crucial for the kinetics of stomatal closure.Taken together,our results support a model in which SCAB1 works as a molecular switch,and directs the microfilament rearrangement through integrating the sequentially generated Ca^(2+) and ABA signals during osmotic stress induced stomatal closure.

Small holes,big impact:Stomata in plant–pathogen–climate epic trifecta

The regulation of stomatal aperture opening and closure represents an evolutionary battle between plants and pathogens,characterized by adaptive strategies that influence both plant resistance and pathogen virulence.The ongoing climate change introduces further complexity,affecting pathogen invasion and host immunity.This review delves into recent advances on our understanding of the mechanisms governing immunity-related stomatal movement and patterning with an emphasis on the regulation of stomatal opening and closure dynamics by pathogen patterns and host phytocytokines.In addition,the review explores how climate changes impact plant–pathogen interactions by modulating stomatal behavior.In light of the pressing challenges associated with food security and the unpredictable nature of climate changes,future research in this field,which includes the investigation of spatiotemporal regulation and engineering of stomatal immunity,emerges as a promising avenue for enhancing crop resilience and contributing to climate control strategies.

Extracellular ATP Promotes Stomatal Opening of Arabidopsis thaliana through Heterotrimeric G Protein α Subunit and Reactive Oxygen Species

In recent years, adenosine tri-phosphate （ATP） has been reported to exist in apoplasts of plant cells as a signal molecule. Extracellular ATP （eATP） plays important roles in plant growth, development, and stress tolerance. Here, extra- cellular ATP was found to promote stomatal opening of Arabidopsis thaliana in light and darkness. ADP, GTP, and weakly hydrolyzable ATP analogs （ATPγS, Bz-ATP, and 2meATP） showed similar effects, whereas AMP and adenosine did not affect stomatal movement. Apyrase inhibited stomatal opening. ATP-promoted stomatal opening was blocked by an NADPH oxidase inhibitor （diphenylene iodonium） or deoxidizer （dithiothreitol）, and was impaired in null mutant of NADPH ox- idase （atrbohD/F）. Added ATP triggered ROS generation in guard cells via NADPH oxidase. ATP also induced Ca^2＋ influx and H ＋ efflux in guard cells. In atrbohD/F, ATP-induced ion flux was strongly suppressed. In null mutants of the heterotrimeric G protein α subunit, ATP-promoted stomatal opening, cytoplasmic ROS generation, Ca^2＋ influx, and ^H＋ efflux were all sup- pressed. These results indicated that eATP-promoted stomatal opening possibly involves the heterotrimeric G protein, ROS, cytosolic Ca^2＋, and plasma membrane H＋-ATPase.

Protein Tyrosine Phosphatases Mediate the Signaling Pathway of Stomatal Closure of Vicia faba L.

The regulation of stomatal movement is one of the most important signaling networks in plants. The H+-ATPase at the plasma membrane of guard cells plays a critical role in the stomata opening, while there are some conflicting results regarding the effectiveness of the plasma membrane H+-ATPase inhibitor, vanadate, in inhibiting stomata opening. We observed that 2 mmol/L vanadate hardly inhibited light-stimulated stomata opening in epidermal peels of Vicia faba L., but significantly inhibited dark- and ABA-induced stomatal closure. These results cannot be explained with the previous findings that H+-ATPase was inhibited by vanadate. In view of the fact that vanadate is an inhibitor of protein tyrosine phosphatases (PTPases), we investigated whether the stomatal movement regulated by vanadate is through the regulation of PTPase. As expected, phenylarsine oxide (PAO), a specific inhibitor of PTPase, has very similar effects and even more effective than vanadate. Typical PTPase activity was found in guard cells of V. faba; moreover, the phosphatase activity could be inhibited by both vanadate and PAO. These results not only provide a novel explanation for conflicting results about vanadate modulating stomatal movement, but also provide further evidence for the involvement of PTPases in modulating signal transduction of stomatal movement.

Nitric Oxide Blocks Blue Light-Induced K^+ Influx by Elevating the Cytosolic Ca^(2+) Concentration in Vicia faba L. Guard Cells

Ca2＋ plays a pivotal role in nitric oxide （NO）-promoted stomatal closure. However, the function of Ca2＋ in NO inhibition of blue light （BL）-induced stomatal opening remains largely unknown. Here, we analyzed the role of Ca2＋ in the crosstalk between BL and NO signaling in Vicia faba L. guard cells. Extracellular Ca2＋ modulated the BL-induced stomatal opening in a dose-dependent manner, and an application of 5 μM Ca2＋ in the pipette solution significantly inhibited BL-activated K＋ influx. Sodium nitroprusside （SNP）, a NO donor, showed little effect on BL-induced K＋ influx and stomatal opening response in the absence of extracellular Ca2＋ , but K＋ influx and stomatal opening were inhibited by SNP when Ca2＋ was added to the bath solution. Interestingly, although both SNP and BL could activate the plasma membrane Ca2＋ channels and induce the rise of cytosolic Ca2＋ , the change in levels of Ca2＋ channel activity and cytosolic Ca2＋ concentration were different between SNP and BL treatments. SNP at 100 μM obviously activated the plasma membrane Ca2＋ channels and induced cytosolic Ca2＋ rise by 102.4%. In contrast, a BL pulse （100 μmol/m 2 per s for 30 s） slightly activated the Ca2＋ channels and resulted in a Ca2＋ rise of only 20.8%. Consistently, cytosolic Ca2＋ promoted K＋ influx at 0.5 μM or below, and significantly inhibited K＋ influx at 5 μM or above. Taken together, our findings indicate that Ca2＋ plays dual and distinctive roles in the crosstalk between BL and NO signaling in guard cells, mediating both the BL-induced K＋ influx as an activator at a lower concentration and the NO-blocked K＋ influx as an inhibitor at a higher concentration.