TheSLAC1Anion Channel

Abstract

AbstractThe balance between CO2uptake for photosynthesis and water loss through transpiration determines plant growth and productivity. Guard cells form stomatal pores in plant epidermis that adjust this balance via volume changes that control stomatal aperture. Guard cell volume is regulated by fluxes of ions and water across the membrane via various ion channels. The slow anion channel 1 (SLAC1) is essential for efficient stomatal regulation, as it is the major mediator of the efflux of anions from guard cells during stomatal closure. The activation of SLAC1 requires phosphorylation and is the end point of signalling pathways initiated by abiotic and biotic triggers that cause stomata to close, such as the phytohormone abscisic acid, elevated CO2levels, darkness, low air humidity, ozone; and pathogen and damage‐associated molecular patterns and hormones. SLAC1 is conserved in evolution, but SLACs in different plant groups have different mechanisms of regulation.Key ConceptsSLAC1 is the major guard cell slow‐type anion channel that needs to be activated by phosphorylation in theN‐ andC‐terminal regions for stomatal closure in response to endogenous and environmental stimuli.The plant hormone abscisic acid (ABA) triggers SLAC1 activation through a signalling pathway that involves ABA receptors and PP2C phosphatases, which regulate the Ca2+‐independent kinase OST1 and several Ca2+‐dependent protein kinases that phosphorylate SLAC1. The same pathway is involved in SLAC1 activation in response to darkness, CO2, ozone and low air humidity.Elevated CO2activates SLAC1 via bicarbonate, which directly enhances ion channel activity. The Raf‐type kinase HT1 and mitogen‐activated protein kinases MPK12 and MPK4 regulate SLAC1 activation in response to CO2, but not ABA, contributing to a CO2‐specific branch of SLAC1 activation.Pathogen‐associated molecules trigger the activation of SLAC1 and its homologue SLAH3 that contributes to guard cell anion currents. SLAH3 is the key anion channel in stomatal response to fungal chitin, whereas SLAC1, SLAH3 and OST1 contribute to bacterial flagellin‐induced stomatal closure.SLAC1 and OST1 are conserved in early land plants, such as mosses, lycophytes and ferns, but their role in these plants needs further study. The regulation of SLAC1 anion channel differs among plant groups – for example, the monocot SLACs, unlike Arabidopsis SLAC1, require external nitrate for activation.