Summary During infection plants recognize microbe‐associated molecular patterns (MAMPs), and this leads to stomatal closure. This study analyzes the molecular mechanisms underlying this MAMP response and its interrelation with ABA signaling.Stomata in intact Arabidopsis thaliana plants were stimulated with the bacterial MAMP flg22, or the stress hormone ABA, by using the noninvasive nanoinfusion technique. Intracellular double‐barreled microelectrodes were applied to measure the activity of plasma membrane ion channels.Flg22 induced rapid stomatal closure and stimulated the SLAC1 and SLAH3 anion channels in guard cells. Loss of both channels resulted in cells that lacked flg22‐induced anion channel activity and stomata that did not close in response to flg22 or ABA. Rapid flg22‐dependent stomatal closure was impaired in plants that were flagellin receptor (FLS2)‐deficient, as well as in the ost1‐2 (Open Stomata 1) mutant, which lacks a key ABA‐signaling protein kinase. By contrast, stomata of the ABA protein phosphatase mutant abi1‐1 (ABscisic acid Insensitive 1) remained flg22‐responsive.These data suggest that the initial steps in flg22 and ABA signaling are different, but that the pathways merge at the level of OST1 and lead to activation of SLAC1 and SLAH3 anion channels.

SUMMARYStomatal closure is known to be associated with early defence responses of plant cells triggered by microbeassociated molecular patterns (MAMPs). However, the molecular mechanisms underlying these guard-cell responses have not yet been elucidated. We therefore studied pathogen-induced changes in ion channel activity in Hordeum vulgare guard cells. Barley mildew (Blumeria graminis) hyphae growing on leaves inhibited light-induced stomatal opening, starting at 9 h after inoculation, when appressoria had developed. Alternatively, stomatal closure was induced by nano-infusion of chitosan via open stomata into the substomatal cavity. Experiments using intracellular double-barreled micro-electrodes revealed that mildew stimulated S-type (slow) anion channels in guard cells. These channels enable the efflux of anions from guard cells and also promote K + extrusion by altering the plasma membrane potential. Stimulation of S-type anion channels was also provoked by nano-infusion of chitosan. These data suggest that MAMPs of mildew hyphae penetrating the cuticle provoke activation of S-type anion channels in guard cells. In response, guard cells extrude K + salts, resulting in stomatal closure. Plasma membrane anion channels probably represent general targets of MAMP signaling in plants, as these elicitors depolarize the plasma membrane of various cell types.

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