Schematic Illustration of the Regulating Processes at the Guard Cells (IMAGE)
Caption
Involvement of the ABA receptors in ABA- and CO2-induced closure of the stoma. When in drought stress mode, the plant produces large amounts of the ABA hormone that acts on the guard cells (left, left guard cell) and binds to the primary player, PYL2, and the secondary player, PYR1. High CO2 levels (left, right guard cells) cause PYL4, 5 and to some extent PYR1 to become more sensitive and bind ABA at low intracellular concentrations. In both cases, the ABI1 phosphatase is bound, enabling the OST1 kinase to first activate itself and then the SLAC1 anion channel. The resulting efflux of anions depolarises the membrane potential, which in turn activates the GORK potassium output channel. The efflux of osmotically active salts causes water to flow out of the guard cells. They shrink and the stoma closes (right).
Credit
Heike Mueller & Peter Ache / University of Wuerzburg
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