Stomatal movements are crucial for the control of plant water status and protection against pathogens. Assays on epidermal peels revealed that, similar to abscisic acid (ABA), pathogen-associated molecular pattern (PAMP) flg22 requires the AtPIP2;1 aquaporin to induce stomatal closure. Flg22 also induced an increase in osmotic water permeability (P_f) of guard cell protoplasts through activation of AtPIP2;1. The use of HyPer, a genetic probe for intracellular hydrogen peroxide (H₂O₂), revealed that both ABA and flg22 triggered an accumulation of H₂O₂ in wild-type but not pip2;1 guard cells. Pretreatment of guard cells with flg22 or ABA facilitated the influx of exogenous H₂O₂ Brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) and open stomata 1 (OST1)/Snf1-related protein kinase 2.6 (SnRK2.6) were both necessary to flg22-induced P_f and both phosphorylated AtPIP2;1 on Ser121 in vitro. Accumulation of H₂O₂ and stomatal closure as induced by flg22 was restored in pip2;1 guard cells by a phosphomimetic form (Ser121Asp) but not by a phosphodeficient form (Ser121Ala) of AtPIP2;1. We propose a mechanism whereby phosphorylation of AtPIP2;1 Ser121 by BAK1 and/or OST1 is triggered in response to flg22 to activate its water and H₂O₂ transport activities. This work establishes a signaling role of plasma membrane aquaporins in guard cells and potentially in other cellular context involving H₂O₂ signaling.

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