Aquaporins Facilitate Hydrogen Peroxide Entry Into Guard Cells To Mediate Aba- And Pathogen-Triggered Stomatal Closure

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. Pre-treatment 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 amechanismwhereby 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 plasmamembrane aquaporins in guard cells and potentially in other cellular context involving H2O2 signaling.