Humidity-driven ABA depletion determines plant-pathogen competition for leaf water

Bacterial phytopathogens, such as Pseudomonas syringae pv. tomato (Pst) DC3000, induce water-soaked lesions in the leaf apoplast under high humidity, facilitating infection. However, it remains largely unclear how plants regulate their resistance to restrict bacterial infection in response to humidity. Here, we demonstrate that abscisic acid (ABA)-catabolizing ABA 8'-hydroxylase, encoded by CYP707A3, plays a critical role in this resistance in Arabidopsis thaliana. Elevated humidity induces CYP707A3 expression, which is essential for reducing ABA levels and promoting stomatal opening, thereby limiting bacterial water-soaking and infection following leaf invasion. High humidity also increases cytosolic Ca²⁺ levels via the Ca²⁺ channels CNGC2 and CNGC4, with partial involvement from CNGC9, activating the calmodulin-binding transcription activator CAMTA3 to drive CYP707A3 induction. However, Pst DC3000 counteracts this defense response using type III secretion effectors, including AvrPtoB, facilitating water-soaking. These findings provide insights into the mechanisms underlying the competition between plants and bacteria for leaf water under elevated humidity.