Genetically determined leaf age-dependent variation in stress hormone crosstalk balances trade-offs between biotic and abiotic stresses in Arabidopsis

In nature plants must respond to multiple stresses simultaneously, which likely demands crosstalk between stress response pathways to minimize fitness costs. Here we provide genetic evidence that biotic and abiotic stress crosstalk is differentially prioritized in leaves of different ages in Arabidopsis thaliana to maintain fitness under combined stresses. Abiotic stress dampened immune responses to bacterial and filamentous eukaryotic pathogens in old rosette leaves through abscisic acid signaling, whereas this antagonistic effect was cancelled in young rosette leaves dependently on PBS3, a signaling component of salicylic acid. Plants lacking PBS3 exhibited enhanced abiotic stress tolerance at the cost of decreased fitness under combined biotic and abiotic stress exposure. Furthermore, PBS3 is indispensable for the establishment of salt stress- and leaf age-dependent phyllosphere bacterial communities. Collectively, our work reveals a mechanism that balances trade-offs upon conflicting stresses and identifies a genetic intersection between immunity, the leaf microbiota, and abiotic stress tolerance.