A sensitive genetic screen reveals that BAK1 kinase activity is required for its functions in plant immunity, development and cell death control

Abstract To dissect the signaling networks that orchestrate immune gene expression, we developed a sensitive genetic screen using an ethyl methanesulfonate (EMS)-mutagenized population of Arabidopsis transgenic plants. These plants carry an early immune responsive gene FRK1 (flg22-induced receptor-like kinase 1) promoter fused with a luciferase (LUC) reporter. A series of mutants with altered pFRK1::LUC activity were identified and named as Arabidopsis g enes g overning i mmune gene e xpression ( aggie ). The aggie5 mutant showed reduced pFRK1::LUC induction in response to multiple MAMPs treatments. Moreover, the activation of the endogenous FRK1 gene by MAMPs was also compromised in aggie5 . Significantly, the bacterial flagellin epitope flg22-induced MAPK activation, ROS production, and immune-related gene expression were reduced in aggie5 . Map-based cloning coupled with next-generation sequencing revealed that aggie5 encodes a kinase-inactive BAK1, which is involved in brassinosteroid responses, innate immunity, and cell death control. Consistently, the aggie5/bak1-16 mutant displayed compromised responses to BR treatment. Furthermore, the aggie5/bak1-16 mutant exhibited seedling lethality when combined with the mutation of its closest homolog BKK1/SERK4. The data suggest that BAK1 kinase activity is required for its multiple functions in plant immunity, development, and cell death control. The isolation and characterization of these Aggie genes will contribute to a better understanding of host immune signaling and provide genetic resources to improve crop resistance.