Characterization of the Role of CSA1 in Maintaining Homeostasis of BAK1-BIR3 Complexes and Triggering Autoimmune Cell Death

Abstract

As a co-receptor of leucine-rich repeat pattern recognition receptors (PRRs), A. tha-liana BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 (BAK1/SERK3) can regulate different signaling pathways including growth and development, immune response, and cell death control by directly interacting with and / or positively regulating multiple ligand binding receptors. The BAK1-interacting receptor kinase BIR3 can prevent the BAK1-ligand binding receptor interactions by directly interacting with both ligand-binding receptors and BAK1 (and all members of the SERK family). The interactome of BIR3 revealed a BIR3 interacting TIR-NBS-LRR (TNL) protein CSA1. Double mutants in bak1 bir3 show a severe dwarf pheno-type and spontaneous cell death. In this study, we found that CSA1 directly interacts with BIR3, and indirectly with BAK1. The double mutant bak1 bir3 shows cell death symptoms and strong dwarfism that are dependent on ETI pathway components ENHANCED DISEASE SUSCEPTIBILITY (EDS1) and PHYTOALEXIN-DEFICIENT 4 (PAD4) and also salicylic acid, but not SENESCENCE-ASSOCIATED GENE 101 (SAG101). We also find that NRG1 has a slight contribution to the growth phenotype of bak1 bir3. CSA1 mediates cell death in bak1-4 and bak1-4 bir3-2 mutants via ef-fector-triggered immunity (ETI) pathways components. Thus, we propose that CSA1 guards homeostasis of BIR3 BAK1 proteins and initiates autoimmune cell death that is observed when BAK1 BIR complexes are impaired. The absence of intact BAK1 and BIR3 activates CSA1-triggering ETI cell death. This suggests that downstream of BAK1 defense responses are activated by both PTI and ETI pathways for efficient plant immunity.