Arabidopsis BAK1/SERK3, a co-receptor of leucine-rich repeat pattern recognition receptors (PRR), mediates pattern-triggered immunity (PTI). Genetic inactivation of BAK1 or BAK1-interacting receptors (BIR) causes cell death. We foundthat the TIR-NBS-LRR protein CONSTITUTIVE SHADE-AVOIDANCE 1 (CSA1) physically interacts with BIR3, but not with BAK1 and mediates ETI-type cell death in the absence of BAK1 and BIR3.