Loss of the common immune coreceptor BAK1 leads to NLR-dependent cell death.

Abstract

Plants utilize a two-tiered immune system consisting of pattern recognition receptor (PRR)-triggered immunity (PTI) and effector-triggered immunity (ETI) to defend themselves against pathogenic microbes. The receptor protein kinase BAK1 plays a central role in multiple PTI signaling pathways in <i>Arabidopsis</i> However, double mutants made by <i>BAK1</i> and its closest paralog <i>BKK1</i> exhibit autoimmune phenotypes, including cell death resembling a typical nucleotide-binding leucine-rich repeat protein (NLR)-mediated ETI response. The molecular mechanisms of the cell death caused by the depletion of BAK1 and BKK1 are poorly understood. Here, we show that the cell-death phenotype of <i>bak1 bkk1</i> is suppressed when a group of <i>NLR</i>s, <i>ADR1</i>s, are mutated, indicating the cell-death of <i>bak1 bkk1</i> is the consequence of NLR activation. Furthermore, introduction of a <i>Pseudomonas syringae</i> effector HopB1, which proteolytically cleaves activated BAK1 and its paralogs via either gene transformation or bacterium-delivery, results in a cell-death phenotype in an <i>ADR1</i>s-dependent manner. Our study thus pinpoints that BAK1 and its paralogs are likely guarded by NLRs.