Atypical Resistance Protein RPW8/HR Triggers Oligomerization of the NLR Immune Receptor RPP7 and Autoimmunity.

In certain plant hybrids, immunity signaling is initiated when immune components interact in the absence of a pathogen trigger. In Arabidopsis thaliana, such autoimmunity and cell death are linked to variants of the NLR RPP7 and the RPW8 proteins involved in broad-spectrum resistance. We uncover the molecular basis for this autoimmunity and demonstrate that a homolog of RPW8, HR4, can trigger the assembly of a higher-order RPP7 complex, with autoimmunity signaling as a consequence. HR4-mediated RPP7 oligomerization occurs via the RPP7 C-terminal leucine-rich repeat (LRR) domain and ATP-binding P-loop. RPP7 forms a higher-order complex only in the presence of HR4 and not with the standard HR4 variant, which is distinguished from HR4 by length variation in C-terminal repeats. Additionally, HR4 can independently form self-oligomers, which directly kill cells in an RPP7-independent manner. Our work provides evidence for a plant resistosome complex and the mechanisms by which RPW8/HR proteins trigger cell death.