An activated form of NB-ARC protein RLS1 functions with cysteine-rich receptor-like protein RMC to trigger cell death in rice

A key feature following pathogen recognition by resistance (R) protein containing NB-ARC (nucleotide-binding adaptor shared by Apaf-1, R proteins, and Ced-4) domain is the hypersensitive response (HR) type cell death accompanied by the accumulation of reactive oxygen species and nitric oxide. However, the underpinning mechanisms integral to this process remain relatively opaque. Here we show that a gain-of-function mutation in the NB-ARC protein, RLS1 (Rapid Leaf Senescence 1), triggers high-light-dependent HR-like cell death in rice. The RLS1-mediated defense response is largely independent of salicylic acid accumulation, NPR1 (Nonexpressor of Pathogenesis-Related Gene 1) activity and RAR1 (Required for Mla12 Resistance 1) function. A screen for suppressors of RLS1 activation identified RMC (Root Meander Curling) is essential for the RLS1 activated defense response. RMC encodes a cysteine-rich receptor-like secreted protein (CRRSP) and functions as a RLS1-binding partner. Intriguingly, their co-expression resulted in a change in the pattern of subcellular localization and was sufficient to trigger cell death accompanied by a decrease in the activity of the antioxidant enzyme, APX1. Collectively, our findings reveal a NB-ARC - CRRSP signaling module that modulates oxidative state, cell death process, and associated immunity responses in rice.