Fig 1.
Schematic model of RNL activation and function during Arabidopsis innate immunity.
Resting state: In the absence of pathogens, RNLs exist likely as inactive monomers and in close proximity to cell surface-localized PRRs (including RLPs and RLKs). RNLs localize at the PM via direct interaction of their CCR domains with the anionic phospholipid Pi4P. Intermediate state: The perception of PAMPs by PRRs leads to the recruitment of a co-receptor and activates immune signaling by auto- and transphosphorylation. Components of the activated PRR core-complex may phosphorylate RNLs and “prime” them prior to full activation. Components of the activated PRR core-complex may also associate and activate downstream TNLs. TNLs, however, are also activated by recognition of pathogen-derived effector proteins. Activated TNLs assemble into a tetrameric complex. TIR domains have enzymatic activity and produce small signaling molecules that can bind directly to EDS1 heterodimers. Binding of these signaling molelcules to the EDS1-PAD4 heterodimer was shown to cause a conformational change in PAD4 that promotes the interaction with RNLs and may drive their full activation. Active state: Activation of RNLs may lead to the exposure of their N-terminal α1-helix that could trigger the dissociation of the EDS1 heterodimer and the oligomerization of RNLs into a PM-associated resistosome. Activated RNLs promote cation influx (directly or indirectly) that leads to immune responses. It may also be possible that activated RNLs are involved in the release of host peptides/molecules that could activate immune signaling in neighboring cells. PAMP, pathogen-associated molecular pattern; PM, plasma membrane; PRR, pattern-recognition receptor; RLP, receptor-like protein; TIR, Toll-like/interleukin-1 receptor resistance.