New insights in plant immunity signaling activation

Abstract

Plant disease resistance can be triggered by specific recognition of microbial effectors by plant nucleotide binding-leucine rich repeat (NB-LRR) receptors. Over the last few years, many efforts have greatly improved the understanding of effector and NB-LRR function, but have left a lot of questions as to how effector perception activates NB-LRR induction of defense signaling. This review describes exciting new findings showing similarities and differences in function of diverse plant NB-LRR proteins in terms of pathogen recognition and where and how resistance proteins are activated. Localization studies have shown that some NB-LRRs can activate signaling from the cytosol while others act in the nucleus. Also, the structural determination of two NB-LRR signaling domains demonstrated that receptor oligomerization is fundamental for the activation of resistance signaling.

Figure 1. Different models for NB-LRR signaling activation

Models for activation and signalling are presented for several R proteins to illustrate the potential variability in the mechanism and subcellular location of these events. The R protein N-terminal TIR and CC domains are represented by purple and green ovals respectively, the NB-ARC domain by an orange crescent and the Leucine Rich Repeats by a series of blue ovals. a) In a resting state, Barley MLA navigates between the cytosol and the nucleus as an inactive homodimer interacting through the CC domain. The presence of its corresponding Barley mildew effector AvrA10 induces the accumulation of MLA in the nucleus, nucleotide exchange and conformational changes allowing the interaction of the CC domain with WRKY factors to derepress defense activation. b) The potato CC-NB-LRR Rx is also present in both the nucleus and cytosol. Its nucleocytoplasmic partitioning depends on the trafficking regulator RanGAP2 (purple), which acts as a cytoplasmic retention factor of Rx. The Potato Virus X Coat Protein (CP) is recognized in the cytosol and signaling is activated in this location. The Rx nuclear pool is required for correct regulation of resistance function. This protein is shown as a monomer as no direct evidence of its oligomerisation state is available. c) In the absence of pathogen, L6 is attached to the Golgi membrane through its N-terminal signal anchor (red rectangle). The protein is kept in an inactive state where the TIR domain dimerization interface is not exposed. Upon recognition of the flax rust effector AvrL567, nucleotide exchange and conformational change exposes the TIR domain for homodimerization and interaction with signaling proteins (yellow circles) to activate defense signalling. d) The Arabidopsis RPM1 protein is kept at the plasma membrane in a complex with the effector target RIN4 (yellow) and the protein kinase RIPK (red). The presence of the Pseudomonas effector AvrB induces RIPK and RIN4 phosphorylation. RIN4 modification leads to RPM1 activation and signaling at the plasma membrane. Again, in the absence of direct evidence otherwise, this protein is shown as a monomer..