The conformational and subcellular compartmental dance of plant NLRs during viral recognition and defense signaling

Abstract

Plant innate immune response against viruses utilizes intracellular Nucleotide Binding domain Leucine Rich Repeat (NLR) class of receptors. NLRs recognize different viral proteins termed elicitors and initiate diverse signaling processes that induce programmed cell death (PCD) in infected cells and restrict virus spread. In this review we describe the recent advances made in the study of plant NLRs that detect viruses. We describe some of the physical and functional interactions these NLRs undertake. We elaborate on the intra-molecular and homotypic association of NLRs that function in self-regulation and activation. Nuclear role for some viral NLRs is discussed as well as the emerging importance of the RNAi pathway in regulating the NLR family.

Figure 1. Activation and regulation of nuclear signaling by some viral NLRs

In the absence of the viral elicitor, NLRs are in an inactive state. In the case of Rx NLR, the association between LRR and CC-NB domain achieves this. For others like N, the mode of inhibition is still unknown. Different NLRs sense the viral elicitor by different mechanisms but in all cases, it is shown or predicted that ATP hydrolysis at the NB domain, shifts its conformation into an open, signaling competent state. In this state the NLR can signal for the activation of defense gene transcription (green arrows) in the nucleus. This is achieved in different ways. a) N is a nucleo-cytoplasmic protein. In the presence of the viral elicitor TMV p50-U1, N associates with the host protein NRIP1-p50 complex in the cytoplasm. Subsequent ATP hydrolysis and homotypic associations between TIR domains lead to N activation. Activated N (either as a monomer or a dimer/oligomer) associates with the SPL6 transcription factor in the nucleus to modulate defense gene transcription Nucleo-cytoplasmic immune modulator EDS1 is required for N function. EDS1 is known to associate with PAD4 and SAG101 in the nucleus. However, EDS1 interaction with N in the cytoplasm and/or its interaction with SPL6 in the nucleus in unknown. b) Rx is also a nucleo-cytoplasmic protein whose localization is regulated by the RanGAP2. The CP of PVX disrupts Rx intramolecular associations thus initiating activation. The activated Rx may associate or regulate the function of unknown transcription factors/transcriptional modulators in the nucleus to promote defense gene transcription. c) RCY1 may regulate nuclear gene transcription by modulating its association with the WRKY70 transcription factor. The location and dynamics of this association are unknown. d) Activated HRT may regulate transcription by modulating the activity of immune modulators like EDS1 (with which it associates) or SAG101 and PAD4 which are required for HRT–mediated immunity.