Plants lack an adaptive immune system, so each cell must mount an individual defence upon infection. A successful defence against pathogens limits damage to a few cells at the site of infection, but if defences fail, pathogens can spread throughout the plant. There are two defence systems in plant cells. The first line of defence comprises transmembrane pattern-recognition receptors, which respond to conserved pathogen signals, such as flagellin. The second line of defence involves cytoplasmic nucleotide-binding leucine-rich-repeat (NB-LRR) proteins, which are related to animal Nod (nucleotide-binding oligomerization domain) proteins and are encoded by plant R (resistance) genes. There are two main classes of NB-LRRs: CC-NB-LRRs have a coiled–coil domain and TIR-NB-LRRs have a Toll-interleukin-1 domain. Although both classes of NB-LRRs can interact directly with pathogen proteins, many NB-LRR proteins 'guard' crucial host proteins and detect interactions that are made between these proteins and pathogen virulence proteins.
Previous research from the Dinesh-Kumar laboratory indicated that TIR-NB-LRR immune-receptor-protein N interacts indirectly with the TMV p50 helicase protein to mediate an innate immune response, and that the TIR domain of N is indispensable for pathogen detection. But how does the N protein detect p50? Caplan et al. first used the immune protein N as bait in a yeast two-hybrid assay to search for host proteins that it might interact with. They pulled out a chloroplast protein that they named N receptor interacting protein 1 (NRIP1). In vitro (yeast two hybrid) and in vivo analyses (co-immunoprecipitation of tagged proteins) confirmed that N binds NRIP1. Silencing NRIP1 with RNA interference rendered plants susceptible to TMV, which showed that NRIP1 was necessary for the N-mediated immune response to this virus. Finally, NRIP1 was shown to interact with p50 in vitro and in vivo, which led the authors to propose that N guards the host protein NRIP1 to mediate an antiviral immune response.
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