The N resistance protein recognizes its tobacco mosaic virus elicitor through a novel mechanism

In their natural environments plants are constantly attacked by a variety of pathogens. Plants have therefore evolved diverse defense mechanisms that allow them to evade the assaults. One strategy used by plants relies on germ-line encoded molecules encoded by Resistance (R) genes that specifically recognize the presence of gene products of corresponding Avirulence (Avr) gene carried by a given pathogen and then initiate defense responses. To date, more than forty R genes, which confer resistance to all classes of plant pathogens, have been cloned. R gene products show striking similarity. They contain a few conserved domains including the TIR, CC, NB and LRR domains. These domains are also found in animal innate immune molecules, especially the TLRs and NODs. The mechanism that R proteins use to recognize pathogen-derived Avr proteins and then to initiate defense signaling was investigated. The tobacco model system, which consists of the tobacco N gene and the tobacco mosaic virus replicase proteins, was used in these studies. N is a TIR-NB-LRR type R protein. Yeast screens with the LRR domain of N and the full length N protein identified several potential interaction partners for N including HSP90, PP5 and SPL transcription. These and proteins that they are known to associate with were examined for their roles in N-mediated resistance using VIGS. All except PP5 were required for N function. N was found to be a cytoplasmic and, to a lesser extent, nuclear protein. The localization of N does not change on infection with TMV or in the presence of its Avr protein. N was also found to associate in vivo with its Avr elicitor, the 50-kDa helicase domain of the viral replicase, p50. This is the first report of an in vivo association between an R and Avr protein. Surprisingly, the TIR domain of N is necessary and sufficient for this association. These data have been integrated into existing models for R-Avr interaction and used to develop a new model for the function of N. The results presented here also suggest novel functions for the conserved TIR domain and indeed for R proteins.