A Novel Role for the TIR Domain in Association with Pathogen-Derived Elicitors

Abstract

Plant innate immunity is mediated by Resistance (R) proteins, which bear a striking resemblance to animal molecules of similar function. Tobacco N is a TIR-NB-LRR R gene that confers resistance to Tobacco mosaic virus, specifically the p50 helicase domain. An intriguing question is how plant R proteins recognize the presence of pathogen-derived Avirulence (Avr) elicitor proteins. We have used biochemical cell fraction and immunoprecipitation in addition to confocal fluorescence microscopy of living tissue to examine the association between N and p50. Surprisingly, both N and p50 are cytoplasmic and nuclear proteins, and N's nuclear localization is required for its function. We also demonstrate an in planta association between N and p50. Further, we show that N's TIR domain is critical for this association, and indeed, it alone can associate with p50. Our results differ from current models for plant innate immunity that propose detection is mediated solely through the LRR domains of these molecules. The data we present support an intricate process of pathogen elicitor recognition by R proteins involving multiple subcellular compartments and the formation of multiple protein complexes. Each year, up to 10% of world agricultural production is lost to pests and diseases caused by a variety of pathogens including bacteria, fungi, nematodes, and viruses. Scientists have understood for nearly a century that plants carry their own immune system that actively engages pathogens and prevents many infections. One aspect of the plant immune system is defined by the gene-for-gene hypothesis: a plant Resistance (R) gene encodes a protein that specifically recognizes and protects against one pathogen or strain of a pathogen carrying a corresponding Avirulence (Avr) gene. In tobacco and its relatives, the N resistance protein confers resistance to infection by the Tobacco mosaic virus (TMV). We have used N, and the TMV elicitor, p50, to investigate the mechanism of gene-for-gene resistance. We show that N and p50 associate in the cytoplasm and nucleus of plant cells and that this association is mediated by N's TIR domain, which is structurally similar to animal innate immunity molecules. Our findings provide novel insight into how R proteins recognize pathogen Avr proteins, and should help in long-term efforts to enhance crop yield.