Interactions between tobacco mosaic virus and the tobacco N gene

The interaction between tobacco mosaic virus (TMV) and tobacco harbouring t he N gene is a classical system for studying gene-for-gene interactions in disease resistance. The N gene confers resistance to TMV by mediating defen ce responses that function to limit viral replication and movement. We isol ated the N gene and determined that N belongs to the nucleotide-binding-sit e-leucine-rich-repeat (NBS LRR) class of plant disease resistance genes, an d encodes both full-length and truncated proteins. Sequence homologies and mutagenesis studies indicated a signalling role for the N protein similar t o that seen for proteins involved in defence responses in insects and mamma ls. The jy gene confers resistance to TMV in transgenic tomato, demonstrati ng the use of the NBS-LRR class of disease resistance genes in engineering crop resistance. From the pathogen side of this interaction, the TMV 126 kD a replicase protein has been implicated as the avirulence factor that trigg ers N-mediated defence responses. We employed Agrobacterium-mediated expres sion strategies to demonstrate that expression of the putative helicase reg ion of the replicase protein is sufficient to elicit N-mediated defences. T he thermosensitivity of the N-mediated response to TMV is retained when ind uced by expression of this replicase fragment. Thus, both components of thi s gene-for-gene interaction are now available for studies that address the molecular mechanisms involved in N-mediated TMV resistance.