A resistance gene product of the nucleotide binding site - leucine rich repeats class can form a complex with bacterial avirulence proteins in vivo

Resistance (R) genes in plants mediate gene-for-gene disease resistance. Th e ligand-receptor model, which explains the gene-for-gene specificity, pred icts a physical interaction between an elicitor, which is directly or indir ectly encoded by an avirulence (avr) gene in the pathogen, and the correspo nding R gene product. The nucleotide binding site (NBS) - leucine rich repe ats (LRR) class of R genes is the largest known class of R genes. Here we r eport that an NBS-LRR R protein and its cognate Avr protein form a complex together in the plant cell. The Arabidopsis thaliana R genes RPS2 and RPM1 confer gene-for-gene disease resistance to strains of the phytopathogenic b acterium Pseudomonas syringae carrying the avr genes avrRpt2 and avrB, resp ectively. Using transient expression of these genes in Arabidopsis leaf mes ophyll protoplasts, we first demonstrated that the protoplast system is app ropriate for the investigation of the gene-for-gene recognition mechanism. Formation of an in vivo complex containing the RPS2 and AvrRpt2 proteins wa s demonstrated by co-immunoprecipitation of the proteins following expressi on of the genes in protoplasts. This complex contained at least one additio nal plant protein of approximately 75 kDa. Unexpectedly, RPS2 also formed a complex with AvrB. We speculate that complex formation between AvrRpt2 and RPS2 is productive and leads to the elicitation of the resistance response , whilst complex formation between AvrB and RPS2 is unproductive and possib ly competes with complex formation between AvrRpt2 and RPS2.