Developments in the interaction of bacterial avirulence genes and plant disease resistance genes

Abstract

We attempted to clone all of the avirulence genes from Pseudomonas syringae pv. tomato that cause it to give a non-host hypersensitive response (HR) in soybean plants. Three different classes of clones were isolated from a P.s. tomato cosmid library which functioned in P. syringae pv. glycinea to elicit the HR in soybean. One clone, avrA, was identical to a gene previously cloned from race 6 of P.s. glycinea. The second locus, avrE, occurs adjacent to the hrpS end of the hrp cluster of P.s. tomato and is complex, requiring two divergent transcriptional units for activity. The third gene, avrD, functions by causing bacterial hosts to secrete a low molecular weight lactone elicitor that is specifically perceived by soybean plants carrying the matching Rpg4 disease resistance gene. Ronald et al.. [J. Bacteriol. 174, 1604 (1992)] showed that P.s. tomato also contains an avirulence gene, avrPto, that functions in tomato cultivars carrying the Pto disease resistance gene and in all tested soybean cultivars except Peking. A P.s. tomato strain mutated in avrA, avrD, avrE and avrPto yielded a null reaction on the soybean cultivar Peking, but gave HR on all other soybean cultivars tested. This mutant also exhibited reduced virulence on the normal host plant, tomato. The results suggest that P.s. tomato carries several avirulence genes which may contribute to virulence on the normal host plant and are also responsible for elicitation of the non-host HR on soybean plants.KeywordsHypersensitive ResponseTobacco Mosaic VirusCoat Protein GeneSoybean CultivarDisease Resistance GeneThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.