PHENOTYPIC-EXPRESSION OF PSEUDOMONAS-SYRINGAE AVR GENES IN ESCHERICHIA-COLI IS LINKED TO THE ACTIVITIES OF THE HRP-ENCODED SECRETION SYSTEM

The specific recognition of elicitors produced by plant pathogenic bac teria carrying avirulence (avr) genes is postulated to initiate cellul ar defense responses in plants expressing corresponding resistance gen es, The biochemical functions of most avr genes, hoc-ever, are not kno wn. A heterologous system was developed to phenotypically express Pseu domonas syringae avr genes in Escherichia coli cells that required the II syringae hip cluster, E, coli MC4100 transformants carrying the pl asmid-borne P. syringae pv, syringae Pss61 hip duster and Il syringae pv. glycinea avrB expressed from a triple lacUV5 promoter gained the a bility to elicit the hypersensitive response in soybean cultivars expr essing Rpg1 and in an Arabidopsis thaliana accession expressing RPM1. Inactivation of energy transducing or outer membrane components of the hip-encoded secretion system blocked phenotypic expression of avrB in E, coli, but deletions abolishing harpin(Pss), production had little effect on the production of the AvrB phenotype by the E. coli transfor mants. Phenotypic expression of avrA, avrPto, avrRpm1, avrRpt2, and av rPph3 in E. coli was also shown to require the hip cluster, The result s indicate that generation of the Avr phenotype in E! syringae strains is specifically dependent on the secretion activities of the hrp clus ter.