Phenotypic Expression of Pseudomonas syringae avr Genes in E. colils Linked to the Activities of the hrp-Encoded Secretion System. Minna U. Pirhonen. Department of Plant Biology, University of Maryland at College Park, College Park, MD 20742 U.S.A. Michael C. Lidell (1), Daniel L. Rowley (1), SeonWoo Lee (2), Songmu Jin(1), Yingqi Liang (1), Sara Silverstone(1), NoelT. Keen(2), and Steven W. Hutcheson(1,3). (1) Department of Plant Biology, University of Maryland at College Park, College Park, MD 20742 U.S.A. (2)Department of Plant Pathology, University of California, Riverside, CA 92521 U.S.A. (3) Center for Agicultural Biotechnology of the University of Maryland Biotechnology Institute, College Park, MD 20742 U.S.A. MPMI 9:252-260. Accepted 5 March 1996. Copyright 1996 The American Phytopathological Society.

The specific recognition of elicitors produced by plant pathogenic bacteria carrying avirulence (avr) genes is postulated to initiate cellular defense responses in plants expressing corresponding resistance genes. The biochemical functions of most avr genes, however, are not known. A heterologous system was developed to phenotypically express Pseudomonas syringae avr genes in Escherichia coli cells that required the P. syringae hrp cluster. E. coli MC4100 transformants carrying the plasmid-borne P. syringae pv. syringae Pss61 hrp cluster and P. syringae pv. glycinea avrB expressed from a triple lacUV5 promoter gained the ability to elicit the hypersensitive response in soybean cultivars expressing Rpg1 and in an Arabidopsis thaliana accession expressing RPM1. Inactivation of en-ergy transducing or outer membrane components of the /ir/7-encoded secretion system blocked phenotypic expression of avrB in E. coli, but deletions abolishing harpinPss production had little effect on the production of the AvrB phenotype by the E. coli transformants. Phenotypic expression of avr A, avrPto, avrRpml, avrRpt2, and avrPph3 in E. coli was also shown to require the hrp cluster. The results indicate that generation of the Avr phenotype in P. syringae strains is specifically dependent on the secretion activities of the hrp cluster.