Bacteria Expressing Avirulence Gene D Produce a Specific Elicitor of the Soybean Hypersensitive Reaction. N. T. Keen. Department of Plant Pathology and Genetics Graduate Group, University of California, Riverside 92521 U.S.A. S. Tamaki, D. Kobayashi, D. Gerhold, M. Stayton, H. Shen, S. Gold, J. Lorang, H. Thordal-Christensen, D. Dahlbeck, and B. Staskawicz. Department of Plant Pathology and Genetics Graduate Group, University of California, Riverside 92521 U.S.A. MPMI 3:112-121. Accepted 3 November 1989. Copyright 1990 The American Phytopathological Society.

Escherichia coli cells carrying expression plasmids with the cloned avirulence (avr) gene D from Pseudomonas syringae pv. tomato produced substantial amounts of the 34-kDa protein product. Infiltration of these E. coli cells into soybean leaves also resulted in a hypersensitive reaction (HR) on soybean cultivars resistant to but not susceptible to P. s. pv. glycinea carrying the cloned avrD gene. However, lysed E. coli cells expressing the avrD gene elicited little or no HR. The partially purified avrD-encoded protein extracted from E. coli cells also appeared to be devoid of HR elicitor activity. However, E. coli cells expressing avrD secreted a low molecular weight factor into the culture medium that elicited the HR only on resistant soybean cultivars. The same elicitor activity was found in culture fluids of wild-type P. s. pv. tomato as well as several other P. s. pv. glycinea pathovars previously shown to contain DNA homologous to avrD. However, P. s. pv. glycinea did not produce significant avrD elicitor despite containing hybridizing DNA sequences. Several P. syringae pathovars that lacked hybridizing DNA and an avrD mutant strain of P. s. pv. tomato also did not produce detectable quantities of the avrD elicitor. However, introduction of the cloned avrD gene on a broad host range plasmid enabled these bacteria to produce the extracellular elicitor. Production of the avrD elicitor by P. s. pv. glycinea cells carrying the cloned avrD gene occurred independently of the hrp genes, considered important for pathogenicity and HR induction by certain P. syringae pathovars. The results indicated that expression of avirulence gene D in P. syringae pathovars and in E. coli causes them to produce a diffusible, elicitor-active molecule which initiates cultivar-specific induction of the HR.