Dramatically Reduced Virulence of Mutants of Pseudomonas solanacearum Defective in Export of Extracellular Proteins Across the Outer Membrane. Yaowei Kang. Plant Protection Institute, Chinese Academy of Agricultural Sciences, Yuan Ming Yuan West Road, 100094. Jianzhong Huang(1) Guozhang Mao(2) Li-yuan He(2) and Mark A. Schell(1). (1)Departments of Microbiology and Plant Pathology, University of Georgia, Athens 30602 U.S.A. and (2)Plant Protection Institute, Chinese Academy of Agricultural Sciences, Yuan Ming Yuan West Road, 100094. MPMI 7:370-377. Accepted 5 January 1994. Copyright 1994 The American Phytopathological Society.

Pseudomonas solanacearum is an important wilt-inducing pathogen that infects a wide variety of important crop plants throughout the world. Studies using artificial inoculation methods suggest that some of its extracellular proteins play a significant, but auxiliary role in production of wilt disease. We isolated mutants of race 1 and race 3 strains of P. solanacearum with Tn5 insertions at a single locus (eep) whose culture supernatants lack all of its •known extracellular enzymes and most other detectable extracellular proteins (EXPs). Analysis of subcellular fractions of eep::Tn5 mutants showed that they still synthesized many of these EXPs but accumulated them inside the cell. Experiments with PhoA fusion proteins showed that export of proteins across the inner membrane was not affected by the eep mutation, suggesting that eep functions only in protein export across the outer membrane. Production of extracellular polysaccharide was not obviously affected by the eep mutation. Analysis of eep mutants in stem-inoculated tomato plants showed that they had lost the ability to cause wilt symptoms or kill the plant, possibly because they colonized stems much more slowly than wild types. Plants grown in soil inoculated with the eep mutants did not develop any visible disease symptoms over a 20-day period, and their stems contained fewer than 10³ P. solanacearum cells, whereas wild types killed plants in 14 days, and more than 10¹⁰ cells were found in their stems. These results suggest that an individual or group of extracellular proteins of P. solanacearum is required for infection via the roots, as well as wilting and killing of host plants.