VIEW ARTICLE | DOI: 10.1094/MPMI-4-198
Genetic Evidence that Extracellular Polysaccharide Is a Virulence Factor of Pseudomonas solanacearum. Timothy P. Denny. Department of Plant Pathology, The University of Georgia, Athens 30602 U.S.A. Seung-Ryel Baek. Department of Plant Pathology, The University of Georgia, Athens 30602 U.S.A. MPMI 4:198-206. Accepted 27 December 1990. Copyright 1991 The American Phytopathological Society.
Additional Keywords: Lycopersicon esculentum.
To test whether the extracellular polysaccharide (EPS) produced by Pseudomonas solanacearum is responsible for the wilt symptoms caused by this plant pathogen, two classes of Tn5-induced, EPS-impaired mutants were further characterized and manipulated. In comparison with wild-type AW1, the class I mutant AW1-1 produced about 95% less EPS on rich and minimal media and slowly wilted one third as many tomato plants in stem and root inoculation assays. In contrast, class II mutants produced nearly wild-type amounts of EPS on minimal medium and largely retained virulence. Eight cosmid clones were identified in a genomic library of wild-type strain AW1 that restored EPS production by the mutants to varying degrees, with one cosmid fully complementing both class I and class II mutants. Southern blot analysis, restriction mapping, subcloning, and Tn3-HoHo1 mutagenesis demonstrated the existence of two neighboring regions involved in EPS production. The minimum sizes for regions I and II were 9 and 2.6 kilobases (kb), respectively. Phenotype conversion, which results in spontaneous EPS‾ mutants, decreased expression of lacZ fusions in both regions. EPS+ merodiploids of AW1-1 were more virulent than AW1-1, but plasmid instability in planta hindered our interpretation of these experiments. However, allelic replacement in certain AW1-1 merodiploids generated strains that were EPS+ and kanamycin sensitive, and these wild-type recombinants were invariably as virulent as AW1. These results provide genetic evidence that EPS is an important virulence factor required by P. solanacearum strain AW1 to wilt tomato.