Colonization of resistant and susceptible maize plants by Erwinia stewartii strains differing in exopolysaccharide production

Abstract

Susceptible (B14A, T220) and relatively resistant (C123) inbred maize lines were inoculated with Erwinia stewartii strains differing in their ability to produce extracellular polysaccharide (EPS) in vitro, and a correlation was found between EPS production and virulence. The EPS-deficient strains grew and spread more slowly in host vascular tissues than wild-type strains, were less likely than their wild-type counterparts to colonize plants systemically and were unable to cause wilting in infected plants. Although the resistant inbred became infected by all strains, lesions expanded more slowly than those in susceptible inbreds. Pathogen populations developed more slowly in the vascular tissues of resistant plants than in susceptible plants, and the resistant plants were rarely colonized systemically and never wilted. Ultrastructural observations indicated that the butyrous mutants were unable to form well-developed capsules in planta, and they were apparently not immobilized or agglutinated in host tissues. In both resistant and susceptible plants, the pathogen was initially restricted to the xylem vessels. Bacteria seemed to exit the vessels in areas where thin primary walls bordered intercellular spaces. Xylem parenchyma cells produced materials that were secreted into the vessels of infected plants, and these materials may have aided in localization of the pathogen. Cell wall breakdown did not occur until the very late stages of pathogenesis, and E. stewartii did not produce cell wall-degrading enzymes in vitro.