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Abstract
Stenotrophomonas maltophifia C3 inhibited the germination of conidia and the growth of germtubes of Bipolaris sorokiniana (Sacc.) Shoemaker on tall fescue (Festuca arundinacea Schreb.) leaves. It reduced the incidence and severity of leaf spot disease caused by B. sorokiniana under growth chamber and field conditions. Chitinolysis was found to be one mechanism of action. Transposon mutants of C3 that are chitinase deficient or have reduced chitinase production colonized grass to lower levels and provided reduced disease protection in comparison to the wild type. Chitinase activity on grass leaves and bean blossoms colonized by C3 was higher than on the non-colonized plant parts; the addition of chitin with C3 further increased chitinase activity. When a chitinase-minus mutant was applied to plant parts with or without chitin, no additional chitinase activity was detected over the controls. Extracts from C3-colonized plant parts had antifungal activity, and this property was related to chitinase activity. Chitinase production by C3 was induced in vitro by chitin or chitin-containing fungal cell walls. Chitinolytic fractions of C3 in broth cultures, that were partially purified by chitin affinity chromatography, were more antifungal than non-chitinolytic fractions. They also exhibited strong exochitinase and slight endochitinase activities. When the major chitinolytic fraction was subjected to SDS-PAGE, five proteins were revealed (25, 32, 48, 65 and 75 KDa). Only the N-terminal amino acid sequence of the 32-Kda protein showed homology to known bacterial chitinases. Five chitinase-active bands were detected when the gels were probed with a fluorescent chitin substrate. These bands appeared only as 32-Kda and 48-Kda proteins when re-electrophoresed in denaturing gels, suggesting that the smaller proteins were cleavage products of the larger proteins. Several methods were identified that increased disease control efficacy: application of C3 cells in combination with chitin, application of cells in combination with fluid from, a chitin-containing broth culture, and use of C3 cells induced to produced chitinase. Thus, improvements in biocontrol can be achieved with knowledge of the modes of action.
