Enzymic adaptation of cereal pathogens to the monocotyledonous primary wall

Abstract

Three cereal pathogens Rhizoctonia cerealis, Fusarium culmorum and Pseudocercosporella herpotrichoides produced a similar sequence of cell wall-degrading enzymes when grown on wheat seedling cell walls. Main activities and order of production were arabanase, xylanase and laminarinase. In contrast to pathogens with dicotyledonous hosts only low levels of pectic enzymes accumulated and then relatively late during growth. Rhizoctonia cerealis produced similar activities (including α- l -arabinofuranosidase) on both seedling cell walls and cell walls from straw, but on potato stem walls xylanase remained low while β-galactosidase and pectin lyase predominated. Extracellular enzymes from R. cerealis released arabinose and xylose from wheat cell walls. Rhizoctonia cerealis-infected wheat seedlings contained increased xylanase in developing (10 day) lesions; after 22 days levels of arabinosidase, arabanase, galactanase, endo β-1,4-glucanase, poly-galacturonase and pectin lyase had also increased significantly. Pectic enzymes and endoglucanase were absent from 14day F. culmorum lesions. Synthesis of most enzymes was inducible but arabanase and laminarinase production was constitutive for all three pathogens, and in some cases was not even controlled by catabolite repression. Production of R. cerealis arabinosidase was induced which confirms that this activity is distinct from arabanase. Results are discussed in terms of adaptation of cereal pathogens to the monocotyledonous primary wall, in particular to the high arabinoxylan and mixed glucan content and low galacturonide levels.