Developmentally regulated conversion of surface-exposed chitin to chitosan in cell walls of plant pathogenic fungi

Conversion of surface-exposed chitin to chitosan in cell walls of in vitro - and in vivo-differentiated infection structures of two rust fungi, the wheat stem rust fungus Puccinia graminis f. sp. tritici and the broad bean rust fungus Uromyces fabae, and of the causal agent of maize anthracnose, Colletotrichum graminicola, were studied. Epi-fluorescence microscopy with the fluorescence-labeled lectin wheat germ agglutinin (WGA) revealed that surfaces of infection structures formed on the plant cuticle expose chitin, whereas surfaces of structures formed after invading the host do not. To identify chitin modification by de-N-acetylation, we raised polyclonal antibodies specifically recognizing de-N-acetylated chitosan. These antibodies labeled only those infection structures that differentiate inside the plant, indicating that chitosan is exposed on cell wall surfaces post penetration. Surface modification of the fungal cell walls by chitin de-N-acetylation is discussed as a fungal strategy to protect cell walls of pathogenic hyphae from enzymatic hydrolysis by host chitinases, and to avoid generation of an auto-catalytic defense response system in the invaded host tissue.