α-1,3-Glucan Is Required for Cell Wall Integrity and Full Virulence of Colletotrichum graminicola, but Not for Evasion of PAMP-Triggered Immunity.

Although α-1,3-glucan represents a major polymer in cell walls of filamentous plant pathogenic fungi, its role in vegetative and pathogenic development is poorly understood. The maize anthracnose fungus Colletotrichum graminicola differentiates elaborate appressoria on the leaf surface and exhibits a hemibiotrophic lifestyle after host invasion. Promoter-eGFP-fusion analyses of all three α-1,3-glucan synthase (AGS) genes, designated as AGS1, AGS2, and AGS3, suggest that AGS3 is expressed in vegetative and pathogenic hyphae, whereas expression of AGS1 and AGS2 was not, or barely, detectable. Targeted gene deletions yielded single and double deletion mutants, as well as triple mutants, which lacked the entire repertoire of α-1,3-glucan synthase genes. Conidia of mutants lacking AGS3 developed multiple germ tubes, and vegetative hyphae had cell walls containing strongly reduced amounts of alkali-soluble glucose, showed hyperbranching, reduced growth rates on all substrata tested, and hypersensitivity to the cell wall-perturbing agents Caspofungin and Nikkomycin Z. Moreover, mutants lacking AGS3 exhibited strong virulence defects and, intriguingly, biotrophic hyphae of these mutants that differentiated in planta were severely misshapen. Increased cell wall pore diameters, increased mobility of uncharged dextran-FITC conjugates through cell walls of AGS3-deficient mutants and lack of PAMP-triggered immune responses in maize leaves inoculated with these mutants suggest that α-1,3-glucan does not play a role in masking of fungal cell walls and immune evasion, but rather controls mobility of fungal cell wall-modifying enzymes and cell wall integrity.