The Cochliobolus carbonum SNF1 gene is required for cell wall-degrading enzyme expression and virulence on maize

The production of cell wall-degrading enzymes (wall depolymerases) by plant pathogenic fungi is under catabolite (glucose) repression. In Saccharomyce s cerevisiae, the SNF1 gene is required for expression of catabolite-repres sed genes when glucose is limiting. An ortholog of SNF1, ccSNF1, was isolat ed from the maize pathogen Cochliobolus carbonum, and ccsnf1 mutants of HC toxin-producing (Tox2(+)) and HC toxin-nonproducing (Tox2(-)) strains were created by targeted gene replacement. Growth in vitro of the ccsnf1 mutants was reduced by 50 to 95% on complex carbon sources such as xylan, pectin, or purified maize cell walls. Growth on simple sugars was affected, dependi ng on the sugar. Whereas growth on glucose, fructose, or sucrose was normal , growth on galactose, galacturonic acid, maltose, or xylose was somewhat r educed, and growth on arabinose was strongly reduced. Production of HC toxi n was normal in the Tox2(+) ccsnf1 mutant, as were conidiation, conidial mo rphology, conidial germination, and in vitro appressorium formation. Activi ties of secreted beta-1,3-glucanase, pectinase, and xylanase in culture fil trates of the Tox2(+) ccsnf1 mutant were reduced by 53, 24, and 65%, respec tively. mRNA expression was downregulated under conditions that induced the following genes encoding secreted wall-degrading enzymes: XYLI, XYL2, XYL3 , XYL4, XYP1, ARF1, MLG1, EXG1, PGN1, and PGX1. The Tox2(+) ccsnf1 mutant w as much less virulent on susceptible maize, forming fewer spreading lesions ; however, the morphology of the lesions was unchanged. The Tox2(-) ccsnf1 mutant also formed fewer nonspreading lesions, which also retained their no rmal morphology. The results indicate that ccSNF1 is required for biochemic al processes important in pathogenesis by C. carbonum and suggest that pene tration is the single most important step at which ccSNF1 is required. The specific biochemical processes controlled by ccSNF1 probably include, but a re not necessarily restricted to, the ability to degrade polymers of the pl ant cell wall and to take up and metabolize the sugars produced.