Journal Article

The Cochliobolus Carbonum SNF1 Gene Is Required for Cell Wall-Degrading Enzyme Expression and Virulence on Maize

Scott-Craig, John S., Tonukari, Nyerhovwo J., Walton, Jonathan D.

2000

Overview

The production of cell wall-degrading enzymes (wall depolymerases) by plant pathogenic fungi is under catabolite (glucose) repression. In Saccharomyces cerevisiae, the SNF1 gene is required for expression of catabolite-repressed genes when glucose is limiting. An ortholog of SNF1, ccSNF1, was isolated from the maize pathogen Cochliobolus carbonum, and ccsnf1 mutants of HC toxin-producing (${\\rm Tox}2^{+}$) and HC toxin-nonproducing (${\\rm Tox}2^{-}$) 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, depending on the sugar. Whereas growth on glucose, fructose, or sucrose was normal, growth on galactose, galacturonic acid, maltose, or xylose was somewhat reduced, and growth on arabinose was strongly reduced. Production of HC toxin was normal in the ${\\rm Tox}2^{+}$ ccsnf1 mutant, as were conidiation, conidial morphology, conidial germination, and in vitro appressorium formation. Activities of secreted β-1,3-glucanase, pectinase, and xylanase in culture filtrates of the ${\\rm Tox}2^{+}$ ccsnf1 mutant were reduced by 53, 24, and 65%, respectively. mRNA expression was downregulated under conditions that induced the following genes encoding secreted wall-degrading enzymes: XYL1, XYL2, XYL3, XYL4, XYP1, ARF1, MLG1, EXG1, PGN1, and PGX1. The ${\\rm Tox}2^{+}$ ccsnf1 mutant was much less virulent on susceptible maize, forming fewer spreading lesions; however, the morphology of the lesions was unchanged. The ${\\rm Tox}2^{-}$ ccsnf1 mutant also formed fewer nonspreading lesions, which also retained their normal morphology. The results indicate that ccSNF1 is required for biochemical processes important in pathogenesis by C. carbonum and suggest that penetration is the single most important step at which ccSNF1 is required. The specific biochemical processes controlled by ccSNF1 probably include, but are not necessarily restricted to, the ability to degrade polymers of the plant cell wall and to take up and metabolize the sugars produced.

• Publisher: American Society of Plant Physiologists,American Society of Plant Biologists
• Subject: Amino Acid Sequence / amino acid sequences / arabinose / ARF1 gene / Ascomycota / Ascomycota - genetics / Ascomycota - growth & development / Ascomycota - pathogenicity / Base Sequence / beta-glucanase / Bipolaris zeicola / carbohydrate metabolism / Carbon / Carbon sources / ccSNF1 gene / Cell Wall / Cell Wall - metabolism / cell wall components / Cell walls / Cochliobolus carbonum / Corn / Crop production / DNA Primers / enzyme activity / Enzymes / EXG1 gene / fructose / fungal diseases of plants / Fungal Proteins / Fungal Proteins - genetics / Fungi / galactose / galacturonic acid / gene expression / genes / Genetic Complementation Test / genetics / growth & development / Hydrolysis / Lesions / maltose / messenger RNA / metabolism / microbiology / MLG1 gene / Molecular Sequence Data / Mutants / Mutation / open reading frames / pathogenicity / pectins / PGN1 gene / PGX1 gene / phenotype / Polymers / Protein Serine-Threonine Kinases / Protein-Serine-Threonine Kinases - genetics / Protein-Serine-Threonine Kinases - metabolism / Saccharomyces cerevisiae / Sequence Homology, Amino Acid / SNF1 gene / sucrose / Sugar / Sugars / Toxins / Virulence / Virulence - genetics / wall depolymerase / XYL1 gene / XYL2 gene / XYL3 gene / XYL4 gene / xylan / xylan endo-1,3-^b-xylosidase / xylose / XYP1 gene / Yeasts / Zea mays / Zea mays - microbiology