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

Abstract

The production of cell wall-degrading enzymes (wall depolymerases) by plant pathogenic fungi is under catabolite (glu-cose) 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 car-bonum , 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, 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 Tox2 ccsnf1 mutant, as were conidiation, conidial morphology, conidial germination, and in vitro appressorium formation. Activities of secreted-1,3-glucanase, pectinase, and xylanase in culture filtrates of the Tox2 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 Tox2 ccsnf1 mutant was 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 non-spreading 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.

A Zinc-Finger-Family Transcription Factor, AbVf19, Is Required for the Induction of a Gene Subset Important for Virulence in Alternaria brassicicola

“…Similarly, the cutinase gene (CutAb) had 73-times fewer transcript in the ∆abvf19 mutant in this study, but cutab mutants in Yao and Kӧller's did not affect virulence (Yao and Köller, 1995). These mutation studies suggested that each enzyme-coding gene either contributed little to virulence, or their functions were redundant as proposed previously based on ccSNF1 and foSNF1 studies (Ospina-Giraldo et al, 2003;Tonukari et al, 2000). The current ∆abvf19 mutant study supports the importance of coordinated regulation of those genes during pathogenesis.…”

“…The importance of CWDE genes in virulence has been supported by identification of mutants with impaired derepression mechanisms of carbon catabolite repression in Colletotrichum carbonum and Fusarium oxysporum (Ospina-Giraldo et al, 2003;Tonukari et al, 2000).…”

“…Relatively little is known about the major nutrient sources of fungal metabolism during the late stage of infection. The regulation of CWDE expression is orchestrated by sucrose nonfermenting 1 (SNF1) homologs in two distantly related fungi (Ospina-Giraldo et al, 2003;Tonukari et al, 2000). A kinase ccSNF1…”

Requirement for either a host- or pectin-induced pectate lyase for infection of Pisum sativum by Nectria hematococca

“…In other cases, multiple-gene disruption failed to significantly decrease virulence (8,12,13). Therefore, conclusive evidence for or against a role for any particular enzyme activity in any aspect of pathogenesis has been difficult to obtain (51). A major decrease in virulence would require disruption of genes that encode all of the functionally redundant barrierdegrading enzymes that can play an essential role in pathogenesis.…”

Application of computers in biochemical education – the african challenge

“…Other information that can be obtained from such question includes the length of the protein, its hydrophobic/hydrophilic nature, subcellular localization, presence/absence of signal peptide, and motifs (Tonukari et al, 2000). Faculty can benefit twice from the time spent preparing for class when they write test questions that require students to access genomic data related to their research.…”