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 (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 (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 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. INTRODUCTIONA major barrier to the penetration and spread of potential pathogenic organisms is the plant cell wall, and all of the major groups of cellular plant pathogens are known to make extracellular enzymes that can degrade cell wall polymers. Although the involvement of wall-degrading enzymes and their genes in penetration, pathogen ramification, plant defense induction, and symptom expression has been studied extensively, conclusive evidence for or against a role for any particular enzyme activity in any aspect of pathogenesis has been difficult to obtain (Walton, 1994).The major obstacle to addressing the function of walldegrading enzymes has been redundancy: all of the pathogens that have been studied in detail have multiple genes for any particular enzyme activity. Thus, most fungal strains mutated in wall-degrading enzyme genes-by either conventional (e.g., Cooper, 1987) or molecular (e.g., Scott-Craig et al., 1990) methods-retain at least som...

The LmSNF1 Gene Is Required for Pathogenicity in the Canola Blackleg Pathogen Leptosphaeria maculans

“…In a SNF1 mutant of C. carbonum , both sporulation and spore germination were similar to the wild-type [10]. In a SNF1 mutant of M. oryzae , the sporulation ability was abolished [19], while in a SNF1 mutant of P. digitatum sporulation was reduced but the spores germinated normally [22].…”

“…To test the role of CWDEs as a whole in fungal pathogenicity, disruption of the elements that control the derepression mechanism could be a more reliable approach than working on single CWDE genes [10]. In Saccharomyces cerevisiae , the SNF1 (sucrose non-fermenting 1) gene has been shown to play a central role in carbon catabolite repression [11].…”

Localization and Transcriptional Responses of Chrysoporthe austroafricana in Eucalyptus grandis Identify Putative Pathogenicity Factors

“…We identified several putative CWDEs that were up-regulated in planta ( Table 1 ). A key candidate involved in the regulation of CWDEs, Snf1 , is an important pathogenicity factor in Magnaporthe oryzae, Fusarium oxysporum, Gibberella zeae , and Cochliobolus carbonum (Tonukari et al, 2000; Ospina-Giraldo et al, 2003; Lee et al, 2009; Yi et al, 2009; Turra et al, 2014). Snf1 was expressed at higher levels in planta than in vitro in the current study ( Table 2 ).…”

How the Necrotrophic Fungus Alternaria brassicicola Kills Plant Cells Remains an Enigma

“…Loss-of-function mutation of its homolog causes a reduced expression of multiple CWDE genes under derepressive conditions in C. carbonum and Fusarium oxysporum (106,108). Mutants of the former exhibited an 80% decrease in virulence, while mutants of the latter show a significant delay in the development of wilt symptoms compared to their wild types.…”

“…For example, the expression levels of about half of the cellobiohydrolases and glucanases in the Magnaporthe oryzae genome are induced during pathogenesis (105). Knocking out a few of these genes, however, does not significantly affect virulence in Cochliobolus carbonum (91,92,106). In contrast, reducing the expression levels of many cellulases in M. oryzae using an RNA interference (RNAi) approach resulted in defects in penetration of the host epidermis and inhibited colonization of tissues, causing a reduction in virulence (105).…”