Necrotrophic Fungus Sclerotinia Sclerotiorum Research Articles

Metabolic processes and carbon nutrient exchanges between host and pathogen sustain the disease development during sunflower infection by Sclerotinia sclerotiorum

Interactions between the necrotrophic fungus Sclerotinia sclerotiorum and one of its hosts, Helianthus annuus L., were analyzed during fungal colonization of plant tissues. Metabolomic analysis, based on (13)C- and (31)P-NMR spectroscopy, was used to draw up the profiles of soluble metabolites of the two partners before interaction, and to trace the fate of metabolites specific of each partner during colonization. In sunflower cotyledons, the main soluble carbohydrates were glucose, fructose, sucrose and glutamate. In S. sclerotiorum extracts, glucose, trehalose and mannitol were the predominant soluble carbon stores. During infection, a decline in sugars and amino acids was observed in the plant and fungus total content. Sucrose and fructose, initially present almost exclusively in plant, were reduced by 85%. We used a biochemical approach to correlate the disappearance of sucrose with the expression and the activity of fungal invertase. The expression of two hexose transporters, Sshxt1 and Sshxt2, was enhanced during infection. A database search for hexose transporters homologues in the S. sclerotiorum genome revealed a multigenic sugar transport system. Furthermore, the composition of the pool of reserve sugars and polyols during infection was investigated. Whereas mannitol was produced in vitro and accumulated in planta, glycerol was exclusively produced in infected tissues and increased during colonization. The hypothesis that the induction of glycerol synthesis in S. sclerotiorum exerts a positive effect on osmotic protection of fungal cells and favors fungal growth in plant tissues is discussed. Taken together, our data revealed the importance of carbon-nutrient exchanges during the necrotrophic pathogenesis of S. sclerotiorum.

Annual Meeting, Québec, Quebec, 2006, The Canadian Phytopathological Society / Réunion annuelle, Québec, Québec, 2006, La Société canadienne de phytopathologie

Annual Meeting, Québec, Quebec, 2006, The Canadian Phytopathological Society / Réunion annuelle, Québec, Québec, 2006, La Société canadienne de phytopathologie

Identification of QTL Associated with White Mold Resistance in Common Bean

White mold, caused by the necrotrophic fungusSclerotinia sclerotiorum(Lib.) de Bary, is a serious disease of common bean (Phaseolus vulgarisL.). The use of resistant cultivars is the preferred control strategy, but the approach has been difficult to implement because of quantitative nature of genetic resistance to white mold. The objectives of this study were to identify quantitative trait loci (QTL) associated with resistance toS. sclerotiorum, identify QTL associated with agronomic traits that contribute to white mold avoidance, and locate putative QTL on the integrated bean map. Bulked segregant analysis using multiple trait bulks that included disease severity index (DSI), yield, and days to flower was used to detect QTL associated with field resistance to white mold in a Middle American recombinant inbred line (RIL) population of common bean. QTL that accounted for 9.2 to 14.7% of the phenotypic variation for DSI were located on linkage groups B2, B5, B7, and B8 of the integrated bean map. In addition, QTL were detected for seed size and yield and agronomic traits associated with disease avoidance: days to maturity, days to flower, and lodging. Heritability estimates for DSI were moderate (0.41) and correlations with agronomic traits that included lodging (0.56**), architecture (0.35**), canopy height (−0.33**), and yield (−0.64**) support the importance of these traits in disease avoidance. Data from the current study provides breeders with critical information on which traits and genomic regions to target as part of an overall strategy to enhance resistance to white mold in common bean.

Ambient pH controls the expression of endopolygalacturonase genes in the necrotrophic fungus Sclerotinia sclerotiorum

In the necrotrophic fungus Sclerotinia sclerotiorum, secretion of polygalacturonases (PGs) and decrease of the environmental pH via oxalic acid production are considered as the main pathogenicity determinants. In order to evaluate the relationship between these two aspects of the infection process, we analyzed the expression of the endoPG-encoding genes pg1–3. Transcription of pg1–3 was not carbon regulated but was strictly controlled by pH and highly favored in a narrow range of acidic pH. During plant infection, a pH gradient was established in relation to oxalic acid secretion. Transcripts of pg1–3 were localized to the zone of colonization of healthy tissues while transcripts of genes encoding other lytic enzymes were restricted to the more acidic zones of the infected tissues. Our results show that progressive acidification of the ambient medium by the fungus is a major strategy for the sequential expression of pathogenicity factors.

Carbon and pH Modulate the Expression of the Fungal Glucose Repressor Encoding Genes

The phytopathogenic necrotrophic fungus Sclerotinia sclerotiorum secretes large amounts of oxalic acid and cell wall-degrading enzymes (CWDE). Enzyme synthesis is pH dependent and subject to carbon catabolite repression. Because of the possible involvement of the glucose repressor CRE1 in the transcriptional control of the CWDE encoding genes, we studied crel expression with respect to the nature of the extracellular carbon source and the value of the external pH. Comparative analysis of the cre1 homologous gene creA from Aspergillus nidulans was performed with pH-deregulated mutants. The data obtained showed that ambient pH controls the expression level of the carbon repressor genes, and this control does not depend on the pH transcription factor PacC.

Differential absorption and localization of two Sclerotinia sclerotiorum endo-polygalacturonases in soybean hypocotyls

The isolate B-24 of the necrotrophic fungus Sclerotinia sclerotiorum produces two major endo-polygalacturonases (PG-II and PG-IV) during infection of soybean seedlings. By a combination of biochemical, immunofluorescence and morphological techniques it was shown that they were differentially absorbed by soybean hypocotyl tissue, becoming located preferentially at different sites. PG-IV was absorbed much more than PG-II and it was able to permeate the unwounded surface of the hypocotyl and accumulate in the cortical cell layer where it produced an increase of autofluorescence and a marked loss of adhesion of cortical cells. When applied to the cut-end surface of the hypocotyl PG-IV remained localized near the site of the treatment. In contrast PG-II was not absorbed through the epidermis but, when applied to the cut surface of hypocotyl it was transported some distance into the tissue possibly via the vascular bundles, The different behaviour of the two PGs together with their different properties suggest they play a different but complementary role in the pathogenic strategy of S. sclerotiorum.