Impact of carbon sources on growth and oxalate synthesis by the phytopathogenic fungus Sclerotinia sclerotiorum

The impact of various supplemental carbon sources (oxalate, glyoxylate, glycolate, pyruvate, formate, malate, acetate, and succinate) on growth and oxalate formation (i.e., oxalogenesis) by Sclerotinia sclerotiorum was studied. With isolates D-E7, 105, W-B10, and Arg-L of S. sclerotiorum, growth in an undefined broth medium (0.1% soytone; pH 5) with 25 mM glucose and 25 mM supplemental carbon source was increased by the addition of malate and succinate. Oxalate accumulation occurred in the presence of glucose and a supplemental carbon source, with malate, acetate, and succinate supporting the most oxalate synthesis. With S. sclerotiorum Arg-L, oxalate-to-biomass ratios, an indicator of oxalogenic potential, were dissimilar when the organism was grown in the presence of different carbon sources. The highest oxalate-to-biomass ratios were observed with pyruvate, formate, malate, acetate, and succinate. Time-course studies with acetate-supplemented cultures revealed that acetate and glucose consumption by S. sclerotiorum D-E7 coincided with oxalogenesis and culture acidification. By day 5 of incubation, oxalogenesis was halted when cultures reached a pH of 3 and were devoid of acetate. In succinate-supplemented cultures, oxalogenesis essentially paralleled glucose and succinate utilization over the 9-day incubation period; during this time period, culture pH declined but never fell below 4. Overall, these results indicate that carbon sources can regulate the accumulation of oxalate, a key pathogenicity determinant for S. sclerotiorum.