| Summary: | <i>Coniothyrium minitans</i> (<i>Cm</i>) is a mycoparasitic fungus of <i>Sclerotinia sclerotiorum</i> (<i>Ss</i>), the causal agent of Sclerotinia stem rot of oilseed rape. <i>Ss</i> can produce oxalic acid (OA) as a phytotoxin, whereas <i>Cm</i> can degrade OA, thereby nullifying the toxic effect of OA. Two oxalate decarboxylase (OxDC)-coding genes, <i>CmOxdc1</i> and <i>CmOxdc2</i>, were cloned, and only <i>CmOxdc1</i> was found to be partially responsible for OA degradation, implying that other OA-degrading genes may exist in <i>Cm</i>. This study cloned a novel OxDC gene (<i>CmOxdc3</i>) in <i>Cm</i> and its OA-degrading function was characterized by disruption and complementation of <i>CmOxdc3</i>. Sequence analysis indicated that, unlike <i>CmOxdc1</i>, <i>CmOxdc3</i> does not have the signal peptide sequence, implying that CmOxDC3 may have no secretory capability. Quantitative RT-PCR showed that <i>CmOxdc3</i> was up-regulated in the presence of OA, malonic acid and hydrochloric acid. Deletion of <i>CmOxdc3</i> resulted in reduced capability to parasitize sclerotia of <i>Ss</i>. The polypeptide (CmOxDC3) encoded by <i>CmOxdc3</i> was localized in cytoplasm and gathered in vacuoles in response to the extracellular OA. Taken together, our results demonstrated that <i>CmOxdc3</i> is a novel gene responsible for OA degradation, which may work in a synergistic manner with <i>CmOxdc1</i>.
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