Degradation of oxalic acid by Coniothyrium minitans and its role in suppression of Sclerotinia sclerotiorum

摘要

Degradation of oxalic acid (OA) by the mycoparasitic fungus Coniothyrium minitans (CM), and the effects of OA degradation on production and activity of β-(1, 3)-glucanase of CM were investigated. Over 85% of OA was degraded by CM grown at 20°C for 15 days in modified potato dextrose broth (PDB) containing OA at 0.8 to 28 mM. Meanwhile, pH of these cultures increased from 3.0-4.8 to 8.3-8.6. Capability of CM to degrade OA was evident in the bioassay experiment, in which toxicity of filtrates of 15-day-old CM cultures grown in PDB containing OA at 8, 16 or 24 mM to seedlings of oilseed rape was significantly reduced in comparison with controls. In dual cultures grown on potato dextrose agar, spread of CM onto colonies of Sclerotinia sclerotiorum (SS) resulted in elevation of pH from 2.9 to 6.6 and accumulation of β-(1, 3)-glucanase. Increase of pH on flower petals of oilseed rape inoculated with CM and SS was observed after incubation on water agar for 6 days. Inoculation of petals with CM and SS reduced petal-mediated infection of leaves of oilseed rape by SS. CM grown in PDB amended with OA at 16 mM for 9, 12 and 15 days increased production of β-(1, 3)-glucanase by 60.7, 22.8 and 51.6%, respectively, compared to untreated control (PDB alone). The activity of β-(1, 3)-glucanase produced by CM was enhanced by OA at 2 mM which conditioned pH at 6.0, but inhibited by OA at 4-32 mM which lowered pH to 3.5-1.5, whereas the pH of the control treatment (without OA) was 7.0. On the other hand, sodium oxalate at 2 to 32 mM, which conditioned pHs at 5.9-6.1, did not inhibit activity of this enzyme. These results suggest that degradation of OA by CM can nullify the pH effect caused by OA and may improve mycoparasitism of CM on SS by stimulating its production of β-(1, 3)-glucanase and/or activity of this enzyme.