Effect of the endophyte Ceratobasidium stevensii on 4-HBA degradation and watermelon seed germination

A preliminary trial indicated that the replanting obstacle of watermelon could be mitigated by the addition of the endophytic fungus Ceratobasidium stevensii B6 to the soil. The mechanism by which this treatment reduced the obstacles to continuous cropping was not clear, although the phenolic acids secreted by watermelon are a known problem. Consequently, we investigated the degradation of phenolic acids by C. stevensii, using 4-hydroxybenzoic acid (4-HBA) as a model. When the concentration of 4-HBA rose to 500 mg/L, it completely inhibited the germination of watermelon seeds. We therefore investigated the optimal conditions for 4-HBA degradation. When active C. stevensii broth was inoculated into degradation culture broth at a density of 5% (v/v), the 4-HBA concentration decreased to a level below the level of detection after 6 days. C. stevensii could also degrade 4-HBA when it was used as a sole carbon source. Glucose, sucrose, starch, and fructose stimulated the degradation of 4-HBA. High concentrations of glucose enhanced 4-HBA degradation. The optimal conditions for the degradation of 4-HBA by C. stevensii were 28°C, initial pH 5.5, and a shaking speed of 150 rpm. The addition of 500 mg/kg of 4-HBA to dried soil lasted for 2 months, followed by addition of C. stevensii which reduced the concentration of 4-HBA in soil below the level of detection after 7 days. Watermelon seedlings cultured in treated soil had a significantly increased number of stems and greater leaf length compared to controls. Therefore, C. stevensii has great potential in effectively lessening the obstacles to watermelon continuous cropping caused by 4-HBA accumulation.   Key words: Ceratobasidum stevensii, 4-hydroxybenzoic acid, watermelon, continuous cropping obstacles, degradation.