Preparation of Active Chitooligosaccharides with a Novel Chitosanase AqCoA and Their Application in Fungal Disease Protection

Enzymes that degrade fungal cell walls and the resulting oligosaccharides are promising weapons to combat plant fungal disease. In this study, we identified a novel endo-chitosanase, AqCoA, from Aquabacterium sp. A7-Y. The enzyme showed a specific activity of 18 U/mg toward 95% deacetylated chitosan at pH 5.0 and 40 °C. AqCoA also showed activity toward sodium carboxymethylcellulose, indicating substrate promiscuity. AqCoA hydrolyzed chitosan into chitooligosaccharides (CoA-COSs) with degrees of polymerization (DPs) of 3–5 but showed no activity toward CoA-COSs with DPs <6, indicating an endo-type activity. At 2.5 μg/mL, AqCoA inhibited appressorium formation of Magnaporthe oryzae; the produced CoA-COSs also inhibited the growth of M. oryzae and Fusarium oxysporum. Furthermore, CoA-COSs acted as immune elicitors in rice by inducing the reactive oxygen species burst and the expression of defense genes. These results demonstrated that AqCoA and its resulting CoA-COSs might be effective tools for protecting plants against pathogenic fungi.