No abstract

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.

The enzyme 4‐α‐glucanotransferase catalyzes the intramolecular and intermolecular transglycosylation of α‐1,4‐glucan, which plays an important role in starch metabolism and starch modification. In this study, the gene encoding a 4‐α‐glucanotransferase from Corallococcus sp. EGB (ccGtase) is cloned and expressed in Escherichia coli BL21 (DE3). CcGtase contains all the conserved amino acid residues of glycoside hydrolase family 77 (GH77), which form the active site environment of the enzymes in the α‐amylase family, while sequence alignments show that 4‐α‐glucanotransferases from myxobacteria constitute an independent group along with the GH77 glucanotransferases. Product analysis shows that CcGtase is able to convert malto‐oligosaccharides and starch into linear maltooligosaccharides with a low degree of polymerization (DP < 12) instead of cycloamylose. The transfer action toward starch is only observed in the presence of glucose, and delayed production of maltose from the transfer reaction is also observed. During the reaction, no apparent hydrolytic activity is observed. Based on the low sequence identity (9–35%) and unique characteristics, CcGtase can be regarded as a new member of the GH77 family.

Corallococcus sp. strain EGB is a Gram-negative myxobacteria isolated from saline soil, and has considerable potential for the biocontrol of phytopathogenic fungi. However, the detailed mechanisms related to development and predatory behavior are unclear. To obtain a comprehensive overview of genetic features, the genome of strain EGB was sequenced, annotated, and compared with 10 other Corallococcus species. The strain EGB genome was assembled as a single circular chromosome of 9.4 Mb with 7916 coding genes. Phylogenomics analysis showed that strain EGB was most closely related to Corallococcus interemptor AB047A, and it was inferred to be a novel species within the Corallococcus genus. Comparative genomic analysis revealed that the pan-genome of Corallococcus genus was large and open. Only a small proportion of genes were specific to strain EGB, and most of them were annotated as hypothetical proteins. Subsequent analyses showed that strain EGB produced abundant extracellular enzymes such as chitinases and β-(1,3)-glucanases, and proteases to degrade the cell-wall components of phytopathogenic fungi. In addition, 35 biosynthetic gene clusters potentially coding for antimicrobial compounds were identified in the strain EGB, and the majority of them were present in the dispensable pan-genome with unexplored metabolites. Other genes related to secretion and regulation were also explored for strain EGB. This study opens new perspectives in the greater understanding of the predatory behavior of strain EGB, and facilitates a potential application in the biocontrol of fungal plant diseases in the future.