Characterization of root endophytic Paenibacillus polymyxa isolates with biocontrol activity against Xanthomonas translucens and Fusarium graminearum

Fusarium crown rot (FCR), Fusarium head blight (FHB), and bacterial leaf streak (BLS) of wheat are important diseases worldwide. The lack of commercial resistant cultivars and harmful effects of using chemical poisons for controlling these diseases prompted scientists to use biological agents as eco-friendly strategies for disease management. Plants carry small community of microbes known as endophytes that have effect on plant growth regulation and also protect plants against phytopathogens. In this study, a total of 35 endophytic bacteria were isolated from wheat root. Two isolates were selected for further study based on their potential ability to control Xanthomonas translucens and Fusarium graminearum in vitro. Both isolates were identified as Paenibacillus polymyxa SeR8 and BR20 (F and B-type, respectively) according to the results of 16S rRNA gene sequencing, morphological, and physiological analysis. Light and scanning electron microscopy showed alternations in the F. graminearum structures in the presence of the endophytes. Production of protease, amylase, pectinase, cellulase, IAA, sensitivity to H2O2, biofilm formation, and phosphate solubilization in the type B was more than the type F. On the other hand, the production of lipase, catalase, ammonia, and siderophore by the F-type was more than the B-types, but they could not produce biosurfactants and ACC deaminase. None of them could produce chitinase. Kernel bioassay and mycotoxin analyses indicated that the F-type had better impact on protecting seeds and decreasing DON production. The percentages of FHB infection reduction, FCR severity reduction, and protection rate against BLS in the type F and B were 76.31, 64.10%; 67.66, 65.41%, and 54.96, 63.57%, respectively. Greenhouse experiments indicated that the biomass, 1000 grains weight, chlorophyll a, b, T, and carotenoids were increased in the plants treated with the endophytic bacteria. In our study, the B-type showed better antibacterial activity in comparison with the F-type, while the F type showed better antifungal activity than the B type. This is the first report that indicates the importance of types F and B of P. polymyxa in the biocontrol of Fusarium crown rot and bacterial leaf streak diseases.