Identification Of Endophytic Bacterium JK-3 From Watermelon Plants And Its Antagonistic Mechanism

JK-3, screened from the watermelon root, is a kind of Gram-positive bacterium with rod-shape(3.0～4.2×0.7～1.0μm), peritrichous flagella, endogenous oval spore and enlarged cyst. The colony is light yellow, round, elevated, non-folding, edge neatly. Many sticky substances are produced on the surface of the colony four days post-incubation. The results showed the postive for the catalase and nitrates, but negative for the oxidase and methylred. The strain, a facultative anaerobe, can hydrolyze starch and casein, but it can't utilize citrate and malonate salts. Only a 1463 bp specific fragment of 16S rDNA sequences was amplified from the genomic DNA of the strain with universal primers. Sequence analysis by BLASTn confirmed that JK-3 has 100% of nucleotide identity with Paenibacillus polymyxa G-14 (Acession No. EU434620.1). The most matching Species ID(Paenibacillus polymyxa)was produced by the Biolog automated microbial identification system. The possibility was 100, and the similarity was 0.867. Based on the data from morphological and molecular identification, conventional physiological and biochemical analysis, JK-3 is identified as Paenibacillus polymyxa.Dual culture and germination tests showed that JK-3 had obvious antagonism to 8 different phytopathogenic fungi with the strongest inhibition to Botrytis cinerea, which indicate that JK-3 is an antagonist with broad spectrum. Both culture suspension of JK-3 and its sterile filtrate have high inhibition to the conidial germination of Fusarium oxysporum. In the pot tests, the strain JK-3 exhibited 74.6% and 94.6% of preventive control effect on seedling incidence and fatality rate of watermelon Fusarium wilt repectively. This study further indicated that the strains JK-3 had good antagonistic ability and a good application prospect.The antifungal protein was obtained by precipitating and dialyzing the JK-3 filtrate through 70% ammonium sulfate. And it has significant inhibition effect on mycelial growth of both Fusarium oxysporum and Didymella bryoniae. The protein is unstable to heat, but it is insensitive to ultraviolet radiation, chloroform and proteinase K. Characters of the antifungal protein was preliminarily studied. The protein was a kind of glycoprotein, but no lipid, withoutα-amino acid and phenyl structure.