Isolation, Identification And Characterization Of Two Kinds Of Autotoxic Compound-degrading Bacterium From Cucumber Rhizosphere

Autotoxicity of cucumber is one of important reasons of cucumber’s continuous cropping obstacle. This study aimed at overcomeing the continuous cropping obstacles of cucumber through the degradation of cucumber autotoxins by actinomyces. Six strains were isolated and screened from cucumber rhizosphere to investigate their effects on the growth of cucumber and their ability in degrading FA or PHBA in liquid culture and soil.(1) CSY-P1and CSY-P3were isolated by M-9PHBA medium and identified as Pseudomonus putida CSY-P1and Pseudomonas toyotomiensis CSY-P3. Four strain able to degrade ferulic acid (FA) were isolated by M-9FA medium and identified as Bacillus methylotrophicus CSY-F1, Brachybacterium phenoliresistens CSY-F3.(2) CSY-P1had a higher PHBA-degrading ability than other strains of Pseudomonus putida reported to date. It degraded PHBA efficiently at pH6-10, and protocatechoic acid (PA) was a metabolite. Results of response surface showed that temperature27.74℃, pH8.98and an initial PHBA concentration of0.63g/L were the optimal conditions. Under which, CSY-P1degraded99.47%PHBA in hydroponic nutrient solutions at72h. In soil containing lmg/g PHBA,0.25mL/g CSY-P1made PHBA remaining to be1.1%at16d.Bacillus methylotrophicus CSY-Fl’s metabolites included4-vinyl guaiacol (4VG), vanillin, vanillic acid and protocatechuic acid. The maximum productivity of4VG by CSY-F1reached683.2mg/L at37℃in presence of0.7g/L FA. CSY-F1can decompose98.9%FA in hydroponic solutions. At42d, four inoculation amounts (0.25,0.125,0.075and0.05mL/g soil) of CSY-F1separately made the percentages of FA remaining to be0.5%,0.8%,2.3%and5.7%.(3) When cucumber seedlings were grown in soil containing FA, soil enzyme activities were decreased, and the malonaldehyde level in cucumber leaves was increased. After CSY-F1was applied to the FA-contained planted soil, the soil enzyme activities were increased and the FA concentration in soil was decreased. Meanwhile, antioxidative enzyme activities were elevated and malonaldehyde level was reduced in cucumber leaves, thus mitigating FA stress to cucumber seedlings. Under FA environments, activities of superoxide dismutase, catalase and monodehydroascorbate reductase were increased more in CSY-F1than in another isolated strain CSY-F3, which degraded FA poorly. During PHBA degradation, catalase activity in CSY-P1was induced more than in CSY-P3and increased the activities of SOD, APX, DHAR, MDHAR and GR in CSY-P1less than in CSY-P3. This suggests that CAT may be involved in PHBA-degradation of bacteria.