Isolation,Identification And Characterization Of P-Hydroxybenzoic Acid-Degrading Strain

Continuous cropping obstacle seriously affects the production and sustainable development of many vegetables.Allelopathic autotoxicity caused by the accumulation of root-secreted phenolic acids in soil is one of the main factors for the continuous cropping obstacle.p-Hydroxybenzoic acid(PHBA)is one important phenolic acid secreted by cucumber roots.Thus,a PHBA-degrading microorganism was isolated to alleviate the continuous cropping obstacle of cucumber in this study.When the isolate was applied to PHBA-contained soil,it not only mitigated PHBA stress to cucumber seedlings but also affectd the activities of antioxidant enzymes in cucumber leaves and soil enzymes.The detailed findings of this paper are as follows:(1)A soil microorganism which had an ability of PHBA degradation was isolated and identified as Streptomyces coelicoflavus GLY-20 according to analyses of the 16 S rDNA gene sequences.(2)S.coelicoflavus GLY-20 decomposed PHBA at pH 6-10 and showed an increased PHBA-degrading ability at 28-32°C.Also,0.1-0.5g/L PHBA made the isolate degrade PHBA better.The optimal values of culture conditions given by the 3D response surface were temperature 30.32°C,pH 7.62 and an initial PHBA concentration of 0.56g/L.Protocatechuic acid(PA)was detected as a secondary metabolite.It reached a maximum accumulation at 60 h and then decreased.(3)S.coelicoflavus GLY-20 was able to degrade 99.8% PHBA in Hoagland nutrient solution at 180 h,and this provided the possibility for recycling nutrient solutions in soilless culture.(4)When 1mL(7.24 log cfu/g soil),2mL(7.54 log cfu/g soil),3mL(7.72 log cfu/g soil),4mL(7.85 log cfu/g soil)of spore suspensions of S.coelicoflavus GLY-20 were respectively inoculated in unplanted soil containing 110μg PHBA per g soil.The PHBA remaining was 50.30,38.79,24.16,and 8.87μg/g soil at 30 d.So S.coelicoflavus GLY-20 can degrade PHBA in unplanted soil.(5)When cucumber seedlings were grown in soil containing PHBA,activities of some soil enzymes were decreased,and the malonaldehyde level in cucumber leaves was increased.After application of GLY-20 to the cucumber-planted and PHBA-contained soil,the activities of these soil enzymes were increased,and the PHBA concentration in rhizosphere soil was decreased.Meanwhile,the activities of some antioxidant enzymes were elevated in cucumber leaves,thus reducing the levels of malonaldehyde,hydrogen peroxide and superoxide radical and mitigating PHBA stress to cucumber seedlings.(6)With glucose or PHBA as the sole carbon source,the growth tendency of S.coelicoflavus GLY-20 was similar.The logarithmic phase of the strain reached at 36 h.(7)After transcriptome sequencing analysis was conducted between glucose-and PHBA-cultured GLY-20,226 differential expression genes were gained.Those genes were in connection with PHBA degradation,and would provide a direction for further studing the molecular mechanism of PHBA-degrading microorganism.