Optimization of Culture Conditions of Paenibacillus polymyxa X-11 and Its Growth-Promotion Effects on Citrus maxima L.Osbeck'Majiayou
Paenibacillus polymyxa X-11(abbreviated as X-11)is a plant rhizosphere growth-promoting bacterium strain screened in the research group's previous studies.This strain has the functions of nitrogen fixation,antimicrobe and growth-promotion,and thus has a potential for development and application.To optimize the culture conditions of strain X-11,a two-factor experimental design was used to study the effects of initial pH of culture medium and cul-ture time,volume of culture medium and culture temperature on the live cell number and the extracellular cellulase(including xylanase)activities of X-11 bacterial suspension through shake flask culture experiments.Then,the growth-promotion effects of X-11 bacterial suspension cultured using the optimized culture conditions on the seed-lings of Citrus grandis L.Osbeck were determined.The results showed that,when the initial pH of potato sucrose broth(PDB)medium was 8 and cultured for 40 hours(h),the X-11 bacterial suspension had the highest live cell number,reaching 4.78×10 6 colony forming units/mL(CFU/mL).When the volume of the composite medium was 35 mL and cultured at 35 ℃,the X-11 bacterial suspension had the highest live cell number,up to 4.47×10 7 CFU/mL.When strain X-11 inoculated in PDB medium with the initial pH7,and cultured for 48 h or 56 h,the bacterial suspension had the highest extracellular cellulase and xylanase activities,reaching 2.26 U(mg/mL/min)and 3.93 U,respectively.When the volume of PDB medium was 20 mL and cultured at 25 ℃,the X-11 bacterial suspension had the highest extracellular cellulase and xylanase activities that reaching 5.14 U and 3.30 U,respectively.When cultured in PDB medium,the live cells number in X-11 bacterial suspension had an interaction effect between initial pH and culture time,while the interaction effect of cellulase or xylanase activity between initial pH and culture time was not significant.When cultured in composite medium,the number of live cells had an interaction effect between the medi-um volume and culture temperature.When cultured in PDB or composite medium,cellulase activity exhibited an in-teraction effect between the medium volume and culture temperature.Only when strain X-11 cultured in PDB medi-um,its extracellular xylanase exhibited an interaction effect between medium volume and culture temperature.To sum up,it's conducive to increasing the live cell number of X-11 bacterial suspension,when cultured at 35 ℃ for 40 h in composite medium with the initial pH8 and the medium volume of 35 mL/100 mL-shake flask;to increase the extracellular cellulase and xylanase activities,strain X-11 should be inoculated in PDB medium with an initial pH7 and medium volume of 20 mL/100 mL-shake flask,and then when cultured at 25 ℃ for 48 h or 56 h.,it's conductive to the increase of X-11 bacterial suspension and xylanase activity.X-11 bacterial suspension cultured with the opti-mized culture conditions for high number of live cells was used to irrigate roots of C.maxima seedlings,promoting the plant growth significantly.Compared with the control,the height of the main stem and the number of branches of C.maxima treated with the X-11 bacterial suspension increased by 63.5％and 3.5 times,respectively.The results may provide a basis for the preparation of X-11 microbial agent and its application in plant growth promotion and plant disease biological control.
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