Screening And Fermentation Of A High-IAA-Yield Microbial Strain And Its Growth-promoting Effection

Plant growth-promoting rhizobacteria(PGPR)are a group of bacteria found in the plant root zone that promote plant growth,control plant diseases and maintain soil fertility.One of the functions of PGPR is the production of indole-3-acetic acid(IAA),a plant growth regulator that plays an important role in plant cell division,seed germination,root development,differentiation and elongationand vegetative growth.The application of IAA can promote crop growth and increase crop yield,and is widely used in agricultural production.In addition to plants that produce a small amount of IAA,many microorganisms can also produce IAA.The use of IAA-producing bacteria in agricultural production not only promotes crop growth and increases crop yield,but also helps to reduce the use of pesticides and chemical fertilisers.In this study,a high-IAA-yield microbial strain was screened,and its fermenting technics optimization and growth-promoting effect on vegetables were studied.The main reserch results are as follows:(1)Screening and identification of a high-IAA-yield microbial strain and its fermentation characteristics.A high-IAA-yield microbial strain FX-02 was isolated from vegetable rhizosp Here soil and identified as Enterobacter hormaechei.The best carbon source for both FX-02 growth and IAA production was glycerol.The best nitrogen source for the growth of FX-02 was peanut meal.Calcium chloride was the most suitable inorganic salt for the growth and IAA production of FX-02.The IAA biosynthetic pathway of FX-02belong to tryptophan-dependent type and FX-02 has a strong ability to transform tryptophan.(2)Study on fermenting technics optimization of Enterobacter hormaechei.In this study,a high IAA-producing microbial strain was screened as FX-02(Enterobacter hormaechei),and its yield reached 2349.40 mg/L.Therefore,FX-02 could be used as an excellent strain for the production of IAA-producing microbial agents,and also had the preliminary potential to produce IAA by fermentation.Therefore,two indicators,FX-02growth and IAA production,were used for fermentation process optimisation.The optimal medium for the fermentation of FX-02 growth was 5 g/L of glycerol,10 g/L of peptone,10 g/L of tryptophan,1.5 g/L of dipotassium hydrogen phosphate and 1.5 g/L of magnesium sulphate.The optimal fermentation medium for the production of IAA was10 g/L of glycerol,5 g/L of peptone,12 g/L of tryptophan and 1.5 g/L of magnesium sulphate.The optimal fermentation conditions for FX-02 growth in 50 L fermentor were as follows:temperature 30℃,initial p H 7.0,inoculation amount 5%(v/v),aeration rate1.0 vvm,stirring speed 400 r/min.Under these conditions,the cell dry weight was 18.1g/L.The optimum culture conditions for IAA production were as follows:temperature30℃,initial p H 7.0,inoculation amount 9%(v/v),aeration rate 0.8 vvm,stirring speed300 r/min.Under these conditions,the yield of IAA could reach 2767.0 mg/L.The results of fed-batch fermentation showed that supplementing water and peptone could increase the growth of FX-02 and IAA production,However,supplementing tryptophan can increase IAA production merely.(3)Studides on the effects of Enterobacter hormaechei.FX-02 agents and fermented liquid(centrifuge supernatant without bacteria)on vegetable growth.FX-02 had remarkable effect on promoting vegetable growth.The dry weight of the overground part of the plant increased up to 70.95%after appling 30 m L/m~2 of FX-02 agents for potted tomatoes.The dry weight of the overground part of the plant increased up to 204.59%after appling 45 m L/m~2of FX-02 agents for potted cucumbers.The dry weight of the overground part of the plant increased up to 140.00%after appling 45 m L/m~2of FX-02agents for potted cabbages.The fermented liquid can promote the growth of tomato,cucumber and cabbage seeds significantly.FX-02 could grow and yield IAA in soil,where the yield amount of IAA reached 346.3 mg/kg after 7 days of culture in mixed soil.