Isolation and Characterization of Indole Acetic Acid Producing Root Endophytic Bacteria and Their Potential for Promoting Crop Growth

Endophytic bacteria colonize in plant tissues and enhance plant growth by a wide variety of mechanisms. The objectives of this study were to examine the population of root endophytic bacteria in soybean and corn, and to identify IAA-producing endophytic bacterial strains and their growth promoting effect. The density of endophytic bacteria varied irrespective of crops, sampling times and soil amendments. A total of 119 and 277 bacterial isolates were isolated from soybean and corn roots, respectively. 39.6% of the total isolates showed IAA production in the range of 1~23 μg mL-1 in culture medium supplemented with tryptophan. Fourteen isolates, designated as S1-S4 from soybean roots and C1-C10 from corn roots, had the capacity of producing IAA over 10 mg L-1. Based on 16S rRNA gene sequence analysis, the fourteen isolates were closely related to Psychrobacillus, Microbacterium, Lysinibacillus and Bacillus. Pot experiment indicated that the growth-promoting effects varied among these 14 bacterial strains and not all of the strains were able to promote growth of the tested soybean and wheat plants. Strains Microbacterium sp. C4 and Lysinibacillus sp. C7 showed better performances in promoting soybean and wheat seedling growth.