Abstract:

Plant growth-promoting rhizobacteria （PGPR） are a category of beneficial bacteria which colonize the plant rhizosphere and promote plant growth and development. In this study， PGPR strains with the capacity for nitrogen fixation， phosphate solubilization， ACC deaminase activity and IAA production were isolated and screened from the rhizosphere of Leymus chinensis in different regions， with a view to providing PGPR resources for the development of microbial fertilizers. Twenty strains possessing high nitrogenase activities （range 8.71 to 11.63 nmol C₂H₄ ·mL^-1·h^-1） were isolated from Lanxi， Xilinhot and Hulun Buir. Twenty six inorganic phosphorus solubilizing bacterial strains with ratios of the diameter of phosphate-solubilizing halo to colony diameter （D/d values） greater than 1.5 were identified. For these， the range of phosphate solubilization was 7.08-82.71 μg·mL^-1. Thirty six organic phosphorus solubilizing bacteria with D/d values greater than 1.5 were also isolated with the range of phosphate-solubilization being 1.56-32.48 μg·mL^-1. Sixty PGPR strains with ACC deaminase activities were screened and the range of activities observed were 0.04-64.31 μmol α-KA·mg^-1 Pr·h^-1. Thirty nine PGPR strains with high activities of nitrogenase， phosphate-solubilizing ability and ACC deaminase activity were further selected for determination of IAA production. The results showed that all 39 PGPR strains in this selection had the ability to secrete IAA， with the secretion amounts ranging from 3.27 to 48.97 μg·mL^-1. Two strains （HPS14 and XPR2） which were observed to increase height and aboveground and belowground biomass of Leymus chinensis were screened out in an in vitro experiment. Compared with control plants （not inoculated with PGPR）， the height， and aboveground and belowground dry weight of Leymus chinensis in a treatment inoculated with a PGPR strain named HPS14 were increased by 49.0%， 89.2% and 243.1%， respectively； and for plants inoculated with a PGPR strain named XPR2 were increased by 16.8%， 28.9% and 240.0%， respectively. 16S rRNA showed that these two strains belong to the species Inquilinus ginsengisoli and Phyllobacterium loti， respectively. These strains could be used for the development of microbial fertilizer.

Key words: Leymus chinensis, plant growth-promoting rhizobacteria, nitrogen fixing bacteria, phosphorus solubilizing bacteria, 1-aminocyclopropane-1-carboxylic acid deminase