Screening for plant growth-promoting rhizobacteria to promote early soybean growth

Although many studies have been conducted to identify the specific traits b y which plant growth-promoting rhizobacteria (PGPR) promote plant growth, u sually they were limited to studying just one or two of these traits. We se lected 116 isolates from bulk soil and the rhizosphere of soybean [Glycine max (L.) Merr.] and examined them for a wide array of traits that might inc rease early soybean growth in nonsterile soil (PGPR traits). A subsample of 23 isolates, all but one of which tested positive for one or more of these PGPR traits, was further screened for traits associated with biocontrol, ( brady)rhizobial inhibition, and rhizosphere competence. Six of eight isolat es positive for 1-aminocgclopropane-1-carboxglate (ACC, a precursor of ethy lene) deaminase production, four of seven isolates positive for siderophore production, three of four isolates positive for beta-1,3-glucanase product ion, and two of five isolates positive for P solubilization increased at le ast one aspect of early soybean growth. One isolate, which did not share an y of the PGPR traits tested in vitro except antagonism to Sclerotium rolfsi i and Sclerotinia sclerotiorum, also promoted soybean growth. One of the 23 isolates changed bradgrhizobial nodule occupancy. Although the presence of a PGPR trait in vitro does not guarantee that a particular isolate is a PG PR, the results suggest that rhizobacteria able to produce ACC deaminase an d, to a lesser extent, beta-1,3-glucanase or siderophores or those able to solubilize P in vitro may increase early soybean growth in nonsterile soil.