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

Although many studies have been conducted to identify the specific traits by which plant growth-promoting rhizobacteria (PGPR) promote plant growth, usually they were limited to studying just one or two of these traits. We selected 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 increase 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 isolates positive for 1-aminocgclopropane-1-carboxglate (ACC, a precursor of ethylene) deaminase production, four of seven isolates positive for siderophore production, three of four isolates positive for beta-1,3-glucanase production, and two of five isolates positive for P solubilization increased at least one aspect of early soybean growth. One isolate, which did not share any of the PGPR traits tested in vitro except antagonism to Sclerotium rolfsii 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 PGPR, the results suggest that rhizobacteria able to produce ACC deaminase and, 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.