The effects of traits of Bacillus megaterium on seed and root colonization and their correlation with the suppression of Rhizoctonia root rot of soybean

Bacillus megaterium strain B153-2-2 is a potential bacterial biocontrol agent against Rhizoctonia solani isolate 2B12 (ISG-2B). To study the role of antagonism (Ant), chemotaxis (Che), motility (Mot), and sporulation (Spo) of the biocontrol agent during seed and root colonization and the correlation between root colonization and the suppression of soybean (Glycine max) root rot caused by R. solani, strain B153-2-2 (Che(+)Mot(+)Ant(++)Spo(++)) and the seven derived mutants with altered antagonism, chemotaxis, motility, and/or sporulation were used. The bacterial cells were introduced into soil separately either as a soybean seed coating or soil application. Two soil mixtures defined as coarse and fine soil were used. The bacterial cell chemotactic response to soybean root and seed exudates and antagonism to R. solani were significantly (p = 0.05) correlated with root and seed colonization in some but not all treatments. The sporulation-defective mutants had low cell populations immediately after application and, therefore, reduced root colonization. The differences in root colonization diminished among the mutants and strain B153-2-2 when R. solani was present in the soil or, as seedlings grew older. Soybean seedling roots grown in coarse soil had significantly greater colonization by B153-2-2 or its mutants and a lower disease index than that in fine soil. There was a significant positive correlation (r(2) = 0.78) between root colonization by strain B153-2-2 or its mutants and suppression of Rhizoctonia root rot.