Assessment of growth-promoting properties of Pseudomonas spp. on soybeans under field conditions

The soybean is recognized worldwide as one of the most important crops due to its protein- and oil-rich seeds. Several beneficial bacterial strains, including those of the genus Pseudomonas, are known to increase plant yield and disease tolerance. The aim of this study was to test the potential of 15 strains of Pseudomonas spp. to promote soybean growth under field conditions. Strains were identified based on 16S rRNA and gyrB sequences, and strains belonging to risk group 1 were further analyzed. Selected strains were tested for plant growth-promoting (PGP) properties, biofilm formation and in vitro antimicrobial activity against various fungal pathogens. PGP activity indicators of the selected strains were evaluated on soybean plants grown in fertilizer-free soil and in soil treated with 70% and 100% fertilizers. Three Pseudomonas spp. strains, P. putida VB131A, P. fulva VB134B and P. rhodesiae VB143S, were selected as PGP candidates. All three strains showed the ability to produce IAA and solubilize phosphorus, while only P. rhodesiae VB143S showed strong lipase and protease activity. Only P. fulva VB134B showed the ability to produce biofilm and hydrogen cyanide. All three strains showed in vitro antifungal activity against Alternaria infectoria. P. fulva VB134B and P. rhodesiae VB143S showed antifungal activity against Alternaria alternata and P. putida VB131A and P. rhodesiae VB143S against Epicoccum nigrum. Only P. fulva VB134B showed activity against Monilinia laxa. Statistically significant differences in plant height were not observed regardless of the strain or soil treatment used. In terms of first pod height, the P. fulva VB134B treatment was most effective when applied to soybeans grown in 100% fertilized soil. In contrast, plants treated with P. rhodesiae VB143S and grown in 100% fertilized soil had the highest number of branches and pods per plant.