APPLIED MICROBIAL AND CELL PHYSIOLOGY Phytohormone production by three strains of Bradyrhizobium japonicum and possible physiological and technological implications

The aim of this work was to evaluate phytohormone biosynthesis, siderophores production, and phosphate solubilization in three strains (E109, USDA110, and SEMIA5080) of Bradyrhizobium japonicum, most commonly used for inoculation of soybean and nonlegumes in USA, Canada, and South America. Siderophore production and phosphate solubilization were evaluated in selective culture conditions, which had negative results. Indole-3-acetic acid (IAA), gibberellic acid (GA3), and abscisic acid (ABA) production were analyzed by gas chromatography–mass spectrometry (GC-MS). Ethylene and zeatin biosynthesis were determined by GS–flame ionization detection and highperformance liquid chromatography (HPLC-UV), respectively. IAA, zeatin, and GA3 were found in all three strains; however, their levels were significantly higher (p<0.01) in SEMIA5080 (3.8 μg ml), USDA110 (2.5 μg ml), and E109 (0.87 μg ml), respectively. ABA biosynthesis was detected only in USDA110 (0.019 μg ml). Ethylene was found in all three strains, with highest production rate (18.1 ng ml h) in E109 cultured in yeast extract mannitol medium plus L-methionine. This is the first report of IAA, GA3, zeatin, ethylene, and ABA production by B. japonicum in pure cultures, using quantitative physicochemical methodology. The three strains have differential capability to produce the five major phytohormones and this fact may have an important technological implication for inoculant formulation.