Improved nodulation and seed yield of soybean (Glycine max) with a new isoflavone-based inoculant of Bradyrhizobium elkanii

Flavonoids produced by roots are secondary metabolites involved in the induction of nod genes in rhizobial cells and nodule organogenesis. We describe the development and quality control of a new formulation for soybean (Glycine max (L.) Merr.) based on Bradyrhizobium elkanii cells grown in a culture medium containing isoflavones. As opposed to B. japonicum, the performance of B. elkanii pre-incubated with isoflavones is unknown. Its evaluation implied in vitro, greenhouse, and field research trials. Greenhouse and in vitro treatments comprised: seeds inoculated with a traditional inoculant based on B. elkanii; seeds inoculated with a new formulation based on isoflavones-cultivated B. elkanii cells, the exogenous application of isoflavones into the growth substrate, and the non-inoculated control. Field treatments included non-inoculated controls with and without N-fertilizer and seeds inoculation with the traditional or the new formulation. No changes in quality parameters (bacterial survival on both seeds and liquid formulation) were observed, as compared with the traditionally produced bradyrhizobial cells. Also, improved plant response (nodulation, plant growth, and seed yield) was observed when seeds were inoculated with the new formulation. The exogenous application of isoflavones directly into the substrate had deleterious effects on soybean growth and nodulation; however, the increased values of agronomical parameters in field conditions show that the addition of isoflavones during the industrial fermentation of B. elkanii is a better option over the common bradyrhizobial inoculant.