Proposed some interactions at molecular level of PGPR coinoculated with Bradyrhizobium diazoefficiens USDA110 and B.japonicum THA6 on soybean symbiosis and its potential of field application

Abstract

The objectives of this research were to select the appropriate plant growth promoting rhizobacteria (PGPR) and evaluate their influence in promoting nodulation and N2-fixing efficiency of soybean (Glycine max) by coinoculation with Bradyrhizobium diazoefficiens USDA110 and B. japonicum THA6 strains. Selected 12 PGPR performed a significant capability of promoting N2-fixation, nodule number, nodule and plant dry weight with both of the commercial bradyrhizobial strains, USDA110 and THA6 (P<0.05). Furthermore, isolates S141 and S222, which are closely related to Bacillus subtilis and Staphylococcus sp., were selected for coinoculation with USDA110 and THA6. The effective coinoculation doses of PGPR:Bradyrhizobium on soybean were 106:106 colony forming unit (CFU)ml−1seed−1. The expression levels of soybean and Bradyrhizobium related genes including Glyma17g07330, otsA, phbC, dctA and nifH in nodule discontinuously triggered both up- and down-regulation at different time frames (2–7 WAI). The transmission electron microscopy (TEM) micrograph of coinoculated soybean nodule showed the compact cluster of bacteroids which was densely packed with poly-ß-hydroxybutyrate (PHB) granules. The amounts of PHBs remained in mature nodule of coinoculation treatments whilst single inoculation nodules were senescence. The induction of soybean root subsequently increased the nodulation signaling and then activated the trehalose accumulation and the transport of carbon that represented an increase in PHB accumulation, resulting in the enhancement of the nodulation and N2-fixation in soybean. These results were accordingly related with phenotypic characters in Leonard’s jar experiment in terms of enhancing the nodulation and N2-fixation in soybean. The effect of coinoculation experiment under field condition could increase 9.7–43.6% of seed yield per hectare which was higher than those of uninoculation or single inoculation of PGPR or Bradyrhizobium. Therefore, the efficiency to enhance soybean N2-fixation by coinoculation of S141 and S222 with Bradyrhizobium strategy could be developed for supreme soybean inoculants.