Co-Inoculation of Bacillus velezensis Strain S141 and Bradyrhizobium Strains Promotes Nodule Growth and Nitrogen Fixation

Abstract

The objective of this research was to evaluate the PGPR effect on nodulation and nitrogen-fixing efficiency of soybean (<i>Glycine</i> max (L.) Merr.) by co-inoculation with <i>Bradyrhizobium</i> <i>diazoefficiens</i> USDA110. Co-inoculation of <i>Bacillus</i> <i>velezensis</i> S141 with USDA110 into soybean resulted in enhanced nodulation and N2-fixing efficiency by producing larger nodules. To understand the role of S141 on soybean and USDA110 symbiosis, putative genes related to IAA biosynthesis were disrupted, suggesting that co-inoculation of USDA110 with S141Δ<i>yhcX</i> reduces the number of large size nodules. It was revealed that <i>yhcX</i> may play a major role in IAA biosynthesis in S141 as well as provide a major impact on soybean growth promotion. The disruption of genes related to cytokinin biosynthesis and co-inoculation of USDA110 with S141Δ<i>IPI</i> reduced the number of very large size nodules, and it appears that IPI might play an important role in nodule size of soybean-<i>Bradyrhizobium</i> symbiosis. However, it was possible that not only IAA and cytokinin but also some other substances secreted from S141 facilitate <i>Bradyrhizobium</i> to trigger bigger nodule formation, resulting in enhanced N2-fixation. Therefore, the ability of S141 with Bradyrhizobium co-inoculation to enhance soybean N2-fixation strategy could be further developed for supreme soybean inoculants.