Rhizobium tropici exopolysaccharides as carriers improve the symbiosis of cowpea-Bradyrhizobium-Paenibacillus

Exopolysaccharides (EPS) may represent a viable inoculant carrier to replace peat and reduce production costs. In addition, the use of plant growth-promoting bacteria (PGPB), as Paenibacillus, in association with rhizobia represents a new technology that can improve the biological nitrogen fixation (BNF) and crop productivity. Thus, the present research aimed to characterize the EPS that are produced by R. tropici and evaluate their effectiveness as carriers for cowpea inoculation. Initially, EPS were defined as a heteropolysaccharide polyanionic inert to bacterial strains. In addition, EPS were successful as vehicles for cowpea inoculation with Bradyrhizobium sp. compared to peat, enhancing the growth parameters and nitrogen fixation of these plants. Thus, EPS were used for cowpea inoculation with Bradyrhizobium sp. and Paenibacillus species to test the effectiveness of these carriers on unsterile soil. Cowpea development and productivity were observed after cowpea inoculation with Bradyrhizobium sp., co-inoculation with Bradyrhizobium sp. and P. graminis or with Bradyrhizobium sp. and P. durus, and simultaneous co-inoculation with Bradyrhizobium sp., P. graminis and P. durus using EPS as carriers. Cowpea plants were collected in two stages, flowering and grain filling, and the plants that were co-inoculated with Bradyrhizobium sp. and Paenibacillus sp. produced better results. Simultaneous co-inoculation with Bradyrhizobium sp., P. graminis and P. durus stood out from all treatments. Overall, the use of EPS that were produced by R. tropici and Paenibacillus species represents an emerging technology for the improvement of cowpea-rhizobia symbiosis due to the positive effects on BNF and crop productivity. Key words: Inoculant, plant growth-promoting bacteria, biological nitrogen fixation.