Metabolism of nitrogen and carbon: Optimization of biological nitrogen fixation and cowpea development

Although the favorable role of plant growth-promoting bacteria (PGPB) in biological nitrogen fixation (BNF) is well established, their involvement in nodule metabolism is unknown. This study was performed to determine the relationship between C and N metabolism in cowpea nodules inoculated with only Bradyrhizobium sp. or with double and triple combinations of Bradyrhizobium sp., and two PGPB, Paenibacillus graminis and Paenibacillus durus during two critical phases of nodule development (flowering and beginning of senescence). The triple inoculation (Bradyrhizobium sp and two PGPB) induced higher N content in nodules, total-N accumulation per plant and shoot dry weight compared with other combinations at the beginning of senescence. This increased N performance was positively correlated with the nodule sucrose content but not with the content of total soluble carbohydrates, reduced sugars and starch. The higher BNF under triple inoculation conditions was not significantly associated with sucrose synthase activity but was slightly associated with soluble acid invertase activity in nodules at the beginning of senescence. These enzymes exhibited opposite responses between flowering and the beginning of senescence in all treatments. Glutamate synthase, glutamine synthetase and glutamate dehydrogenase were stimulated by double (Bradyrhizobium sp. + P. durus) and triple inoculation compared with other treatments. Our data revealed that inoculation with Bradyrhizobium sp. and PGPB is favorable for BNF activity in cowpeas. This positive interaction requires a complex balance involving enzymes and metabolites related to C and N metabolism.