Minimization of oxidative stress in cowpea nodules by the interrelationship between Bradyrhizobium sp. and plant growth-promoting bacteria

Abstract

Oxidative damage can result in various degenerative processes, including membrane lipid peroxidation, a process promoted by chain reactions initiated by reactive oxygen species that can rapidly affect many lipid molecules, resulting in damage to cellular structures. This study aimed to evaluate the symbiotic performance of cowpea plants inoculated and co-inoculated with Bradyrhizobium sp. and different combinations of plant growth-promoting bacteria (PGPB), Paenibacillus graminis or P. durus, based on biochemical variables related to protection/oxidative stress and senescence during and after the establishment of symbiosis. The experiment was conducted under greenhouse conditions. Cowpea seeds were disinfected and inoculated as follows: Bradyrhizobium sp.; co-inoculated with Bradyrhizobium sp. and P. graminis or Bradyrhizobium sp. and P. durus; or co-inoculated with Bradyrhizobium sp. and the two PGPB (P. graminis and P. durus). The plants were grown in Leonard jars containing washed and autoclaved sand. Uninoculated plants were used as an absolute control. The cowpea nodules were harvested at flowering point and the beginning of senescence. The results showed significant differences in the variables related to antioxidant metabolism in response to the treatments and harvest times. Although there was reduced ascorbate peroxidase, catalase and phenol peroxidase activities were observed in the plants co-inoculated with Bradyrhizobium sp. and P. graminis and Bradyrhizobium sp. and P. durus at flowering point, no accumulation of hydrogen peroxide or increase in lipid peroxidation was observed, indicating that the enzymatic activity was effective in controlling the possible oxidative damage in these plants. The cowpeas co-inoculated with Bradyrhizobium sp. and the two PGPB (P. graminis and P. durus) showed a delay of the deleterious effects of senescence and, therefore, a better symbiotic performance.