Maize growth and yield promoting endophytes isolated into a legume root nodule by a cross-over approach

Rhizobia can colonize the roots of grasses in the Brazilian semiarid region; however, studies to isolate these bacteria for growth promotion of maize via unconventional isolation approaches have not been conducted. This study aimed to isolate, characterize (biochemically and molecularly) and assess the potential of cowpea nodulating/maize associated rhizobia for plant growth promotion. Extracts of surface-disinfected maize root were inoculated onto cowpea seeds in axenic conditions. Rhizobia were isolated and classified by sequencing of the 16 S rRNA and nodC genes, and recA and atpD were specifically assayed for Bradyrhizobium. The in vitro ability of rhizobia to produce auxin and siderophores, solubilize calcium phosphate, exhibit antagonism against the growth of the pathogen Fusarium verticillioides, and undergo endophytic colonization were evaluated. The ability of rhizobia to promote the growth of both potted cowpea and maize and to promote the growth and yield of maize under field conditions was also evaluated. Thirteen isolates were obtained, including six Rhizobium, four Agrobacterium, and three Bradyrhizobium isolates, which exhibited high genetic variability and low similarity with the type strains. All bacteria showed at least one plant growth promotion mechanism. Six bacteria stood out regarding the promotion of maize growth, and one bacterium showed the best performance in cowpea. The results of the field assay with maize indicated that Bradyrhizobium sp. ESA 114 and Rhizobium sp. ESA 116 induced the same maize grain yields as those observed in plants inoculated with the commercial strain of Azospirillum brasilense Ab-V5. Maize harbors a community of putative facultative endophytic rhizobia that are genetically diverse and have the potential to be used in inoculant production.