Potential Plant-Growth-Promoting and Nitrogen-Fixing Bacteria Associated with Pioneer Plants Growing on Mine Tailings

Some pioneer plants are able to grow on mine tailings, an extreme environment characterized by high heavy metal (HM) content and lack of nutrients. Plant-growth-promoting rhizobacteria (PGPB) play an important role by improving conditions for plant adaptation, production, and establishment. Particularly, in this nitrogen-deficient environment, biological nitrogen fixation provides the major source of nitrogen available for plants. In this work, bacteria associated with six plant species growing on mine tailings in Zacatecas, Mexico, were isolated and selected for nitrogen fixation, phosphate solubilization, germination stimulation, auxin production, and HM resistance. The major nitrogen-fixing bacteria distributed among rhizospheres were members of the genus Paenibacillus, although Azospirillum lipoferum and Bradyrhizobium japonicum were also isolated. Particularly, Paenibacillus azotofixans BR_30 displayed the higher nitrogen-fixing activity and PGP activities, but was highly sensitive to HMs; meanwhile, Paenibacillus graminis BR_35 and Paenibacillus caespitis BR_32 were moderate nitrogen fixers, but maintained nitrogen-fixing activity in the presence of small concentrations of HMs. All Paenibacillus species were positive for acetylene reduction and capable of growing successfully in liquid nitrogen-free media. Among the non-nitrogen-fixing PGPBs, Agrobacterium, Olivibacter, Inquilinus, Sphingomonas, Brevundimonas, Pantoea, and Enterobacter species were identified. Agrobacterium, Azospirillum, and Rhizobium exhibited acid indole acetic acid production and Paenibacillus spp. and Olivibacter spp. were capable of phosphate solubilization. The majority of non-nitrogen-fixing PGPBs grew in moderate concentrations of HMs (up to 2 nM).