Characterization of rhizosphere and endophytic bacteria from roots of maize (Zea mays L.) plant irrigated with wastewater with biotechnological potential in agriculture

The aim of this study was to characterize culturable rhizosphere and endophytic bacterial isolates isolated from rhizosphere soil and roots of maize plant irrigated with industrial and municipal wastewater in terms of resistance to heavy metals and salinity and plant growth promoting (PGP) traits. Results illustrated that both rhizosphere isolates and endophytic ones had various PGP characteristics in terms of both the number and the production amount of these characteristics. A substantial number of the bacterial isolates (both endophytic isolates and rhizosphere isolates) were tolerant to heavy metals (multi-metal resistant bacteria). Compared to endophytic isolates, rhizosphere isolates had greater resistance to heavy metals. Both endophytic isolates and rhizosphere ones showed remarkable resistance to salinity (7% NaCl). Based on comparison of 16S rRNA sequences and biochemical tests, the effective isolates, based on having multiple PGP characteristics and higher resistance to heavy metals and salinity, were identified. Isolates N5 and R7 were closely related to Bacillus cereus and Enterobacter cloacae, respectively. In addition, the ability of rhizosphere strain R7, as a multi-metal resistant bacterium, in the removal of cadmium (Cd) and lead (Pb) by its biomass and colonization of maize roots in the presence of these metals was evaluated. This strain could remove these metals from the solution (46.5-88.95%) and colonize both root surface and inside root of maize (4-7 Log10 CFU (colony-forming unit) g^-1 fresh root weight) under heavy metal stress. Therefore, it can be concluded that maize plant irrigated with industrial and municipal wastewater harbors salinity and heavy metals-resistant bacteria and may be potential reservoirs for isolating bacteria effective at alleviating heavy metal stress in the plant, reducing accumulation of heavy metals in crops such as maize, and removing heavy metals in aqueous media (bioremediation of heavy metal-contaminated wastewater system).