Characterization of plant growth-promoting bacteria isolated from the rhizosphere of Robinia pseudoacacia growing in metal-contaminated mine tailings in eastern Morocco.

Mining activity in the Touissit district of Eastern Morocco has led to an unprecedented accumulation of heavy metals, mainly lead and zinc, in the tailing ponds of the open-air mines. This poses a real danger to both the environment and local population. The goal of this work was to characterize the P lant G rowth P romoting R hizobacteria (PGPR) isolated from the rhizosphere soil of R. pseudoacacia plants grown wild in the abandoned Pb- and Zn-contaminated tailing ponds in the mining district of Touissit, in Eastern Morocco. One hundred bacterial strains were isolated from the rhizosphere of black locust (Robinia pseudoacacia L.) plants growing naturally in the Touissit mine tailings. Quantitative determination of indole-acetic and siderophores production, inorganic phosphate solubilization, hydrolysis of 1-aminocyclopropane-1-carboxylic acid (ACC deaminase activity), and ability to act as a biocontrol agent allowed selection of the 3 strains, 7MBT, 17MBT and 84MBT with improved PGP properties. The three strains grew well in the presence of high concentration of Pb-acetate and ZnCl 2; and the addition of Pb or Zn to the culture medium differently affected the PGP properties analyzed. Inoculation of black locust grown with the 3 selected strains, in the presence 1000 μg ml−1 of Pb-acetate, produced varying effects on the plant dry weight. The strain 84MBT alone or in combination with strains 7MBT and 17MBT increased significantly the dry weight of the plants by 91, 62, and 85% respectively. The 16S rRNA gene sequence analysis of each strain showed that the strains 7MBT 17MBT and 84MBT had 99.34, 100, and had 99.72% similarity with Priestia endophytica (formerly B. endophyticus), B. pumilus NBRC 12092T, and B. halotolerans NBRC 15718T, respectively. • Out of 100 strains, three were characterized as the best preforming in the PGP activities. • The three strains were identified as Bacillus pumilus, B. endophyticus, and B. halotolerans. • The strains tolerate high lead and zinc concentrations. • The metal stress enhanced the PGP activities of the three strains. • The inoculation by the strains improved significantly the growth of black locust by 85–91%. [ABSTRACT FROM AUTHOR]