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

Background: 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. Objectives: The goal of this work was to characterize the Plant Growth Promoting Rhizobacteria (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. Main results: 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 and the addition of Pb or Zn to the culture medium differently affected the PGP properties analyzed. Novelty statement: Inoculation of black locust grown with the 3 selected strains, in the presence 1000 μg ml 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 12092, and B. halotolerans NBRC 15718, respectively.This work was supported by the Moroccan ministry of Higher Education and Scientific research within the frame of PPR2 project “Utilization des biotechnologies microbiennes et Végétales pour la réhabilitation des sites miniers abandonnés” (BOMIVER). Miss Meryeme Bennis was granted a Khadija Baddouri fellowship in 2019 from HORIZON and France Foundation. Financial support was also obtained from the European Regional Development Fund (ERDF) cofinanced Grant AGL2013-45087-R from Ministerio de Economía y Competitividad (Spain)