Alleviation of Salinity and Metal Stress Using Plant Growth-Promoting Rhizobacteria Isolated From Semiarid Moroccan Copper-Mine Soils

Phytoremediation is an eco-friendly method for rehabilitation of mine tailing using plants and their associated rhizosphere microorganisms. Some heavy metal and salt-tolerant plant growth promoting rhizobacteria (PGPR) could be beneficial in alleviating soil salinity and heavy metal stress. The aim of this work is to select PGPR that could be used in phytoremediation process. Twenty-nine rhizobacteria were examined for their ability to grow at increasing concentrations of NaCl and high concentrations of Zn, Pb, Cu and Cd. The results showed that seventeen rhizobacteria displayed high salinity and metal tolerance (up to 100g L − 1 NaCl, up to 5 mM Cd, 9 mM Pb, 10 mM Zn, and Cu up to 6 mM). This work showed also that salinity and metallic stress has affected bacterial growth and metabolism by increasing intracellular proline, soluble sugars, free amino-acids and exopolysaccharides production. Moreover, almost all tested bacteria maintained their PGP traits under 10 % of NaCl and multi-metal stress. Four strains exhibiting the best PGP activities namely Mesorhizobium tamadayense , Enterobacter xiangfangensis , Pseudomonas azotifigens and Streptomyces Caelestis were selected for root elongation bioassay. The consortium of these rhizobacteria improves significantly the root elongation of Peganum harmala and Lactuca sativa under metallic and salt stress. Thus, the rhizobacteria with beneficial traits as well as tolerance to abiotic stress could be useful to stimulate plants establishment under different environmental stresses.