Single and dual inoculation with rhizobacteria on alfalfa (Medicago sativa L.) growth under lead stress conditions

Tailings from the Zeida mining region, located in the Middle Atlas Mountains of Morocco, contain high levels of lead and zinc which have many adverse effects, regarding both the environment and the health of the local human population. Finding practical methods to limit heavy metal dispersion and subsequent pollution of ecosystems in this area is therefore critical. This study aims to evaluate lead-tolerant rhizobacteria with an aim of exhibiting multiple plant growth-promoting traits. Thus, the growth of Medicago sativa may be improved and its resistance under lead stress conditions and may be subsequently used for the phytostabilization of lead-contaminated soils. Forty bacteria were isolated from the rhizospheric soil of Astragalus armatus plants growing wildly in the Zeida mine tailings. After preventing the duplicates of obtained isolates, the resistance to various heavy metals at high levels allowed the selection of two strains (i.e. AaR114 and AaR72). These strains were evaluated in vitro for characteristics that promote plant development, such as the synthesis of 1-aminocyclopropane-1-carboxylic acid deaminase, indoleacetic acid, hydrogen cyanide, siderophore, phosphate solubilization, and antifungal activity. Inoculation of M. sativa plants with rhizobacteria AaR114 and AaR72, in the presence of 100 mu g mL(-1) of lead-acetate, was shown to significantly improve plant tolerance, increase aerial and root biomass, and diminished the negative impacts of heavy metals on plants. The 16S rRNA sequences analyses of the bacteria revealed that the strains AaR114 and AaR72 were linked to Bacillus subtilis DSM 10( T) and Neobacillus niacini NBRC 15566( T), respectively.