Phytotoxicity and genotoxicity as a new approach to assess heavy metals effect on Medicago sativa L.: Role of metallo-resistant rhizobacteria

Abstract The present work deals with the phytotoxicity and the genotoxicity of three heavy metals, namely copper, lead, and zinc on Medicago sativa seeds germination. Indeed, the role of metallo-resistant bacteria in lessening heavy metal toxicity was investigated. For this purpose, solutions of Cu, Pb, and Zn were used at the rate of 0.5, 1, 1.5, 2, and 2.5 mM for 4 d, along with untreated control using distilled water. The obtained findings displayed the stronger inhibitory effect of heavy metals on the germination and rhizogenesis process as well as a significant mito-depressive effect in the meristematic cells. For instance, Cu at the rate of 2.5 mM inhibited seed germination by 95 %, root elongation fell from 2.25 (in the control) to 0.14, while the mitotic index was reduced by 155%. Moreover, inoculation with metallo-resistant bacteria attenuated heavy metal toxicity, increased germination, rhizogenesis, mitotic index, and thus plant early growth. Metallo-resistant rhizobacteria inoculation improved the germination percentage by 40 % in the case of Cu and by 24 % in the case of Zn. The overall results suggest the cytotoxicity and the genotoxicity potential of Cu, Pb, and Zn on plants even at low concentrations. Furthermore, metallo-resistant bacteria application lessened heavy metal toxicity. This study confirms the suitability of M. sativa as a plant for the phytoremediation processes.