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ABSTRACT
Plants on contaminated mining soils often show a reduced growth due to nutrient depletion as well as trace elements (TEs) toxicity. Since those conditions threat plant's survival, plant growth-promoting rhizobacteria (PGPRs), such as rhizobia, might be of crucial importance for plant colonization on TE-contaminated soils. Native rhizobia from mining soils are promising candidates for bioaugmented phytoremediation of those soils as they are adapted to the specific conditions. In this work, rhizobia from Zn- and Cd-contaminated mining soils were in vitro screened for their PGP features [organic acids, indole-3-acetic acid (IAA), and siderophore (SID) production; 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity; and Ca3(PO4)2 solubilization] and Zn and Cd tolerance. In addition, some type and reference rhizobia strains were included in the study as well. The in vitro screening indicated that rhizobia and other native genera have great potential for phytoremediation purposes, by exerting, besides biological N2 fixation, other plant growth-promoting traits. Leucaena leucocephala–Mesorhizobium sp. (UFLA 01-765) showed multielement tolerance and an efficient symbiosis on contaminated soil, decreasing the activities of antioxidative enzymes in shoots. This symbiosis is a promising combination for phytostabilization.
Acknowledgments
The authors are very grateful to André Janssen and Jorik Janssen for their helpful assistance on soil sampling in Lommel; to Ariadna S. Lopez, Iva Cholakova, Hendrik Fourier, and Ms Carine Put for their helpful assistance on harvesting; to Ms Ann Wijgaerts and Ms Carine Put for their kind support with enzymatic analysis; and to Ann Sofie Stevens for helping with the pictures of the nodules.
Funding
This research was supported by the National Council for the Scientific and Technological Development (CNPq), the Commission for Improvement of Higher Education Staff (CAPES), and the Foundation for Research of the State of Minas Gerais (FAPEMIG). W.M. Rangel thanks CAPES for the Doctoral training sandwich abroad (BEX: 13079/2013-01). F.M.S. Moreira thanks CNPq for the research productivity fellowship and grant. The authors also thank CNPq, FAPEMIG, and CAPES for students' fellowship. This work also has been financially supported by the Hasselt University Methusalem project 08M03VGRJ. The authors gratefully acknowledge partial funding collaboration from Rede de Pesquisas em Áreas Afetadas por Mineração (RECUPERAMINA) through its coordinator Luiz R. G. Guilherme.