Leguminosae native nodulating bacteria from a gold mine As-contaminated soil: Multi-resistance to trace elements, and possible role in plant growth and mineral nutrition

ABSTRACT

Efficient N₂-fixing Leguminosae nodulating bacteria resistant to As may facilitate plant growth on As-contaminated sites. In order to identify bacteria holding these features, 24 strains were isolated from nodules of the trap species Crotalaria spectabilis (12) and Stizolobium aterrimum (12) growing on an As-contaminated gold mine site. 16S rRNA gene sequencing revealed that most of the strains belonged to the group of α-Proteobacteria, being representatives of the genera Bradyrhizobium, Rhizobium, Inquilinus, Labrys, Bosea, Starkeya, and Methylobacterium. Strains of the first four genera showed symbiotic efficiency with their original host, and demonstrated in vitro specific plant-growth-promoting (PGP) traits (production of organic acids, indole-3-acetic-acid and siderophores, 1-aminocyclopropane-1-carboxylate deaminase activity, and Ca₃(PO₄)₂ solubilization), and increased resistance to As, Zn, and Cd. In addition, these strains and some type and reference rhizobia strains exhibited a wide resistance spectrum to β-lactam antibiotics. Both intrinsic PGP abilities and multi-element resistance of rhizobia are promising for exploiting the symbiosis with different legume plants on trace-element-contaminated soils.

KEYWORDS:

• plant growth-promoting
• biological N₂ fixation
• trace elements multi-resistance
• β-lactam antibiotics resistance

Acknowledgments

The authors are very grateful to Teotônio S. de Carvalho for kindly preparing the dendrograms using R software. This research was supported by the Brazilian National Council for the Scientific and Technological Development (CNPq), the Brazilian 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).

Funding

F.M.S. Moreira thanks CNPq for the research productivity fellowship and grant (304574/2010-4). We also thank CNPq, FAPEMIG, and CAPES for students' fellowship. This work also has been financially supported by the Hasselt University Methusalem project 08M03VGRJ. We gratefully acknowledge partial funding contribution from the Rede de Pesquisas em Áreas Afetadas por Mineração (RECUPERAMINA) through its coordinator Luiz R. G. Guilherme.