Symbioses of Plants with Rhizobia and Mycorrhizal Fungi in Heavy Metal-Contaminated Tropical Soils

Abstract

Symbioses of plants with rhizobia and mycorrhizal fungi have been poorly studied in heavy metal-contaminated tropical soils. Studies found were mainly in areas subject to Zn processing, coal and gold mining activities and sewage sludge application. Although no data were found about the occurrence of rhizobia in these soils, tolerance of rhizobia from diverse origins (plant species and ecosystems) revealed strains with high tolerance to Zn, Cd and Cu. Cupriavidus necator, recently described as a nitrogen-fixing legume symbiont, is among the high-tolerant species. The arbuscular mycorrhizal fungi (AMF) genera most commonly reported in heavy metal (HM)-contaminated areas were Glomus and Acaulospora. Some tropical legume trees have been identified as promising for programmes to recover soils degraded by excess HMs, such as Enterolobium contortisiliquum and Acacia mangium, which are able to form an efficient symbiosis with Bradyrhizobium, Mimosa caesalpiniaefolia and Leucaena leucocephala, which form symbioses with Cupriavidus necator. AMF were shown to improve the tolerance of hosting plants to As in contaminated soils. Leguminous species had a low As translocation index from the roots to shoots when inoculated with AMF species.