Increased heavy metal tolerance of cowpea plants by dual inoculation of an arbuscular mycorrhizal fungi and nitrogen-fixer Rhizobium bacterium

Through biological inoculation technology, the bacterial-mycorrhizal-legume tripartite symbiosis in artificially heavy metal polluted soil was documented and the effects of dual inoculation with arbuscular mycorrhizal (AM) fungus and Rhizobium (N-fixing bacteria, NFB) on the host plant cowpea (Vigna sinensis) in pot cultures were investigated at six concentrations of Zn (0.0 – 1000 mg/kg dry soil) and Cd (0.0 – 100 mg/kg dry soil). From a number of physiological indices measured in this study, microsymbionts significantly increased dry weight, root : shoot ratios, leaf number and area, plant length, leaf pigments, total carbohydrates, N and P content of infected plants as compared with non infected controls at all levels of heavy metal concentrations. Tolerance index of cowpea plants was increased in the presence of microsymbionts than in their absence in polluted soil. Microsymbionts dependencies of cowpea plants tended to be increased at higher levels of Zn and Cd in polluted soil. Metals accumulated by microsymbionts-infected cowpea plant were mostly distributed in root tissues, suggesting that an exclusion strategy for metal tolerance widely exists in them. This study provides evidence for benefits of NFB to AM fungi in the protection of host plants against the detrimental effects of heavy metals. If so, bacterial-AM-legume tripartite symbiosis could be a new approach to increase the heavy metal tolerance of legumes plants under heavy metal polluted soil.Keyword: Mycorrhiza, Vigna sinesis, heavy metals, microsymbiosis, Rhizobiu