Synergic effect of arbuscular mycorrhizal fungi and bradyrhizobia on biomass response, element partitioning and metallothionein gene expression of soybean-host under excess soil zinc

The synergic effect of rhizobia (R) and arbuscular mycorrhizal (AM) fungi on the bioproduction, trace element partitioning and metallothionein gene expression of soybean-host was investigated under normal and elevated soil zinc conditions. In a completely randomized 3×4 factorial design, the experimental treatments - Zn addition (0, 200, and 400 mg Zn kg−1 soil) and inoculation (uninoculated control, R, AM, and RAM dual inoculation) were set up in the greenhouse for nine weeks. While the inoculants effectiveness was decreased in 400 mg Zn kg−1 soils, RAM induced significantly higher biomass production under all soil Zn treatments. The biomass response indicated that AM modulated stem and root bioproduction in favor of leaf/pod, while rhizobium favored root production and potentiated AM effect in dual inoculation. The partitioning of Zn and Mn in the hosts indicated synergic effects between AM and R, in RAM plants. Compared with control, AM lowered leaf Zn concentrations by reducing root Zn concentrations and modulating root-to-stem and stem-to-leaf Zn translocations. Compared to AM, RAM plants achieved lower leaf and pod Zn concentrations by mainly reducing root-to-stem Zn translocation. Zn treatment increased leaf and pod Mn in control plants, but symbionts countered this by regulating root-to-stem Mn translocation, especially in RAM. Type 1 metallothionein gene expression in roots was highest in RAM and lowest in control plants, but Zn effects were not dose-dependent. Synergisms in symbionts root colonization, number and greenness of leaves, element partitioning and metallothionein gene expression are indicated as important mechanisms underlying the effective partnership between AM and R, in the dual inoculation.