Contents lists available at ScienceDirect Rhizosphere journal homepage: www.elsevier.com/locate/rhisph Synergic effect of arbuscular mycorrhizal fungi and bradyrhizobia on biomass response, element partitioning and metallothionein gene expression of soybean-host under excess soil zinc Young Bassey Ibiang a,b , Kazunori Sakamoto a, ⁎ a Graduate School of Horticulture, Chiba University, Matsudo, Chiba 271-8510, Japan b Department of Genetics and Biotechnology, University of Calabar, PMB 1115 Calabar, Nigeria ARTICLE INFO Keywords: Biofertilization Symbionts effectiveness Zn-Mn homeostasis Phytostimulation GmMT1 expression Plant-microbe symbioses ABSTRACT 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 fa- vored 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 transloca- tions. 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 sym- bionts root colonization, number and greenness of leaves, element partitioning and metallothionein gene ex- pression are indicated as important mechanisms underlying the effective partnership between AM and R, in the dual inoculation. 1. Introduction Rhizobia (R) and arbuscular mycorrhizal (AM) fungi colonize le- gume roots and maintain symbiosis with the host. Utilized as bio- fertilizers and bioprotectants, these microorganisms support host growth and responses to biotic and abiotic conditions, while deriving shelter and photosynthates in return (Smith and Read, 2008; Polacco and Todd, 2011). Generally, improvement in host bioproduction is an important outcome expected of their utilization in plant production systems, including in sub-optimal conditions such as drought, deficient or excessive soil trace elements, salinity, etc. (Gamalero et al., 2009). One of the problems with the deployment of bioinoculants, however, is that the biomass response of the host may be positive, negative, or nil (Nogueira and Cardoso, 2003; Smith and Smith, 2011). It has been argued that aside insufficient time for the “maturity” of plant-AM symbiosis during short-term studies (Smith and Smith, 2011), evaluating inoculant effectiveness using only total biomass (as is com- monly observed in literature) may mask biomass allocations between plant parts, and obscure a precise microsymbiont effect on host bio- production (Jayne and Quigley, 2014). To evaluate the biomass parti- tioning effects of symbionts, the biomass response calculation for dis- tinct parts of host-plant had been suggested (Poorter and Nagel, 2000; Veresoglou et al., 2012). Many factors may contribute to a negative biomass response of a host to inoculants, as host response is a complex issue involving the plant-microbe-environment interaction (Smith and Smith, 2011). Un- derstanding the factors underlying host response enhances the effi- ciency of the biological approach to optimizing crop productivity while minimizing the use of agrochemicals (Meena et al., 2017). Several re- ports indicate that due to synergic effects, dual inoculation with AM fungi (AMF) and R improved host performance more than single sym- biont inoculations (Antunes et al., 2006; Chalk et al., 2006). But this is https://doi.org/10.1016/j.rhisph.2018.03.002 Received 3 February 2018; Received in revised form 9 March 2018; Accepted 9 March 2018 ⁎ Corresponding author. E-mail addresses: youngangale@yahoo.com (Y.B. Ibiang), ksakamoto@faculty.chiba-u.jp (K. Sakamoto). Rhizosphere 6 (2018) 56–66 Available online 17 March 2018 2452-2198/ © 2018 Elsevier B.V. All rights reserved. T