VrNramp5 is responsible for cadmium and manganese uptake in Vigna radiata roots

As a non-essential mineral, Cadmium inhibits the growth and development of plants. A cultivar (Sulu, SL as wild type) of mungbean is more sensitive to Cd toxicity than its mutant line (20#). However, there is little information about the underlying mechanism. In this study, mutant line (20#) of mungbean exhibited lower Cd accumulation than wild type SL based on the time-course and dose-dependent Cd content in mungbean roots. Meanwhile, compared to 20# roots, the root tips of SL possess a higher capacity for transient Cd influx during exposure to Cd alone or co-treatment with Cd and manganese (Mn). Cd exposure reduced Mn contents of roots in both SL and 20#. Likewise, the addition of Mn in solution or in Cd-polluted soils inhibited Cd uptake in roots. Such reciprocal inhibition shows that Cd competes with Mn for uptake in roots of mungbean. Comparative transcriptomic of roots in SL and 20# were analyzed by Illumina sequencing to elucidate the molecular mechanisms of competitive uptake between Cd and Mn for mungbean. The transcriptomic analysis demonstrated that 28 metal transporters genes showed the log 2 fold changes under Cd stress, including ABC, ZIP, CAX, CCX, MTP, and Nramp family members. Expression abundance of VrABCC15–1 and VrZIP8 genes were significant up-regulated, and VrNramp5 was down-regulated in roots of mungbean under Cd stress. Thus, the three genes were complementarily expressed in Cd sensitive yeast (Δycf1). Expression of VrNramp5 resulted in hypersensitivity to excess Cd owing to the increment of Cd uptake in yeasts during exposure to Cd, but not by VrABCC15–1 or VrZIP8. Expressing-VrNramp5-SL or -VrNramp5–20# yeasts also accumulate higher Mn than that of the expressing-empty vector yeasts. The findings of this study demonstrated that the differences of Cd accumulation between SL and 20# was due to Cd uptake differences, and VrNramp5 is involved in Mn and Cd uptake in mungbean roots.