Effect of Wheat NAM Genes on Remobilization of Fe and Zn and Translocation of Minerals to Grain During Grain Fill.

We are interested in understanding mineral translocation to seeds to improve their nutritional value.  We compared a transgenic wheat (NAM RNAi knock-down) with low grain Fe and Zn concentrations with its isogenic control to quantify the effects of NAM genes on mineral remobilization from vegetative tissues.  We define remobilization as the net loss of stored mineral content from an organ over time. RNAi and control wheat were harvested at several time points from anthesis to maturity, and separated into vegetative tissues and grain.  Mineral concentrations and contents were determined.  When grown in soil, total vegetative Fe and Zn content decreased in controls, indicating remobilization.  Vegetative Fe content decreased 39%, which could account for 47% of grain Fe, while Zn content decreased 57%, accounting for 43% of grain Zn.  In the RNAi line, only Zn was remobilized (25% decrease), accounting for 23% of grain Zn.  Continued uptake during grain filling was the major contributor of Fe and Zn content. Iron and Zn grain concentrations and contents were substantially lower in the RNAi line, although grain size and yield were unaffected.  When grown hydroponically with ample nutrients, remobilization from total vegetative tissue was not seen, but grain Fe and Zn contents remained lower in the RNAi line.  However, when Fe was withheld post-anthesis, grain size and yield were unaffected, but both control and RNAi lines demonstrated Fe remobilization, indicating that Fe-limited plants increased their remobilization percentage.  Hydroponically vs. soil-grown wheat grain had two-fold higher Fe and four-fold higher Zn concentrations, indicating that grain can accumulate more of these metals, if uptake and translocation processes can be increased.  In summary, under limiting mineral supply, the wheat NAM genes increase Fe and Zn net remobilization from leaves. Under abundant Fe and Zn supply they also increase the translocation of minerals absorbed during grain fill.