| Iron and zinc deficiency commonly limit plant growth and crop yields. Furthermore, deficiencies in both of these micronutrients are common throughout the world, particularly in developing countries where people may only consume simple diets consisting primarily of staple foods. Therefore, engineering plants with an increased ability to obtain zinc or iron from the soil offers a way to increase crop yields and possibly to fortify foods pre-harvest. In order to successfully obtain fortified plants, however, we must first understand the molecular mechanisms controlling the accumulation and distribution of metals throughout the plant. The goal of this thesis was to begin to dissect the mechanisms controlling uptake and distribution of iron and zinc.; To this end, we characterized the putative zinc transporter, ZIP4, and demonstrated that ZIP4 functions as a zinc transporter in planta. ZIP4 is able to mediate the transport of zinc in yeast, and ZIP4 mRNA accumulates to high levels in zinc deficient shoots. ZIP4 protein localizes to Golgi vesicles under zinc-replete conditions; however, when cells are depleted of zinc, ZIP4 accumulates at the plasma membrane. Thus, the subcellular localization of ZIP4 is regulated by zinc. This is the first report of metal-regulated subcellular localization of a ZIP family member in plants.; Although the mechanisms of iron uptake in plants are relatively well understood, little is known about the translocation of iron from the root to the shoot. More specifically, the transporters responsible for effluxing iron from the cells surrounding the vascular bundle in order to enter the xylem or the phloem have not yet been identified. Therefore, we identified two putative iron exporters, FPT1 and FPT2, which belong to a family of iron exporters first identified and characterized in vertebrates. Consistent with roles in iron distribution, FPT2 mRNA accumulates under iron-deficient conditions. Furthermore, FPT1 mRNA, although not iron-regulated, localizes to the interior of the root and along the veins in the shoot suggesting a role for FPT1 in supplying the phloem with iron. However, mutations in either FPT1 or FPT2 have no dramatic affect on iron homeostasis suggesting that the function of these two proteins at least partially overlaps. |
