| Iron and zinc are essential metal micronutrients that most humans acquire from plant-based diets. Unfortunately, plants are not a good source of these micronutrients, so billions of people worldwide suffer from deficiencies of these two elements. If the molecular mechanisms behind metal homeostasis in plants were understood, plants could be engineered with increased micronutrient content to improve human health. In addition, understanding how plants acquire metals from the soil may allow the engineering of plants that are better able to grow on marginal soils, thus leading to overall increases in crop productivity. Another problem facing the industrial world where increased knowledge of metal homeostasis in plants could be quite useful involves heavy metal contamination of soil due to mining, metal smelting, and other human activities. Plants can be engineered to have enhanced metal uptake capabilities as well as enhanced metal tolerance mechanisms, and then used to clean up contaminated soils. For these reasons, I have sought to further characterize members of a family of metal transporters in Arabidopsis thaliana known to be involved in the transport of various transition metals, including iron and zinc.; The proteins ZIP5, ZIP6, and ZIP9 were identified by sequence similarity to other ZIP metal transporters but were otherwise uncharacterized. I have used mRNA expression data, reverse genetics techniques, overexpression studies and yeast complementation studies to demonstrate that ZIP5 functions as a zinc transporter in both yeast and plants. ZIP5 is able to complement a yeast zinc uptake mutant, supporting a role for ZIP5 in zinc transport. ZIP5 mRNA is induced in the roots and shoots in response to zinc deficiency. The zip5 loss-of-function mutant, zip5-1 , has retarded growth relative to wild-type and cannot be rescued by supplying high levels of zinc, suggesting that ZIP5 is not involved in uptake of zinc from the soil. Because zip5-1 plants have wild-type levels of zinc in their shoots, ZIP5 is also not likely to be involved in loading the xylem. The observed growth defect of the zip5 loss-of-function mutant may be due to a failure to properly distribute zinc to growing tissues. Consistent with this, zip5-1 seeds have 50% less zinc than wild-type seeds, and seeds from plants overexpressing ZIP5 have 40% more zinc. (Abstract shortened by UMI.)... |
