UC San Diego

Heavy metals such as iron, zinc and manganese are essential for many biological processes. However, these essential heavy metals are toxic when present in excess, and certain heavy metals, such as cadmium and arsenic, have no biological function and are toxic even in trace amounts. In plants, heavy metals are taken up from the environment by the roots, transported and distributed through the plant via the xylem and phloem and ultimately accumulated in seeds, which serve as a primary dietary source for humans. Understanding metal loading into the seed is thus an integral part of producing seed-based foods of high nutritional quality with relatively low quantities of toxic, non-essential metals. In the first part of this thesis, a collection of Arabidopsis thaliana mutants were screened to identify molecular components underlying long-distance metal transport and seed loading by detecting altered metal accumulation in seeds. In the second part of this thesis, an A. thaliana mutant overaccumulating cadmium in seeds, opt3-2, is characterized. opt3-2 exhibits differential accumulation of iron, zinc, manganese and cadmium among leaves, roots and seeds, suggesting that metal-specific processes are responsible for long-distance transport of these metals. opt3-2 also overaccumulates the thiol peptide glutathione in leaves but underaccumulates glutathione in roots. Furthermore, OPT3 is shown to play a role in the regulation of the iron deficiency response and may mediate the shoot-to-root transport of a signaling molecule, possibly glutathione, regulating metal uptake in the roots in response to metal accumulation in leaves