Iron homeostasis is hierarchically regulated by multiple inputs : evidence for the role of reactive oxygen species and iron-zinc cross talk

Abstract

Iron (Fe) is a heavy metal micronutrient vital for all forms of life. In plants, Fe deficiency results in chlorosis and reduced growth, while Fe excess results in lipid peroxidation through the generation of reactive oxygen species. Hence, Fe homeostasis must be tightly regulated. Plants have been shown to use multiple sensing mechanisms to regulate whole plant Fe demand (systemically) and at through protein level changes at the root epidermis (locally). The companion cell of the phloem has recently been strongly implicated as the site of systemic Fe sensing. In this work I demonstrate that leaves and roots are subject to multiple regulatory inputs which modulate Fe dependent gene expression in a hierarchical fashion, and was able to separate these responses into reactive oxygen species (ROS) dependent and independent groups. Excess heavy metal has been shown to generate ROS, hence plants must also balance relative abundances of each heavy metal to prevent deficiency/toxicity. We identified bZIP23, which was previously described as an inducer of Zn uptake, as a likely candidate to mediate the mediate Fe-Zn crosstalk through the characterization of the double mutant bzip23-1/opt3-2 which suppresses opt3 dependent induction of Fe deficiency responses, likely by directly regulating the Fe uptake machinery. To facilitate the identification of time dependent changes in root growth phenotypes, such as under heavy metal stress, I designed and constructed a Small Plant Imaging Platform (SPIP) which able to capture high quality images for automated time course analysis which we aim to distribute throughout the plant science community. Finally, I have performed the two complementary experiments to first identify Fe dependent changes in gene translation in companion cells which is paired with the identification of transcription factor which directly regulate OPT3. Initial results indicate a novel mechanism of Fe release from the cell wall in the leaf vasculature during Fe deficiency, and implicate transcription factors known to mediate Fe deficiency responses as being responsible for the rapid induction of OPT3 upon Fe deficiency.