UNCOVERING NEW ELEMENTS OF IRON HOMEOSTASIS IN MAIZE

This research aimed to contribute to the current understanding of the molecular mechanisms of iron homeostasis in plants, specifically in grasses. Iron (Fe) is a micronutrient essential for plant growth and development, involved in processes such as photosynthesis and respiration. Iron is also crucial in the human diet, as insufficient iron intake leads to iron deficiency anemia, which affects about 25% of the world\u27s population. Plants acquire iron from the soil through the roots and translocate it to other tissues, including the edible grains. These processes are highly regulated through multi-component networks, to ensure that potential toxic effects of excess iron are managed. Because iron is naturally insoluble in the soil, plants have developed mechanisms to increase its uptake. There are two distinct uptake mechanisms, Strategy I, performed by non-grass plant species and Strategy II, performed by grass species. Our knowledge of the mechanisms taking place in grasses is less detailed than in non-grasses. Most cereal crops are grasses, highlighting the need to uncover more aspects of their iron response. My research focused on gene discovery, a common approach for the nutritional improvement of crops. During this time, I aimed to identify new components of iron homeostasis in maize and understand their role in iron uptake. As a first step, I analyzed the maize transcriptomic response to iron deficiency and established a set of iron-responding genes in maize. To determine their possible roles in iron homeostasis, individual genes were investigated for similarity to those in other, better-characterized species. Additionally, I analyzed novel iron-deficient maize mutants that expand the list of critical iron-homeostasis genes. These were identified by screening mutant stocks for the iron-deficient (yellow-striped) phenotype. Mutants were analyzed using genomic approaches for variations that cause detrimental gene effects. One of these mutants, ivy (interveinal yellow), was determined to be iron deficient, and the underlying gene was confirmed as ZmCCD8, a key enzyme in the biosynthesis of strigolactones. Even though these hormones have been found to participate in various plant processes, they have not been previously linked to iron homeostasis. Mutant plants had altered iron-regulated gene expression. Comparison of the transcriptomes of wild-type iron-deficient plants and maize ccd8 mutants revealed suppression of several genes of the iron uptake machinery in ccd8. These genes are normally upregulated during iron deficiency and include the iron-regulated transcription factor IRO2 and genes involved in the biosynthesis of iron chelators and transporters. Additionally, external supply of synthetic strigolactone in the mutants alleviated chlorosis and returned iron-regulated gene expression to basal levels. The gene expression in the ccd8 mutants responded normally during limited iron conditions, indicating that strigolactones are not required for the iron deficiency response. I have hypothesized that strigolactones contribute to normal expression of iron-regulated genes in maize under sufficient iron conditions and may act to enhance iron uptake during iron deficiency