Gene Discovery for Improved Iron Nutrition in Maize

The micronutrient iron (Fe) is essential for photosynthesis, respiration, and many other processes, but Fe is only sparingly soluble in aqueous solution, making adequate acquisition by plants a serious challenge. Fe is a limiting factor for plant growth on approximately 30% of the world's arable lands. Furthermore, iron is highly reactive and, if over-accumulated, can cause cellular damage. As a response to these key properties of iron, plants have evolved highly regulated mechanisms to ensure efficient and tightly controlled acquisition from the soil, translocation from roots to shoots, and accumulation in developing seeds.
The global demand for crops with high concentrations of nutrients in edible tissues is increasing due to current trends in population growth, global climate change, and decreasing arable land resources . Fe deficiency in humans is a global health issue, affecting 1.62 billion people, or about 25% of the world's population. It is imperative that we gain a better understanding of the mechanisms that plants use to regulate iron homeostasis, since these will be important targets for future biofortification strategies.
Current knowledge of the molecular mechanisms governing plant iron uptake and translocation is limited, as is our knowledge of how these processes are controlled at the molecular level. During this project, we will use molecular, biochemical, and physiological approaches to better understand mechanisms of nutrient (i.e., iron) uptake, a stated goal of the National Institute for Food and Agriculture (NIFA). The focus of this proposal is on gene discovery, an engine for crop improvement in two important ways. Most obviously, understanding of the molecular mechanisms responsible for iron uptake and homeostasis is a requirement for genetic engineering approaches to crop improvement. Without knowledge of the genes involved, we cannot know what engineered approaches could be taken. However, public acceptance of engineering approaches is limited, and partly because of this, breeding approaches have been extremely important in currently used efforts to enhance the iron concentration in the edible parts of plants. Many studies have identified quantitative trait loci (QTL) that have small effects.
Discovery of additional genes will be essential in identifying the genes underlying these QTL and in understanding their function. At present, limited mechanistic knowledge limits our ability to understand these genes. Although gene discovery efforts are sometimes dismissed as having insufficient immediate impact on agriculture, we maintain that such efforts will be critical for solving roadblocks to successful iron biofortification that have now been encountered by many projects. Specifically, it is now clear that simply increasing iron uptake does not cause increased iron concentration in the plant.  To overcome this, discovery of the mechanisms that plants use to sense and signal iron levels within tissues will need to be better understood. These are the focus of our work, but virtually none of the genes involved in this sensing and signaling are known, particularly in cereal species. The genetic approaches employed to discover new genes, destroy efficient uptake through mutation but then, by identifying the mutant genes, reveal the molecular mechanism(s) that were responsible for regulating iron homeostasis. This is how we identify novel, previously uncharacterized genes and their functions. At the end of this project, three new genes relating to iron will be discovered.