Enhancing Iron Content in Maize by Gene Regulation

In my first blog on Brassica oleracea, I introduced that variants of these crops are rich in vitamins, dietary fiber, and many minerals. Among minerals, iron content is an essential micronutrient for crops and humans, which plays a crucial role in various biological processes necessary for growth and development. Here, I will discuss the importance of iron in crops and how to enhance the iron content, especially in maize, by gene regulation. 

Iron is an Essential Micronutrient for Crops

Iron, an essential micronutrient, playing an important role in various physiological and biochemical processes in crops. Most importantly, it is involved in the synthesis of chlorophyll, the pigment responsible for capturing light energy, which is necessary for photosynthesis. Without sufficient iron, plants cannot produce chlorophyll effectively, leading to reduced photosynthetic activity and impaired growth. And the crops with insufficient iron content exhibit yellow leaves, particularly in younger leaves.

Iron is also a key component of enzymes involved in energy transfer and metabolism. One of the examples is its important role in respiration. Iron is involved in various enzymatic reactions related to respiration, where energy is released from organic compounds to fuel cellular activities. Enzymes containing iron facilitate electron transport chains, which are essential for ATP production and metabolic processes. Speaking of metabolism, iron plays a role in the synthesis and metabolism of nitrogen compounds within crops. It is involved in the enzymatic conversion of nitrate to ammonium, which is an essential step in nitrogen assimilation and protein synthesis. Besides protein synthesis, iron-containing enzymes are also involved in DNA replication and repair, as well as cell cycle regulation. Moreover, iron uptake and transport mechanisms are critical for root development and function. Deficient iron content in crops can impair root growth, reduce the crop's ability to acquire water and nutrients from the soil.

Iron is indispensable for various biochemical and physiological processes in crops, influencing their growth, development, and productivity. Iron deficiency can lead to yellow or pale leaves, stunted growth, reduced biomass accumulation, and poor crop yield. Addressing iron deficiencies is essential for maintaining crop health and achieving optimal agricultural yields. 

Iron Uptake in Crops

As iron limitation severely affects crop growth, iron is often a component of agricultural fertilizers used to improve crop yields. Although iron is abundant on Earth, it is usually present in insoluble forms not available for crop uptake. To solubilize iron, roots release protons (H⁺) that convert insoluble iron into soluble ions for crops to absorb. Most crops preferentially absorb ferrous iron (Fe²⁺) due to its higher solubility and bioavailability. To enhance iron uptake by roots, crops release root-secreted compounds like phenolics and flavonoids to reduce ferric iron (Fe³⁺) to ferrous iron and organic chelators to bind to iron ions.

Once in the root vicinity, ferrous iron ions are taken up by specific transporter proteins embedded in the plasma membrane of root cells, such as the Iron-Regulated Transporters (IRTs) and Yellow Stripe-Like (YSL) proteins, which enables the uptake of ferrous iron across the root cell membrane and into the root cells' cytoplasm. Then iron ions are transported internally via the crop's vascular system to various tissues, including leaves, stems, and reproductive organs. This iron transport within the crops is regulated by particular proteins and chelators to ensure the optimal utilization of iron in different tissues and organs. (Figure 1).

It's important to note that the efficient iron uptake by crops can be influenced by various factors, including soil pH, soil iron availability, and the presence of other soil nutrients, etc. Iron deficiency in the soil can affect plant growth and development, highlighting the importance of maintaining efficient iron levels for crop productivity. In the following section, we will discuss how to influence the iron uptake by crops at the genetic level.

Enhancing Iron Content in Maize

As we know, iron is not only essential to crop productivity, but also a very important microelement for human health. Iron deficiency still remains a global health issue, especially among people in developing countries. Just like crops, iron deficiency can cause many health problems in humans, such as affecting physical and cognitive development, immune function, etc. One of the most common treatments is iron supplementation from iron-rich food in the diet. Here's a question... What if we increase the iron content in a specific crop and keep it in our daily diet or use it as treatment to iron deficiency? Researchers from Chinese Academy of Agricultural Sciences recently published a paper where they identified a gene in maize regulating iron content in maize kernels. The study provides an approach to develop iron-enriched maize. 

They first conducted a genome-wide association study (GWAS) for Fe concentrations in maize kernels and found 11 genes significantly correlated with iron content in maize kernels. By investigating the mRNA abundances of the 11 candidate genes, they found that only Zm00001d027400 [ZmNAC78 (NAM/ATAF/CUC DOMAIN TRANSCRIPTION FACTOR 78)] had consistently higher expression in high iron content maize kernels than in low ones. The expression level of ZmNAC78 is positively correlated with iron content in the random selected maize kernel inbred lines. Through further study on the upstream and downstream sequence of ZmNAC78, the researchers identified a major promoter of the target gene defined as the 500-bp region upstream of the transcription start site (TSS). They then successfully cultivated maize with high yield and high iron concentrations in their kernels by using a molecular marker developed from a 42-base pair insertion or deletion in the major promoter of ZmNAc78. (Figure 2) This confirmed their hypothesis that ZmNAC78 protein directly regulates messenger RNA abundance of iron transporters, resulting in higher iron content in maize kernels.

Conclusions and Future Perspectives

Micronutrient deficiency is a global problem for human health, as well as crop health. Even though there are some reported iron-enhanced crops, it hardly meets the increasing need for iron content in human diet, especially in developing countries. In this study, researchers significantly increased the maize kernel iron content and grain yield by using molecular assisted breeding to develop maize varieties with higher quality. It also provides insight into understanding and regulating iron content in gene level for the development of other iron-enriched crops.

References

Ramzan Y, Hafeez MB, Khan S, Nadeem M, Saleem-ur-Rahman, Batool S, et al. Biofortification with Zinc and Iron Improves the Grain Quality and Yield of Wheat Crop. Int J Plant Prod. 2020 Sep;14(3):501–10.

Connorton JM, Balk J, Rodríguez-Celma J. Iron homeostasis in plants - a brief overview. Metallomics. 2017 Jul 19;9(7):813–23.

Yan P, Du Q, Chen H, Guo Z, Wang Z, Tang J, et al. Biofortification of iron content by regulating a NAC transcription factor in maize. Science. 2023 Dec 8;382(6675):1159–65. 

Note: Images in this post have been created with BioRender and Tome.