Scientists from the Zhejiang University College of Life Sciences reveal the function of a transcription factor that could help alleviate iron deficiency in our diets.
Iron is a mineral crucial to the body’s growth and development. However, staple food crops like rice, wheat, and maize, have relatively little iron. The importance of these crops to the human diet has led them to become the main cause of iron deficiency today. According to the World Health Organization, more than 30 per cent of the world’s human population suffers from iron deficiency anaemia, a condition where there is a shortage of healthy red blood cells. As plants are the primary source of dietary iron and an iron deficiency can seriously affect one’s health, there is a need to understand how iron is maintained in plants to improve crop production and human nutrition.
It is previously known that plants store iron in their seeds to aid the growth of seedlings after germination. During the fruit-bearing process, mother plants store iron in seeds. However, too much iron in the seed is toxic to embryos. Thus, how much iron is accumulated needs to be tightly regulated during seed development. Despite the importance of seed iron loading, how plants control this process and its biological function remains to be known.
In this study led by Professor Zheng Shao-Jian from the Zhejiang University College of Life Sciences, researchers looked into the underlying mechanism and biological significance of seed iron accumulation and found that the protein INO promotes reproductive success by protecting seeds from the toxic effects of excessive iron accumulation.
They carried out a mutant screen and isolated a T-DNA insertion mutant that displayed increased tolerance to iron deficiency when grown on an iron-free medium. The causal T-DNA was found to be inserted in a gene called INO, which codes for a transcription factor INO that was previously known to be crucial for Arabidopsis reproductive development.
By taking a closer look at INO, the researchers ran a series of tests and observed that in INO gene knockdown mutants, there was increased embryonic iron accumulation, suggesting that iron is transported to the seeds through the phloem and the outer layer of the seed coat called the outer integument. This observation further puts forth the idea that nutrient movement through the phloem and outer integument to the developing seed is a common pathway.
The team also found that increased expression of the gene NRAMP1, which encodes an iron transporter, in INO knockdown mutants. Conversely, when INO was overexpressed, the expression of NRAMP1 was downregulated. The lack of NRAMP1 was observed to reduce iron accumulation and increase sensitivity to iron deficiency, thereby suggesting that NRAMP1 is involved in seed iron loading.
Furthermore, as excessive iron loading is toxic to seeds, the study was able to demonstrate that such excessive iron loading in INO mutants could be rescued by NRAMP1 mutation. Taken together, their work suggests that INO plays a crucial role in protecting reproduction by reducing iron loading into developing seeds through the repression of NRAMP1 gene.
From this research, a molecular basis for improving iron content in crops may be established and we can explore the possibility of manipulating INO expression levels to increase seed iron loading without affecting embryogenesis. “The promotion of seed anti-oxidative capacity should also be considered in future crop breeding strategies aimed at increasing seed iron accumulation,” said Zheng.
Source: Sun, L. et al. (2021). Restriction of iron loading into developing seeds by a YABBY transcription factor safeguards successful reproduction in Arabidopsis. Molecular Plant.