Hong-Qing Ling, Chunlin Chen, Huilan Wu, Yue Zhang, Ning Wang and Yan Cui
The State Key Laboratory of Plant Cell and Chromosome Engineering, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, No. 1 West Beichen Road, Chaoyang District, Beijing 100101, China
Iron is an essential micronutrient for virtually all organisms. Its shortage or excess causes severe nutritional disorder. In plants, disrupted iron homeostasis will affect plant growth and development, leading to reduction of crop yield and quality. Additionally, iron deficiency is also a major malnutrition in human health (http://www.harvestplus.org/iron.html), increasing iron density in staple foods is considered to be the most effective and economic method to alleviate the problem of Fe deficiency in humans. To cope with the iron deficiency, plants have developed two main strategies, namely strategy I and strategy II, to effectively acquire iron from the rhizosphere. All plants except the gramineae use the strategy I mechanism (also called reduction mechanism) to acquire Fe from soil under iron deficiency, while the gramineous plants take up iron via the strategy II mechanism (also called chelation-based mechanism). The uptake, translocation, internal distribution and utilization of iron are genetically controlled. During the past decade, the molecular regulation mechanism of iron uptake and homeostasis has been intensively studied. Many genes involved in the regulation process have been isolated and their functions have been illustrated. Tomato FER, encoding a bHLH protein, is the first identified regulatory gene involved in the iron uptake and homeostasis. The ortholog of FER in Arabidopsis is FIT. FER/FIT is a key gene in the regulation of iron uptake and homeostasis in strategy I plants because loss of FER/FIT function plants are unable to activity the typical iron deficiency responses and the expression of iron uptake genes under iron limitation. In Arabidopsis, FIT interacted with the Ib subgroup bHLH (Ib bHLH) proteins, which contains bHLH38, bHLH39, bHLH100 and bHLH101, functioning in the transcriptional activation of iron uptake genes FRO2 and IRT1. The expression of the four Ib bHLH genes was negatively regulated by SKB1, which as an epigenetic negative modulator associated with chromatin of the promoters of the four Ib bHLH genes and mediated their histone H4R3 dimethylation according to iron status of plants. The four Ib bHLH proteins, possessing functional redundancy, dimerized with FIT. Further, MED16 subunit of the Mediator complex transmits signals to the Pol II complex by associating with FIT in the FIT/Ib BHLH complex for activating the expression of its regulated genes, such as FRO2 and IRT1. Additionally, it was confirmed that bHLH34 and bHLH104 were involved in iron uptake and homeostasis in Arabidopsis by controlling the expression of Ib bHLH and PYE genes. Recently, we also identified other genes functioning in the regulation of FIT expression and its protein stability. All show that the regulation mechanism of iron uptake and homeostasis in plants is very complicated and there are many genes involved in the regulation process. Here, we present the molecular regulation mechanisms of iron uptake and homeostasis by reviewing the progress obtained recently in plants, and prospect future development in this region.