Submitted to: Journal of Plant Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2005
Publication Date: 6/1/2006
Citation: Charlson, D.V., Shoemaker, R.C. 2006. Evolution of iron acquisition in higher plants. Journal of Plant Nutrition. 29:1109-1125. Interpretive Summary: Iron is a nutrient that plants need to survive. Plants acquire iron from the soil using one of two different processes. The grasses use a strategy (Strategy II) that is generally different from the strategy used by dicots such as soybean (Strategy I). The authors searched public databases looking for DNA sequences of the genes thought to be associated with the two different strategies. As expected, the dicots and gymnosperms did not have genes associated with the strategy used by grasses. However, surprisingly, the grasses appear to have genes associated with both Strategy I and Strategy II. This suggests that Strategy II mechanisms arose after the grasses evolved away from the non-grasses. This project demonstrated the usefulness of public databases in the study of evolution and metabolism. The information will be of interest to evolutionary biologists and to scientists studying the basic biology of nutrient uptake.
Technical Abstract: During iron-deficiency stress (IDS), Strategy I plants induce an iron-stress response (ISR) characterized by increased iron reduction at the root surface, acidification of the rhizosphere, and secretion of iron reductants. Although having many Strategy I activities, grasses employ the Strategy II mechanism, which involves the synthesis, secretion, and uptake of phytosiderophores the chelate Fe3+ from soil. In recent years, several genes involved in Strategies I and II have been identified in plants. We examined the phylogenetic distribution of iron-acquisition genes among plant species using publically available expressed sequence tages (ESTs) of five dicots (Arabidopsis, Medicago, potato, soybean, and tomato), five grasses (barley, corn, rice, sorghum, and wheat) and one gymnosperm (pine). In general for Strategy I genes, all plant species examined possessed ESTs with amino acid similarity to Arabidopsis genes for an iron reductase (FRO2), iron transporter (IRT1), and MATE protein (FRD3) implicated in iron homeostasis. Expressed sequence tags for a tomato bHLH transcription factor (FER) associated with regulation of the ISR were identified only in potato and tomato. For Strategy II genes, all examined plant species expressed ESTs with amino acid similarity to a phytosiderophore transporter (YS1) from corn and nicotianamine synthase. However, only the five grasses expressed genes for phytosiderophore synthesis; nicotianamine aminotransferase (NAAT-B) and mugineic acid synthase (IDS3) from barley. Due to the conservation of Strategy I genes among both Strategy I and II species and absenve of Strategy II genes in dicots and gymnosperm species, we concluded that Strategy II in grasses was derived relative to Strategy I. In addition, the conservation of iron-acquisition genes across the plant kingdom indicates the potential of modifying crop species via genetic engineering to improve tolerance to IDS and iron nutritional quality.