Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2009
Publication Date: 9/1/2009
Citation: Rogers, E.E., Wu, X., Stacey, G., Nguyen, H.T. 2009. Two MATE Proteins Play a Role in Iron Efficiency in Soybean. Journal of Plant Physiology. 166:1453-1459. Interpretive Summary: Iron is a necessary nutrient for plants; however, it is not very soluble. Like other nutrients obtained from the soil, it must first be taken into the plant roots and then moved to the above ground portions of the plant where it is needed for photosynthesis and other processes. Iron’s low solubility makes both these steps difficult. Citrate is a compound that binds to iron and increases its solubility. This work describes GmFRD3a and GmFRD3b, two proteins that transport citrate into the soybean vascular system, where the citrate can bind to iron, increase its solubility and allow more iron to move to the leaves. A soybean line that is very efficient at acquiring and translocating iron expresses higher levels of GmFRD3b than a soybean line that is not efficient at using iron. The iron efficient line also has higher citrate levels. It is likely that the higher expression of GmFRD3b causes the higher citrate levels and is one of the things that causes this line’s iron efficiency. To date, it has not been possible to combine the traits of high yield and iron efficiency in one soybean line, which makes modern elite varieties highly susceptible to iron deficiency and the associated yield losses. This work gives soybean breeders another target, high levels of GmFRDb, to incorporate into modern high-yielding varieties to improve iron efficiency.
Technical Abstract: Iron is a necessary but often limiting nutrient for plant growth and development. Soybeans grown on the high-pH calcareous soils are especially prone to developing iron deficiency chlorosis and suffering the resultant yield losses. Once iron is transported into the root, it must be translocated from the root to the shoot where it is needed for photosynthesis and other processes. Previous work has indicated that iron is likely to move through the xylem as ferric citrate. In Arabidopsis thaliana, citrate is effluxed into the xylem by the FRD3 protein. Here we present the identification and characterization of two soybean genes, GmFRD3a and GmFRD3b, with similar sequence and function to AtFRD3. The expression of both GmFRD3a and GmFRD3b is induced by iron deficiency in the iron-efficient reference cultivar Williams 82. GmFRD3b, but not GmFRD3a, is expressed at higher levels in the iron-efficient cultivar Clark than in its iron-inefficient near isogenic line iso-Clark, likely accounting for the higher xylem citrate levels in Clark. Increased xylem citrate levels lead to increased solubility of ferric iron in Clark xylem exudate as compared to iso-Clark exudate. These results support the hypothesis that high xylem citrate levels are needed for efficient root to shoot translocation of iron. High expression levels of FRD3 genes are also proposed as a target for future iron efficiency breeding projects.