Two MATE proteins play a role in iron efficiency in soybean

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 ferric reductase defective3 (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 (iso), 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. Along with efficient ferric chelate reductase activity and root iron uptake activity, high expression levels of FRD3 genes are also proposed as a target for future iron efficiency breeding projects.