ARS | Publication request: Can an increase in leaf iron reductase activity enhance seed iron accumulation in soybean?

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: March 31, 2008
Publication Date: October 11, 2008
Citation: Vasconcelos, M.W., Clemente, T.E., Grusak, M.A. 2008. Can an increase in leaf iron reductase activity enhance seed iron accumulation in soybean? [abstract]. XIV International Symposium on Iron Nutrition and Interactions in Plants, October 11-16, 2008, Beijing, China. p. 134.

Technical Abstract: Iron is an important micronutrient for human nutrition, with plant foods providing a significant amount of dietary iron in certain population groups, and in some cases, providing the sole source of dietary iron. Because iron deficiency is unfortunately common in many human populations, we have been interested in exploring all possible means to increase the iron concentration of edible plant products. Thus, our focus in this study was to analyze the role of leaf iron reductase activity in the movement of iron from leaves to developing seeds of soybean [Glycine max (L.) Merr.]. Iron is transported to soybean leaves as ferric citrate, but we don’t know if a reduction step is required before iron is phloem loaded and subsequently exported from leaves or other source tissues. For this work, we studied a transgenic soybean line in which the ectopic expression of AtFRO2 (the root iron reductase from Arabidopsis thaliana) confers iron reductase activity throughout the plant. Soybean cultivar Thorne was transformed with a construct containing the Arabidopsis FRO2 gene. Quantitative RT-PCR revealed that the CaMV 35S promoter drove AtFRO2 expression in different organs of the transgenic plants, including roots, leaves, and seeds. Plants were grown in hydroponic conditions and were provided either 10, 32 or 100 uM Fe(III)EDDHA until plant maturity. Protoplasts isolated from leaf cells of 35S-FRO2 and wild-type plants showed that AtFRO2 expression increased leaf iron reduction capacity up to 3-fold. Iron analysis of different shoot tissues revealed increased iron concentrations in the transgenic plants, including up to a 556% increase in leaves, a 250% increase in pod walls, but only a 10% increase in seeds, relative to wild-type controls. The lower increase in seed iron levels suggests that other factors besides leaf reductase capacity may be limiting the translocation of excess vegetative iron to the seeds. The 35S-FRO2 plants also had a higher expression of the ferritin gene in roots and leaves, suggesting that at least some of the excess iron was stored in the ferritin form. We will use these and other results to discuss possible mechanisms of iron mobilization within leaves, and the potential of using enhanced leaf iron reduction to increase iron concentration in soybean or other seeds.