Evaluation of constitutive iron reductase (AtFRO2) expression on mineral accumulation and distribution in soybean (Glycine max L.)

Authors: VASCONCELOS, MARTA - Universidade Catolica Portuguesa; CLEMENTE, THOMAS - University Of Nebraska; Grusak, Michael

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2014
Publication Date: 4/3/2014
Citation: Vasconcelos, M.W., Clemente, T.E., Grusak, M.A. 2014. Evaluation of constitutive iron reductase (AtFRO2) expression on mineral accumulation and distribution in soybean (Glycine max L.). Frontiers in Plant Science. 5:112.

Interpretive Summary: Iron is one of the most important micronutrients in human and plant nutrition, and a suitable level of iron nutrition in plants is vital to providing adequate concentrations of this mineral in harvestable plant organs for human food or animal feed. Researchers have been interested in creating plant foods that are nutrient-dense in iron and other minerals, using an approach referred to as biofortification. Soybeans, being an important plant food in several parts of the world, would be a suitable target for biofortification programs. In this study, we tested whether plants that were modified in their ability to absorb and modify the form of iron within the plant, would demonstrate higher levels of iron in seeds. We also looked to see if this modification led to changes in the levels of other minerals. Surprisingly, although leaf and pod wall levels of iron were increased dramatically, seed iron levels were only increased slightly in the modified plants, relative to the controls. The concentrations of several other minerals were also increased in vegetative tissues of the iron modified plants. These results show that changes focused on one mineral can lead to much broader nutritional changes. This type of approach could be used to make more nutritious crop plants.

Technical Abstract: Iron is an important micronutrient in human and plant nutrition. Adequate iron nutrition during crop production is central for assuring appropriate iron concentrations in the harvestable organs, for human food or animal feed. The whole-plant movement of iron involves several processes, including the reduction of ferric to ferrous iron at several locations throughout the plant, prior to transmembrane trafficking of ferrous iron. In this study, soybean plants that constitutively expressed the AtFRO2 iron reductase gene were analyzed for leaf iron reductase activity, as well as the effect of this transgene's expression on root, leaf, pod wall, and seed mineral concentrations. High Fe supply, in combination with the constitutive expression of AtFRO2, resulted in significantly higher concentrations of different minerals in roots (K, P, Zn, Ca, Ni, Mg and Mo), pod walls (Fe, K, P, Cu and Ni), leaves (Fe, P, Cu, Ca, Ni and Mg) and seeds (Fe, Zn, Cu and Ni). Leaf and pod wall iron concentrations increased as much as 500% in transgenic plants, while seed iron concentrations only increased by 10%, suggesting that factors other than leaf and pod wall reductase activity were limiting the translocation of iron to seeds. Protoplasts isolated from transgenic leaves had three-fold higher reductase activity than controls. Expression levels of the iron storage protein, ferritin, were higher in the transgenic leaves than in wild-type, suggesting that the excess iron may be stored as ferritin in the leaves and therefore unavailable for phloem loading and delivery to the seeds. Also, citrate and malate levels in the roots and leaves of transgenic plants were significantly higher than in wild-type, suggesting that organic acid production could be related to the increased accumulation of minerals in roots, leaves and pod walls, but not in the seeds. All together, these results suggest a more ubiquitous role for the iron reductase in whole-plant mineral accumulation and distribution.