Whole-root iron(III)-reductase activity throughout the life cycle of iron-grown Pisum sativum L. (Fabaceae): relevance to the iron nutrition of developing seeds

To understand the whole-plant processes which influence the Fe nutrition of developing seeds, we have characterized root Fe(III)-reductase activity and quantified whole-plant Fe balance throughout the complete 10-week (10-wk) life cycle of pea (Pisum sativum L., cv. Sparkle). Plants were grown hydroponically in complete nutrient solution with a continuous supply of chelated Fe; all side shoots were removed at first appearance to yield plants with one main shoot. Root Fe(III)-reductase activity was assayed with Fe(III)-EDTA. Flowering of the experimental plants began on wk 4 and continued until wk 6; seed growth and active seed import occurred during wks 5-10. Vegetative growth terminated at wk 6. Iron(III) reduction in whole-root systems was found to be dynamically modulated throughout the plant's life cycle, even though the plants were maintained on an Fe source. Iron(III)-reductase activity ranged from 1-3 micromol Fe reduced . g-1 DW . h-1 at early and late stages of the life cycle to 9.5 micromol Fe reduced . g-1 DW . h-1 at wk 6. Visual assays demonstrated that Fe(III)-reductase activity was localized to extensive regions of secondary and tertiary lateral roots during this peak activity. At midstages of growth (wks 6-7), root Fe(III)-reductase activity could be altered by changes in internal shoot Fe demand or external root Fe supply: removal of all pods or interruption of phloem transport from the reproductive portion of the shoot (to the roots) resulted in lowered root Fe(III)-reductase activity, while removal of Fe from the nutrient solution resulted in a stimulation of this activity. Total shoot Fe content increased throughout the 10-wk growth period, with Fe content in the non-seed tissues of the shoot declining by 50% of their maximal level and accounting for 35% of final seed Fe content. At maturity, total seed Fe represented 74% of total shoot Fe; total Fe in the roots (apoplasmic and symplasmic Fe combined) was minimal. These studies demonstrate that the root Fe(III)-reductase system responds to Fe status and/or Fe requirements of the shoot, apparently through shoot-to-root communication involving a phloem-mobile signal. During active seed-fill, enhanced root Fe(III)-reductase activity is necessary to generate sufficient Fe2+ for continued root Fe acquisition. This continuing Fe supply to the shoot is essential for the developing seeds to attain their Fe-content potential. Increased rates of root Fe(III) reduction would be necessary for seed Fe content to be enhanced in Pisum sativum.