Iron transport and storage within the seed coat and embryo of developing seeds of pea (Pisum sativum L.)

To understand the cellular processes related to iron transport and sequestration within the developing pea seed (Pisum sativum), total iron and ferritin iron were analysed in seed coat and embryo tissues of the iron-hyperaccumulating pea mutant, Sparkle [dgl, dgl], and its wild-type parent, cv. Sparkle. For plants grown hydroponically with 2 mu M Fe, embryo Fe concentrations averaged 65 mu g g(-1) dry weight in mature wild-type seeds and 163 mu g g(-1) dry weight in mature dgl seeds; iron concentrations were also higher in dgl seed coats. Extracted and electrophoretically separated seed proteins were probed with a polyclonal antibody raised against pea seed ferritin. In both genotypes, ferritin was detected in the embryo, but not in the seed coat. Ferritin iron accounted for 92% of the total iron in mature wildtype embryos, but only 42% of the total iron in mature dgl embryos. Radiotracer studies using Fe-59 were used to characterize the movement of iron within the seed coat. Unequal distribution of 59Fe in opposing sections taken from the two hemispheres of the seed coat demonstrated that iron was symplastically phloem unloaded. These results suggest that iron resides transiently within the nonvascular seed coat cells and that all cells at the inner surface of the seed coat may be involved in the release of iron to the embryo apoplast. However, the form of iron resident within the seed coat and/or taken up by the embryo is presently unknown.