Submitted to: Seed Science Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/1998
Publication Date: N/A
Interpretive Summary: Because pea seeds can serve as a human dietary source of the essential nutrient iron, we wanted to better understand how iron is transported to and stored within the developing pea seed. Our long-term goal is to get more iron into seeds for nutritional purposes. In this study we focused on two sections of the seed: the seed coat (outside tissues)and embryo (inner seed tissues; that portion of the seed normally found in split pea soup), and also studied these tissues in two types of pea: a normal pea variety and a mutant pea that can overaccumulate iron. We examined iron levels in the seed coat and embryo, iron movement throughout the seed, and the levels of iron in the storage protein, ferritin. Growth of plants on moderate levels of iron resulted in a nearly three-fold increase in seed iron content of the mutant pea, relative to normal pea. In both plant types, we found ferritin protein in the embryo, but not in the seed coat; this tells us that the seed coat is not involved in long-term iron storage. Ferritin iron made up nearly all the iron in the normal pea, but only 42 percent of the total iron in the mutant embryos. In studies with radioactive iron, we found an unequal distribution of labeled iron in the two sides of the seed coat. This suggests that iron is transported throughout the seed coat cells via cellular connections, rather than crossing cell membranes. This means that iron can move faster through the seed coat and that all the cells at the inner surface of the seed coat may be involved in the release of iron to the embryo. These results will help us identify the physiological processes that control iron content and storage in seeds.
Technical Abstract: To understand the cellular processes related to iron transport and sequestration within the developing pea seed (Pisum sativum L.), 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 uM Fe, embryo Fe concentrations averaged 65 ug/g dry weight in mature wild-type seeds an 163 ug/g dry weight in mature dgl seeds; iron concentrations also were elevated 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 percent of the total iron in mature wild-type embryos, but only 42 percent of the total iron in mature dgl embryos. Radiotracer studies using 59-Fe were used to characterize the movement of iron within the seed coat. Unequal distribution of 59-Fe 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 compartment 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.