Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/1995
Publication Date: N/A
Interpretive Summary: Iron is an important mineral nutrient for plants, and many plants in certain soil types suffer from iron-deficiency stress. This can lead to reductions in crop yield and low iron content of plant foods. We wanted to study the root, shoot, and iron-nutritional factors that control how iron is acquired by the roots of dicotyledonous plants, which are plants with two seed leaves. We studied a mutant type of pea (named "dgl") to find out why this mutant can overaccumulate iron to toxic levels. We studied an enzyme in the root cells, the iron reductase enzyme, that functions to "reduce" iron in the soil so it can be absorbed by the plant. Iron reduction is a process whereby the electrical charge on the iron atom is changed from +3 to +2; only the +2 form of iron can be absorbed by pea roots. We learned that the dgl mutant absorbs too much iron because its roots have very high reductase activity; normal plants regulate reductase activity in their roots so they don't absorb too much iron. We also studied whether this activity in dgl roots was associated with the root tissues directly, or perhaps was influenced by factors within the shoot tissues. In experiments where we grafted shoots of the dgl mutant onto roots of a normal pea, we demonstrated that the shoots were responsible for the heightened activity in the roots. Apparently, the shoots transmit a chemical signal to the shoots to influence root processes. Our results are important because an understanding of shoot-to-root communication may enable us to improve the iron content of important food crops.
Technical Abstract: To understand the root, shoot, and Fe-nutritional factors which regulate root Fe acquisition processes in dicotyledonous plants, Fe(III) reduction and net proton efflux were quantified in root systems of an Fe-hyperaccumulating mutant (dgl) and a parental (cv Dippes Gelbe Viktoria) genotype of Pisum sativum. Plants were grown with (+Fe-treated) or without (-Fe-treated) added Fe(III)-N,N'-ethylenebis[2-(2-hydroxyphenyl)-glycine] (0.002 mM); root Fe(III) reduction was measured in solutions containing growth nutrients, 0.1 mM Fe(III)-ethylenediaminetetraacetic acid and 0.1 mM Na2-bathophenanthrolinedisulfonic acid. Daily measurements of Fe(III) reduction (d 10 to 20) revealed initially low rates in +Fe-treated and -Fe-treated dgl, followed by a nearly 5-fold stimulation in rates by d 15 for both growth types. In DGV, root Fe(III) reductase activity rose only minimally by d 20 in +Fe-treated plants, and about 3-fold in -Fe-treated plants, beginning on d 15. Net proton efflux was enhanced in roots of -Fe-treated DGV and both dgl growth types, relative to +Fe-treated DGV. In dgl, the enhanced proton efflux occurred prior to the increase in root Fe(III) reductase activity. Reductase studies using plants with reciprocal shoot:root grafts demonstrated that shoot expression of the dgl gene leads to the generation of a transmissible signal which enhances Fe(III) reductase activity in roots. The dgl gene product may alter, or interfere with, a normal component of a signal transduction mechanism regulating Fe homeostasis in plants.