HANOVER, N.H. – Researchers have isolated a gene that helps plants to move the iron in soil into their roots. The finding could enable scientists to develop plants that are more efficient in extracting this essential element from poor soils and to that are richer sources of dietary iron. The study, led by biologists Mary Lou Guerinot, of Dartmouth College, and Nigel Robinson, of Newcastle University, England, is reported in the Feb. 25 issue of the journal Nature.
A lack of nutrients that is not associated with famine conditions is known as "hidden hunger" and is recognized as the world's biggest malnutrition problem, says Guerinot. "Because plants are the principle source of iron for most people, the generation of iron-fortified crops could have a significant impact on human health." The World Health Organization estimates that more than 3 billion people suffer from iron deficiency.
Although iron is as essential to healthy plants as it is to healthy humans, more than a third of the world's soils are deficient in ferrous iron – the form of iron that plants need for growth. The ferric reductase gene, cloned by Guerinot and colleagues from the common weed Arabidopsis, produces an enzyme that transforms ferric iron, the most common form of iron found in soils, into ferrous iron. The ferric reductase enzyme straddles the surfaces of the plant's root cells in a series of loops. Each molecule of reductase collects electrons from chemicals manufactured inside the root cell and passes them onto ferric compounds in the soil around the root. This converts the ferric compounds into ferrous compounds which can then be absorbed by the root cell.
To isolate the gene that encodes the reductase enzyme, the Dartmouth investigators produced large numbers of randomly-mutated specimens of Arabidopsis and screened them until they were able to identify two specimens unable to produce the reductase enzyme. Meanwhile, the Newcastle team used molecular techniques to clone several genes that might encode reductase enzymes. By mapping the genes and the mutations, the two groups realized that one of the genes identified by the Newcastle team was probably the gene mutated in the plants identified by the Dartmouth team. This was confirmed by sequencing the gene, called FRO2, in the two mutant plant specimens. Graduate student Catherine Procter of the Newcastle team found that in both cases, the gene had mutated to a form which would render it non-functional.
Postdoctoral researcher Erin Connolly of the Dartmouth team moved a working copy of the FRO2 gene back into the mutated plants, which then began to reduce and absorb iron in the normal way, confirming that the gene was responsible for production of reductase.
"Enhancing the nutrient content value of plants for human consumption has rarely been an objective of plant research, in part because it was presumed that nutrient-enhanced plants would be lower yielding," says Guerinot. "Scientists now realize that breeding or genetically engineering plants for improved nutrient content is a worthwhile objective."
The investigators have begun research on several closely-related genes that may be responsible for concentrating the iron in specific parts of plants, such as the leaves or fruits, or may be involved in uptake of other metals such as copper. Manipulating these genes could boost "green" technologies such as phytoremediation, a method of removing pollutants from industrial wasteland by growing plants on it.
The work by the US based team was funded by the National Science Foundation; the UK-based team was funded by the Biotechnology and Biological Sciences Research Council.
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