Iron presents a quandary in biology. Its chemical properties have made it an essential metal for life, forming a vital part of cytochromes, globins and various redox enzymes. However, in its inorganic state it is relatively insoluble, resulting in the development of specialized systems for iron uptake and transport around living organisms. Iron is mainly taken up into the biosphere through the roots of plants, and in the 18 January issue of Nature, Elsbeth Walker and colleagues report the identification of a protein that is responsible for moving iron across this barrier in the world's major crop plants.

One strategy adopted by plants to absorb iron is to extrude from their roots small organic compounds called phytosiderophores. These bind tightly to the sparingly soluble iron in the soil, and are then reabsorbed into the roots, taking their chelated iron with them. Walker and colleagues have discovered the transporter for phytosiderophores back into the root in the classic mutant of maize known as yellow stripe 1 (ys1).

Yellow stripe 1 was first identified in 1929 by George W. Beadle, through its distinctive phenotype in which iron deficiency leads to incomplete pigmentation of leaves and alternating yellow and green stripes running their length (left in the figure, beside a wild-type leaf). By the early 1990s it was known that ys1 plants are defective in iron–phytosiderophore uptake, making the YS1 gene product a good candidate for the elusive phytosiderophore transporter.

To identify the YS1 gene, Walker's group used the Ac transposon to produce a library of mutant plants, one of whose disruptions proved allelic with ys1. The Ac-disrupted gene was then sequenced and its identity as yellow stripe confirmed by sequencing the allele from three separate ys1 mutants.

The gene encodes a protein that contains twelve putative transmembrane regions, but is this really a phytosiderophore transporter? As a test, the YS1 gene was introduced into a mutant yeast (Saccharomyces cerevisiae ) strain defective in extracting iron from the surrounding medium. This did not, on its own, rescue the yeast phenotype unless maize phytosiderophores were added to the growth medium.

At least one mystery remains. Maize's YS1 shows similarity to genes of unknown function from species throughout the plant kingdom — monocots, dicots, gymnosperms and mosses — many of which do not use phytosiderophores to absorb iron. Among those lacking phytosiderophores is Arabidopsis thaliana , but its recently sequenced genome contains no fewer than eight homologues of YS1. Perhaps these cousins are involved in the transport of iron within plants; for example by acting as a receptor for nicotianamine, an iron chelator found in all plants, which has been proposed to aid the long-range movement of iron through phloem tubes.

Iron deficiency is a leading nutritional disorder in the developing world, and this study brings us one step closer to staple crop varieties better able to accumulate bioavailable iron. Coming so close on the heels of the completion of the Arabidopsis genome sequence it also highlights the valuable genetic heritage of other, intensely studied plants. After all, Barbara McClintock discovered transposons, on which this and many other molecular biology studies are reliant, from work done entirely in maize.