Plants take up zinc from the soil into the roots, from where it then travels in the xylem to the shoot and from here in the phloem to the developing seeds. During this journey zinc has to cross a number of apoplastic barriers, where export from one symplast is needed before import into another. In Arabidopsis roots, the heavy metal ATPases AtHMA2 and AtHMA4 are localized to the pericycle cells and are important for the export of zinc, in order for zinc to enter the xylem and get to the shoot.

I have identified a new novel role for AtHMA2 and AtHMA4 in the developing seed. The Arabidopsis seed consists of three genetically distinct tissues (seed coat, endosperm and embryo), between which apoplastic barriers exist and where export-import systems are needed. I have localized AtHMA2 and AtHMA4 to the seed coat, the maternal tissue that surrounds the endosperm and embryo.

I have shown that AtHMA2 and AtHMA4 at this location actively export zinc from the mother plant seed coat. Mutant plants that lack AtHMA2 and AtHMA4 accumulate zinc in the seed coat, and consequently have vastly reduced amounts of zinc inside the seed.

The finding that AtHMA2 and AtHMA4 are involved in pumping zinc out of the mother plant seed coat, so that it can enter the endosperm and embryo, is an important step towards designing future biofortification strategies for increasing the zinc content of cereal seeds.