Fragile bones, diarrhoea, reduced mental capacity, infection-prone skin and immune system disorders – the list of symptoms of zinc deficiency is very long. Unfortunately, it is also particularly relevant for many people. Up to 60 per cent of the population in the third world countries are at risk of suffering from zinc deficiency – particularly women and children. Zinc is an important co-worker for 300 enzymes in the body, so if the level of zinc is too low, there is a whole range of processes in the body that do not function optimally.

The problem can be solved in different ways. Scientists from the Faculty of Agricultural Sciences at Aarhus University are tackling the challenge from the biotechnological point of view by developing crops with a higher zinc content that will be more easily absorbed in the digestive system.

A number of foods are naturally rich in zinc – meat and nuts, for example, and oysters, in particular. However, meat and oysters are not what usually finds its way into the food bowls of the world’s most impoverished people. People in developing countries get most of their nutrition from plant products.

Cereals contain reasonable amounts of zinc, but most of it is in a form not readily absorbed by the body and to a large extend it disappears when the cereal is ground. This is because it is mainly found in the outer husk, which is usually removed when the grain is ground to flour. The absorption of zinc in the gut is also inhibited by phytic acid – an acid that is present in cereals.

Transport at the genetic level

As part of the initiative to increase zinc absorption in vulnerable people, scientists at the Faculty of Agricultural Sciences are investigating the transport mechanisms that zinc uses in the plant. The aim is to relocate some of the zinc content in the husks to the endosperm, which is the starchy part of the cereal that people mainly eat. By understanding the mechanisms responsible, if will be easier to adjust them.

The technique that scientists are using is called Laser Capture Microdissection (LCM), and involves cutting ultra-thin slices of the material – in this case tissue samples from barley grains. You then locate the cell or cells in the slices that you would like to examine further. These are cut out by a laser and subsequently transferred to a small plastic tube. In this way, you obtain very pure samples of the different types of cells that a grain contains.

- By using the LCM technology we have been able to isolate the specific tissues from the grain that are involved in import and deposition of zinc, says post-doc Birgitte Tauris from the Department of Genetics and Biotechnology at the Faculty of Agricultural Sciences. We have subsequently examined how strongly the different genes are expressed – a so-called gene expression profiling.

- On the basis of the expression data we can identify the specific genes that may be important for the transport and deposition of zinc in the grain. In this way we have been able to establish a model of how zinc is absorbed, transported and deposited in the grain and which genes are involved. This is, as far as we know, the first time that anyone tries to determine how many genes are involved. We can then test if we are right in our predictions by, for example, creating genetically modified plants in which we have altered the genetic activity of certain genes. We have started testing one transport protein that appears to be very important for the transport of zinc into the grain, explains Birgitte Tauris.

Just as a car will not start until you have turned the key in the ignition, then all genes have to be activated before they can be decoded. This happens when a particular area on the gene, the so-called promoter region, becomes activated. It is therefore the unique structure of the gene’s promoter that determines the extent to which the gene is decoded. Scientists have found a promoter from a gene that is very active in the transport cells of barley grains. By joining the promoter from this gene to the gene they presume codes for a zinc transporter (HMA2), they may be able to increase the amount of zinc that is transported into the kernel.

Zinc poised for action

With both car and key available, the transport of zinc into the barley grain should soon be ready to go, but so far it is only at the experimental stage. Improving the transport is not without problems, though. Chemically, zinc is very similar to the toxic metal cadmium.

- This means that if you increase the transport of zinc into the kernel, you also risk increasing the transport of cadmium. We therefore have to make the transport more zinc-specific, says Birgitte Tauris.

The work on increasing the content of zinc as well as its bioavailability has caught the interest of the organisation HarvestPlus (HarvestPlus.com). Their aim is to relieve micronutrient deficiency by normal plant breeding processes and genetic modification using primary cultured plants such as wheat, maize, rice, potato, beans and cassava.

The Molecular Biology and Biotechnology research unit to which Birgitte Tauris belongs has thus set up a research contract with HarvestPlus on the development of fortified wheat with a higher iron content that is more readily absorbed in the human digestive system.

The work on zinc is supported by The Danish Council for Technology and Innovation and the EU through the integrated project PHIME that focuses on the role of heavy metals for human health. Read more about PHIME on www.phime.org .

Text: Janne Hansen