A zinc-binding mechanism for health

Zinc is an essential micronutrient that humans must obtain in their diet. With around 25 % of the world's population deficient in zinc, increasing this mineral in food crops may be an effective way to improve global health.

The EU-funded STABLE PLANT (A new tool to study in-situ micronutrient uptake and translocation in plants) project looked at ways to improve zinc uptake by plants in zinc-deficient soils. Since many developing countries depend on rice as a staple food, STABLE PLANT developed a model system to analyse how rice takes up zinc.

Researchers already know that when plants are grown in iron-deficient soils, they release metal-binding agents called phytosiderophores into the soil. These form soluble complexes with iron that can be more readily absorbed by roots.

STABLE PLANT wanted to see whether rice plants use the same mechanism to absorb zinc. Researchers grew different rice varieties in zinc-deficient soils in the Philippines and investigated which forms of zinc are present in soils and plant stems.

Researchers detected a heavy form of zinc in plant stems rather than a lighter, more soluble form. This indicated that the plants absorbed the heavy, low-soluble zinc with the aid of phytosiderophores.

To further study the mechanism, researchers used another metal-binding agent that works in the same way as phytosiderophores. They found that it indeed formed complexes with heavy zinc rather than the lighter form.

Confirming this association, the more metal-binding agent they added, the more zinc was dissolved. This mechanism, which is particularly effective in the typical soil pH range, therefore allows plants to dissolve zinc in soils containing very little soluble zinc.

These results can help rice breeders develop high-zinc crop varieties by producing plants with high levels of phytosiderophores. This work may also be extended to other micronutrients and other crop systems.