Functional characterisation of the barley ZIP7 zinc transporter.

Abstract

Zinc (Zn) is an essential micronutrient for the function of many biological processes. Zn deficiency therefore affects normal growth and development in all living organisms. Many cropping soils are low in Zn, leading to widespread Zn deficiency in cereal crops. This reduces crop yield and grain nutrition value, which increases the risk of Zn deficiency-related health problems, especially for women and children in developing countries. Increasing Zn density in cereal grains (biofortification) is a potentially effective strategy to alleviate widespread Zn malnutrition in humans. However, grain Zn biofortification is restricted by the translocation of Zn from roots to shoots and from shoots to grains. So far, little is known about the metal transporters involved in these two limiting processes. Identification and characterisation of these transporters are crucial for Zn biofortification. The Zn regulated, Iron regulated-like Protein (ZIP) family is a group of transporters controlling cellular Zn influx, and could be important for Zn uptake and translocation in plants. In this study, barley (Hordeum vulgare L.) was used as a model plant to identify and characterise new ZIP transporters. Nine new HvZIP members were identified, and transcript analyses of 13 HvZIP genes revealed that eight of them were induced in Zn-deficient plants. HvZIP7 was further characterised as there is no available functional information for this transporter. HvZIP7 is primarily expressed in vascular tissues of roots and shoots and localises in the plasma membrane when expressed in onion epidermal cells. The over-expression of HvZIP7 resulted in a specific increase of Zn translocation from roots to shoots and Zn accumulation in shoots and grains when Zn is abundant in the growth media, suggesting that HvZIP7 mediates Zn translocation and/or retranslocation. The enhanced Zn accumulation in plants did not affect plant growth or grain yield. Furthermore, the over-expression of HvZIP7 did not enhance Cd accumulation in shoots, which differs from the over-expression of heavy metal ATPases. In addition, transgenic HvZIP7 plants could achieve 30% higher Zn concentration in grains than that of controls when grown with a low dose of Zn fertilisers under conditions similar to the field, indicating that our findings would have direct applications to Zn biofortification and the improvement of plant Zn nutrition in cereals.