The Regulatory Mechanisms Controlling Zinc Content in Wheat

Cereals such as wheat serve as staples for a large proportion of the world’s population. However, they contain relatively low concentrations of essential micronutrients such as zinc (Zn) in their edible tissues. This is a major issue for human nutrition and food security. The process of Zn uptake and partitioning in plants is highly controlled, with systems present for sensing and responding to Zn status. In the model plant, Arabidopsis thaliana, two transcription factors, bZIP19 and bZIP23, are thought to act as Zn sensors mediating the increased expression of Zn membrane transporters, ZIPs (Zrt/Irt-like proteins), in response to low Zn status. In this thesis the identification and characterisation of homologous bZIP transcription factors and ZIP transporters in wheat are described. TabZIP sequence analysis confirmed the presence of motifs characteristic to the F-group of bZIP transcription factors. Expression of these wheat bZIPs in an Atbzip19 bzip23 line showed a conservation of function between the Arabidopsis and wheat group F bZIPs. A key question is whether the wheat bZIP transcription factors and ZIP transporters are regulated by Zn. Gene-expression analysis indicated that the wheat transcription factors TabZIP1, 3a, 3b, 4 & 5, and the wheat ZIP transporters, TaZIP1, 4, 5, 6 & 7 are induced by Zn-deficient conditions. The Zn-transport capability of TaZIP1, 5, 6, 7 & 8 was confirmed using heterologous yeast expression. Additionally, the binding ability of TabZIPs to regulatory-elements in the promoters of TaZIPs was demonstrated. This links TabZIPs and TaZIPs in the Zn-regulatory mechanism of wheat. This research has identified key genes involved in the regulation, uptake and distribution of Zn in wheat. The molecular mechanisms elucidated will be important in the development of Zn biofortified wheat varieties as well as cultivars which maintain high yield in Zn-deficient conditions. These may prove vital in achieving global food security.