Regulation and function of the zeaxanthin epoxidase

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The zeaxanthin epoxidase (ZE) catalyzes the conversion of zeaxanthin to violaxanthin. This reaction represents not only one of the two reactions of the so-called xanthophyll cycle (which serves essential photoprotective functions in photosystem II) but is furthermore an important step of carotenoid biosynthesis leading to the formation of neoxanthin and the phytohormone abscisic acid (ABA), which is involved in plant development and stress responses. So far, little is known about (i) the regulation and biochemical properties of the ZE in general and (ii) the regulation of the dual function of the ZE in the xanthophyll cycle and ABA biosynthesis. Our preliminary work indicated that two different isoforms of the ZE are active in plants, which may serve different functions with respect to xanthophyll cycle activity and ABA biosynthesis. The goal of the planned work is to understand the regulation and function of the ZE in relation to its dual function in energy dissipation and ABA biosynthesis. For this purpose, two approaches will be followed: 1. Over-expression and characterisation of the two isoforms of the ZE in Arabidopsis, to understand the dual role of the ZE in the xanthophyll cycle and ABA biosynthesis. 2. The heterologous over-expression of the ZE from diatoms with different substrate specificities and regulation properties, to understand the biochemical properties of the ZE, particularly with respect to energy dissipation.