Carotenoids: Localization and Function

Abstract

Carotenoids are components of every pigment-protein complex in the photosynthetic apparatus of higher plants. These pigments, previously referred to as ‘accessory,’ are now recognized to fulfill indispensable functions in light harvesting, protection against photooxidation, and regulation of Photosystem II efficiency. The wealth of information accumulated in recent years dealing with the closely related questions of carotenoid organization and functions are summarized in this chapter. In the first section the distribution of carotenoids in the different pigment proteins is reported showing that each photosystem subunit has its characteristic composition. The organization of the different xanthophylls within the antenna complexes is discussed on the basis of recent structural and biochemical evidence. In the second section, advances in photophysical mechanisms through which carotenoids perform their classical light harvesting and protective functions are discussed. In addition, particular attention is given to discussion of the xanthophyll cycle which, in conjunction with the transthylakoid ΔpH1 down-regulates Photosystem II photochemical efficiency by non-radiative dissipation of energy in the light-harvesting complexes. Down-regulation helps to keep PS II traps open, thereby helping to maintain electron transport and to protect the reaction center from photoinhibition.