The xanthophyll cycle of higher plants: influence of antenna size and membrane organization

Abstract

The development of the photosynthetic apparatus of intermittent light grown pea plants under continuous illumination has been investigated. We determined the formation of antenna proteins and the synthesis of pigments at different stages of greening and compared the data with the changes in the xanthophyll cycle reactions. The limited convertibility of violaxanthin in the de-epoxidation reactions of the cycle was found to be closely related to the presence of antenna proteins and could be attributed to direct (pigment binding) and indirect (grana formation) functions of antenna proteins. The reduced epoxidation rate in intermittent light plants was found to be accelerated with increasing amounts of antenna proteins. However, the changes in the epoxidation rates were not consistent with the assignment of the epoxidase activity to LHC II, the major light harvesting complex protein of photosystem II. This interpretation was further supported by an unchanged epoxidase activity in – also LHC II depleted – bundle sheath cells of the C 4 plant Sorghum bicolor and stroma fractions of isolated spinach thylakoids. We assume that the basic function of antenna proteins in the xanthophyll cycle of higher plants is mainly related to the binding of the substrate and/or to interactions with the de-epoxidase/epoxidase. By that antenna proteins seem to be responsible for the limited violaxanthin convertibility as well as they are required for highest epoxidation rates.