Dynamics of photosynthetic complexes in the thylakoid membranes from higher plants

Abstract

Photosynthetic machinery in higher plants is localised in the thylakoids enclosed in a chloroplast. To optimise and regulate the photosynthetic efficiency under different, rapidly changeable environmental conditions the dynamics of the thylakoid membrane components is required. It has been invoked in several contexts, for example during assembly and turnover of the photosynthetic apparatus, regulation of light-harvesting and photosynthetic electron transport. In this study, by employing a confocal FRAP technique combined with freeze-fracture electron microscopy, I addressed a fundamental problem of visualising the mobility and distribution of photosynthetic complexes in a direct way close to the situation in vivo - the thylakoids of intact chloroplasts isolated from green plants. Firstly, I provided direct evidence that the dynamic changes in the distribution of photosynthetic complexes are involved in two high-light related physiological phenomena, namely photoinhibition and non-photochemical quenching. My study indicates that the photoinhibited membranes exhibit an elevated level of protein mobility accompanied by a decreased spacing between the complexes with the opposite effect observed in the photoprotective state. Secondly, my work allowed the identification of some key elements that are responsible for controlling the mobility under different physiological conditions such as: (1) phosphorylation of PSII core complexes after photoinhibition, (2) PsbS protein enhancing the membrane fluidity in a dark-adapted state and decreasing it after light treatment, (3) different xanthophyll composition of light-harvesting antenna with particular attention being paid to zeaxanthin which decreases the size of mobile fraction, (4) the degree of macromolecular crowding and the organisation of PSII-LHCII supercomplexes in the grana membranes which is dependent strongly on individual light-harvesting proteins, the minor antenna 4 complexes in particular. Lastly, a completely new approach of visualising the mobility of photosynthetic machinery in intact leaves has been introduced as a useful tool to study different aspects of plant acclimation and physiology under natural conditions.