FACTORS INFLUENCING PS-II PARTICLE ARRAY FORMATION IN ARABIDOPSIS-THALIANA CHLOROPLASTS AND THE RELATIONSHIP OF SUCH ARRAYS TO THE THERMOSTABILITY OF PS-II

Chloroplasts of the thermal-stable fatty acid desaturase mutants JB67 and LK3 of Arabidopsis thaliana are characterised by the presence of r egular arrays of freeze-fracture particles associated with the core an d light-harvesting antennae of Photosystem II (Tsvetkova et al. (1994) Biochim. Biophys. Acta 1192, 263-271). Similar arrays were found to b e induced in the membranes of chloroplasts isolated from wild-type pla nts by resuspending the chloroplasts in media containing Tricine and/o r high concentrations of compatible co-solutes such as sorbitol. The t hermal stability of their chloroplasts was also increased under such c onditions. The increased tendency to form PS II particle arrays, and t he enhanced thermal stability of PS II, in chloroplasts isolated from the mutants and wild-type chloroplasts suspended in different media, a ppear to be reflections of the increased stability of protein-protein interactions between and within PS II units, respectively. The role of lipids in determining the formation of freeze-fracture particle array s in the mutants is discussed in terms of the observed changes in lipi d composition and their possible role in the control of lipid/protein synthesis.