Changes in the expression of photosynthetic genes precede loss of photosynthetic activities and chlorophyll when glucose is supplied to mature spinach leaves

Abstract

To study the molecular mechanism of ‘sink’ regulation on photosynthesis, thylakoid protein synthesis and content of photosynthetic proteins were compared with changes in photosynthetic activities and in leaf chlorophyll (chl) content in detached leaves kept on a diluted nutrient solution in the presence or absence of 50 mM glucose for 4 days at 60 μmol m −2 s −1. During the experiment, leaf chl content decreased in glucose-supplied leaves about 50%, in detached control leaves about 20%. The degradation of the photosynthetic apparatus was not connected with any substantial changes in photosynthetic parameters, when determined by pulse-modulated chl fluorescence at growth light conditions. However, at saturating light intensities of 1000 μmol m −2 s −1, leaf oxygen production was inhibited about 43% in leaves supplied with glucose, leading to a substantial increase in membrane energetization, as indicated by the increase of the non-photochemical quench, q n, and a down-regulation of photosystem (PS) II activity in the light, as determined by ΔF F′ m (yield of variable fluorescence under steady state conditions/fluorescence yield when all reaction centres are closed (after a saturating flash) under steady-state conditions). No substantial changes were observed in the detached control leaves. The reduction of leaf oxygen production under external glucose supply was probably caused by a 60% loss of ribulose-1,5-bisphosphat-carboxylase, as revealed by Western blotting. In the detached control leaves, the content of Rubisco stayed stable. The content of D1 protein of the PS II reaction centre was reduced by 43% in the glucose-supplied leaves, when related to chl. However, a substantial loss of D1 protein of 30% was observed in the detached control leaves. As in other systems, the specific loss of D1 protein initiated the degradation of PS II and via the breakdown of light harvesting complex (LHC) II the loss of leaf chl. By measuring thylakoid protein synthesis, it could be shown that the changes in D1 protein content were caused by an inhibition of its synthesis, together with an increase in its degradation. In the glucose-supplied leaves, the expression of the 25 kDa polypeptide of the LHC II was also inhibited.