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Background  We have studied the impact of carbohydrate-starvation on the acclimation response to high light using  Arabidopsis thaliana  double mutants strongly impaired in the day- and night path of photoassimilate export from the chloroplast. A complete knock-out mutant of the triose phosphate/phosphate translocator (TPT;  tpt-2  mutant) was crossed to mutants defective in (i) starch biosynthesis ( adg1-1, pgm1  and  pgi1-1;  knock-outs of ADP-glucose pyrophosphorylase, plastidial phosphoglucomutase and phosphoglucose isomerase) or (ii) starch mobilization ( sex1-3 , knock-out of glucan water dikinase) as well as in (iii) maltose export from the chloroplast ( mex1-2 ).    Results  All double mutants were viable and indistinguishable from the wild type when grown under low light conditions, but - except for  sex1-3/tpt-2  - developed a high chlorophyll fluorescence (HCF) phenotype and growth retardation when grown in high light. Immunoblots of thylakoid proteins, Blue-Native gel electrophoresis and chlorophyll fluorescence emission analyses at 77 Kelvin with the  adg1-1/tpt-2  double mutant revealed that HCF was linked to a specific decrease in plastome-encoded core proteins of both photosystems (with the exception of the PSII component cytochrome b 559 ), whereas nuclear-encoded antennae (LHCs) accumulated normally, but were predominantly not attached to their photosystems. Uncoupled antennae are the major cause for HCF of dark-adapted plants. Feeding of sucrose or glucose to high light-grown  adg1-1/tpt-2  plants rescued the HCF- and growth phenotypes. Elevated sugar levels induce the expression of the glucose-6-phosphate/phosphate translocator2 (GPT2), which in principle could compensate for the deficiency in the TPT. A triple mutant with an additional defect in GPT2 ( adg1-1/tpt-2/gpt2-1 ) exhibited an identical rescue of the HCF- and growth phenotype in response to sugar feeding as the  adg1-1/tpt-2  double mutant, indicating that this rescue is independent from the sugar-triggered induction of GPT2.    Conclusions  We propose that cytosolic carbohydrate availability modulates acclimation to high light in  A. thaliana . It is conceivable that the strong relationship between the chloroplast and nucleus with respect to a co-ordinated expression of photosynthesis genes is modified in carbohydrate-starved plants. Hence carbohydrates may be considered as a novel component involved in chloroplast-to-nucleus retrograde signaling, an aspect that will be addressed in future studies.

bmc plant biology

Defects in leaf carbohydrate metabolism compromise acclimation to high light and lead to a high chlorophyll fluorescence phenotype in Arabidopsis thaliana