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

Jessica Schmitz,Mark Aurel Schöttler,Kirsten Bell,Tatjana Kleine,Anja Schneider,Dario Leister,Stephan Krueger,Ulf-Ingo Flügge,Rainer E Häusler,Stefan Geimer

doi:10.1186/1471-2229-12-8

Abstract

BackgroundWe 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).ResultsAll 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 b559), 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.ConclusionsWe 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.

Highlights

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
Most strikingly the abundance of PSII and PSI supercomplexes as well as of PSII monomers was severely diminished in adg1-1/tpt-2 compared to the wild type and the single mutants, whereas ATPase and LHCII trimers were not affected (Figure 4D)
The major outcome of our investigations can be summarized as follows: (i) double mutants impaired in the day- and night path of photoassimilate export are still viable, (ii) the growth and high chlorophyll fluorescence (HCF) phenotypes develop only when the mutant plants were grown in high light (HL) and were virtually absent in low light (LL)-grown plants. (iii) Growth in HL resulted in severe changes in the composition of thylakoid proteins involved in photosynthetic electron transport characterized most prominently by a decrease in the plastome-encoded core components of both photosystems leading to increased contents of uncoupled, highly fluorescent nuclear encoded PSII and PSI antennae

Summary

Introduction

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. The majority of the CO2 assimilated in the CalvinBenson cycle is eventually converted into carbohydrates These can be retained inside the chloroplast in form of transitory starch or exported from the chloroplast and further transported via the phloem in form of sucrose to supply sink tissues such as roots, flowers and developing seeds with carbon and energy. The triose phosphate/ phosphate translocator (TPT) of the inner envelope membrane of chloroplasts is the major interface for the day path of photoassimilate export from the stroma [1]. Maltose is further metabolized via DPE2 [8,9], a cytosolic heteroglycan [10] and cytosolic glucan phosphorylase (in A. thaliana PSH2) resulting in glucose and glucose-1phosphate (Glc1P), which enter further metabolism [11]

Methods

Results

Discussion

Conclusion