Plants must be able to adjust to the variable environmental conditions that they are exposed to. For this a very flexible metabolism is needed. In this project, plant metabolism is investigated with the aim to obtain a better understanding of the metabolic pathways involved, their regulation and compartmentation in photosynthetic cells. The core of the project is on the interaction of respiratory and photosynthetic metabolic pathways. Special emphasis is given to metabolic adjustments related to low temperature stress and acclimation.

Respiration/photorespiration
The conclusion from studies during the last decade is that mitochondrial oxidative phosphorylation is important for the supply of ATP to the cytosol of photosynthetic cells under most conditions. Other important functions for leaf mitochondria are to oxidise excessive redox equivalents from the chloroplasts and to supply carbon skeletons for biosynthetic purposes. Still, many questions remain regarding specific aspects of these interactions. We study the contribution of respiration and photorespiration to the redox balance of the plant cell. Chlorophyll fluorescence is used to estimate the reduction state of chloroplasts. The redox state of NAD(H) and NADP(H) pools and content of key metabolites is estimated in chloroplasts, mitochondria and cytosol using protoplasts and the redox state of the mitochondrial ubiquinone pool is determined both in protoplasts and isolated mitochondria. The plants used are both wild-type and mutants/ antisense of barley and potato with decreased levels of mitochondrial glycine decarboxylase.

Low temperature
During the last years it has become clear that cold acclimation in many plants is associated with an increased capacity for photosynthesis. This is linked to increased levels of soluble carbohydrates and generally increased metabolite pools. The activities of key enzymes of CO₂ fixation and sucrose metabolism is also upregulated. A clear distinction in response is observed between short-term cold stress and long-term cold acclimation. For full cold acclimation development at low temperature is required. This include altered effects on gene expression from accumulated carbohydrates, which has interesting implications for gene regu-lation with respect to carbohydrates as signals or modulators of gene expression. We study stress and acclimation to low temperature both with respect to the chloroplast electron transport chain and adjustments of carbohydrate metabolism. The main experimental system is Arabidopsis where antisense technique is an important tool for the questions asked.

Exposure of plants to low temperature is an example where overreduction of the chloroplast electron transport chain may occur. A possible mechanism to export excess electrons from the chloroplast is via the ”malate valve” which involves malate/oxaloacetate exchange over the envelope membrane. The redox equivalents exported could, via a number of shuttle mechanisms, be taken into the mitochondria and thus made available for their electron transport chain. We are initiating investigations of this question using antisense to alter levels of key enzymes in the proposed shuttle mechanisms.

We are initiating studies on low temperature stress and acclimation in Poplar using genomic and proteomic methods.

Selected publications:

Gardeström, P. and Wigge, B. () The influence of photorespiration on ATP/ADP ratios in the chloroplasts, mitochondria and cytosol studied by rapid fractionation of barley (Hordeum vulgare) protoplasts. Plant Physiol. 88: 69-76.

Strand, Å, Hurry, V., Gustafsson, P. and Gardeström, P. () Development of Arabidopsis thaliana leaves at low temperature releases the suppression of photosynthesis and gene expression despite the accumulation of soluble carbohydrates. Plant J. 12(3): 605-614.

Igamberdiev, A, Krömer, S., Hurry, V.M. and Gardeström, P. () The role of mitochondrial electron transport during photosynthetic induction. A study with barley (Hordeum vulgare) protoplasts incubated with rotenone and antimycin. Physiol Plant 104: 431-439.

Strand, Å., Hurry, V., Henkes, S., Huner, N., Gustafsson, P., Gardeström, P. and Stitt, M. () Acclimation of Arabidopsis leaves developing at low temperatures. Increasing cytoplasmic volume accompanies increased activities of enzymes in the Calvin cycle and in the sucrose-biosynthesis pathway. Plant Physiol 119:.

Strand, Å., Zrenner, R., Trevanion, S., Stitt, M., Gustafsson, P. and Gardeström, P. () Decreased experssion of two key enzymes in the sucrose biosynthesis pathway, cytosolic fructose-1,6-bisphosphate and sucrose-phosphate synthase, has remarcable different consequences for photosynthetic carbon metabolism in transgenic Arabidopsis thaliana. Plant J 23(6): 759-770.

Heineke, D., Bykova, N., Gardeström, P. and Bauwe, H. () Metabolic responces of potato plants to an antisense reduction of the P-protein of glycine decarboxylase. Planta 212: 880-887

Igamberdiev, A.I., Bykova, N., Lea, P.J. and Gardeström, P. () The role of photorespiration in redox regulation and energy balance of photosynthetic plant cells: a study with a barley mutant deficient in glycine decarboxylase. Physiol Plant 111: 427-438.

Savitch, L., Barker-Åström, J., Ivanov, A., Hurry, V., Öquist, G., Huner, N. and Gardeström, P. () Cold acclimation of Arabidopsis thaliana results in incomplete recovery of photosynthetic capacity, associated with an increased reduction of the chloroplast stroma. Planta 214: 295-303.