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 and to respiration during leaf senescence.

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. From estimations of subcellular redox states of NAD(H) and NADP(H) pools and ATP/ADP ratios we believe that the function of leaf mitochondria are different in the light as compared to darkness. The TCA cycle is reorganized in the light so that it changes from being the main source of energy in the cell to become a flexible mechanism that enables the cell to sustain the photosynthetic process, both through the production of carbon skeletons and by contributing to the redox homeostasis of the cell.

Low temperature
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.

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 possible involving the mitochondrial alternative oxidase. We are investigating this question using antisense to alter levels of key enzymes in the proposed shuttle mechanisms and by comparing ozon stress to low temperature stress.

Senescence
We are initiating studies on respiration and mitochondrial activities during leaf senescence. Comparison of ESTs from a cDNA library prepared from autumn poplar leaves to a library perpared from young leaves indicate that respiration is important during the process of leaf senescence. Further studies are currently initiated using both Arabidopsis and Poplar as model systems.

Selected publications:

 

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.

 

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.U., 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.

 

Strand, Å., Foyer, C.H., Gustafsson, P., Gardeström, P. and , Hurry V. () Increased expression of sucrose-phosphate synthase in transgenic Arabidopsis thaliana results in improved photosynthetic performance and increased freezing tolerance at low temperatures. Plant Cell & Environm. 26:523-535.

 

Bhalerao, R., Keskitalo, J., Sterky, F., Erlandsson, R., Björkbacka, H., Jonsson Birve, S., Karlsson, J., Gardeström, P. , Gustafsson, P., Lundeberg, J., Jansson, S., () Gene expression in autumn leaves. Plant Physiol 131:1-13

 

Igamberdiev, A.U. and Gardeström, P. () Regulation of NAD- and NADP-dependent isocitrate dehydrogenases by reduction levels of pyridine nucleotides in mitochondria and cytosol of pea leaves. BBA :117-125