PARENT SESSION

Symposium S6C Photosynthesis, respiration alternative electron sinks
Thursday September 2nd, 2004 10:20 AM-12:20 PM Room 510B
Chair: Greg Vanlerberghe
Co-Chair: Allan Rasmusson

Tetrahydrofolate, C1 metabolism and photosynthesis. Fabrice Rébeillé^*,1, Stéphane Ravanel¹, Maryse Block¹, Samuel Jabrin¹, Bernadette Gambonnet¹, Roland Douce¹, ¹ Physiologie Cellulaire Végétale, Grenoble, France

ABSTRACT- During the course of photorespiration, 2-phosphoglycolate is converted to 3-phosphoglycerate. A key step of this pathway occurs in mitochondria where 2 molecules of glycine are converted into 1 molecule of serine through the combined action of the glycine decarboxylase complex (GDC) and the serine hydroxymethyltransferase (SHMT). These two systems represent in a mature leaf up to 40 % of the soluble proteins of the matrix. Because they interact through a common pool of tetrahydrofolate (THF), the question of how and where THF is synthesized arises. THF is involved in the transfer of C1 units and is the central cofactor of C1 metabolism (DNA and methionine synthesis, methylation reactions). It is synthesized in the mitochondria through five reactions. During plant development, THF biosynthesis is more active in highly dividing tissues such as embryos and root tips than in mature tissues. This probably reflects the high C1 metabolism activity required in the former tissues for DNA and methionine synthesis. The exception is the leaf: folate accumulates during leaf development and remains high in the mature tissue, suggesting that THF synthesis is required for photosynthesis. Indeed, folate accumulation in leaves is triggered by light. However, the light-dependent accumulation takes place essentially in the cytosol, suggesting that the high folate level measured in leaves is required for other purposes than photorespiration. The high folate content found in the cytosol is probably linked to the methylation cycle. This pathway requires CH3-THF and is exclusively located in the cytosol where it is involved in the synthesis and regeneration of S-adenosylmethionine (SAM). SAM, in turn, is the source of methyl units for a very large number of methylation reactions, including the synthesis of lignins, lipids, chlorophylls. For example, we calculate that the rate of chlorophyll synthesis during greening requires that the cytosolic pool of CH3-THF was renewed in less than one minute. SAM produced in the cytosol has to be transported in the chloroplast for chlorophyll synthesis. In this connection, we characterized on the envelope membrane a specific transporter capable to catalyze a net import of SAM, as well as a counter exchange between SAM and S-adenosylhomocysteine.

KEY WORDS: C1 metabolism, Photorespiration, Folate