Peroxiredoxins are ubiquitous thioredoxin-or glutaredoxin-dependent peroxidases, the function of which is to destroy peroxides. Peroxiredoxin Q, one of the four plant subtypes, is a homolog of the bacterial bacterioferritin comigratory proteins. We show here that the poplar (Populus tremula x Populus tremuloides) protein acts as a monomer with an intramolecular disulfide bridge between two conserved cysteines. A wide range of electron donors and substrates was tested. Unlike type II peroxiredoxin, peroxiredoxin Q cannot use the glutaredoxin or cyclophilin isoforms tested, but various cytosolic, chloroplastic, and mitochondrial thioredoxins are efficient electron donors with no marked specificities. The redox midpoint potential of the peroxiredoxin Q catalytic disulfide is Ϫ325 mV at pH 7.0, explaining why the wild-type protein is reduced by thioredoxin but not by glutaredoxin. Additional evidence that thioredoxin serves as a donor comes from the formation of heterodimers between peroxiredoxin Q and monocysteinic mutants of spinach (Spinacia oleracea) thioredoxin m. Peroxiredoxin Q can reduce various alkyl hydroperoxides, but with a better efficiency for cumene hydroperoxide than hydrogen peroxide and tertiary butyl hydroperoxide. The use of immunolocalization and of a green fluorescence protein fusion construct indicates that the transit sequence efficiently targets peroxiredoxin Q to the chloroplasts and especially to those of the guard cells. The expression of this protein and of type II peroxiredoxin is modified in response to an infection by two races of Melampsora larici-populina, the causative agent of the poplar rust. In the case of an hypersensitive response, the peroxiredoxin expression increased, whereas it decreased during a compatible interaction.

Depending on the sites of accumulation within the SAM and on the stage of the annual growth cycle, lipids, starch and tannins may be involved in different processes. In spring, energy and structural materials released by lipid hydrolysis may contribute to stem elongation and/or cell-to-cell communication. During organogenesis, energy and structural materials released by starch hydrolysis may influence developmental programmes in the SAM and adjacent cells. Tannins may be involved in cellular detoxification. At the end of the growing season, accumulation of lipid and starch is positively correlated with the onset of dormancy.

The Paxillus involutus (Fries) Karsten-Betula pendula Roth association was studied during the early stages of formation. Cytological studies revealed fungal colonization behind the root cap and gradually around the entire root apex. Ultrastructural investigations were carried out and insoluble polysaccharide distribution was followed. The density of starch grains increased in plant cells especially after 4 d of contact between the two partners, but later on decreased strongly in the root cap. Large amounts of glycogen were revealed in the hyphae in certain mycorrhizal regions after 6 d of contact : in the Hartig net, in the inner sheath but only near the net, and all along the outer sheath surrounding the mycorrhiza. Thickenings of the epidermal cell walls were detected as early as 2 d after contact and then varied according to the distance from the root tip. Such polysaccharide distributions are assumed to show a transfer of carbohydrates from the root to the fungus and are discussed in terms of carbon requirements for both partners.Key words : Ectomycorrhiza, root tip, starch, glycogen, cell-wall thickenings. The development of ectomycorrhiza involves the differentiation of structurally specialized fungal tissues, interfaces between symbionts, and a highly co-ordinated metabolic interplay. Some investigations on carbon and nitrogen metabolism have shown the ability of fungi to improve the N nutrition of host plants (Chalot et al.,

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