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.

A sequence coding for a peroxiredoxin (Prx) was isolated from a xylem/phloem cDNA library from Populus trichocarpa and subsequently inserted into an expression plasmid yielding the construction pET-Prx. The recombinant protein was produced in Escherichia coli cells and purified to homogeneity with a high yield. The poplar Prx is composed of 162 residues, a property that makes it the shortest plant Prx sequence isolated so far. It was shown that the protein is monomeric and possesses two conserved cysteines (Cys). The Prx degrades hydrogen peroxide and alkyl hydroperoxides in the presence of an exogenous proton donor that can be either thioredoxin or glutaredoxin (Grx). Based on this finding, we propose that the poplar protein represents a new type of Prx that differs from the so-called 2-Cys and 1-Cys Prx, a suggestion supported by the existence of natural fusion sequences constituted of a Prx motif coupled to a Grx motif. The protein was shown to be highly expressed in sieve tubes where thioredoxin h and Grx are also major proteins. ; fax 33383912243.Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/

Amino acids are the currency of nitrogen exchange between source and sink tissues in plants and constitute a major source of the components used for cellular growth and differentiation. The characterization of a new amino acid transporter belonging to the amino acid permease (AAP) family, AAP11, expressed in the perennial species Populus trichocarpa is reported here. PtAAP11 expression analysis was performed by semi-quantitative RT-PCR and GUS activity after poplar transformation. PtAAP11 function was studied in detail by heterologous expression in yeast. The poplar genome contains 14 putative AAPs which is quite similar to other species analysed except Arabidopsis. PtAAP11 was mostly expressed in differentiating xylem cells in different organs. Functional characterization demonstrated that PtAAP11 was a high affinity amino acid transporter, more particularly for proline. Compared with other plant amino acid transporters, PtAAP11 represents a novel high-affinity system for proline. Thus, the functional characterization and expression studies suggest that PtAAP11 may play a major role in xylogenesis by providing proline required for xylem cell wall proteins. The present study provides important information highlighting the role of a specific amino acid transporter in xylogenesis in poplar.

This paper investigates the uptake, transport, state and self-diusion of water in twigs and buds of Picea abies L. (Karst.) trees in winter until reactivation in spring. The presence or absence of xylem in embryonic shoots, as well as the intensity and type of bud dormancy were also studied. Three clones of P. abies were grown in a forest in northeastern France. The clones diered in their timing of bud-burst, with the two clones from the Vosges showing dierent degrees of early bud-burst and the clone from Poland showing late bud-burst. One-bud cuttings grown in standard forcing conditions showed a great dierence in dormancy between the two provenances, but only a small dierence between the two Vosges clones. Earliness of bud-burst was not strongly linked to the intensity of dormancy. A dye experiment combined with light microscopy indicated increased velocity of water transport in stems up to a maximum in April, initial entry of water into embryonic shoots, protoxylem dierentiation in embryonic shoots from April, and then, shortly before bud-burst, water transport through the newly-formed protoxylem almost up to the meristem. Nuclear magnetic resonance measurements of the transverse relaxation time (T 2 ) and the self-diusion coecient of water con®rmed these observations and showed water availability in embryonic shoots. The sequence of water-related phenomena occurring in early spring was the same in the three clones, but was earliest in the Vosges clone with the earliest bud-burst and latest in the Polish clone with late bud-burst. The results imply that this sequence of water-related phenomena leads to bud-burst.# 2000 Annals of Botany Company