SummaryPhotorespiration is a light-dependent source of H 2 O 2 in the peroxisomes, where concentrations of this signalling molecule are regulated by catalase. Growth of Arabidopsis knock-out mutants for CATALASE2 (cat2) in ambient air caused severely decreased rosette biomass, intracellular redox perturbation and activation of oxidative signalling pathways. These effects were absent when cat2 was grown at high CO 2 levels to inhibit photorespiration, but were re-established following a subsequent transfer to air. Growth of cat2 in air at different daylengths revealed that photoperiod is a critical determinant of the oxidative stress response. Decreased growth was observed in 8-h, 12-h and 16-h photoperiods, but lesion development was dependent on long days. Experiments at different light fluence rates showed that cell death in cat2 was linked to long days and not to total light exposure or the severity of oxidative stress. Perturbed intracellular redox state and oxidative signalling pathway induction were more prominent in short days than in long days, as evidenced by glutathione status and induction of defence genes and oxidative stress-responsive transcripts. Similar daylength-dependent effects were observed in the response of mature plants transferred from short days in high CO 2 conditions to ambient air conditions. Prior growth of plants with short days in air alleviated the cat2 cell-death phenotype in long days. Together, the data reveal the influence of photoperiodic events on redox signalling, and define distinct photoperiod-dependent strategies in the acclimation versus cell-death decision in stress conditions.

The plant plastidial thioredoxins (Trx) are involved in the light-dependent regulation of many enzymatic activities, owing to their thiol-disulfide interchange activity. Three different types of plastidial Trx have been identified and characterized so far: the m-, f-, and x-types. Recently, a new putative plastidial type, the y-type, was found. In this work the two isoforms of Trx y encoded by the nuclear genome of Arabidopsis (Arabidopsis thaliana) were characterized. The plastidial targeting of Trx y has been established by the expression of a Trx::GFP fusion protein. Then both isoforms were produced as recombinant proteins in their putative mature forms and purified to characterize them by a biochemical approach. Their ability to activate two plastidial light-regulated enzymes, NADP-malate dehydrogenase (NADP-MDH) and fructose-1,6-bisphosphatase, was tested. Both Trx y were poor activators of fructose-1,6-bisphosphatase and NADP-MDH; however, a detailed study of the activation of NADP-MDH using site-directed mutants of its regulatory cysteines suggested that Trx y was able to reduce the less negative regulatory disulfide but not the more negative regulatory disulfide. This property probably results from the fact that Trx y has a less negative redox midpoint potential (2337 mV at pH 7.9) than thioredoxins f and m. The y-type Trxs were also the best substrate for the plastidial peroxiredoxin Q. Gene expression analysis showed that Trx y2 was mainly expressed in leaves and induced by light, whereas Trx y1 was mainly expressed in nonphotosynthetic organs, especially in seeds at a stage of major accumulation of storage lipids.Thioredoxins (Trx) are small (approximately 12 kD) ubiquitous proteins involved in thiol-disulfide exchange reactions (Buchanan, 1980;Holmgren, 1989). In higher plant cells, they are found in cytosol, mitochondria, and plastids (Schü rmann and Jacquot, 2000). All are nuclear encoded irrespective of their subcellular localization. The availability of the Arabidopsis (Arabidopsis thaliana) complete genome sequence revealed that higher plant Trxs belonged to a multigene family. Several different Trx types have been distinguished on the basis of sequence identity and intron positions, with several isoforms within each type (Meyer et al., 2002). The situation is particularly complex for plastidial Trx (cpTrx), where four different types of Trx have been found. Besides the long-time known thioredoxins m and f comprising 4 and 2 members respectively, a third type of cpTrx, named the x-type, has been recently studied and found to be a very good reductant of plastidial 2-Cys peroxiredoxins (Prxs), whereas having almost no activity with the classical target enzymes of cpTrx m and f, i.e. NADPmalate dehydrogenase (NADP-MDH) and Fru-1,6-bisphosphatase (FBPase). The fourth type has been identified quite recently (Lemaire et al., 2003) during the analysis of the genomes of Chlamydomonas reinhardtii and of the cyanobacterium Synechocystis. While only one isoform is present in the green alga and the cyanoba...