Glutaredoxins (GRXs) belong to the antioxidant and signalling network involved in the cellular response to oxidative stress in bacterial and eukaryotic cells. In spite of the high number of GRX genes in plant genomes, the biological functions and physiological roles of most of them remain unknown. Here the functional characterization of the Arabidopsis GRXS13 gene (At1g03850), that codes for two CC-type GRX isoforms, is reported. The transcript variant coding for the GRXS13.2 isoform is predominantly expressed under basal conditions and is the isoform that is induced by photooxidative stress. Transgenic lines where the GRXS13 gene has been knocked down show increased basal levels of superoxide radicals and reduced plant growth. These lines also display reduced tolerance to methyl viologen (MeV) and high light (HL) treatments, both conditions of photooxidative stress characterized by increased production of superoxide ions. Consistently, lines overexpressing the GRXS13.2 variant show reduced MeV- and HL-induced damage. Alterations in GRXS13 expression also affect superoxide levels and the ascorbate/dehydroascorbate ratio after HL-induced stress. These results indicate that GRXS13 gene expression is critical for limiting basal and photooxidative stress-induced reactive oxygen species (ROS) production. Together, these results place GRXS13.2 as a member of the ROS-scavenging/antioxidant network that shows a particularly low functional redundancy in the Arabidopsis GRX family.

NPR1 is a master regulator of plant response to pathogens that confers immunity through a transcriptional cascade mediated by salicylic acid (SA) and TGA transcription factors. Little is known, however, about its implication in plant response to abiotic stress. Here, we provide genetic and molecular evidence supporting that Arabidopsis NPR1 plays an essential role in cold acclimation by regulating cold-induced gene expression independently of SA and TGA factors. Our results demonstrate that, in response to low temperature, cytoplasmic NPR1 oligomers release monomers that translocate to the nucleus where they interact with heat shock transcription factors HSFA1 to promote the induction of HSFA1regulated genes and cold acclimation. Accordingly, Arabidopsis mutants deficient in HSFA1 factors display reduced capacity to cold acclimate, and cold induction of heat stressresponsive genes is required for correct development of cold acclimation. All these findings unveil an unexpected function for NPR1 in plant response to low temperature, reveal a new regulatory pathway for cold acclimation mediated by NPR1 and HSFA1 factors, and place NPR1 as a central hub integrating cold and pathogen signaling for a better adaptation of plants to an ever-changing environment.3 Low temperature is a major environmental stress that adversely affects growth and development of plants, determines their geographic distribution and limits crop productivity. Many plants from temperate regions, including Arabidopsis, have evolved an adaptive process whereby their constitutive freezing tolerance increases after being exposed to low nonfreezing temperatures 1 . This process, named cold acclimation, is complex and involves many physiological and biochemical changes 2 . Research over the last decades has shown that most of these changes are controlled by low temperature through changes in gene expression. Arabidopsis, for instance, reprograms its transcriptome during cold acclimation involving more than 3000 genes 3 . Unfortunately, however, the role that the vast majority of these genes play in this adaptive process and the signaling pathways through which they operate remain to be elucidated. The best characterized pathway is mediated by a family of three transcription factors, named CBF1 to CBF3, that are estimated to control the induction of around 12% of the Arabidopsis cold-responsive genes 4 . Plant hormones also have a significant function in cold signaling. Low temperature induces changes in the levels of abscisic acid (ABA), ethylene (ET), jasmonic acid (JA) and gibberellins (GA), and it has been shown that ABA, ET, JA, GA and brassinosteroids (BRs) signaling contribute to the regulation of cold acclimation 5 . Salicylic acid (SA) accumulates in response to cold in several species 6 but its function in the process still needs further investigation.In plants, accumulating evidence indicate that low temperature interacts with other environmental cues. Interestingly, several studies support that cold signals are closely associated with defen...