Day length is a key regulator of transcriptomic responses to both CO 2 and H 2 O 2 in Arabidopsis

Growth day length, CO 2 levels and H 2 O 2 all impact plant function, but interactions between them remain unclear. Using a whole‐genome transcriptomics approach, we identified gene expression patterns responding to these three factors in Arabidopsis Col‐0 and the conditional catalase‐deficient mutant, cat2 . Plants grown for 5 weeks at high CO 2 in short days (hCO 2 ) were transferred to air in short days (SD air) or long days (LD air), and microarray data produced were subjected to three independent studies. The first two analysed genotype‐independent responses. They identified 1549 genes differentially expressed after transfer from hCO 2 to SD air. Almost half of these, including genes modulated by sugars or associated with redox, stress or abscisic acid (ABA) functions, as well as light signalling and clock genes, were no longer significant after transfer to air in LD. In a third study, day length‐dependent H 2 O 2 ‐responsive genes were identified by comparing the two genotypes. Two clearly independent responses were observed in cat2 transferred to air in SD and LD. Most H 2 O 2 ‐responsive genes were up‐regulated more strongly in SD air. Overall, the analysis shows that both CO 2 and H 2 O 2 interact with day length and photoreceptor pathways, indicating close networking between carbon status, light and redox state in environmental responses.