It recently has been shown that 12-hydroxyjasmonate and its sulfonated derivative occur naturally in A. thaliana (Gidda et al. , 2003). The enzyme catalyzing the sulfonation of 12-OHJA is encoded by the AtST2a gene, one of the 18 sulfotransferase (ST)-coding genes present in the A. thaliana genome (Gidda et al. , 2003). We demonstrate that 12-OHJA induces floral evocation in A. thaliana plants growing under inductive photoperiods and that this inducing activity is abolished by sulfonation under non-inductive photoperiods. We also demonstrate that CONSTANS , a putative transcription factor that accelerates flowering in response to long photoperiod, and TERMINAL FLOWER 2, a homolog of Drosophila heterochromatin-associated protein 1, which acts on meristem identity genes to repress the initiation of flowering, promote or repress flowering through the direct or indirect regulation of AtST2a expression. We, therefore, propose that AtST2a is a member of the photoperiod-dependent flower induction pathway downstream from CO and TFL2. In Nicotiana tabaccum 12-OHJA is found to participate in the determination of flower structures and regulates the expression of NtPLE36 , a flower organ identity gene. The A. thaliana genome contains a sequence ( AtST2b ) that is closely related to AtST2a . The deduced amino acid sequences of the two genes share 85% identity and 92% similarity. Despite this high level of similarity AtST2b did not accept 12-OHJA as substrate. Transgenic plants overexpressing AtST2b in sense or antisense orientation, as well as AtST2b knock out mutant plants did not show a visible phenotype suggesting that the role of AtST2b in Arabidopsis thaliana is different from AtST2a
