Transcriptional control of Set Domain Group 8 and carotenoid isomerase during Arabidopsis development

Carotenoids are pigments required for photosynthesis, photoprotection and the production of carotenoid-derived hormones such as ABA and strigolactones. The carotenoid biosynthetic pathway bifurcates after lycopene to produce epsilon- and beta-carotenoids and this branch is critical for determining carotenoid composition. Here, we show how the branch point can be regulated by the chromatin-modifying histone methyltransferase, Set Domain Group 8 (SDG8) targeting the carotenoid isomerase (CRTISO). SDG8 is required to maintain permissive expression of CRTISO during seedling development, in leaves, shoot apex, and some floral organs. The CRTISO and SDG8 promoters show overlapping tissue-specific patterns of reporter gene activity. Interestingly, CRTISO showed atypical reporter gene expression in terms of greater variability between different lines compared to the Cauliflower Mosaic Virus 35S promoter (CaMV35s) and εLCY promoters, potentially due to chromosomal position effects. Regulation of the CRTISO promoter was dependent in part upon the presence or absence of SDG8. Knockouts of SDG8 (carotenoid and chloroplast regulation (ccr1)) and CRTISO (ccr2) result in altered carotenoid composition and this could be restored in ccr2 using the CaMV35s or CRTISO promoters. In contrast, varying degrees of GUS expression and carotenoid complementation by CRTISO overexpression using CaMV35S or CRTISO promoters in the ccr1 background demonstrated that both the CRTISO promoter and open reading frame are necessary for SDG8-mediated expression of CRTISO.