Role of carotenoid cleavage dioxygenase 1 (CCD1) in apocarotenoid biogenesis revisited

Abstract

Oxidative tailoring of C(40) carotenoids by double bond-specific cleavage enzymes (carotenoid cleavage dioxygenases, CCDs) gives rise to various apocarotenoids. AtCCD1 generating C(13) and C(14) apocarotenoids and orthologous enzymes in other plants are the only CCDs acting in the cytosol, while the hitherto presumed C(40) substrate is localized in the plastid. A new model for CCD1 action arising from a RNAi-mediated CCD1 gene silencing study in mycorrhizal hairy roots of Medicago truncatula may solve this contradiction. This approach unexpectedly resulted in the accumulation of C(27) apocarotenoids but not C(40) carotenoids suggesting C(27) as the main substrates for CCD1 in planta. It further implies a consecutive two-step cleavage process, in which another CCD performs the primary cleavage of C(40) to C(27) in the plastid followed by C(27) export and further cleavage by CCD1 in the cytosol. We compare the specificities and subcellular locations of the various CCDs and propose the plastidial CCD7 to be the first player in mycorrhizal apocarotenoid biogenesis.

Keywords: C13 apocarotenoids; abscisic acid; arbuscular mycorrhiza; aroma compounds; carotenoid cleavage dioxygenase CCD1; norisoprenoids; strigolactones.

Figure 1

Comparison of substrates, enzymes and their compartmentation in three carotenoid cleavage pathways. (A) ABA biosynthesis involves cleavage of cis-carotenoid substrates by NCEDs in the plastid, C₁₅ xanthoxin export to the cytosol followed by further metabolization steps and transport. (B) Strigolactone biosynthesis is assumed to start from β-carotene and to proceed via two consecutive cleavage steps (CCD7 and CCD8) inside the plastid as exemplified by the MAX3 and MAX4 proteins of Arabidopsis. The C₁₈ cleavage product of CCD8 or a derivative of it is predicted to serve as mobile strigolactone precursor undergoing export to the cytosol, further modification steps, transport and eventually perception as a regulator of shoot branching. (C) Proposed organization of local C₁₃ and C₁₄ apocarotenoid biosynthesis in a mycorrhizal root cell. Lactucaxanthin as the tentatively proposed C₄₀ carotenoid precursor containing two α-ionone rings is tailored by two consecutive cleavage steps in the plastid (CCD7) and subsequently, following export of the C₂₇ intermediate, in the cytosol (CCD1). The C₂₇ intermediate has only been detected upon silencing CCD1 expression. Additional modification steps in the cytosol lead to the various C₁₃ cyclohexenone and C₁₄ mycorradicin derivatives accumulating in mycorrhizal roots. Abbreviations: MAX, more axillary branching; Gly, glycoside.