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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

RNA Interference-Mediated Repression of MtCCD1 in Mycorrhizal Roots of Medicago truncatula Causes Accumulation of C₂₇ Apocarotenoids, Shedding Light on the Functional Role of CCD1^1,[W],[OA]

Leibniz-Institut für Pflanzenbiochemie, Abteilung Sekundärstoffwechsel (D.S.F., W.S., D.S., M.H.W.) and Abteilung Natur- und Wirkstoffchemie (J.S.), D–06120 Halle (Saale), Germany

Tailoring carotenoids by plant carotenoid cleavage dioxygenases (CCDs) generates various bioactive apocarotenoids. Recombinant CCD1 has been shown to catalyze symmetrical cleavage of C₄₀ carotenoid substrates at 9,10 and 9',10' positions. The actual substrate(s) of the enzyme in planta, however, is still unknown. In this study, we have carried out RNA interference (RNAi)-mediated repression of a Medicago truncatula CCD1 gene in hairy roots colonized by the arbuscular mycorrhizal (AM) fungus Glomus intraradices. As a consequence, the normal AM-mediated accumulation of apocarotenoids (C₁₃ cyclohexenone and C₁₄ mycorradicin derivatives) was differentially modified. Mycorradicin derivatives were strongly reduced to 3% to 6% of the controls, while the cyclohexenone derivatives were only reduced to 30% to 47%. Concomitantly, a yellow-orange color appeared in RNAi roots. Based on ultraviolet light spectra and mass spectrometry analyses, the new compounds are C₂₇ apocarotenoic acid derivatives. These metabolic alterations did not lead to major changes in molecular markers of the AM symbiosis, although a moderate shift to more degenerating arbuscules was observed in RNAi roots. The unexpected outcome of the RNAi approach suggests C₂₇ apocarotenoids as the major substrates of CCD1 in mycorrhizal root cells. Moreover, literature data implicate C₂₇ apocarotenoid cleavage as the general functional role of CCD1 in planta. A revised scheme of plant carotenoid cleavage in two consecutive steps is proposed, in which CCD1 catalyzes only the second step in the cytosol (C₂₇ C₁₄ + C₁₃), while the first step (C₄₀ C₂₇ + C₁₃) may be catalyzed by CCD7 and/or CCD4 inside plastids.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Michael H. Walter (mhwalter{at}ipb-halle.de).

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.125062

^* Corresponding author; e-mail mhwalter{at}ipb-halle.de.

Received June 20, 2008; accepted September 8, 2008; published September 12, 2008.

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