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Published on September 12, 2008; 10.1104/pp.108.125062

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Received June 20, 2008
Accepted September 8, 2008

RNAi-mediated Repression of MtCCD1 Carotenoid Cleavage Dioxygenase in Mycorrhizal Roots of Medicago truncatula Causes Accumulation of C27 Apocarotenoids Shedding Light on the Functional Role of CCD1

Daniela S. Floß , Willibald Schliemann , Jurgen Schmidt , Dieter Strack , and Michael H. Walter *

Leibniz-Institut fur Pflanzenbiochemie (IPB), Abteilung Sekundarstoffwechsel and Abteilung Natur- und Wirkstoffchemie, D-06120 Halle (Saale), Germany

* Corresponding author; email: mhwalter{at}ipb-halle.de.

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, are still unknown. In this study we have carried out RNAi-mediated repression of a Medicago truncatula CCD1 (MtCCD1) 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 3-6% of the controls, while the cyclohexenone derivatives were only reduced to 30-47%. Concomitantly, a yellow-orange color appeared in RNAi roots. Based on UV spectra and MS 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.




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