Comparison of wild-type and carotenoid accumulating Arabidopsis roots

The amount of carotenoids in plant tissues is the net result of the rate of biosynthesis and degradation. While the complex network of enzymes involved in biosynthesis was extensively characterized in recent years, knowledge on oxidative carotenoid degradation, which is predominantly initiated by non-enzymatic rather than enzymatic cleavage into apocarotenoids, is sparse. Moreover, subsequent metabolism of the formed apocarotenoids has so far only been characterized for apocarotenoids being precursors of carotenoid-derived plant hormones, but remains unknown for the majority of apocarotenoids. Notably, non-enzymatic carotenoid degradation generates substantial losses of nutritionally important carotenoids such as provitamin A carotenoids, zeaxanthin and lutein in many crops, fruits and vegetables. In order to identify novel processes involved in apocarotenoid metabolization in plants, we characterized the transcriptomic response of Arabidopsis roots accumulating high levels of ß-carotene and ß-apocarotenoids due to overexpression of the rate-limiting carotenogenic enzyme phytoene synthase. Transcriptome analysis revealed feedback regulation on carotenoid biosynthesis aiming at reducing the amount of ß-carotene, which might suggest involvement of specific apocarotenoid signaling molecules originating directly from ß-carotene degradation. Moreover, the transcriptome response showed a large overlap with detoxification responses observed upon plant treatment with xenobiotics and fatty acid-derived reactive carbonyl species (RCS). Notably, metabolite analysis revealed that the response was not due to lipid stress, a potential secondary effect of carotenoid accumulation, and is therefore distinct from an RCS response. Nonetheless, a set of known RCS detoxification enzymes was found to be induced as part of the response. In agreement with structural similarities between RCS and ß-apocarotenoids, the latter representing reactive electrophile species (RES), we report for the first time that RCS detoxification enzymes are also capable of converting apocarotenoids derived from ß-carotene and xanthophylls into apocarotenols and apocarotenoic acids in vitro. Moreover, several glutathione-S transferases, glycosyltransferases and transporters were induced in carotenoid-accumulating Arabidopsis roots. In view of similarities to mechanisms found in crocin biosynthesis in Crocus sativus and RCS metabolism in plants, our data might suggest glycosylation, glutathionylation and compartmentalization as key processes in (apo)carotenoid metabolism in plants and open the way to further investigations in this field.