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Abstract
BackgroundCarotenoids are plant metabolites which are not only essential in photosynthesis but also important quality factors in determining the pigmentation and aroma of flowers and fruits. To investigate the regulation of carotenoid metabolism, as related to norisoprenoids and other volatile compounds in peach (Prunus persica L. Batsch.), and the role of carotenoid dioxygenases in determining differences in flesh color phenotype and volatile composition, the expression patterns of relevant carotenoid genes and metabolites were studied during fruit development along with volatile compound content. Two contrasted cultivars, the yellow-fleshed 'Redhaven' (RH) and its white-fleshed mutant 'Redhaven Bianca' (RHB) were examined.ResultsThe two genotypes displayed marked differences in the accumulation of carotenoid pigments in mesocarp tissues. Lower carotenoid levels and higher levels of norisoprenoid volatiles were observed in RHB, which might be explained by differential activity of carotenoid cleavage dioxygenase (CCD) enzymes. In fact, the ccd4 transcript levels were dramatically higher at late ripening stages in RHB with respect to RH. The two genotypes also showed differences in the expression patterns of several carotenoid and isoprenoid transcripts, compatible with a feed-back regulation of these transcripts. Abamine SG - an inhibitor of CCD enzymes - decreased the levels of both isoprenoid and non-isoprenoid volatiles in RHB fruits, indicating a complex regulation of volatile production.ConclusionsDifferential expression of ccd4 is likely to be the major determinant in the accumulation of carotenoids and carotenoid-derived volatiles in peach fruit flesh. More in general, dioxygenases appear to be key factors controlling volatile composition in peach fruit, since abamine SG-treated 'Redhaven Bianca' fruits had strongly reduced levels of norisoprenoids and other volatile classes. Comparative functional studies of peach carotenoid cleavage enzymes are required to fully elucidate their role in peach fruit pigmentation and aroma.
Highlights
Carotenoids are plant metabolites which are essential in photosynthesis and important quality factors in determining the pigmentation and aroma of flowers and fruits
An increasing number of dioxygenase enzymes that cleave carotenoid compounds to form volatile norisoprenoids, abscisic acid (ABA) and regulators of plant growth and development has been characterized. These enzymes have been referred to as carotenoid cleavage dioxygenases (CCDs) and 9-cis-epoxycarotenoid dioxygenases (NCEDs) [12] and represent a plant multienzyme family: Arabidopsis has nine CCD/NCED members, of which four have been classified as CCDs (AtCCD1, AtCCD4, AtCCD7 and AtCCD8) and the remaining as ABArelated NCEDs [13]
Functional analysis of CCD enzymes determined that CCD7 and CCD8 are mostly related to the synthesis of norisoprenoid plant hormones, while CCD1 and CCD4 are preferentially involved in volatile production, by using different carotenoid substrates with variable specificity and cleavage site, which probably contributes to the diversity of norisoprenoids found in nature [[14,15,16,17] and references therein]
Summary
Carotenoids are plant metabolites which are essential in photosynthesis and important quality factors in determining the pigmentation and aroma of flowers and fruits. Following the formation of the first carotenoid phytoene from the general isoprenoid pathway, the pathway bifurcates after lycopene with respect to the ring type, giving rise to carotenes and xanthophylls with either b-b or b-ε rings (Figure 1, Additional File 1) In addition to their roles in plants as photosynthetic accessory pigments and colorants, carotenoids are precursors to norisoprenoids ( called apocarotenoids). An increasing number of dioxygenase enzymes that cleave carotenoid compounds to form volatile norisoprenoids, abscisic acid (ABA) and regulators of plant growth and development has been characterized. These enzymes have been referred to as carotenoid cleavage dioxygenases (CCDs) and 9-cis-epoxycarotenoid dioxygenases (NCEDs) [12] and represent a plant multienzyme family: Arabidopsis has nine CCD/NCED members, of which four have been classified as CCDs (AtCCD1, AtCCD4, AtCCD7 and AtCCD8) and the remaining as ABArelated NCEDs [13]. Comparative genetics studies have indicated that carotenoid pigmentation patterns have
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