## 4 Discussion
The expression of _MiZDS_ and _MiLCYb_ genes corresponded with carotenoid accumulation in the peel and pulp of mango during fruit development and ripening. The expression levels of those two genes were higher in 'Kaituk' than the other two cultivars. It was previously found that _ZDS_ and _LCYb_ played a vital role in carotenoid biosynthesis in fruit mainly accumulating \(\beta\)-carotene. _ZDS_ catalyzed the dehydrogenation of \(\zeta\)-carotenoid to pro-lycopene, which was subsequently involved in \(\beta\)-carotene biosynthesis. In apricot and tomato, _ZDS_ was considered to be one of the key determining genes for carotenoid regulation [45]. _LCYb_ played a significant role in controlling the \(\beta\),\(\beta\)-carotenoid synthesis. In loquat and kiwi, the expression of _LCYb_ during the ripening process was closely related to the accumulation of carotenoids [46; 47]. Those observations were similar to the present results in mango, which predominately accumulated \(\beta\)-carotene, accounting for almost 60% of total carotenoids in the fruits [48; 49]. This suggested that the coordinated expression of _MiZDS_ and _MiLCYb_ in the carotenoid biosynthesis pathway might be a crucial step to control carotenoid accumulation in the three mango cultivars. Beside carotenoid biosynthetic genes, carotenoid catabolic genes also played a vital role in regulating carotenoid accumulation during fruit maturation [50]. This has illustrated that the carotenoid pool was controlled by the rate of carotenoid catabolism. The accumulation of carotenoids in various plants has been shown to be inversely correlated with the expression of carotenoid catabolic genes, particularly in members of _NCED_, _CCD1_, and _CCD4_ subfamilies [51]. The high _CCDs_ transcript level indicated that most of the carotenoids accumulated in plants were degraded. In the previous findings, the role of carotenoid catabolism in controlling carotenoid accumulation in several plants was clearly described. In chrysanthemum, the levels of carotenoid biosynthesis gene expression in white and yellow petals were similar. In the meantime, _CCD4_ was highly expressed in white petal to degrade carotenoids, but almost undetectable in the yellow petal [52]. In white and yellow fleshed peach, the differences in carotenoid biosynthetic genes were undetected, but the high transcript level of the _CCD4_ gene in white flesh peach was found, accounting for the higher carotenoid degradation [53]. In strawberry, which contained a small amount of carotenoids, the induction of _CCD1_ transcript level enhanced the degradation of carotenoids nearly 50% during fruit ripening. In mango, little information is available about the role of carotenoid catabolism in fruit pigmentation compared to carotenoid biosynthesis. In this study, the results were similar to the earlier study. It was found that the transcript level of _MiCCD1_, _MiNCED2_, and _MiNCED3_ was lower in 'Kaituk' than the other two mango cultivars in both tissues. This might have contributed to the low carotenoid degradation and led to the largest amount of carotenoids accumulated in 'Kaituk'. A similar mechanism of carotenoid regulation was also found in mango cv. 'Jinhuang'. The high expression of _CCD_ was closely related to the degradation of carotenoids synthesized by 'Jinhuang' [5]. These results might conclude that the differential regulation of _MiCCD1_, _MiNCED2_, and _MiNCED3_ appeared to be a critical mechanism responsible for determining the carotenoid accumulation in different mango cultivars.