## 4 Discussion
In addition to the differences in fruit development and cultivar, it is well documented that carotenoid accumulation varied significantly among plant tissues [37]. In this study, the differential accumulation of carotenoids in both tissues was detected, with a greater amount of carotenoids in the peel than in the pulp. These results corresponded with previous findings in various fruits. For example, in citrus, carotenoid accumulation in the peel was 5-10 times higher than in the pulp [38]. Similar results were also found in apple, where carotenoid accumulation in the peel was 3-5 times higher than in the pulp [14]. Apart from that, the carotenoid compositions could also vary between plant tissues [39]. However, the variation in carotenoid contents and compositions in plant tissues could not always be described by the transcriptional regulation. The total amount of carotenoids was considered to be associated with the sink capacity of plastids, where carotenoids were synthesized. In some plants, carotenoid accumulation could be influenced by the presence/absence of chloroplasts/chromoplasts [9]. In cauliflower orange (Or) mutant, enhancing the sink capacity by activating the differentiation of chloroplasts or other colorless plastids into chromoplasts enabled an increase in carotenoid accumulation in various tissues [40]. This might suggest that the regulatory mechanism of carotenoids was different in the peel and pulp. The accumulation of carotenoids and their regulatory mechanism should be separately investigated. Due to the critical importance of carotenoids in plants and human health, the regulatory mechanism of carotenoids has been characterized in a variety of plant species at multiple molecular levels, including transcription, post-transcription, and post-translation [41]. However, the understanding of carotenoid regulation has not been achieved in mango fruits. To clarify the carotenoid metabolism pathway in mango, the expression of crucial regulatory genes in the carotenoid biosynthesis pathway (_MiPSY_, _MiPDS_, _MiZDS_, _MiCRTISO_, _MiLCYb_, _MiLCYe_, _MiHYb_, and _MiZEP_) and degradation pathway (_MiCCD1_,MiNCED2, and MiNCED3) was determined in the three mango cultivars during fruit development and ripening. In this study, the relationship between the expression of carotenoid biosynthesis genes and the accumulation of carotenoids was observed. In mango peel, the expression of _MiPSY_, _MiZDS_, _MiLCYb_, and _MiZEP_ rapidly increased during fruit development and ripening. Meanwhile, in mango pulp, the expression of _MiZDS_ and _MiLCYb_ continuously increased during fruit development and ripening. The highest transcript level of those genes was detected in 'Kaituk' during fruit development and ripening, followed by 'Nam Dok Mai No.4' and 'Nam Dok Mai Sithong' in both peel and pulp. These results indicated that the variations in carotenoid content in developmental stages and mango cultivars might be attributed to the expression of _MiPSY_, _MiZDS_, _MiLCYb_, and _MiZEP_ in peel and _MiZDS_, and _MiLCYb_ in pulp. The expression of _PSY_ has been described as a crucial step influencing carotenoid accumulation in higher plants [42]. To clarify the role of _PSY_ in regulating carotenoid biosynthesis, tomato fruits were transformed with an antisense _PSY1_, which reduced carotenoid accumulation. It was found that carotenoid biosynthesis was inhibited and resulted in ripe fruit with white color. This indicated that _PSY_ was a rate-limiting gene in regulating carotenoid biosynthesis in plants. The considerable increment of _PSY_ expression during ripening was described in many fruits and vegetables, such as banana [43], citrus [4], and pepper [7]. It was described that the increment in the expression of the _PSY_ gene and carotenoid content coincidentally occurred with the elevated expression of relevant genes in the ethylene biosynthesis pathway [44]. Different metabolic steps also presented as being related to carotenoid biosynthesis.