背景回顾：Carotenoids are important natural pigments that give bright colors to plants. The difference in the accumulation of carotenoids is one of the key factors in the formation of various colors in carrot taproots. Carotenoid cleavage dioxygenases (CCDs), including CCD and nine-cis epoxycarotenoid dioxygenase (NCED), are the main enzymes involved in the cleavage of carotenoids in plants. Seven CCD genes have been annotated from the carrot genome. 

结果1-表达模式：In this study, through expression analysis, we found that the expression level of DcCCD4 was significantly higher in the taproot of white carrot (low carotenoid content) than orange carrot (high carotenoid content). 

结果2-过表达：The overexpression of DcCCD4 in orange carrots caused the taproot color to be pale yellow, and the contents of α-carotene and β-carotene decreased sharply. 

结果3-功能缺失突变：Mutant carrot with loss of DcCCD4 function exhibited yellow color (the taproot of the control carrot was white). The accumulation of β-carotene was also detected in taproot. 

结果4-体外检测酶活：Functional analysis of the DcCCD4 enzyme in vitro showed that it was able to cleave α-carotene and β-carotene at the 9, 10 (9’, 10’) double bonds.

结果5-有色体：In addition, the number of colored chromoplasts in the taproot cells of transgenic carrots overexpressing DcCCD4 was significantly reduced compared with that in normal orange carrots. 

结论：Results demonstrated that DcCCD4 affects the accumulation of carotenoids through cleavage of α-carotene and β-carotene in carrot taproot.

 摘 要 

类胡萝卜素是重要的自然色素，赋予植物以亮丽的颜色。类胡萝卜素的差异性积累是颜色各异的胡萝卜主根形成的主要因素之一。类胡萝卜素裂解双加氧酶CCDs包括了CCD和9-顺式环氧类胡萝卜素双加氧酶（NCED），参与了植物中类胡萝卜素的裂解。胡萝卜基因组中共注释到了7个CCD基因。本文中，通过表达分析，作者发现DcCCD4基因在类胡萝卜素含量较低的白胡萝卜主根中的表达要显著高于类胡萝卜素含量较高的橙胡萝卜。在橙胡萝卜中过表达DcCCD4基因会导致其主根变为苍黄色，并且其中α-胡萝卜素和β-胡萝卜素的含量急剧降低。而白胡萝卜素的DcCCD4基因功能丢失突变后，其主根变成黄色，对照的主根仍为白色。并且在其主根中检测到了β-胡萝卜素的积累。通过体外对DcCCD4酶的功能检测，发现其能够在(9’, 10’)双键裂解α-胡萝卜素和β-胡萝卜素。另外，与正常的橙胡萝卜相比，在过表达DcCCD4基因的橙胡萝卜主根细胞中，有色体的数量大大降低。本文的研究结果表明，DcCCD4通过裂解胡萝卜主根中的α-胡萝卜素和β-胡萝卜素影响类胡萝卜素的积累。