State Key Laboratory of Crop Genetics and Germplasm Enhancement, Ministry of Agriculture and Rural Affairs Key Laboratory of Biology and Germplasm Enhancement of Horticultural Crops in East China, College of Horticulture, Nanjing Agricultural University, 1 Weigang, Nanjing, 210095, China.
Plant J. 2021 Nov;108(4):1116-1130. doi: 10.1111/tpj.15498. Epub 2021 Oct 28.
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 9-cis epoxycarotenoid dioxygenase, are the main enzymes involved in the cleavage of carotenoids in plants. Seven CCD genes have been annotated from the carrot genome. 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). The overexpression of DcCCD4 in orange carrots caused the taproot color to be pale yellow, and the contents of α- and β-carotene decreased sharply. 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. Functional analysis of the DcCCD4 enzyme in vitro showed that it was able to cleave α- and β-carotene at the 9, 10 (9', 10') double bonds. 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. Results showed that DcCCD4 affects the accumulation of carotenoids through cleavage of α- and β-carotene in carrot taproot.
类胡萝卜素是赋予植物鲜艳颜色的重要天然色素。类胡萝卜素积累的差异是胡萝卜肉质根形成各种颜色的关键因素之一。类胡萝卜素裂解双加氧酶（CCD），包括 CCD 和 9-顺式环氧类胡萝卜素双加氧酶，是植物中类胡萝卜素裂解的主要酶。从胡萝卜基因组中注释了 7 个 CCD 基因。在这项研究中，通过表达分析，我们发现白色胡萝卜（类胡萝卜素含量低）的肉质根中 DcCCD4 的表达水平明显高于橙色胡萝卜（类胡萝卜素含量高）。DcCCD4 在橙色胡萝卜中的过表达导致肉质根颜色呈淡黄色，α-和 β-胡萝卜素含量急剧下降。DcCCD4 功能丧失的突变型胡萝卜表现为黄色（对照胡萝卜的肉质根为白色）。在肉质根中也检测到 β-胡萝卜素的积累。DcCCD4 酶的体外功能分析表明，它能够在 9、10（9'、10'）双键处裂解 α-和 β-胡萝卜素。此外，与正常橙色胡萝卜相比，过表达 DcCCD4 的转基因胡萝卜肉质根细胞中有色质体的数量明显减少。结果表明，DcCCD4 通过裂解胡萝卜肉质根中的 α-和 β-胡萝卜素影响类胡萝卜素的积累。
