Liu Xiaogang, Lin Cailing, Ma Xiaodi, Tan Yan, Wang Jiuzhao, Zeng Ming
College of Horticulture and Landscape Architecture, Southwest University, Chongqing, China.
Key Laboratory of Horticulture Science for Southern Mountainous Regions, Ministry of Education, Chongqing, China.
Front Plant Sci. 2018 Feb 15;9:166. doi: 10.3389/fpls.2018.00166. eCollection 2018.
Fruits of sweet orange (), a popular commercial species, contain high concentrations of flavonoids beneficial to human health. These fruits predominantly accumulate -glycosylated flavonoids, in which the disaccharides [neohesperidose (rhamnosyl-α-1,2-glucose) or rutinose (rhamnosyl-α-1,6-glucose)] are linked to the flavonoid aglycones through the 3- or 7-hydroxyl sites. The biotransformation of the flavonoid aglycones into -rutinosides or -neohesperidosides in the plants usually consists of two glycosylation reactions involving a series of uridine diphosphate-sugar dependent glycosyltransferases (UGTs). Although several genes encoding flavonoid UGTs have been functionally characterized in the plants, full elucidation of the flavonoid glycosylation process remains elusive. Based on the available genomic and transcriptome data, we isolated a with a high expression level in the sweet orange fruits that possibly encodes a flavonoid glucosyltransferase and/or rhamnosyltransferase. Biochemical analyses revealed that a broad range of flavonoid substrates could be glucosylated at their 3- and/or 7-hydrogen sites by the recombinant enzyme, including hesperetin, naringenin, diosmetin, quercetin, and kaempferol. Furthermore, overexpression of the gene could significantly increase the accumulations of quercetin 7--rhamnoside, quercetin 7--glucoside, and kaempferol 7--glucoside, implying that the enzyme has flavonoid 7--glucosyltransferase and 7--rhamnosyltransferase activities .
甜橙是一种广受欢迎的商业品种，其果实含有高浓度的对人体健康有益的黄酮类化合物。这些果实主要积累β-糖基化黄酮类化合物，其中二糖[新橙皮糖（鼠李糖基-α-1,2-葡萄糖）或芸香糖（鼠李糖基-α-1,6-葡萄糖）]通过3-或7-羟基位点与黄酮苷元相连。植物中黄酮苷元向β-芸香糖苷或β-新橙皮糖苷的生物转化通常由涉及一系列尿苷二磷酸-糖依赖性糖基转移酶（UGTs）的两个糖基化反应组成。尽管已经在植物中对几个编码黄酮类UGTs的基因进行了功能表征，但黄酮类糖基化过程的全面阐明仍然难以实现。基于现有的基因组和转录组数据，我们在甜橙果实中分离出一个高表达水平的基因，该基因可能编码一种黄酮类葡萄糖基转移酶和/或鼠李糖基转移酶。生化分析表明，重组酶可以在其3-和/或7-氢位点对多种黄酮类底物进行糖基化，包括橙皮素、柚皮素、香叶木素、槲皮素和山奈酚。此外，该基因的过表达可以显著增加槲皮素7-β-鼠李糖苷、槲皮素7-β-葡萄糖苷和山奈酚7-β-葡萄糖苷的积累，这意味着该酶具有黄酮类7-β-葡萄糖基转移酶和7-β-鼠李糖基转移酶活性。
