Institute of Chemical, Environmental and Bioscience Engineering, Technische Universität Wien, 1060, Vienna, Austria.
Fruit Science, Technical University of Munich, 85354, Freising, Germany.
BMC Plant Biol. 2018 Oct 3;18(1):216. doi: 10.1186/s12870-018-1424-0.
Commercially available poinsettia (Euphorbia pulcherrima) varieties prevalently accumulate cyanidin derivatives and show intense red coloration. Orange-red bract color is less common. We investigated four cultivars displaying four different red hues with respect to selected enzymes and genes of the anthocyanin pathway, putatively determining the color hue.
Red hues correlated with anthocyanin composition and concentration and showed common dark red coloration in cultivars 'Christmas Beauty' and 'Christmas Feeling' where cyanidin derivatives were prevalent. In contrast, orange-red bract color is based on the prevalent presence of pelargonidin derivatives that comprised 85% of the total anthocyanin content in cv. 'Premium Red' and 96% in cv. 'Harvest Orange' (synonym: 'Orange Spice'). cDNA clones of flavonoid 3'-hydroxylase (F3'H) and dihydroflavonol 4-reductase (DFR) were isolated from the four varieties, and functional activity and substrate specificity of the corresponding recombinant enzymes were studied. Kinetic studies demonstrated that poinsettia DFRs prefer dihydromyricetin and dihydroquercetin over dihydrokaempferol, and thus, favor the formation of cyanidin over pelargonidin. Whereas the F3'H cDNA clones of cultivars 'Christmas Beauty', 'Christmas Feeling', and 'Premium Red' encoded functionally active enzymes, the F3'H cDNA clone of cv. 'Harvest Orange' contained an insertion of 28 bases, which is partly a duplication of 20 bases found close to the insertion site. This causes a frameshift mutation with a premature stop codon after nucleotide 132 and, therefore, a non-functional enzyme. Heterozygosity of the F3'H was demonstrated in this cultivar, but only the mutated allele was expressed in the bracts. No correlation between F3'H-expression and the color hue could be observed in the four species.
Rare orange-red poinsettia hues caused by pelargonidin based anthocyanins can be achieved by different mechanisms. F3'H is a critical step in the establishment of orange red poinsettia color. Although poinsettia DFR shows a low substrate specificity for dihydrokaempferol, sufficient precursor for pelargonidin formation is available in planta, in the absence of F3'H activity.
市面上常见的一品红（Euphorbia pulcherrima）品种普遍积累了飞燕草色素衍生物，呈现出鲜艳的红色。橙色-红色苞片颜色则较为少见。我们研究了四个具有不同红色色调的品种，涉及花青素途径的选定酶和基因，这些酶和基因可能决定了颜色色调。
红色色调与花青素组成和浓度相关，在以飞燕草色素衍生物为主的“圣诞美”和“圣诞情”品种中表现出常见的暗红色。相比之下，橙色-红色苞片颜色基于普遍存在的天竺葵色素衍生物，其在 cv. “优质红”中占总花青素含量的 85%，在 cv. “收获橙”（同义词：“橙香料”）中占 96%。从四个品种中分离出了类黄酮 3'-羟化酶（F3'H）和二氢黄酮醇 4-还原酶（DFR）的 cDNA 克隆，并研究了相应重组酶的功能活性和底物特异性。动力学研究表明，一品红 DFR 优先选择二氢杨梅素和二氢槲皮素而不是二氢山奈酚，因此有利于飞燕草色素的形成而不是天竺葵色素。而品种“圣诞美”、“圣诞情”和“优质红”的 F3'H cDNA 克隆编码功能活性酶，cv. “收获橙”的 F3'H cDNA 克隆则包含 28 个碱基的插入，其中部分是靠近插入位点的 20 个碱基的重复。这导致第 132 个核苷酸后出现移码突变和过早的终止密码子，因此是一种无功能的酶。在该品种中证明了 F3'H 的杂合性，但只有突变等位基因在苞片中表达。在这四个品种中，没有观察到 F3'H 表达与颜色色调之间的相关性。
由天竺葵色素衍生的花青素导致的罕见橙色-红色一品红色调可以通过不同的机制实现。F3'H 是建立橙色-红色一品红颜色的关键步骤。尽管一品红 DFR 对二氢山奈酚的底物特异性较低，但在缺乏 F3'H 活性的情况下，植物体内仍有足够的前体用于形成天竺葵色素。
