Development of molecular markers linked to quantitative and qualitative assessment of bitterness in lettuce

莴苣是一种重要的菊科叶菜作物，是大多数沙拉的主要成分。由于公众健康运动提倡“每天吃5份”，新鲜沙拉的消费量有所增加，但与水果相比，这一增长幅度很小，因为消费者发现蔬菜的味道不如水果有吸引力。生菜种植者和零售商面临的一个问题是，许多消费者不愿食用生菜，因为他们认为生菜的味道苦而难吃。造成生菜苦味的主要成分是类异戊二烯，如倍半萜内酯（SLs），它们在生菜中含量很高。这些次生代谢物是由植物合成的，作为对病原体的防御，SLs被认为具有镇痛、止咳和镇静的特性。先前的研究表明，许多SL在L. sativa和L. serriola中都是共同的，8-硫酸盐衍生物被鉴定为新颖而稳定的SL偶联物，具有品种和组织特异性。此外，几种不稳定的SL草酸盐也被确定，这也可能有助于乳胶乳的苦味感官品质。最近对L. sativa和L. serriola的代谢组学分析和酶分析证实（Dixon et al, in prep.）表明，驯化品种的葡萄糖、果糖和蔗糖水平高出3-7倍。叶子中糖含量的升高也与甜味有关，因此我们有多种目标来减少生菜叶子的苦味和增加甜味。该项目将采用分子遗传学和代谢学相结合的方法来实现以下目标：1 .确定生菜基因组中调节SL偶联物和糖生物合成的性状位点。2将分子标记与鉴定出的苦、甜性状QTL相关联。候选基因的多态性鉴定，这些基因与QTL相关，并可能成为QTL的基础，这些QTL可以在定位群体和商业品种中进行评分。评估消费者对8-硫酸盐缀合物和糖中定量改变的苦味的感知。5 .货架期代谢组学和感官变化的量化。该项目将利用学术和工业合作伙伴的专业知识，并将与英国和海外的其他合作伙伴建立合作关系。工业主管是AL Tozer有限公司的植物育种主任，该公司是一家被认为是世界上领先的蔬菜种子育种企业之一的中小企业。AL Tozer有限公司拥有广泛的生菜育种计划，并生产了80多个品种。该公司寻求开发一项针对作物品质特征（如苦味）的育种计划，并在标记辅助育种方面建立内部专业知识，以开发这些性状的分子标记。首席学术导师可以从Richard Michelmore教授的实验室获得生菜ril (L. sativa x L. serriola)，并在此之前绘制了该种群的质量特征。与沃里克人力资源研究所的戴夫·平克教授合作，可以获得适合精细测绘的进一步种群。这些群体将成为代谢物性状QTL研究的基础。代谢物分析可以在雷丁大学的生物中心进行，尽管该项目也能够利用洛桑研究所的MetRo工作人员（Mike Beale教授和Jane Ward博士）的专业知识，他们除了对这些物种的SLs进行表征外，还对L. sativa和L. serriola进行了初步代谢物分析。雷丁大学的监督将由丽莎·梅芬博士提供支持，她在食品生物科学部门管理着一个感官研究小组，在设计和分析感官试验方面具有专业知识。因此，学生将接受一些分子遗传学、生物化学和感官技术方面的最先进的培训。

Lettuce (Lactuca spp) is an important leafy vegetable crop from the Asteraceae that is the principal component of most salads. Consumption of fresh salads has increased in response to the public health campaign to eat '5-a-day' but the increase is small compared to that seen for fruit crops as consumers find the taste of vegetables less attractive than fruit. One of the problems facing lettuce growers and retailers is that many consumers are reluctant to consume lettuce as they perceive the taste to be bitter and unpalatable. Major components contributing towards the bitter taste of lettuce are isoprenoids such as sesquiterpenoid lactones (SLs) which are highly concentrated within lettuce. These secondary metabolites are synthesised by the plant as a defence against pathogens and the SLs are thought to have analgesic, antitussive and sedative properties. Previous work has shown that a number of SLs are common to both L. sativa and L. serriola, with 8-sulphate derivatives identified as novel and major stable SL conjugates that are variety and tissue specific. Additionally, several unstable SL oxalates were identified that may also contribute to the bitter sensory qualities of Lactuca latex. Recent metabolomics analysis and confirmation by enzyme assays (Dixon et al, in prep.) of L. sativa and L. serriola has shown that the levels of glucose, fructose and sucrose are 3-7 fold higher in the domesticated cultivar. Elevated levels of sugars in the leaf are also linked to a sweet flavour and thus we have multiple targets for reducing bitterness and increasing sweetness of lettuce leaves. The project will use a combined molecular genetic and metabolic approach to address the following aims: 1 Identification of trait loci in the lettuce genome that regulate SL conjugates and sugar biosynthesis. 2 Associate molecular markers with the QTL identified for bitterness and sweetness traits. 3 Identification of polymorphisms within candidate genes that are associated with, and potentially underpin, the QTL which can be scored in the mapping population and commercial cultivars. 4 Assessment of the consumer perception of bitterness in lines that are quantitatively altered in 8-sulfate conjugates and sugars. 5 Quantification of metabolomic and sensory changes that occur during shelf life. The project will draw on the expertise of both the academic and industrial partners, and will establish collaborations with other partners in the UK and overseas. The industrial supervisor is the Director of Plant Breeding at AL Tozer Ltd, an SME regarded as one of the leading breeders of vegetable seeds in the world. AL Tozer Ltd has an extensive lettuce breeding programme and has produced over 80 cultivars. The company seeks to develop a breeding programme targeted at quality traits, such as bitterness, in the crop and to build on in-house expertise in marker assisted breeding to develop molecular markers for these traits. The lead academic supervisor has access to the lettuce RILs (L. sativa x L. serriola) from Prof. Richard Michelmore's lab and has previously mapped quality traits in this population. Further populations suitable for fine mapping are available in collaboration with Prof. Dave Pink at Warwick HRI. The populations will be the basis of QTL studies of metabolite traits. Metabolite analysis can be performed in the BioCentre at the University of Reading, although the project is also able to draw on the expertise of MetRo staff (Prof. Mike Beale and Dr Jane Ward) at Rothamsted Research who performed the initial metabolite analysis of L. sativa and L. serriola in addition to characterising SLs in these species. Supervision at the University of Reading will be supported by Dr Lisa Methven who runs a sensory research group within Food Biosciences and has expertise in designing and analysing sensory trials. Thus the student will receive state-of-the-art training in a number of molecular genetic, biochemical and sensory techniques.