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

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.