Mapping taste and flavour traits to genetic markers in lettuce Lactuca sativa

Abstract

Lettuce is the most highly consumed raw leafy vegetable crop eaten worldwide, making it nutritionally important in spite of its comparatively low nutrient density in relation to other vegetables. However, the perception of bitterness caused by high levels of sesquiterpenoid lactones and comparatively low levels of sweet tasting sugars limits palatability. To assess variation in nutritional and taste-related metabolites we assessed 104 members of a Lactuca sativa cv. Salinas x L. serriola (accession UC96US23) mapping population. Plants were grown in three distinct environments, and untargeted NMR and HPLC were used as a rapid chemotyping method, from which 63 unique Quantitative Trait Loci (QTL) were identified. We were able to identify putative regulatory candidate genes underlying the QTL for fructose on linkage group 9, which accounted for up to 36 % of our population variation, and which was stable across all three growing environments; and for 15-p-hydroxyyphenylacetyllactucin-8-sulfate on linkage group 5 which has previously been identified for its low bitterness, while retaining anti-herbivory field effects. We also identified a candidate gene for flavonoid 3',5'- hydroxylase underlying a polyphenol QTL on linkage group 5, and two further candidate genes in sugar biosynthesis on linkage groups 2 and 5. Collectively these candidate genes and their associated markers can inform a route for plant breeders to improve the palatability and nutritional value of lettuce in their breeding programmes.

Keywords: Breeding; Flavour; Lactuca sativa; NMR; Quantitative trait loci; Sesquiterpene.

Fig. 1

Biochemical variation in the ¹H NMR spectral profiles of L. serriola and L. sativa. A) Scores plot (PC1 v PC3) from the principal components analysis (PCA) model comparing the species grown under controlled, high nitrogen and residual nitrogen conditions. B) Scores plot (PC1 v PC2) from the PCA model comparing all offspring grown under controlled, high nitrogen and residual nitrogen conditions. C) Orthogonal projection to latent structures-discriminant analysis (OPLS-DA) model comparing the metabolic profiles of L. serriola and L. sativa grown under all conditions (Q²Y=0.69; p = 0.001). Coefficient plot indicates how metabolites covary with species with red peaks indicating those that are significantly associated with species (p < 0.05). D) Loadings plot highlighting the metabolic features contributing to the PC1 scores in the PCA model comparing offspring from different growing conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Percentage of L. serriola compound concentration relative to L. sativa in each tested environment. Differences in concentration of each compound identified in each L. sativa and L. serriola in each experimental condition. Plants showed most extreme phenotypes in the controlled environment, with up to ten times difference in metabolite concentration (Pheophytin 9.9 times more concentrated in L. serriola). L. serriola consistently contained more polyphenols than L. sativa. Bars to the right of centre represent higher concentration of metabolite in L. sativa relative to L. serriola, bars to the left of the centre show lower concentrations of metabolites in L. sativa relative to L. serriola. Scores for each parental genotype was derived from averages of all NMR run peak areas.

Fig. 3

Fold change in selected compounds between the controlled environment and the two respective field trials for each of the parents. Scores for each parental genotype was derived from averages of all NMR run peak areas. Shown is the fold change of each compound identified in each the low (residual) nitrogen trial and the high nitrogen trial, relative to the controlled environment, for each genotype. Bars to the right of centre represent higher concentration of metabolite in the field trial relative to the controlled environment, bars to the left of the centre show lower concentrations of metabolites in the field trial relative to the controlled environment.

Fig. 4

QTL map showing all markers. Red bars represent traits within the high nitrogen trial. Green bars represent traits within the low nitrogen trial. Black bars represent traits within the controlled environment trial. Blue bars represent traits within the controlled environment trial which were not identified by NMR but by the appropriate assay. QTL driven by L. sativa are shown as bars labelled with plus (+) symbols before the name and those driven by the wild parent L. serriola are shown with a minus (−). Map positions are given in cM, listed to the right of each linkage group. QTL are listed by the ¹H NMR peak ppm, and where applicable the identity. Where multiple peaks co-locate exactly they are listed on the same bar; these are typically resultant from multiple NMR peaks corresponding to a single compound. Bars represent 1 LOD interval, with the whiskers representing a 2 LOD interval.

Fig. 4

QTL map showing all markers. Red bars represent traits within the high nitrogen trial. Green bars represent traits within the low nitrogen trial. Black bars represent traits within the controlled environment trial. Blue bars represent traits within the controlled environment trial which were not identified by NMR but by the appropriate assay. QTL driven by L. sativa are shown as bars labelled with plus (+) symbols before the name and those driven by the wild parent L. serriola are shown with a minus (−). Map positions are given in cM, listed to the right of each linkage group. QTL are listed by the ¹H NMR peak ppm, and where applicable the identity. Where multiple peaks co-locate exactly they are listed on the same bar; these are typically resultant from multiple NMR peaks corresponding to a single compound. Bars represent 1 LOD interval, with the whiskers representing a 2 LOD interval.

Fig. 4

QTL map showing all markers. Red bars represent traits within the high nitrogen trial. Green bars represent traits within the low nitrogen trial. Black bars represent traits within the controlled environment trial. Blue bars represent traits within the controlled environment trial which were not identified by NMR but by the appropriate assay. QTL driven by L. sativa are shown as bars labelled with plus (+) symbols before the name and those driven by the wild parent L. serriola are shown with a minus (−). Map positions are given in cM, listed to the right of each linkage group. QTL are listed by the ¹H NMR peak ppm, and where applicable the identity. Where multiple peaks co-locate exactly they are listed on the same bar; these are typically resultant from multiple NMR peaks corresponding to a single compound. Bars represent 1 LOD interval, with the whiskers representing a 2 LOD interval.