Genetic dissection of vitamin E biosynthesis in tomato

Abstract

Vegetables are critical for human health as they are a source of multiple vitamins including vitamin E (VTE). In plants, the synthesis of VTE compounds, tocopherol and tocotrienol, derives from precursors of the shikimate and methylerythritol phosphate pathways. Quantitative trait loci (QTL) for α-tocopherol content in ripe fruit have previously been determined in an Solanum pennellii tomato introgression line population. In this work, variations of tocopherol isoforms (α, β, γ, and δ) in ripe fruits of these lines were studied. In parallel all tomato genes structurally associated with VTE biosynthesis were identified and mapped. Previously identified VTE QTL on chromosomes 6 and 9 were confirmed whilst novel ones were identified on chromosomes 7 and 8. Integrated analysis at the metabolic, genetic and genomic levels allowed us to propose 16 candidate loci putatively affecting tocopherol content in tomato. A comparative analysis revealed polymorphisms at nucleotide and amino acid levels between Solanum lycopersicum and S. pennellii candidate alleles. Moreover, evolutionary analyses showed the presence of codons evolving under both neutral and positive selection, which may explain the phenotypic differences between species. These data represent an important step in understanding the genetic determinants of VTE natural variation in tomato fruit and as such in the ability to improve the content of this important nutriceutical.

Fig. 1.

VTE biosysnthesis pathway. The MEP, SK, and tocopherol core pathways are highlighted in red, green, and blue, respectively. Candidate genes of the VTE-related pathways (carotenoid, chlorophyll, tryptophan and folate metabolism, and the SEC14 protein) are not highlighted. Enzymes are named according to their abbreviations in Table 1. Genes for which wild alleles were cloned, sequenced, and analysed are underlined.

Fig. 2.

Genomic localization of tocopherol biosynthesis and candidate genes. All genes were localized in the Tomato-EXPEN 2000 genetic map available at the Solanaceae Genomics Network (http://solgenomics.net/index.pl). Markers and genes are indicated on the left side of the chromosomes. Tocopherol QTL are indicated on the right side of the chromosomes. Gene colour code is in accordance with Fig. 1.

Fig. 3.

Tocopherol content. Tocopherol content was determined by HPLC. Grey bars indicate means of six biological replicates. Significant differences compared with the M82 control cultivar (black bars) according to Dunnett test (P<0.05) and/or Kruskal–Wallis test (P<0.05 ** and P<0.1*) are indicated.

Fig. 4.

Evolutionary rates at synonymous (dS) and non-synonymous (dN) sites of candidate genes linked to tocopherol QTL. Genes belonging to MEP, SK, and tocopherol core pathways are highlighted in red, green, and blue, respectively. VTE-related pathway candidates are not highlighted. Lines indicated estimated means.