Research progress on metabolites of lettuce

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(Contributed by  Li Bo)

Background

       Lettuce (Lactucasativa L.) is a main leafy vegetable for human consumption and aneconomically important food crop worldwide[1]. Vegetables areimportant sources of energy, dietary fibers, minerals, and other beneficialphytochemicals such as antioxidants[2]. Metabolites are sources ofenergy and nutrition for humans and are also essential for plant growth,development and reproduction[3,4]. Understanding the naturalvariation and genetic bases of plant metabolism will thus ultimately contributeto biofortification of plants in a manner that is beneficial to humans in termsof both food security and quality[5] . In the past years, forlettuce, large-scale or essentially untargeted LCMS-based metabolomicsapproaches have previously been used to investigate metabolite differencesamong babyleaf, romaine and iceberg cultivar types[6] and betweengreen and red oakleaf lettuce[7], as well as to obtain insight inmetabolites related to browning of fresh-cut romaine lettuce[8].However, comprehensive metabolomics studies of the lettuce gene pool are stillvery little. Here we reviewed recent advances in studies on lettucemetabolites.

Research progress

       Kim et al.[9]exploredthe genetic potential of baby leaves of 23 diverse lettuce cultivars for thehealth-beneficial metabolites. The results showed that the composition andcontents of the studied metabolites in lettuces varied significantly betweencultivars and were principally dependent on leaf color. All red-leaf cultivarswere rich in carotenoids, cyanidin, polyunsaturated fatty acids (PUFAs,primarily in the form of α-linolenic and linoleic acid,) total phenoliccontents (TPC), and antioxidant potential. Among carotenoids, all-E-lutein wasfound in highest amount, followed by all-E-violaxanthin and all-E-lactucaxanthin,accounting for an average of 30%, 28% and 15% of total carotenoids,respectively. The content of total folate was recorded in the range of 6.51(cv. Caesar Green) to 9.73 μg/g (DW) (cv. Asia Heuk Romaine). The principalcomponent analysis (PCA) showed that the cyanidin and other phenolic compoundsare the most potent scavengers of ABTS and DPPH radicals. The overall resultssuggested that all red-leaf lettuce cultivars have a distinct profile ofphytoconstituents, which can be used as a nutrient-dense food.

       Zhang et al.[10] investigated 77 primarymetabolites in 189 accessions including all major horticultural types and wildlettuce L. serriola, showed that the metabolites in L. serriola were differentfrom those in cultivated lettuce. The findings were consistent with thedemographic model of lettuce and supported a single domestication event forthis species. Selection signals among these metabolic traits were detected.Specifically, galactinol, malate, quinate and threonate were significantlyaffected by the domestication process and cultivar differentiation of lettuce.Galactinol and raffinose might have been selected during stem lettucecultivation as an adaption to the local environments in China. Furthermore, identified154 loci significantly associated with the level of 51 primary metabolites.Three genes (LG8749721, LG8763094 and LG5482522) responsible for the levels ofgalactinol, raffinose, quinate and chlorogenic acid were further dissected,which may have been the target of domestication and/or affected by localadaptation. Additionally, these results suggest that human selection resultedin reduced quinate and chlorogenic acid levels in cultivated lettuce.

Treuren et al.[1] investigated 150 Lactucagenebank accessions. A hierarchical cluster analysis of the variation inrelative abundance of 2026 phytochemicals, revealed by untargeted metabolicprofiling, strongly resembled the known lettuce gene pool structure, indicatingthat the observed variation was to a large extent genetically determined. Manyphytochemicals appeared species-specific, of which several are generallyrelated to traits that are associated with plant health or nutritional value.For a large number of phytochemicals the relative abundance was eitherpositively or negatively correlated with available phenotypic data onresistances against pests and diseases, indicating their potential role inplant resistance. Particularly the more primitive lettuces and the closelyrelated wild relatives showed high levels of (poly)phenols and vitamin C, thusrepresenting potential genetic resources for improving nutritional traits inmodern crop types. This study demonstrated the ample availability of suitablegenetic resources for the development of improved lettuce varieties with highernutritional quality and more sustainable production.

       Yang et al.[11] used combined GC ×GC-TOF/MS and UPLC-IMS-QTOF/MS to detect and relatively quantify metabolites in30 lettuce cultivars representing large genetic diversity and identification of171 metabolites. Sixteen of these 171 metabolites (including phenolic acidderivatives, glycosylated flavonoids, and one iridoid) were present atsignificantly different levels in leaf and head type lettuces, which suggestedthe significant metabolomic variations between the leaf and head types oflettuce are related to secondary metabolism. A combination of the results andmetabolic network analysis techniques suggested that leaf and head typelettuces contain not only different levels of metabolites but also have significantvariations in the corresponding associated metabolic networks. The novellettuce metabolite library and novel non-targeted metabolomics strategy devisedin this study could be used to further characterize metabolic variationsbetween lettuce cultivars or other plants. Moreover, the findings of this studyprovide important insight into metabolic adaptations due to natural and humanselection, which could stimulate further research to potentially improvelettuce quality, yield, and nutritional value.

Reference:

[1]Treuren R V, Eekelen H V, Wehrens R, et al. (2018) Metabolite variationin the lettuce gene pool: towards healthier crop varietiesand food. Metabolomics, 14:146.

[2]Slavin, J.L. and Lloyd, B. (2012) Health benefits of fruits and vegetables. Advances in Nutrition, 3, 506–516.

[3]Rojas, C.M., Senthil-Kumar, M., Tzin, V. and Mysore, K.S. (2014) Regulation ofprimary plant metabolism during plant-pathogen interactions and itscontribution to plant defense. Frontiers in Plant Science, 5, 17.

[4]Sulpice, R. and McKeown, P.C. (2015) Moving toward a comprehensive map ofcentral plant metabolism. Annual Review of Plant Biology, 66, 187–210.

[5]Fernie, A.R. and Tohge, T. (2017) The genetics of plant metabolism. AnnualReview of Genetics, 51, 287–310.

[6]Abu-Reidah, I. M., Contreras, M. M., Arráez-Román, D., SeguraCarretero, A.,& Fernández-Gutiérrez, A. (2013). Reversedphase ultra-high-performanceliquid chromatography coupled to electrosprayionization-quadrupole-time-of-flight mass spectrometry as a powerful tool formetabolic profiling of vegetables: Lactuca sativa as an example of itsapplication. Journal of Chromatography A, 1313, 212–227.

[7]Viacava, G. E., Roura, S. I., Berrueta, L. A., Iriondo, C., Gallo, B., &Alonso-Salces, R. M. (2017). Characterization of phenolic compounds in greenand red oak-leaf lettuce cultivars by UHPLCDAD-ESI-QToF/MS using MSE scan mode.Journal of Mass Spectrometry, 52, 873–902.

[8]García, C. J., García-Villalba, R., Gil, M. I., & Tomas-Barberan, F. A.(2017). LC-MS untargeted metabolomics to explain the signal metabolitesinducing browning in fresh-cut lettuce. Journal of Agricultural and FoodChemistry, 65, 4526–4535.

[9]Kim DE, Shang X, Assefa AD, Keum YS, Saini RK. (2018). Metabolite profiling ofgreen, green/red, and red lettuce cultivars: Variation in health beneficialcompounds and antioxidant potential. Food Research International, 105:361-370.

[10] Zhang W, Alseekh S,Zhu X, Zhang Q, Fernie AR, Kuang H, Wen W. (2020). Dissection of thedomestication-shaped genetic architecture of lettuce primary metabolism. PlantJournal, 104(3):613-630.

[11]Yang X, Wei S, Liu B, Guo D, Zheng B, Feng L, Liu Y, Tomás-Barberán FA, Luo L,Huang D.  (2018). A novel integratednon-targeted metabolomic analysis reveals significant metabolite variationsbetween different lettuce ( Lactuca sativa. L) varieties. HorticultureResearch, 5(1):33.