Color regulation mechanism of lettuce leaves

admin

(Contributed by Li Bo)

       Lettuce (Lactuca sativa) is one of the most importantvegetable crops worldwide[1]. Cultivated lettuce shows dramaticvariations in leaf colour. Most lettuce cultivars have green leaves, but othersmay have red leaves. The red color is caused by the accumulation of flavonoids,which contribute to both plant fitness and to the nutrition of consumers[2].However, genetic studies underlying the phenotypic variation of lettuce arelimited[3].

Robinson et al.[4] studies on theinheritance of anthocyanin in lettuce suggested 6 loci controlling anthocyaninaccumulation.

       Zhang et al.[5] identified 12expression quantitative trait loci (eQTL) that regulate the expression of 24genes associated with flavonoid biosynthesis and six candidate loci controllingthe accumulation of anthocyanins in lettuce leaves.

       Zhang et al.[6] identified thirty-fourstructural genes were found to cover the main steps of the anthocyanin pathway,including chalcone synthase, chalcone isomerase, flavanone 3-hydroxylase,flavonoid 3'-hydroxylase, flavonoid 3', 5'-hydroxylase, dihydroflavonol4-reductase, and anthocyanidin synthase. Seven MYB, three bHLH, and two WD40genes, considered anthocyanin regulatory genes, were also identified.

       However, there is no functional analysis of thesecandidate genes on the accumulation of anthocyanins. In addition, their evolutionand their impact on the regulatory mechanisms that drive the accumulation ofanthocyanins remained to be studied.

       Su et al.[7] cloned four genes (RedLettuce Leaves 1 to 4: RLL1 to RLL4) that contribute to colour variationsin lettuce. The RLL1 gene encodes a bHLH transcription factor, and a5-bp deletion in some cultivars abolishes its function to activate theanthocyanin biosynthesis pathway. The RLL2 gene encodes an R2R3-MYBtranscription factor, which was derived from a duplication followed bymutations in its promoter region. The RLL3 gene encodes an R2-MYBtranscription factor, which down-regulates anthocyanin biosynthesis throughcompeting with RLL2 for interaction with RLL1; a mis-sense mutationcompromises the capacity of RLL3 to bind RLL1. The RLL4gene encodes a WD-40 transcription factor, homologous to the RUP genessuppressing the UV-B signal transduction pathway in Arabidopsis; a mis-sensemutation in rll4 attenuates its suppressing function, leading to a highconcentration of anthocyanins. Sequence analysis of the RLL1-RLL4 genesfrom wild and cultivated lettuce showed that their function-changing mutationsoccurred after domestication. The mutations in rll1 disrupt anthocyaninbiosynthesis, while the mutations in RLL2, rll3 and rll4activate anthocyanin biosynthesis, showing disruptive selection for leaf colourduring domestication of lettuce.

Reference:

[1]Funk, V., Susanna, A., Stuessy, T. and Bayer, R.J. Systematics, Evolution, andBiogeography of Compositae. Vienna: International Association for PlantTaxonomy. 2009.

[2]Xu, W., Dubos, C. & Lepiniec, L. Transcriptional control of flavonoidbiosynthesis by MYB-bHLH-WDR complexes. Trends Plant Sci. 2015, 20, 176–185.

[3]Waycott, W., Fort, S. B., Ryder, E. J. & Michelmore, R. W. Mappingmorphological genes relative to molecular markers in lettuce (Lactuca sativaL.). Heredity (Edinb.)1999, 82, 245–251.

[4] Robinson, R.W.,McCreight, J.D. and Ryder, E.J. The Genes of Lettuce and Closely RelatedSpecies. Plant Breed. Rev. 1983,1, 267–293.

[5] Zhang L, Su W, TaoR, et al. RNA sequencing provides insights into the evolution of lettuce andthe regulation of flavonoid biosynthesis. Nat. Commun. 2017, 8(1):2264.

[6] Zhang Y Z, Xu S Z,Cheng Y W, et al. Transcriptome analysis and anthocyanin-related genes in redleaf lettuce. Genet. Mol. Res. 2016, 15(1).

[7] Su W Q, Tao R, LiuW, et al. Characterization of four polymorphic genes controlling red leafcolour in lettuce that have undergone disruptive selection since domestication.Plant Biotechnol. J. 2020, 18, 479–490.