Characterization of four polymorphic genes controlling red leaf colour in lettuce that have undergone disruptive selection since domestication.

Wenqing Su,Weiyi Zhang,Rong Tao,Dean Lavelle,Qun Hu,Jiongjiong Chen,Robert M Larkin,Yu Zhang,Zhen Yue,Shuping He,Wenye Liu,Hanhui Kuang,Lei Zhang,Richard W Michelmore,Changchun Yu,Guanghui An

doi:10.1111/pbi.13213

Abstract

SummaryAnthocyanins protect plants from biotic and abiotic stressors and provide great health benefits to consumers. In this study, we cloned four genes (Red Lettuce Leaves 1 to 4: RLL1 to RLL4) that contribute to colour variations in lettuce. The RLL1 gene encodes a bHLH transcription factor, and a 5‐bp deletion in some cultivars abolishes its function to activate the anthocyanin biosynthesis pathway. The RLL2 gene encodes an R2R3‐MYB transcription factor, which was derived from a duplication followed by mutations in its promoter region. The RLL3 gene encodes an R2‐MYB transcription factor, which down‐regulates anthocyanin biosynthesis through competing with RLL2 for interaction with RLL1; a mis‐sense mutation compromises the capacity of RLL3 to bind RLL1. The RLL4 gene encodes a WD‐40 transcription factor, homologous to the RUP genes suppressing the UV‐B signal transduction pathway in Arabidopsis; a mis‐sense mutation in rll4 attenuates its suppressing function, leading to a high concentration of anthocyanins. Sequence analysis of the RLL1‐RLL4 genes from wild and cultivated lettuce showed that their function‐changing mutations occurred after domestication. The mutations in rll1 disrupt anthocyanin biosynthesis, while the mutations in RLL2, rll3 and rll4 activate anthocyanin biosynthesis, showing disruptive selection for leaf colour during domestication of lettuce. The characterization of multiple polymorphic genes in this study provides the necessary molecular resources for the rational breeding of lettuce cultivars with distinct levels of red pigments and green cultivars with high levels of health‐promoting flavonoids.

Highlights

Anthocyanins are naturally occurring flavonoid chromophores that are largely responsible for the variation in colour among flowers and fruits, an important adaptive trait in plants
Flavonoid biosynthetic genes can be divided into early biosynthetic genes (EBGs), which catalyse the production of dihydroflavonols and late biosynthetic genes (LBGs), which lead to the biosynthesis of proanthocyanidins and anthocyanins (Ferreyra et al, 2012; Xu et al, 2015)
The RUP1 and RUP2 proteins are two WD40repeat proteins that serve as negative regulators of UV-B signalling and anthocyanin biosynthesis by down-regulating the expression of HY5, which regulates the expression of genes in the anthocyanin biosynthesis pathway (Gruber et al, 2010)

Summary

Introduction

Anthocyanins are naturally occurring flavonoid chromophores that are largely responsible for the variation in colour among flowers and fruits, an important adaptive trait in plants. The flavonoid biosynthetic pathway is mainly regulated by a complex of three transcription factors: MYB, bHLH and WD40 (MBW) (Hichri et al, 2011). In Arabidopsis, MYBL2 negatively regulates the MBW complex by competing with R2R3-MYBs for interactions with the bHLH component of the MBW complex (Dubos et al, 2008; Matsui et al, 2008). The R2R3-MYB subunit of the MBW complex may be regulated by other proteins such as nitrate-responsive proteins and RING E3 ligase (An et al, 2017; Gruber et al, 2010; Wang et al, 2018b). The expression of genes associated with anthocyanin biosynthesis is often regulated by biotic and abiotic stress and by environmental factors, such as light and temperature

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