Abstract
Anthocyanins are the primary pigments contributing to the variety of flower colors among angiosperms and are considered essential for survival and reproduction. Anthocyanins are members of the flavonoids, a broader class of secondary metabolites, of which there are numerous structural genes and regulators thereof. In western European populations of Lysimachia arvensis, there are blue- and orange-petaled individuals. The proportion of blue-flowered plants increases with temperature and daylength yet decreases with precipitation. Here, we performed a transcriptome analysis to characterize the coding sequences of a large group of flavonoid biosynthetic genes, examine their expression and compare our results to flavonoid biochemical analysis for blue and orange petals. Among a set of 140 structural and regulatory genes broadly representing the flavonoid biosynthetic pathway, we found 39 genes with significant differential expression including some that have previously been reported to be involved in similar flower color transitions. In particular, F3′5′H and DFR, two genes at a critical branchpoint in the ABP for determining flower color, showed differential expression. The expression results were complemented by careful examination of the SNPs that differentiate the two color types for these two critical genes. The decreased expression of F3′5′H in orange petals and differential expression of two distinct copies of DFR, which also exhibit amino acid changes in the color-determining substrate specificity region, strongly correlate with the blue to orange transition. Our biochemical analysis was consistent with the transcriptome data indicating that the shift from blue to orange petals is caused by a change from primarily malvidin to largely pelargonidin forms of anthocyanins. Overall, we have identified several flavonoid biosynthetic pathway loci likely involved in the shift in flower color in L. arvensis and even more loci that may represent the complex network of genetic and physiological consequences of this flower color polymorphism.
Highlights
Flower color plays a fundamental role in pollinator attraction and plant reproduction (Fenster et al, 2004)
We focus on the flavonoid genes and compounds underlying the blue and orange flower color variation of Lysimachia arvensis
We investigated single nucleotide polymorphisms (SNPs) frequencies that correlate with flower color in two candidates within the narrowly defined anthocyanin biosynthetic pathway (ABP) genes
Summary
Flower color plays a fundamental role in pollinator attraction and plant reproduction (Fenster et al, 2004). Changes in flower color may produce pollinator shifts leading to speciation (Rausher, 2008; Ortiz-Barrientos, 2013; Van der Niet et al, 2014). Abiotic factors have received far less attention, but they may impose selection on flower color (e.g., temperature, precipitation, solar radiation; Strauss and Whittall, 2006; Rausher, 2008; Van der Kooi et al, 2019; Dalrymple et al, 2020; Koski et al, 2020). Large differences in flower color are mainly controlled by the underlying pigments such as chlorophylls, carotenoids, betalains and flavonoids (Tanaka et al, 2008). Anthocyanins are the most common group of flavonoid pigments present in flowers, and differences in the number of hydroxyl groups on their B-ring characterize the three main types: pelargonidin-derived anthocyanins (orange or red; one hydroxyl group), cyanidin-derived anthocyanins (magenta; two hydroxyl groups) and delphinidin-derived anthocyanins (including malvidin conferring blue or purple; three hydroxyl groups; Davies, 2009; Wessinger and Rausher, 2012, 2013; Ng et al, 2018)
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