How to make a red flower: the combinatorial effect of pigments.

Julienne Ng,Stacey D Smith

doi:10.1093/aobpla/plw013

Abstract

Red flowers have evolved repeatedly across angiosperms and are frequently examined in an ecological context. However, less is known about the biochemical basis of red colouration in different taxa. In this study, we examine the spectral properties, anthocyanin composition and carotenoid expression of red flowers in the tomato family, Solanaceae, which have evolved independently multiple times across the group. Our study demonstrates that Solanaceae typically make red flowers either by the sole production of red anthocyanins or, more commonly, by the dual production of purple or blue anthocyanins and orange carotenoids. In using carotenoids to modify the effect of purple and/or blue anthocyanins, these Solanaceae species have converged on the same floral hue as those solely producing red anthocyanins, even when considering the visual system of pollinators. The use of blue anthocyanins in red flowers appears to differ from other groups, and suggests that the genetic changes underlying evolutionary shifts to red flowers may not be as predictable as previously suggested.

Highlights

Flower colour plays a central role in plant ecology, mediating signalling to pollinators (Bradshaw and Schemske 2003; Fenster et al 2004) and contributing to thermoregulation (Anderson et al 2013) and defence against herbivores (Strauss et al 2004)
We examine spectral properties, anthocyanin composition and carotenoid expression in 27 species that represent 24 of the estimated 30 independent origins of red colouration in Solanaceae to address three specific questions: (i) Does floral hue vary depending on the type of pigments present, i.e. do different pigment pathways lead to different shades of red? (ii) In red-flowered species that produce anthocyanins, what is the range of anthocyanin types in terms of hydroxylation? (iii) Does the type of anthocyanin depend on the presence or absence of carotenoids? We predict that yellow – orange carotenoids may commonly combine with blue and purple anthocyanins to result in an overall red floral hue
Our study reveals that the convergent evolution of red flower colouration in Solanaceae is due to the recruitment of different pigment pathways and different classes of pigments

Summary

Introduction

Flower colour plays a central role in plant ecology, mediating signalling to pollinators (Bradshaw and Schemske 2003; Fenster et al 2004) and contributing to thermoregulation (Anderson et al 2013) and defence against herbivores (Strauss et al 2004). Three main classes of pigments contribute to flower colour: anthocyanins, carotenoids and betalains. Anthocyanins are water-soluble flavonoid compounds that are stored in plant vacuoles, and contribute to red, pink, blue and purple colours. Carotenoids are hydrophobic yellow, orange and red compounds that are produced and stored in plastids. These two common plant pigments are known to occur both alone and in combination with floral tissue (e.g. van Raamsdonk 1993; Schemske and Bradshaw 1999; Irwin and Strauss 2005), species whose flowers are solely coloured by carotenoids appear to be relatively rare (Ohmiya 2011)

Objectives

Methods

Results

Discussion

Conclusion