Identification and Characterization of CYC-Like Genes in Regulation of Ray Floret Development in Chrysanthemum morifolium.

Di Huang,Tengxun Zhang,Huitang Pan,Jia Wang,Tangren Cheng,Ming Sun,Xiaowei Li,Qixiang Zhang

doi:10.3389/fpls.2016.01633

Abstract

Chrysanthemum morifolium, one of the most economically important ornamental crops worldwide, is well-known for the elaborate and complex inflorescence which is composed of both bilaterally symmetrical ray florets and radially symmetrical disc florets. Despite continuing efforts, the molecular mechanisms underlying regulation of the two flower types are still unclear so far. CYC-like proteins have been shown to control flower symmetry or regulate flower-type identity in several angiosperm plant lineages. In this study, we conducted comparative analysis of the CmCYC2 genes in two chrysanthemum cultivars and their F1 progenies with various whorls of ray florets. Six CmCYC genes were identified and sequenced, all of which were grouped into the CYC2 subclade. All the six CmCYC2 genes were predominantly expressed in reproductive organs, and in particular in the petal of ray florets. Of these genes, the transcription level of CmCYC2c was highly up-regulated in ray florets of the double-ray flowered heads. In addition, the result that CmCYC2c was highly expressed at key developing stages indicates its role in regulating petal development. Furthermore, overexpression of CmCYC2c in C. lavandulifolium, one of the original species of C. morifolium, led to significant increase in flower numbers and petal ligule length of ray florets. Besides CmCYC2c, the expression of CmCYC2f was also significantly up-regulated in transgenic lines, implying a possible role in regulating development of ray florets. Both results of expression patterns and transgenic phenotypes suggest that CmCYC2c is involved in regulating ray floret identity in the chrysanthemum. This study will be useful for genetic manipulation of flower shape in chrysanthemum and hence promote the process of molecular breeding.

Highlights

Asteraceae is characterized for a highly compressed capitulum which superficially resembles a large single flower but is composed of numerous individual flowers (Funk, 2009)
C. morifolium ‘Guoqing xiaoliuhao’ (GQ) and ‘Mao xiangyu’ (MXY) (Figure 2B) are two hexaploid (2n = 54) ground-cover chrysanthemum cultivars with different whorls of ray florets
To better understand the morphologic traits of ray and disc florets (DF) of MXY, we normalized the development process into four phases: (I) initiation of floral primordia; (II) differentiation of floral organs; (III) growth of floral organs; and (IV) maturation of inflorescence

Summary

Introduction

Asteraceae is characterized for a highly compressed capitulum which superficially resembles a large single flower but is composed of numerous individual flowers (Funk, 2009). Besides ray and DF, some chrysanthemum cultivars have a third flower type: trans floret (TF) which is morphologically similar to ray floret but with smaller or abnormal ventral petal ligule (Figure 1). The presence of the showy ray florets in the capitulum has been shown to be associated with pollinator-mediated speciation, outcrossing rage and genetic diversity, and may lead to the evolutionary success of the Asteraceae (Marshall and Abbott, 1984; Sun and Ganders, 1990; Endress, 1999; Sargent, 2004; JuntheikkiPalovaara et al, 2014) Both classical and modern molecular genetic studies have indicated that the presence or absence of ray florets is mainly under the control of one or two major genes and some other modifier genes (Gillies et al, 2002; Andersson, 2008)

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Results

Conclusion