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

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.

Patterning the Asteraceae Capitulum: Duplications and Differential Expression of the Flower Symmetry CYC2-Like Genes

“…The naming system of the CYC2 -like genes of the present study basically follows Chapman et al ( 2008 ) in sunflower, that is, five clades from CYC2a to CYC2e. The sixth clade was newly named as CYC2g by the present study (“f” was skipped because, Huang et al, 2016 reported a gene called CmCYC2f in C. morifolium , which is actually a CYC2e -like gene according to our study).…”

“…In the radiate capitula of Helianthus annuus, Gerbera hybrida, Senecio vulgaris and Chrysanthemum morifolium, CYC2 -like genes regulate the morphogenesis of florets and differentiation of the marginal and central layer(s) of florets (Broholm et al, 2008 ; Kim et al, 2008 ; Chapman et al, 2012 ; Juntheikki-Palovaara et al, 2014 ; Garcês et al, 2016 ; Huang et al, 2016 ). The differentiated expression patterns and selection regimes of CYC2 -like paralogs showed close correlations with functional divergence (Chapman et al, 2008 ; Tähtiharju et al, 2012 ).…”

“…The differentiated expression patterns and selection regimes of CYC2 -like paralogs showed close correlations with functional divergence (Chapman et al, 2008 ; Tähtiharju et al, 2012 ). Changes in the expression of some CYC2 -like members led to gradient transitions between ray and disc florets, including modifications of ray-specific characteristics, such as the zygomorphy of the corolla, length of the ligule and development of the stamen (Fambrini et al, 2003 ; Broholm et al, 2008 ; Chapman et al, 2012 ; Juntheikki-Palovaara et al, 2014 ; Huang et al, 2016 ); some other CYC2 -like members influence only the morphs of the marginal ray florets (Kim et al, 2008 ; Garcês et al, 2016 ).…”

“…Thus far, however, the history of duplication and the relationships of the CYC2 clade genes in the Asteraceae remain unresolved due to sampling problems. On the one hand, previous studies have focused on radiate species only or involved a few distantly related species samples in the family; on the other hand, their phylogenetic analyses were often based on a region between the TCP and R domains of CYC2 -like genes (Kim et al, 2008 ; Chapman et al, 2012 ; Tähtiharju et al, 2012 ; Huang et al, 2016 ; Bello et al, 2017 ). Moreover, the evidence that co-option of different CYC2 -like copies occurred independently in less-related species per se is inadequate for determining the reason why various Asteraceae capitula exhibit a complex pattern of evolution.…”

Quantitative Classification of the Morphological Traits of Ray Florets in Large-flowered Chrysanthemum

“…As mentioned previously, ray floret shape also includes other variations. This may be the reason that the genetic regulation model of floral organ development in higher plants cannot fully explain the development of the capitulum and different types of ray florets (Huang et al, 2016), a factor that limits novel flower shapes in chrysanthemum breeding.…”