Evolution of Double Positive Autoregulatory Feedback Loops in CYCLOIDEA2 Clade Genes Is Associated with the Origin of Floral Zygomorphy

Abstract

Members of the CYCLOIDEA2 (CYC2) clade of the TEOSINTE BRANCHED1, CYCLOIDEA, and PCF transcription factor genes are widely involved in controlling floral zygomorphy, a key innovation in angiosperm evolution, depending on their persistently asymmetric expression in the corresponding floral domains. However, it is unclear how this asymmetric expression is maintained throughout floral development. Selecting Primulina heterotricha as a model, we examined the expression and function of two CYC2 genes, CYC1C and CYC1D. We analyzed the role of their promoters in protein-DNA interactions and transcription activation using electrophoresis mobility shift assays, chromatin immunoprecipitation, and transient gene expression assays. We find that CYC1C and CYC1D positively autoregulate themselves and cross-regulate each other. Our results reveal a double positive autoregulatory feedback loop, evolved for a pair of CYC2 genes to maintain their expression in developing flowers. Further comparative genome analyses, together with the available expression and function data of CYC2 genes in the core eudicots, suggest that this mechanism might have led to the independent origins of floral zygomorphy, which are associated with plant-insect coevolution and the adaptive radiation of angiosperms.

Distinct Regulatory Changes Underlying Differential Expression of TEOSINTE BRANCHED1-CYCLOIDEA-PROLIFERATING CELL FACTOR Genes Associated with Petal Variations in Zygomorphic Flowers of Petrocosmea spp. of the Family Gesneriaceae

“…For example, in A. majus, the dorsalspecific expression of CYC and DICH produces typical zygomorphic flowers, while their expansion or loss of expression gives rise to backpetals mutants or ventralized peloric flowers (Luo et al, 1996(Luo et al, , 1999. Similar phenomena have been widely observed in other angiosperms (Cubas et al, 1999b;Citerne et al, 2006;Busch and Zachgo, 2007;Pang et al, 2010;Yang et al, 2012). Furthermore, morphological transitions in zygomorphic flowers and between zygomorphy and actinomorphy in closely related species are usually related to the expression changes of CYC-like genes (Zhang et al, 2010;Howarth et al, 2011;Busch et al, 2012;Zhong and Kellogg, 2015).…”

“…Both CYC and DICH encode proteins belonging to the CYC/TB1 (for TEOSINTE BRANCHED1) or ECE (named after a conserved Glu-Cys-Glu motif) lineage in the plant-specific TCP transcription factor family that is named after TB1 from maize (Zea mays), CYC from A. majus, and the PROLIFERATING CELL FACTORS (PCF) from rice (Oryza sativa; Cubas et al, 1999a;Howarth and Donoghue, 2006). A growing body of evidence has shown that CYC-like TCP genes function in controlling floral zygomorphy widely in eudicots (Feng et al, 2006;Busch and Zachgo, 2007;Broholm et al, 2008;Kim et al, 2008;Wang et al, 2008;Yang et al, 2012). The spatial-temporal expression changes of CYC-like genes usually bring about transformations of floral symmetry and modifications of floral morphology, mainly reflected in size and shape changes of the dorsal petals in the second floral whorl (Cubas et al, 1999b;Hileman et al, 2003;Busch and Zachgo, 2007;Gao et al, 2008;Song et al, 2009;Pang et al, 2010;Zhang et al, 2010;Howarth et al, 2011;Busch et al, 2012;Yang et al, 2012;Zhong and Kellogg, 2015).…”

“…As one of the earliest diverging groups in Lamiales, Gesneriaceae has diverse forms of zygomorphic flowers (Cubas, 2004). Recent studies have shown that the spatial-temporal expression alterations of CYC-like genes are highly correlated with the diverse morphologies of zygomorphic flowers in Gesneriaceae (Du and Wang, 2008;Gao et al, 2008;Song et al, 2009;Yang et al, 2012). The genus Petrocosmea, comprising 42 species, is a midsized group in Gesneriaceae with very similar vegetative traits but extremely diverse zygomorphic flowers that are mainly reflected in size and shape variations of the dorsal petals in the second floral whorl (Li and Wang, 2004;Qiu et al, 2011Qiu et al, , 2015.…”

Control of Floret Symmetry by RAY3, SvDIV1B, and SvRAD in the Capitulum of Senecio vulgaris

“…Furthermore, CYC activity was shown to be unaltered in dich mutants, indicating that the regulation of these CYC genes is independent during dorsal petal development (Luo et al, 1999). However, in Primulina heterotricha, CYC1C regulates CYC2D and vice versa, showing the presence of regulatory relationships between CYC genes as seen here (Yang et al, 2012).…”

The Maize PI/GLO Ortholog Zmm16/sterile tassel silky ear1 Interacts with the Zygomorphy and Sex Determination Pathways in Flower Development

“…In the eudicots, there is evidence that the CYCLOIDEA (CYC)-like TCP transcription factors have been repeatedly recruited to control the development of zygomorphy (Luo et al, 1996(Luo et al, , 1999Feng et al, 2006;Yang et al, 2012). In the monocots, the role of the CYC-like genes in floral development (if any) remains undefined.…”