EVOLUTION OF ASTERACEAE INFLORESCENCE DEVELOPMENT AND CYC/TB1-LIKE GENE FUNCTIONS

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http://urn.fi/URN:ISBN:978-951-51-4772-1
Title: EVOLUTION OF ASTERACEAE INFLORESCENCE DEVELOPMENT AND CYC/TB1-LIKE GENE FUNCTIONS
Author: Zhao, Yafei
Contributor: University of Helsinki, Faculty of Agriculture and Forestry, Department of Agricultural Sciences
Doctoral Program in Plant Sciences
Publisher: Helsingin yliopisto
Date: 2019-01-09
Belongs to series: URN:ISSN:2342-5431
URI: http://urn.fi/URN:ISBN:978-951-51-4772-1
http://hdl.handle.net/10138/275476
Thesis level: Doctoral dissertation (article-based)
Abstract: Flowers and their number and arrangement within inflorescences are essential for the reproductive fitness and adaptive success of plants as well as for human sustenance. Comparative studies using species representing various plant lineages are a prerequisite for a comprehensive understanding of the development of diverse flower forms during the evolution of plants. In addition to a few classical model plants, new models, including core eudicots Asteraceae and basal eudicots Papaveraceae, have emerged to enhance our current understanding of the genetic networks in the regulation of plant growth and development from an evolutionary developmental biology perspective. This thesis seeks to understand the evolutionary origin of the Asteraceae capitulum and the regulatory networks of flower type specification as well as to illustrate the ancestral functions of CYCLOIDEA/TEOSINTE BRANCHED1 (CYC/TB1)-like (CYL) genes in basal eudicots. The Asteraceae inflorescence (capitulum, or flower head) superficially resembles a solitary flower, but it is a tightly packed structure composed of different types of flowers, including marginal ray flowers and central disc flowers. Such evolutionary innovations, as studied in Gerbera hybrida here, are owing to the novel functions of the gerbera orthologues of flower meristem identity (FMI) genes LEAFY (GhLFY) and UNUSUAL FLORAL ORGANS (GhUFO) as well as the regulatory networks involving MADS-box and TCP transcription factors (TFs). The expression domain of GhLFY at the inflorescence meristem mimics that of a single flower meristem, suggesting that the Asteraceae capitulum resembles a solitary flower not only morphologically but also at the molecular level. This expression pattern defines the capitulum as a determinate structure that can display floral fate upon ectopic GhUFO expression. Additionally, suppression of GhLFY and GhUFO led to a loss of FMI and suppressed MADS-box floral organ identity genes in a flower type-dependent manner. In particular, GhLFY regulates the early ontogeny of ray flowers, providing the first molecular evidence for how this structure has evolved. We speculate that the differentiation of flower types in Asteraceae is associated with their independent evolutionary origins from separate branching systems. Furthermore, the establishment of the regulatory networks amongst TCP and MADS-box gene family members in flower type specification provides additional support for distinct genetic origins of flower types. As trans-acting regulators of GhCYC3, the strongest CYC-like TCP gene in specifying ray flower identity, the CINCINNATA (CIN)-like TCP GhCIN1 co-localises with GhCYC3 in ray primordia and specifies the development of ray ligules. Moreover, we discovered that the whorl-specific MADS-box TFs control GhCYC3 expression during flower organ development: SEPALLATA3-like GERBERA REGULATOR OF CAPITULUM DEVELOPMENT5 (GRCD5) specifies ray ligule elongation, and C-class GERBERA AGAMOUS-LIKE1 (GAGA1) controls staminode development in ray flowers. Papaveraceae, belonging to a lineage basal to all other eudicots, represents a phylogenetically important family to understand the origin, evolution, and diversification of CYL genes in eudicots. Comparative studies between Eschscholzia californica and Cysticapnos vesicaria revealed their conserved functional roles in shoot branching, petal size, flower symmetry, and stamen growth, although in a species-specific manner. Nonetheless, our studies revealed a novel function in perianth development for CyveCYLs that were shown to specify the distinction of sepal and petal identity by suppressing B-class genes expression in C. vesicaria. These data indicate that instead of recruiting completely new regulators, the evolution of divergent morphologies employs conserved regulatory genes that show varying spatial/temporal expression patterns to perform their functions within modified regulatory networks, thereby contributing to the morphological diversification and origin of evolutionary innovations.Mykerökukinnon rakenteen ja CYC/TB1-tyyppisten säätelygeenien evoluutio kasveilla
Subject: Plant Science
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