Developmental Patterning of Asteraceae Flower Heads

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http://urn.fi/URN:ISBN:978-951-51-7422-2
Title: Developmental Patterning of Asteraceae Flower Heads
Author: Zhang, Teng
Contributor: University of Helsinki, Faculty of Agriculture and Forestry, Department of Agricultural Sciences
Doctoral Program in Plant Sciences
Publisher: Helsingin yliopisto
Date: 2021-08-20
Language: en
Belongs to series: DISSERTATIONES SCHOLA DOCTORALIS SCIENTIAE CIRCUMIECTALIS, ALIMENTARIAS, BIOLOGICAE - URN:ISSN:2342-5431
URI: http://urn.fi/URN:ISBN:978-951-51-7422-2
http://hdl.handle.net/10138/332768
Thesis level: Doctoral dissertation (article-based)
Abstract: Inflorescences are flower bearing structures that display remarkable diversity in plants. Their architecture, referring to the number and arrangement of flowers, is considered as a key attribute to the reproductive success of plants. Asteraceae is one of the largest plant families, and the evolutionary success of this family has been largely attributed to their showy inflorescence structure, the flower head (or capitulum). A flower head combines up to a thousand individual florets and numerous leaf-like bracts onto a single receptacle, and the overall structure superficially mimics a giant solitary flower. Geometrically, the individual florets are arranged in left and right turning spirals following the consecutive numbers of the Fibonacci sequence. Such a pattern has fascinated interdisciplinary researchers over centuries. Elaborating Gerbera hybrida as a study system, this thesis aims at elucidating the molecular mechanisms underlying three key aspects in the development of a flower head: the phyllotactic patterning, the inflorescence patterning, and the patterning of floral organs. This thesis first combined data from diverse microscopic methods with computational modeling and illustrated how the phyllotactic pattern is established during the growth of Gerbera flower heads. The patterning process was governed by the expansion and contraction of the organogenetic zone where new primordia arise. Earliest bract initia were found to pattern on a ‘naked’ head meristem, and to guide the emergence of Fibonacci spiral numbers. A critical character for the patterning process is the lateral displacement of auxin maxima pointing towards the older neighbor. Results from this thesis provided the first experimental basis for understanding how phyllotactic patterns are transited on a growing meristem. This thesis then demonstrated how flower meristem identity genes GhLFY and GhUFO are co-opted to regulate flower head development. While GhUFO acts as the master regulator of flower identity, GhLFY has evolved two novel functions to regulate the determinacy of inflorescence meristem and the early ray floret initiation. The results provided novel insights to explain how the flower head structures are evolved and diversified. This thesis lastly dissected functions of the SEPALLATA-like GRCDs in regulation of Gerbera flower and inflorescence development. In this study, the GRCDs were shown to have evolved specialized functions in regulating floral organ identities, among which, GRCD4 and GRCD5 are two indispensable regulators for petal development. Moreover, GRCD2 and GRCD7 show redundant functions at the inflorescence level maintaining their determinacy. The results provide an example on how gene duplications could lead to specialized and redundant functions in regulation of a highly elaborated inflorescence structure.Kasveilla kukat muodostuvat kukintoihin, joiden rakenne vaihtelee suuresti. Kukintojen arkkitehtuuri, toisin sanoen kukkien lukumäärä ja niiden järjestäytyminen kukinnoissa, vaikuttavat kasvien lisääntymisbiologiaan ja sopeutumiseen. Mykerökukkaisten kasvien heimo, Asteraceae, on yksi maailman suurimmista kasviheimoista. Sen ainutlaatuisen ja näyttävän kukinnon rakenteen on ajateltu olevan syynä tämän kasviheimon menestykseen. Mykerökukinnossa sadat yksittäiset kukat kehittyvät suuresta, laajentuneesta kasvupisteestä (meristeemistä) ja muodostavat säännönmukaisen rakenteen, jota ympäröivät lukuisat suojuslehdet. Kokonaisuudessaan tämä rakenne muistuttaa yhtä suurta kukkaa. Yksittäiset kukat ovat järjestäytyneet geometrisen tarkasti oikealle ja vasemmalle kaartuviin spiraaleihin, joiden lukumäärä noudattaa kahta perättäistä lukua Fibonacci lukujonosta. Tämä säännönmukaisuus on kiehtonut eri alojen tutkijoita vuosisatojen ajan. Tässä väitöskirjassa käytettiin mallikasvina Gerbera hybridaa, jonka avulla selvitettiin mykerökukinnon kehitystä keskittyen erityisesti tunnistamaan niitä molekyylitason mekanismeja, jotka ohjaavat kukkien järjestäytymistä spiraaleihin, kukintomeristeemin järjestäytymistä sekä kukkaelinten kehitystä.
Subject: plant Biology/Plant Breeding
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