The large, expanded inflorescence meristem (IM) is characteristic of the Asteraceae plant family. It generates up to hundreds of flowers that are arranged in regular left- and right-curving spirals whose numbers follow the two consecutive numbers in mathematical Fibonacci sequence. Our aim is to understand early patterning of the IM, establishment of the spiral phyllotaxis as well as signaling that regulates the development of ray and disc flowers. We combine molecular, biomechanical and gene functional studies with computational modeling to understand meristem patterning at spatial and temporal scales.
We currently focus on identifying key mechanisms of the expansion growth of the IM, known to drive spiral organ patterning. Patterning occurs in a ring-like active zone, the size and position of which is dynamically changing. We explore the molecular nature of this active zone and how the spatiotemporal dynamics of it is coordinated by the IM growth. We are also focusing on understanding how mechanical forces (such as wounding and compression) generate a new active zone, re-establish the patterning and trigger new organ fates. We apply transgenic reporter lines with versatile imaging approaches and genome-wide molecular studies.
Literature:
Zhang T, Cieslak M, Owens A, Wang F, Broholm SK, Teeri TH, Elomaa P, Prusinkiewicz P: Phyllotactic patterning of gerbera flower heads. PNAS 118: e2016304118 (2021).
Zhang T, Wang F, Elomaa P: Repatterning of the inflorescence meristem in Gerbera hybrida after wounding. Journal of Plant Research 134: 431-440 (2021).
Prusinkiewicz P, Zhang T, Owens AR, Cieslak M, Elomaa P: Phyllotaxis without symmetry: what can we learn from flower heads? Journal of Experimental Botany 73: 3319-3329 (2022).
Owens A, Zhang T, Gu P, Hart J, Stobbs J, Cieslak M, Elomaa P, Prusinkiewicz P: The hidden diversity of vascular patterns in flower heads. New Phytologist 243: 423-439 (2024).
Asteraceae is one of the largest families of plants. Capitulum is considered one of the key innovations behind the evolutionary success of the family. However, it is still unclear how this unique structure evolved - from single meristems or through step-wise changes from branched inflorescences of their sister families. We are testing the distinct evolutionary hypotheses through gene functional studies and by genome-wide approaches such as single cell and spatial transcriptomics. We also conduct comparative evo-devo studies in Acicarpha, a representative of the Calyceraceae plant family, closest sister family of Asteraceae.
Literature:
Zhao Y, Zhang T, Broholm SK, Tähtiharju S, Mouhu K, Albert VA, Teeri TH, Elomaa P: Evolutionary co-option of floral meristem identity genes for patterning of the flower-like Asteraceae inflorescence. Plant Physiology 172: 284-296 (2016).
Elomaa P, Zhao Y, Zhang T: Flower heads in Asteraceae – recruitment of conserved developmental regulators to control the flower-like inflorescence architecture. Horticulture Research 5: 36 (2018)
Zhang T, Elomaa P. 2024: Development and evolution of the Asteraceae capitulum. Tansley Review. New Phytologist 242: 33-48.
In heterogamous capitula, distinct flower types can be separated. They differ in their sex, symmetry and often also in size, and are specialized in their functions. The role of the showy, marginal ray flowers is to attract pollinators while the more conspicuous, perfect disc flowers, located in the center of the capitulum, ensure efficient reproduction. Our aim is to characterize gene regulatory networks that define the identity of distinct flower types as well as their morphological differentiation.
Literature:
Broholm SK, Tähtiharju S, Laitinen RAE, Albert VA, Teeri TH, Elomaa P: A TCP domain transcription factor controls flower type specification along the radial axis of the Gerbera (Asteraceae) inflorescence. PNAS 105: 9117-9122 (2008).
Tähtiharju S, Rijpkema A, Broholm S, Teeri TH, Albert VA, Elomaa P: Evolution and diversification of the CYC/TB1 gene family in Asteraceae – a comparative study in gerbera (Mutisieae) and sunflower (Heliantheae). Molecular Biology and Evolution 29: 1155-1166 (2012).
Juntheikki-Palovaara I, Tähtiharju S, Lan T, Broholm SK, Rijpkema AS, Ruonala R, Kale L, Albert VA, Teeri TH, Elomaa P: Functional diversification of duplicated CYC2 clade genes in regulation of inflorescence development in Gerbera hybrida (Asteraceae). The Plant Journal 79: 783-796 (2014).
Zhao Y, Broholm SK, Wang F, Rijpkema AS, Lan T, Albert VA, Teeri TH, Elomaa P: TCP and MADS-box transcription factor networks regulate heteromorphic flower type identity in Gerbera hybrida. Plant Physiology 184: 1455–1468 (2020).
Regulation of enzymatic pathways of flavonoid biosynthesis give distinct gerbera varieties widely varying pigmentation patterns. In addition, gerbera harbors unique polyketide-derived compounds showing activity against herbivores and phytopathogens; the bitter tasting triketide-derived glucosides, gerberin and parasorboside, as well as a rare pentaketide-derived coumarin, 4-hydroxy-5-methylcoumarin (HMC). These antimicrobial compounds are synthesized by 2-pyrone synthases (2PS), gerbera specific type III polyketide synthases that show high sequence similarity with the first dedicated enzyme of the flavonoid pathway, chalcone synthase (CHS). Our aim is to discover the as yet unknown enzymatic steps of the biosynthetic pathways for these important compounds.
Literature:
Zhu L, Pietiäinen M, Kontturi J, Turkkelin A, Elomaa P, Teeri TH: Polyketide reductases in defence-related parasorboside biosynthesis in Gerbera hybrida share processing strategies with microbial polyketide synthase systems. New Phytologist 236: 296-308 (2022).