How has the unique inflorescence in Asteraceae, the capitulum, evolved?
What are the early signals that define flower type identity?
How are the major biosynthetic patways to flavonoids and polyketides diversified?
How does the classical ABCE model of flower development apply in Asteraceae?
The presence of three distinct types of flowers in its inflorescence (ray, trans and disc flowers) makes Gerbera a unique target for evolutionary developmental studies. The classical ABCE model of flower development has been built using model species Arabidopsis and Antirrhinum with only single flower forms in their inflorescences. Our studies have focused on evolution of the ABCE functions in Asteraceae. We have identified more than 20 MADS box genes, and made detailed phylogenetic, expression and protein-protein interaction analyses for them as well as transgenic gerbera plants for functional studies. We have discovered both conserved functions in flower organ identity regulation but also examples gene duplications that have led to neo- and subfunctionalization.
What are the major factors and mechanisms that lead to formation of different types of flowers in a single genotype?
Recent studies have indicated that the conserved CYC/TB1-like TCP domain transcription factor genes are involved in regulation of flower type identity in Asteraceae. The CYC/TB1 gene family has expanded in Asteraceae and harbors ten gene family members in gerbera and sunflower, respectively. Of the three subclades found in core eudicots, especially the CYC2 clade genes are differentially expressed between the developing flower types, and are upregulated in ray flower primordia. We have shown that in gerbera GhCYC2, GhCYC3 and GhCYC4 act redundantly to regulate ray flower identity as ectopic expression of these genes converts disc flowers into ray-like. Additionally, these genes show late functions during ray flower petal growth by affecting cell proliferation until the final size and shape of the petals is reached. Our data show functional diversification for the GhCYC5 gene. Ectopic activation of GhCYC5 increases flower density in the inflorescence, suggesting that GhCYC5 may promote the flower initiation rate during expansion of the capitulum. Our general aim is to understand evolution of TCP gene functions as well as to characterize the molecular network (coregulators, target genes, upstream regulators) connected with them.
How has the striking geometric regularity of flower heads evolved?
The individual flowers in the large and expanded capitulum are arranged in left and right winding spirals, parastichies, whose number follow a famous mathematical rule - the Fibonacci numbers. We want to understand early patterning of the inflorescence meristem; establishment of the spiral phyllotaxis as well as signaling that is translated into distinct genetic programs in ray and disc flower development. Our recent data have indicated that the capitulum resembles a solitary flower not only morphologically but also at molecular level. We have shown that the highly conserved floral meristem identity gene, LEAFY (GhLFY) is required for correct patterning of the inflorescence meristem. Furthermore, GhLFY was shown to uniquely regulate the early ontogeny of marginal ray flowers, but not the inner disc flowers. Loss of GhLFY led to conversion ray flowers into branched structures, similar to those found in Calyceraceae, the closest relatives of Asteraceae. Our data suggests that the presence of distinct flowers types relates to their different ontogenic origin.
Functional diversification of type III polyketide synthases (PKSs)
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.