The Duplicated B-Class Heterodimer Model: Whorl-Specific Effects and Complex Genetic Interactions in Petunia hybrida Flower Development

Abstract

In both Antirrhinum (Antirrhinum majus) and Arabidopsis (Arabidopsis thaliana), the floral B-function, which specifies petal and stamen development, is embedded in a heterodimer consisting of one DEFICIENS (DEF)/APETALA3 (AP3)-like and one GLOBOSA (GLO)/PISTILLATA (PI)-like MADS box protein. Here, we demonstrate that gene duplications in both the DEF/AP3 and GLO/PI lineages in Petunia hybrida (petunia) have led to a functional diversification of their respective members, which is reflected by partner specificity and whorl-specific functions among these proteins. Previously, it has been shown that mutations in PhDEF (formerly known as GREEN PETALS) only affect petal development. We have isolated insertion alleles for PhGLO1 (FLORAL BINDING PROTEIN1) and PhGLO2 (PETUNIA MADS BOX GENE2) and demonstrate unique and redundant properties of PhDEF, PhGLO1, and PhGLO2. Besides a full homeotic conversion of petals to sepals and of stamens to carpels as observed in phglo1 phglo2 and phdef phglo2 flowers, we found that gene dosage effects for several mutant combinations cause qualitative and quantitative changes in whorl 2 and 3 meristem fate, and we show that the PHDEF/PHGLO1 heterodimer controls the fusion of the stamen filaments with the petal tube. Nevertheless, when the activity of PhDEF, PhGLO1, and PhGLO2 are considered jointly, they basically appear to function as DEF/GLO does in Antirrhinum and to a lesser extent as AP3/PI in Arabidopsis. By contrast, our data suggest that the function of the fourth Bclass MADS box member, the paleoAP3-type PETUNIA HYBRIDA TM6 (PhTM6) gene, differs significantly from the known euAP3-type DEF/AP3-like proteins; PhTM6 is mainly expressed in the developing stamens and ovary of wild-type flowers, whereas its expression level is upregulated in whorls 1 and 2 of an A-function floral mutant; PhTM6 is most likely not involved in petal development. The latter is consistent with the hypothesis that the evolutionary origin of the higher eudicot petal structure coincided with the appearance of the euAP3-type MADS box genes.

Deciphering the Physalis floridana Double-Layered-Lantern1 Mutant Provides Insights into Functional Divergence of the GLOBOSA Duplicates within the Solanaceae

“…The two duplicates for each lineage redundantly partitioned the role of the B function with a variable subfunctionalization process in Petunia hybrida, Nicotiana benthamiana, and Solanum lycopersicum. This supposition is supported by the observations that a single mutation in each lineage in a species shows a partially mutated B-type phenotype in these Solanaceous species (Vandenbussche et al, 2004;de Martino et al, 2006;Rijpkema et al, 2006;Geuten and Irish, 2010).…”

“…Nonetheless, PFGLO2 might have retained a potential role in corolla identity but was masked by PFGLO1. Such a complete functional separation of GLO-like genes from stamen organ identity was not observed in Petunia hybrida, S. lycopersicum, or N. benthamiana (Vandenbussche et al, 2004;Geuten and Irish, 2010). However, a similar functional separation was observed for the C-class paralogous genes in Antirrhinum majus, in which FARINELLI (FAR) is involved in male fertility, while PLENA exerts roles in organ identity specification of the stamen and carpel (Davies et al, 1999).…”

“…We found that PFGLO1, instead of PFGLO2, had its primary role in complete B function, while PFGLO2 alone exerted a role in male fertility, but controlled the organ number and tip development of the second floral whorl in a PFGLO1-dependent manner. Such a diverging pattern of the GLO duplicates is distinct from the purely redundant patterns observed in Petunia hybrida, S. lycopersicum, or N. benthamiana (Vandenbussche et al, 2004;Geuten and Irish, 2010).…”

“…PFSEP3, a SEPALLATA-like protein, is required for normal floral organ development (Pelaz et al, 2000;Immink et al, 2009). However, the direct interactions between B-class proteins and E-class (PFSEP3) proteins were observed in tomato (Solanum lycopersicum) but not in Arabidopsis, Antirrhinum majus, Petunia hybrida, or Eschscholzia californica (Davies et al, 1996;Vandenbussche et al, 2004;de Folter et al, 2005;de Martino et al, 2006;Leseberg et al, 2008;Lange et al, 2013), indicating that the dimerization pattern of the homeotic orthologous proteins is variable among angiosperms. However, our findings on protein-protein interactions provide a common basis underlying the role of PFGLO1 and PFGLO2 in floral development in Physalis floridana.…”

“…The role of PFGLO2 in the tip development might be a residual of corolla organ identity. However, such functional divergence and genetic interaction of two GLO genes for the phenotypic determination were not observed in Petunia hybrida, S. lycopersicum, or N. benthamiana (Vandenbussche et al, 2004;Geuten and Irish, 2010).…”