Functional characterization of B class MADS-box transcription factors in Gerbera hybrida.

According to the classical ABC model, B-function genes are involved in determining petal and stamen development. Most core eudicot species have B class genes belonging to three different lineages: the PI, euAP3, and TM6 lineages, although both Arabidopsis and Antirrhinum appear to have lost their TM6-like gene. Functional studies were performed for three gerbera (Gerbera hybrida) B class MADS-box genes--PI/GLO-like GGLO1, euAP3 class GDEF2, and TM6-like GDEF1--and data are shown for a second euAP3-like gene, GDEF3. In phylogenetic analysis, GDEF3 is a closely related paralogue of GDEF2, and apparently stems from a duplication common to all Asteraceae. Expression analysis and transgenic phenotypes confirm that GGLO1 and GDEF2 mediate the classical B-function since they determine petal and stamen identities. However, based on assays in yeast, three B class heterodimer combinations are possible in gerbera. In addition to the interaction of GGLO1 and GDEF2 proteins, GGLO1 also pairs with GDEF1 and GDEF3. This analysis of GDEF1 represents the first functional characterization of a TM6-like gene in a core eudicot species outside Solanaceae. Similarly to its relatives in petunia and tomato, the expression pattern and transgenic phenotypes indicate that GDEF1 is not involved in determination of petal identity, but has a redundant role in regulating stamen development.

Transcriptional analysis of petal organogenesis in Gerbera hybrida.

Understanding of the molecular interplay, which determines early steps of flower formation has grown considerably during last years. In contrast, genetic actions responsible for how flower organs acquire their size and shape at later phases of organogenesis are still poorly understood. We have exploited the large and anatomically simple Gerbera (Gerbera hybrida var. Terra regina) ray flower petals to describe transcriptional changes during organogenesis. Gerbera 9 K cDNA microarray was utilized to profile gene expression at six different developmental stages of petal organogenesis, at the earliest stage expansion of petals is starting and at the latest stage petals have reached their final size and shape. Genes potentially participating in petal opening were identified based on the similarity in expression with a known marker gene. Our results showed characteristic sets of genes expressed during the cell division and cell expansion phases of petal development. Interestingly, there was a transition stage during which neither cell division nor cell expansion marker genes were abundantly expressed. Moreover, constitutive expression of late petal specific genes indicates that they participate in petal organogenesis throughout the development and they are not involved in stage specific switch points.

Mining plant diversity: Gerbera as a model system for plant developmental and biosynthetic research.

Gerbera hybrida is a member of the large sunflower family (Asteraceae). Typical of Asteraceae, Gerbera bears different types of flowers in its inflorescence. The showy marginal flowers comprise elongate, ligulate corollas that are female, whereas the central and inconspicuous disc flowers are complete, with both male and female organs. As such, Gerbera offers great potential for comparative developmental research within a single genotype. Moreover, different Gerbera varieties show an impressive spectrum of color patterns, directly displaying responses to developmental cues at all important morphological levels (flower type, flower organ and within organs). Further, Gerbera harbors an arsenal of Asteraceae-type secondary metabolites, not present in other model plants. With powerful reverse genetics methods, a large collection of EST sequences and a new cDNA microarray, Gerbera has become a model plant of the sunflower family.

Reproductive meristem fates in Gerbera.

Flowering plants go through several phases between regular stem growth and the actual production of flower parts. The stepwise conversion of vegetative into inflorescence and floral meristems is usually unidirectional, but under certain environmental or genetic conditions, meristems can revert to an earlier developmental identity. Vegetative meristems are typically indeterminate, producing organs continuously, whereas flower meristems are determinate, shutting down their growth after reproductive organs are initiated. Inflorescence meristems can show either pattern. Flower and inflorescence development have been investigated in Gerbera hybrida, an ornamental plant in the sunflower family, Asteraceae. Unlike the common model species used to study flower development, Gerbera inflorescences bear a fixed number of flowers, and the architecture of the flowers differ in that Gerbera ovaries are inferior (borne below the perianth). This architectural difference has been exploited to show that floral meristem determinacy and identity are spatially and genetically distinct in Gerbera, and we have shown that a single SEPALLATA-like MADS domain factor controls both flower and inflorescence meristem fate in the plant. Although these phenomena have not been directly observed in Arabidopsis, the integrative role of the SEPALLATA function in reproductive meristem development may be general for all flowering plants.

Analysis of the floral transcriptome uncovers new regulators of organ determination and gene families related to flower organ differentiation in Gerbera hybrida (Asteraceae).

Development of composite inflorescences in the plant family Asteraceae has features that cannot be studied in the traditional model plants for flower development. In Gerbera hybrida, inflorescences are composed of morphologically different types of flowers tightly packed into a flower head (capitulum). Individual floral organs such as pappus bristles (sepals) are developmentally specialized, stamens are aborted in marginal flowers, petals and anthers are fused structures, and ovaries are located inferior to other floral organs. These specific features have made gerbera a rewarding target of comparative studies. Here we report the analysis of a gerbera EST database containing 16,994 cDNA sequences. Comparison of the sequences with all plant peptide sequences revealed 1656 unique sequences for gerbera not identified elsewhere within the plant kingdom. Based on the EST database, we constructed a cDNA microarray containing 9000 probes and have utilized it in identification of flower-specific genes and abundantly expressed marker genes for flower scape, pappus, stamen, and petal development. Our analysis revealed several regulatory genes with putative functions in flower-organ development. We were also able to associate a number of abundantly and specifically expressed genes with flower-organ differentiation. Gerbera is an outcrossing species, for which genetic approaches to gene discovery are not readily amenable. However, reverse genetics with the help of gene transfer has been very informative. We demonstrate here the usability of the gerbera microarray as a reliable new tool for identifying novel genes related to specific biological questions and for large-scale gene expression analysis.

Integration of reproductive meristem fates by a SEPALLATA-like MADS-box gene.

Reproductive transition, inflorescence architecture, meristem patterning, and floral organ identity have been studied as distinct research areas in plant science. By using the ornamental plant Gerbera, we demonstrate that all of these keystone aspects of reproductive meristematic fate are integrated genetically by a single SEPALLATA-like MADS-box gene from a functional class designated previously as "floral homeotic" or "organ identity." This extended regulatory network has not been elaborated in the model plant systems, which have a floral design and inflorescence-determinacy state that obscures these relationships.