Floral Developmental Genetics of Gerbera (Asteraceae)

Inflorescence development in the angiosperm family Asteraceae has distinct features not found in the traditional model systems (e.g., Arabidopsis, Petunia, and Zea). In Gerbera hybrida, inflorescences are composed of morphologically different types of flowers tightly packed into a flower head (capitulum) that overtly resembles a single flower. Individual floral organs, such as pappus bristles (sepals), are developmentally specialized, petals and anthers form fused structures, stamens are aborted in marginal flowers, and ovaries are located inferior to other floral organs. These specific features have made Gerbera a rewarding target for comparative studies. We have shown that Gerbera MADS-box genes that group phylogenetically with B- and C-function genes from Arabidopsis and Antirrhinum function in organ determination in a very similar manner to their respective homologs from these model plants. However, MADS-box genes encoding proteins that interact with those of the ABC-genes behave differently. In Arabidopsis, three SEPALLATA (SEP) genes have redundant functions and are needed for development of petals, stamens, and carpels. Homologs of these SEP genes are found in Gerbera (GRCD1, GRCD2), but they show functional specialization. GRCD1 is necessary for stamen development, but not for petal or carpel development. Similarly, GRCD2 has a homeotic function restricted to carpel development. Remarkably, downregulation of the latter also results in floral reversion (which occurs in ovaries) and alters inflorescence architecture by switching off terminal, determinate growth. This integrated SEP-like control over reproductive meristem fate has not been detected in the well-known model systems, which have a different carpel design and normally bear indeterminate inflorescences. Moreover, the organization of flowers on the Gerbera capitulum reveals the presence of a radial morphogenetic gradient that appears to regulate ABC and other MADS-box genes differentially in a cell-nonautonomous manner. As such, there is some commonality in gene regulatory features between Gerbera flowers and inflorescences, which suggests that Gerbera capitula are more than simple analogs of the flowers they bear.