Figure 5.

A flower-type specific ABC(D)E model in Gerbera. In Arabidopsis, the genes AP1, AP2, AP3, PI and AG encode homeotic functions that are needed to determine identity of floral organs according to the ABC model [59]n addition to these genes, development of Arabidopsis floral organs requires the activity of a fourth function, labeled E in the extended 'quartet' model of flower development [74], which redundantly encoded by four related MADS box genes, SEP1, SEP2, SEP3 and SEP4. In Gerbera, the genes GGLO1 and GDEF2 are required for the B function. The C function is redundantly encoded by two highly similar genes, GAGA1 and GAGA2, but for the E function, we have observed subfunctionalization among SEP-like gene paralogs. In Gerbera, GRCD1 is needed for correct flower organ identity in whorl 3 of marginal female flowers only. Correspondingly, GRCD2 is necessary only for carpel development, with stronger loss-of-function phenotypes observed in the central disk flowers. Based on these observations, we predict that still unidentified (possibly redundant) E function MADS box genes are required in whorls 1 and 2, and in disc flower whorl 3. Furthermore, we have not yet identified Gerbera genes that might code for an A function active in whorls 1 and 2. These whorls react differentially to ectopic expression of GAGA1 or GAGA2, raising the possiblity that the A function may itself be split in Gerbera. Relative MADS box gene upregulation phenomena observed in our microarray experiments, organized along the capitulum radius (arrow), are summarized below (and formatted according to) the gerbera disc and ray flower ABCE models above. Z stands for the TM3-like gene G0000100021A03. GSQUA1 and GRCD1 expression in disc flowers is shown offset between whorls 2 and 3 to show their uncertain whorl-specificity regarding function.