Expression patterns of floral organ identity genes in petaloid structure of Mussaenda spp. (Rubiaceae)

Flowers are functional irreplaceable and above all, the most important structure of angiosperms. Some of the inflorescences replace the function of a flower in a way forming pseudanthium, meanwhile, some organs replace the function of petals by enlarged and being petaloid. The enlarged showy calyx-lobes in Mussaenda (Rubiaceae) are good examples. The calyx-lobes, so called calycophylls, are mostly leaf-shaped, but have bright color, papillate epidermis, and loose mesophylls, features generally only found in petals. In higher eudicots, floral organs are typically arranged in four different whorls, namely sepals, petals, stamens and carpels. According to the ABCDE model proposed by Theißen, organ identities are specified by these five class of floral homeotic genes solely or in a combinatory way. According to this model, petal identity is specified by A, B and E class genes. Research in Arabidopsis showed overexpression of A, B and E class genes caused the formation of petaloid leaves, suggested a general mechanism that might be found in other petaloid structures in plants. My thesis is to examine the correlation between the formations of petaloid sepals in Mussaenda and the petal identity genes, i.e. ABE class genes.
Five floral organ identity genes of Mussaenda pubescens were cloned and their expression patterns among different organs were detected by RT-PCR. The AP1 homologue, MupAP1, is expressed in enlarged and small sepals, petals and stamens, carpels and fruits; the SEP3 homologue, MupSEP is strongly expressed in all floral organs and weak in leaves as well. Two of the B class gene, AP3 homologue MupDEF and TM6 homologue MupTM6, were detected expressed in all floral organs, however, the other B class gene, MupGLO, homologous to PI, were detected expressed in small sepals, petals and stamens, carpels and fruits, but not in enlarged sepals. Though there might be other factors involved, results suggest the formation of petaloid sepals in Mussaenda still required A, E, and some B class genes, but further survey for addition factors and functional assay for identified genes are needed.