The orchid flower, remarkable for its zygomorphic syndrome, is closely related to the evolutionary history of Orchidaceae. The evolution of the lip is one of the major factors for the orchid flower exhibiting zygomorphy to be derived from the actinomorphy lilylike flower. This chapter investigates the B- and E-class MADS genes in determining orchid floral organ identities, and the homeotic orchid tepal (HOT) model is proposed for explicating floral morphogenesis in Orchidaceae. Twenty-seven AP3-like and 14 PI-like genes from 12 species of orchids were widely identified. Homologous genes of the PI clade and four duplicated AP3A1, AP3A2, AP3B1, and AP3B2 subclades were conserved in the orchid family. Among the four AP3 subclades, the lip specificity of orchid AP3A2 orthologs was demonstrated by the differential transcript between the petal and the lip of normal flowers and by the ectopic expression in the lip-like petal of orchids with peloric flowers. Based on these results, the HOT model was proposed, in which dualistic characteristics of duplicated B-class MADS-box genes are involved in orchid perianth development and growth, especially the formation of the modified petal - the lip. The PeSEP genes that were uniformly expressed in all floral organs were functionally characterized in respect of protein-protein interaction and gene expression by using yeast two-hybrid and VIGS. PeSEPs themselves and possibly via regulating their downstream B-class genes involved in stimulating cell shape, cellular ultrastructures, floral pigmentation, and cutin biosynthesis, but inhibiting features of leaf-like structures to determine the development of orchid sepals, petals, and lip. Together, these studies have clarified the relationship of orchid B- and E-class MADS-box genes involved in the regulation of orchid floral organ identities and uncovered the critical roles in the obscure mystery of orchid perianth morphogenesis.
All Science Journal Classification (ASJC) codes
- Biochemistry, Genetics and Molecular Biology(all)
- Agricultural and Biological Sciences(all)
- Environmental Science(all)