Recent Progress in the Molecular Regulation of Floral Shape and Structure in Orchids

Orchid flowers exhibit remarkable morphological diversity, arising from the interplay between a conserved MADS-box regulatory framework and lineage-specific developmental innovations. This review examines recent molecular advances in orchid floral organ identity, zygomorphy, labellum morphogenesis, and developmental plasticity. Functional perturbation and protein-protein interaction studies have refined classical floral identity models by providing experimental support for higher-order MADS-box protein complexes involving B-class, AGL6, and SEPALLATA proteins, while the roles of C- and D-class genes in reproductive organ development and canonical A-class genes in floral organ identity remain incompletely resolved. We distinguish whole-flower zygomorphy from labellum identity, highlighting MADS-box complexes as key determinants of perianth organ identity, particularly labellum specification, while the molecular basis of orchid floral symmetry remains less clearly understood. Additional regulatory factors, including TCP, MYB, WOX, and NAC family members, have been implicated in regional growth, cell differentiation, and floral organ morphogenesis, although their hierarchical relationships with MADS-box regulators remain incompletely understood. Hormonal, epigenetic, and small RNA-mediated regulation may provide additional layers of developmental control and contribute to floral morphological plasticity. Despite these advances, the molecular links between early floral organ identity specification and subsequent morphogenesis remain incompletely understood. A major challenge is to determine how early identity regulators establish spatially restricted patterns of cell proliferation, expansion, and differentiation that ultimately generate the complex three-dimensional architecture of orchid flowers.