A hierarchically organized ARF-MADS module integrates auxin signaling into floral organ identity networks

Floral organ identity specification and subsequent organ differentiation are generally regarded as distinct developmental processes, yet the mechanisms connecting these programs remain poorly understood. In orchids, the P code explains how higher-order MADS-box complexes specify sepal, petal, and lip identities; however, how these complexes coordinate downstream developmental outputs remains unknown. Here, we identify an ARF-MADS regulatory module that integrates auxin signaling into the orchid floral identity network. Simultaneous silencing of PaARF6/8/19 causes defects in perianth identity establishment, organ growth, pigmentation, and pedicel abscission that closely resemble those resulting from disruption of P-code complexes. Among these ARFs, PaARF6 functions as the predominant regulator, whereas PaARF8 and PaARF19 provide partially redundant contributions. Mechanistically, PaARF proteins physically interact in vivo with the P-code A/E-class MADS-box proteins OAGL6-1, OAGL6-2, PaSEP1, and PaSEP3. Both genetic and biochemical evidence support a hierarchical organization of this module, with PaARF6 and OAGL6 proteins constituting its principal regulatory axis. Furthermore, conserved interactions between Arabidopsis ARFs and SEP proteins, together with petal identity defects in arf mutants, suggest evolutionary conservation of this mechanism. These findings establish the ARF-MADS module as a mechanistic bridge linking floral organ identity specification with differentiation and maturation.