PhYAB4 directly activates PhAGL6‐2 to regulate lip‐related organ curvature morphology in Paphiopedilum henryanum

The orchid lip is a highly specialized floral organ that contributes to floral architecture and pollinator interaction. However, the developmental and molecular mechanisms underlying lip-related organ curvature in Paphiopedilum remain poorly understood. Using Paphiopedilum henryanum and its Type A peloric mutant, we found that visible divergence between wild-type petals and mutant lip-like petals begins at Stage 4. Curvature was associated with adaxial-abaxial epidermal cell number imbalance in the distal region, whereas average epidermal cell area remained largely unchanged. Transcriptome and expression analyses identified PhAGL6-2 as a lip-associated AGL6 homolog enriched in lips and lip-like petals during Stages 4-5. PhAGL6-2 was preferentially expressed on the abaxial side, particularly in regions associated with curvature. Virus-induced gene silencing-mediated silencing of PhAGL6-2 in Dendrobium Chao Praya Smile and P. henryanum affected perianth organ curvature, suggesting a broader role in floral organ morphogenesis. The YABBY transcription factor PhYAB4 directly bound the AGTGTG motif in the PhAGL6-2 promoter and activated its transcription. Silencing PhYAB4 reduced PhAGL6-2 expression and caused stronger floral curvature, lip outgrowths, and partial disruption of floral symmetry. These findings identify a PhYAB4-PhAGL6-2 pathway associated with epidermal cell number imbalance and lip-related organ curvature.