Toward understanding inflorescence development and architecture in Passiflora: insights from comparative anatomy and expression of APETALA1

Premise The inflorescence of Passiflora species originates from a bud complex that derives from an initially undivided meristem and ultimately produces flowers and tendrils. Because the development of the inflorescence structures derived from such meristems has been variously interpreted, we investigated the ontogeny of the bud complex and the expression of APETALA1 (AP1) in Passiflora species. Methods The anatomical development of 15 species of Passiflora was analyzed using light and scanning electron microscopy. We localized AP1 expression in tissues during inflorescence initiation in two Passiflora species using in situ hybridization. Results In most species, the first primordium to differentiate from the bud complex is a bract, which develops laterally to what will become the inflorescence first-order axis, in this case, the tendril. The bract axillary meristem originates the second-order inflorescence axis meristem, which produces two bracteoles, subsequently developing into a floral meristem. AP1 is uniformly expressed in the initially undivided meristem, with expression maintained in the organ primordia derived from the bud complex. Signal is particularly strong in tendril tips. Conclusions We concluded that what is often understood as the first bract produced by a floral meristem actually is produced by the original axillary meristem. Bracteoles develop from the meristem in the bract axil; bracteoles plus floral meristem constitute the inflorescence second-order axis. Comparison of inflorescence early developmental stages in different subgenera indicates flowers are arranged in a modified cyme, with the tendril representing the inflorescence terminal portion. PasAP1 has a broad expression pattern and may have an important role during inflorescence development.