Exploring carpel evolution: morphological, anatomical, and STM gene expression evidence from Illicium carpel

Abstract The origin of the carpel is crucial for understanding the evolution of angiosperms. A key unresolved debate centres on whether ovules originate from leaf-like structures or shoot structures. We conducted morphological, anatomical, and gene expression investigations on floral buds of Illicium henryi (Schisandraceae) at various developmental stages. A model of the carpel vascular system was established. Our results show that early development of the ovule exhibits traits similar to those of the shoot. The position of ovule initiation is analogous to that of an axillary bud. The floral meristem retains some meristematic cells that have the potential to differentiate into ovules. The placenta contains vascular bundles similar to those present in the primary shoot. The transcription factor SHOOT MERISTEMLESS (STM), which is crucial for the maintenance of shoot apical meristem, has been detected in the placentas. Our findings imply that the carpel of I. henryi is not a simple cauline or foliar structure, but rather a composite structure that originates from the fusion of a shoot-like ovule with a leaf-like carpel wall. These findings provide new evidence for elucidating the origin of the carpel in angiosperms.