B-class floral homeotic gene MapoAPETALA3 may play an important role in the origin and formation of multi-tepals in Magnolia polytepala

In angiosperms, floral architecture diversity reflects its significance in exploring plant evolution. , an endemic and ancient species in China, possesses a unique multi-tepal trait. Notably, the origin and formation of these multi-tepals are poorly understood. In this study, we investigated the origin and formation of multi-tepals from the inner floral whorl and elucidated the underlying molecular regulatory mechanisms by combining phenotypic analysis, sequencing, and molecular experiments. We found that the multi-tepals exhibited morpho-anatomical characteristics similar to normal tepals but differed from petaloid and normal stamens. The temporal dynamics of a large number of differentially expressed genes (DEGs) involved in multiple signaling (transduction) pathways contributed to multi-tepal primordia initiation during early floral differentiation. In particular, the dynamic expression of , , , and might be responsible for floral meristem activation and maintenance, while and potentially regulated floral organ initiation. Floral homeotic genes, such as , contributed to subsequent organ identity specialization. We further isolated a nucleus-localized APETALA3 homolog from , terming it the () gene, which was expressed in almost all vegetative and reproductive tissues. Ectopically expressing in resulted in altered phenotypes of rosette leaves, inflorescences, and florets, particularly generating extra petals instead of undergoing homeotic organ conversion. This discovery revealed an additional function of in regulating organ initiation in addition to its conserved B-function in floral architecture plasticity. In summary, the multi-tepals of originated from the early tepal primordia initiation event rather than stamen petalody. The formation of the multi-tepal trait was attributed to the coordinated regulation of several vital DEGs, with the gene playing an important role. These results provide additional insight into the regulation underlying the floral architecture formation in ancient species and suggest that manipulating the gene may hold promising potential for genetic breeding in ornamental plants.