Full-length floral transcriptome analysis reveals a modified ABCDE model with D-class gene loss and the molecular mechanism of double flower formation in Narcissus tazetta var. chinensis.

The ABCDE model serves as the fundamental framework for understanding floral organ development in angiosperms, yet its systematic transcriptome-wide characterization and functional validation remain lacking in Chinese narcissus (Narcissus tazetta var. chinensis), a traditional ornamental flower with high cultural and economic value. In this study, we identified 16 ABCDE model genes from the PacBio full-length floral transcriptome of the single-flower cultivar 'Jinzhanyintai', including nine A-class, two B-class, one C-class, and four E-class homologs. No canonical D-class genes were detected across multiple independent transcriptome datasets, and phylogenetic and expression evidence support that the C-class gene NtAG may undertake a dual C/D functional role in reproductive organ and ovule development. Based on spatiotemporal expression patterns, we constructed a modified ABCDE model for Chinese narcissus floral development, revealing expanded expression domains of B- and C-class genes underlying the formation of the unique paracorolla organ. Comparative analysis between 'Jinzhanyintai' and the double-flower cultivar 'Yulinglong' demonstrated that abnormal upregulation of B-class genes NtAP3 and NtPI in the stamen whorl drives stamen petaloidy and double flower formation. Subcellular localization and heterologous overexpression assays confirmed that NtPI is a nuclear-localized transcriptional regulator with a conserved function in floral organ morphogenesis. This study provides a foundational framework for understanding floral morphogenesis and key key target genes for molecular breeding in Chinese narcissus.