The transcription factor BnaA9.NST3 promotes flowering via the photoperiod pathway in <i>Brassica napus</i> L.

Flowering and secondary growth are 2 important biological processes in plant development, and there is a close association between flowering and xylem expansion in stem and root secondary growth. Here, we report the discovery of NAM-ATAF1/2-CUC2 (NAC) secondary wall thickening promoting factor 3 (BnaA9.NST3), a key regulator of secondary cell wall formation in xylem fiber cells that affects Brassica napus (B. napus L.) flowering in a photoperiod pathway-dependent manner. We show that the subfunctionalized BnaA9.NST3 directly binds to the BnaC8.NF-YC9 and Bna.CO promoters to regulate their transcription. BnaC8.NF-YC9 interacts with Bna.CO, forming NF-CO complexes that regulate Bna.FT transcription, thereby affecting flowering. Moreover, we found that florigen upregulated secondary cell wall biogenesis (SCWB)-related genes including BnaA9.NST3, accelerating SCWB in stems and promoting vascular maturation and adaptation of the shoot system to meet the plant's mechanical support needs during the reproductive phase. Our results provide insights into the connection between flowering and secondary growth and suggest gene targets for cultivating B. napus lines that flower at the optimal time.