Transcription Factors BnaC09.FUL and BnaC06.WIP2 Antagonistically Regulate Flowering Time under Long-day Conditions in Brassica napus

Flowering time stands as a pivotal agronomic trait in rapeseed (Brassica napus L.). Timely flowering enables rapeseed to avoid weather-related losses, plant diseases, and insect pests. However, the molecular mechanisms governing flowering time remain largely unknown in B. napus. Here, a genome-wide association analysis of 165 diverse rapeseed accessions pinpointed BnaC09.FUL, a MADS-box transcription factor, as a promising candidate gene regulating flowering time in B. napus. Expression analysis revealed that the expression of BnaC09.FUL increased sharply in B. napus shoot apices close to bolting. Overexpression of BnaC09.FUL resulted in early flowering, while mutation of all six paralogs of BnaFUL led to delayed flowering in B. napus. Additionally, a zinc finger transcription factor, BnaC06.WIP2, was identified as an interaction protein of BnaC09.FUL, and overexpression of BnaC06.WIP2 delayed flowering in B. napus, with RNA-sequencing revealing its influence on a plethora of flowering-associated genes. Chromatin immunoprecipitation and transient dual-luciferase reporter assays demonstrated that BnaC06.WIP2 directly repressed the expression of BnaA05.SOC1, BnaC03.SOC1, BnaC04.SOC1, BnaC06.FT, BnaA06.LFY, BnaA08.CAL, BnaC03.CAL, and BnaC07.FUL and indirectly inhibited the expression of several other flowering-related genes. Genetic and molecular investigations highlighted an antagonistic relationship between BnaC09.FUL and BnaC06.WIP2 in regulating flowering in B. napus, primarily through direct regulation of BnaC03.SOC1, BnaA08.CAL, and BnaC03.CAL expression. Taken together, our findings provide a mechanism by which the BnaC09.FUL-BnaC06.WIP2 transcriptional regulatory module controls flowering time in B. napus