BnaA02.TOP1α and BnaC02.TOP1α redundantly regulate floral organ development in Brassica napus L

Floral organ development is a crucial process in plant life, and its precise regulation is essential for crop yield. In this study, we demonstrate that BnaA02.TOP1α and BnaC02.TOP1α, type IB DNA topoisomerases, play a pivotal role in regulating floral organ development in Brassica napus. The double mutant of these two genes, s5, exhibited an increased probability of developing malformed floral organ structures, along with a reduced number of second and third whorl floral organs and an increased number of first and fourth whorl floral organs. These phenotypic defects were primarily attributed to the downregulation of the floral meristem identity genes BnaC03.LFY, BnaA08.UFO, and BnaC08.UFO, and to the altered expression of several B- and E-class genes in the s5 mutant. BnaA02.TOP1α physically bound to the genomic regions of BnaC03.LFY, BnaA08.UFO, and BnaC08.UFO, promoting the recruitment of the RNA polymerase II complex to their transcription start sites. These changes were associated with alterations in histone modifications and the accumulation of DNA:RNA hybrids. Our findings demonstrate that BnaA02.TOP1α regulates DNA topology, thereby facilitating the recruitment of RNA polymerase II at BnaC03.LFY, BnaA08.UFO, and BnaC08.UFO. In conjunction with histone modifications and R-loops, this recruitment promotes the expression of these genes, thereby ensuring normal floral organ development. These findings contribute to our understanding of BnaTOP1α function and provide valuable insights into the regulatory network controlling floral organ development in B. napus.