The Moment Symmetry Breaks: Spatiotemporal Dynamics of CYCLOIDEA Expression During Early Floral Development

The establishment of bilateral symmetry in flowers depends on the precise regulation of CYCLOIDEA (CYC) gene expression along the dorsal-ventral axis. Although auxin and BLADE-ON-PETIOLE (BOP) have been implicated as regulators of CYC, when, where, and how they affect CYC expression remains unclear. Here, we combined transgenic manipulation and fluorescent confocal imaging to capture the spatiotemporal dynamics of the Mimulus parishii CYC genes (MpCYC2a and MpCYC2b) in relation to FM growth and auxin activity maxima in both wild type (WT) and bop mutants (mpbop). Strikingly, MpCYCs have already gained dorsal expression in the FM prior to any detectable auxin maxima, and no difference in MpCYC expression was observed between the wild type and mpbop during this initiation phase. We observed highly dynamic auxin maxima during FM expansion, when MpCYC expressions remained dorsally restricted in WT but expanded broadly in mpbop FMs. These findings suggest that early symmetry breaking in the FM is guided by positional cues independent of auxin or BOP, which are instead essential for refining and maintaining dorsal-specific CYC expression during later FM enlargement. Our work illustrated how combining advanced imaging with emerging model systems can yield fresh insights into long-standing questions in development and evolution, and laid the groundwork for further elucidating the mechanisms underlying the repeated symmetry breaking in FMs.