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 () gene expression along the dorsal-ventral axis. Although auxin and () have been implicated as regulators of , when, where, and how they affect expression remains unclear. Here, we combined transgenic manipulation and fluorescent confocal imaging to capture the spatiotemporal dynamics of the genes ( and ) in relation to FM growth and auxin activity maxima in both wild type (WT) and mutants (). Strikingly, have already gained dorsal expression in the FM prior to any detectable auxin maxima, and no difference in expression was observed between the wild type and during this initiation phase. We observed highly dynamic auxin maxima during FM expansion, when expressions remained dorsally restricted in WT but expanded broadly in 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 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.