Hibiscus bullseyes reveal mechanisms controlling petal pattern proportions that influence plant-pollinator interactions

Colourful flower patterns are key signals to attract pollinators. To produce such motifs, plants specify boundaries dividing petals into subdomains where cells develop distinctive pigmentations, shapes and textures. While some transcription factors and biosynthetic pathways behind these characteristics are well-studied, the upstream processes restricting their activities to specific petal regions remain enigmatic. Here, we unveil that the petal surface of Hibiscus trionum, an emerging model featuring a bullseye on its corolla, is pre-patterned as the bullseye boundary position is specified long before it becomes visible. Using a computational model, we explore how pattern proportions are maintained while petals experience a 100-fold size increase. Exploiting transgenic lines and natural variants, we show that plants can regulate boundary position during the pre-patterning phase or modulate growth either side of this boundary later in development to vary bullseye proportions. Such modifications are functionally relevant, as buff-tailed bumblebees (Bombus terrestris) can reliably identify food sources based on bullseye size and prefer certain pattern proportions.

Methods
See Methods in our manuscript