CUC/auxin patterning of decanalised petal number in <i>Cardamine hirsuta</i>

Petal number is highly canalised in the four-petalled flowers of Arabidopsis. This trait is decanalised in the closely related species <i>Cardamine hirsuta</i>, such that petal number varies from zero to four between individual flowers and in response to natural genetic and environmental variation. Loss of robustness was traced to divergence of the MADS-box transcription factor <i>APETALA1</i> in <i>C. hirsuta</i>, resulting in loss of epistasis over alleles that cause petal number to vary. How petal formation is patterned in these decanalised flowers is an open question. Here we use genetics and quantitative imaging to investigate how a key patterning module, comprising CUP-SHAPED COTYLEDON1,2 (CUC) transcription factors and auxin, regulates petal formation in <i>C. hirsuta</i>. We show that auxin activity maxima are positioned in inter-sepal boundaries, rather than on the floral meristem, rendering petal initiation sensitive to the space available between sepals, such that growth variation influences petal number variation.