SHOOTMERISTEMLESS autoactivation: A prerequisite for fruit metamorphosis

In animals and plants, organ shape is primarily determined during primordium development by carefully coordinated growth and cell division ^1-3 . Rare examples of post-primordial change in morphology (reshaping) exist that offer tractable systems to study mechanisms required for organ-shape determination and diversification. One such example is the heart-shape formation of Capsella fruits that occurs by reshaping the ovate spheroid gynoecium upon fertilization ⁴ . Here we use whole-organ live-imaging to show that dynamic changes in growth and cell division coupled with local maintenance of meristematic identity drives Capsella fruit-shape formation. At the molecular level, we reveal an auxin-induced mechanism ultimately descending on a single cis regulatory element to mediate morphological alteration. This element resides in the promoter of the Capsella rubella SHOOTMERISTEMLESS ⁵ ( CrSTM ) gene. The CrSTM meristem identity factor positively regulates its own expression through binding to this element thereby providing a feed-forward loop at the position and time when protrusions emerge to form the heart. Independent evolution of the STM-binding element in STM promoters across Brassicaceae species correlates with those undergoing a gynoecium-to-fruit metamorphosis. Accordingly, genetic and phenotypic studies showed that the STM-binding element is required to facilitate the shape transition and reveals a conserved molecular mechanism for organ morphogenesis.