Format: ORAL

Yang Dong1,2,3*, Zhi-Cheng Hu1,3, Mateusz Majda4, Hao-Ran Sun1,3 and Lars stergaard5,6

1 State Key Laboratory of Plant Diversity and Specialty Crops, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Xiangshan, Beijing 100093, China 2 China National Botanical Garden, Beijing 100093, China 3 University of Chinese Academy of Sciences, Beijing 100049, China 4 Department of Plant Molecular Biology, University of Lausanne, CH-1015 Lausanne, Switzerland 5 Crop Genetics Department, John Innes Centre, Norwich NR4 7UH, UK 6 Department of Biology, University of Oxford, South Parks Road, OX1 3RB, UK

In animals and plants, organ shape is primarily determined during primordium development by carefully coordinated growth and cell division. 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 fertilization4. 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 SHOOTMERISTEMLESS5 (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.