The developmental transition from a fertilized ovule to a dispersed diaspore (seed or fruit) involves complex differentiation processes of the ovule's integuments leading to the diversity in mature seed coat structures in angiosperms. In this study, comparative imaging and transcriptome analysis were combined to investigate the morph-specific developmental differences during outer seed coat differentiation and mucilage production in Aethionema arabicum, the Brassicaceae model for diaspore dimorphism. One of the intriguing adaptations of this species is the production and dispersal of morphologically distinct, mucilaginous and non-mucilaginous diaspores from the same plant (dimorphism). The dehiscent fruit morph programme producing multiple mucilaginous seed diaspores was used as the default trait combination, similar to Arabidopsis thaliana, and was compared with the indehiscent fruit morph programme leading to non-mucilaginous diaspores. Synchrotron-based radiation X-ray tomographic microscopy revealed a co-ordinated framework of morph-specific early changes in internal anatomy of developing A. arabicum gynoecia including seed abortion in the indehiscent programme and mucilage production by the mucilaginous seed coat. The associated comparative analysis of the gene expression patterns revealed that the unique seed coat dimorphism of Ae. arabicum provides an excellent model system for comparative study of the control of epidermal cell differentiation and mucilage biosynthesis by the mucilage transcription factor cascade and their downstream cell wall and mucilage remodelling genes. Elucidating the underlying molecular framework of the dimorphic diaspore syndrome is key to understanding differential regulation of bet-hedging survival strategies in challenging environments, timely in the face of global climatic change.

Botanischer Garten der Ruhr Universität Bochum Universitätsstraße 150 Bochum D 44780 Germany

Department of Biological Sciences Royal Holloway University of London Egham TW20 0EX UK

Department of Earth Sciences Royal Holloway University of London Egham TW20 0EX UK

Laboratory of Growth Regulators Palacký University Institute of Experimental Botany Czech Academy of Sciences Olomouc CZ 78371 Czech Republic

Matthias Schleiden Institute Genetics Friedrich Schiller University Jena Jena D 07743 Germany

Plant Cell Biology Department of Biology University of Marburg Marburg D 35043 Germany

Swiss Light Source Paul Scherrer Institute Villigen CH 5232 Switzerland

The Royal Botanic Gardens Kew Wellcome Trust Millennium Building Wakehurst Place Ardingly West Sussex RH17 6TN UK

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