Brassinosteroids (BRs) are steroidal phytohormones that are essential for plant growth, development and adaptation to environmental stresses. BRs act in a dose-dependent manner and do not travel over long distances; hence, BR homeostasis maintenance is critical for their function. Biosynthesis of bioactive BRs relies on the cell-to-cell movement of hormone precursors. However, the mechanism of the short-distance BR transport is unknown, and its contribution to the control of endogenous BR levels remains unexplored. Here we demonstrate that plasmodesmata (PD) mediate the passage of BRs between neighboring cells. Intracellular BR content, in turn, is capable of modulating PD permeability to optimize its own mobility, thereby manipulating BR biosynthesis and signaling. Our work uncovers a thus far unknown mode of steroid transport in eukaryotes and exposes an additional layer of BR homeostasis regulation in plants.

Center for Plant Systems Biology VIB Ghent Belgium

Department of Biology Duke University Durham NC USA

Department of Organic and Macromolecular Chemistry Ghent University Ghent Belgium

Department of Plant Biotechnology and Bioinformatics Ghent University Ghent Belgium

Faculty of Science Hokkaido University Sapporo Japan

Howard Hughes Medical Institute Duke University Durham NC USA

Laboratoire de Biogenèse Membranaire Unité Mixte de Recherche 5200 Université de Bordeaux Centre National de la Recherche Scientifique Villenave d'Ornon France

Laboratory of Growth Regulators Institute of Experimental Botany The Czech Academy of Sciences and Palacký University Olomouc Czech Republic

Plant Molecular and Cellular Biology Laboratory and Integrative Biology Laboratory Salk Institute for Biological Studies La Jolla CA USA

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