Even though Sugars Will Eventually be Exported Transporters (SWEETs) have been found in every sequenced plant genome, a comprehensive understanding of their functionality is lacking. In this study, we focused on the SWEET family of barley (Hordeum vulgare). A radiotracer assay revealed that expressing HvSWEET11b in African clawed frog (Xenopus laevis) oocytes facilitated the bidirectional transfer of not only just sucrose and glucose, but also cytokinin. Barley plants harboring a loss-of-function mutation of HvSWEET11b could not set viable grains, while the distribution of sucrose and cytokinin was altered in developing grains of plants in which the gene was knocked down. Sucrose allocation within transgenic grains was disrupted, which is consistent with the changes to the cytokinin gradient across grains, as visualized by magnetic resonance imaging and Fourier transform infrared spectroscopy microimaging. Decreasing HvSWEET11b expression in developing grains reduced overall grain size, sink strength, the number of endopolyploid endosperm cells, and the contents of starch and protein. The control exerted by HvSWEET11b over sugars and cytokinins likely predetermines their synergy, resulting in adjustments to the grain's biochemistry and transcriptome.

Centre of Region Haná for Biotechnological and Agricultural Research Czech Advanced Technology and Research Institute Palacký University Olomouc 78371 Olomouc Czech Republic

Faculty of Science Department of Plant and Environmental Sciences DynaMo Center of Excellence University of Copenhagen Thorvaldsensvej 40 1871 Frederiksberg C Denmark

IBG 4 Bioinformatics Forschungszentrum Jülich 52428 Jülich Germany

Institute of Experimental Physics 5 University of Würzburg Am Hubland 97074 Würzburg Germany

Leibniz Institute of Plant Genetics and Crop Plant Research Corrensstrasse 3 06466 Gatersleben Germany

Scotland's Rural College Kings Buildings West Mains Road Edinburgh EH9 3JGUK

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