By Volodymyr Radchuk and Ljudmilla Borisjuk
Leibniz-Institute of Plant Genetics and Crop Plant Research (IPK), Gatersleben, Germany
Background: Sugars Will Eventually be Exported Transporter (SWEET) is a large family of proteins, which have been found in every sequenced plant genome. The main function of SWEET proteins is the transport of sugars like sucrose and glucose. This makes SWEETs important for various processes during a plant’s growth and development. However, the SWEET family in barley was not characterized so far either in terms of its capabilities to transport specific substrates or their functional roles in grain’s growth and development.
Questions: What is the physiological role of SWEETs in barley grain development? Which substrates are transported by the SWEET proteins present in the seed?
Findings: Of 23 barley SWEET genes, HvSWEET11b, HvSWEET15a and HvSWEET4 are predominantly active in the developing grains. HvSWEET11b protein functions not only as a sugar transporter but is able also to transport the phytohormone cytokinin. Plants carrying a knockout homozygous mutation of HvSWEET11b failed to set any viable grain. The partial repression of HvSWEET11b transcription altered the allocation of both sucrose and cytokinin in the grain and resulted in fewer endosperm cells, lower starch and protein accumulation, and a reduction of the grain size at maturity. The dual substrate capacity of a single transporter protein provides the plant with an efficient means of coordinating the grain’s development and filling.
Next steps: This study highlights the relevance of SWEET proteins as multifunctional transporters. It would be of great interest to explore the molecular mechanisms of how cytokinin transport and metabolism towards and in the grains influence their development.
Volodymyr Radchuk, Zeinu M. Belew, Andre Gündel, Simon Mayer, Alexander Hilo, Goetz Hensel, Rajiv Sharma, Kerstin Neumann, Stefan Ortleb, Steffen Wagner, Aleksandra Muszynska, Christoph Crocoll, Deyang Xu, Iris Hoffie, Jochen Kumlehn, Joerg Fuchs, Fritz F. Peleke, Jedrzej J. Szymanski, Hardy Rolletschek, Hussam H. Nour-Eldin, Ljudmilla Borisjuk. (2023). SWEET11b transports both sugar and cytokinin in developing barley grains. https://doi.org/10.1093/plcell/koad055