Carbohydrate import into seeds directly determines seed size and must have been increased through domestication. However, evidence of the domestication of sugar translocation and the identities of seed-filling transporters have been elusive. Maize ZmSWEET4c, as opposed to its sucrose-transporting homologs, mediates transepithelial hexose transport across the basal endosperm transfer layer (BETL), the entry point of nutrients into the seed, and shows signatures indicative of selection during domestication. Mutants of both maize ZmSWEET4c and its rice ortholog OsSWEET4 are defective in seed filling, indicating that a lack of hexose transport at the BETL impairs further transfer of sugars imported from the maternal phloem. In both maize and rice, SWEET4 was likely recruited during domestication to enhance sugar import into the endosperm.

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We are grateful to D. Ehrhardt and H. Cartwright for confocal microscopy. For the rice experiments, we are grateful to T. Li for constructing the TALEN vector targeting OsSWEET4, B. Liu for rice transgenics, C. Ji for the isolation and transfection of rice protoplasts in the laboratory of B.Y. and X. Li for help with domestication analysis of OsSWEET4. We thank M. Greenfield, A. Grimault and K.M. Wong for plant care, Y. Gong for the yeast complementation assay, M. Evans for providing teosinte plant material and C. Stefan for renaming ZmSWEET4 with “c” for her initial. Work performed on maize in the laboratory of W.B.F. was made possible by support from the Office of Basic Energy Sciences of the US Department of Energy under grant DE-FG02-04ER15542, and work on rice was supported by the National Science Foundation under grant IOS-1258018 (B.Y. and W.B.F.); the other laboratories were supported by the National Science Foundation (IOS-1116561 to D.R.M. and K.E.K., IOS-1025976 to K.E.K. and IOS-1238014 to J.R.-I.), as well as USDA-NIFA 2010-04228 (K.E.K., D.R.M. and M.S.).

Author information


  1. Department of Plant Biology, Carnegie Science, Stanford, California, USA.

    • Davide Sosso
    • , Joelle Sasse
    •  & Wolf B Frommer
  2. Department of Genetics, Development and Cell Biology, Iowa State University, Ames, Iowa, USA.

    • Dangping Luo
    •  & Bing Yang
  3. US Department of Agriculture, Agricultural Research Service (USDA-ARS), Gainesville, Florida, USA.

    • Qin-Bao Li
    •  & Prem S Chourey
  4. Department of Plant Sciences, University of California, Davis, Davis, California, USA.

    • Jinliang Yang
  5. Ecole Normale Supérieure de Lyon, Université Lyon 1, Unité Reproduction et Développement des Plantes, Lyon, France.

    • Ghislaine Gendrot
    •  & Peter M Rogowsky
  6. Plant Molecular and Cellular Biology, Agronomy Department, University of Florida, Gainesville, Florida, USA.

    • Masaharu Suzuki
    • , Karen E Koch
    • , Donald R McCarty
    •  & Prem S Chourey
  7. Department of Plant Sciences, Center for Population Biology and Genome Center, University of California, Davis, Davis, California, USA.

    • Jeffrey Ross-Ibarra


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D.S., P.S.C., P.M.R., J.R.-I., B.Y. and W.B.F. conceived and designed experiments. D.S., D.L., Q.-B.L., J.S., J.Y., G.G., M.S., K.E.K., D.R.M. and J.R.-I. performed experiments. D.S., P.S.C., J.R.-I., B.Y., K.E.K., J.Y., D.R.M. and W.B.F. analyzed the data. D.S. and W.B.F. wrote the manuscript, and J.S., M.S., K.E.K., D.R.M., P.S.C., P.M.R., J.R.-I. and B.Y. revised it.

Competing interests

Patent applications regarding the use of SWEET genes have been filed.

Corresponding author

Correspondence to Davide Sosso.

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