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Sucrose Efflux Mediated by SWEET Proteins as a Key Step for Phloem Transport

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Science  08 Dec 2011:
1213351
DOI: 10.1126/science.1213351

Abstract

Plants transport fixed carbon predominantly as sucrose, which is produced in mesophyll cells and imported into phloem cells for translocation throughout the plant. It is not known how sucrose migrates from sites of synthesis in the mesophyll to the phloem or which cells mediate efflux into the apoplasm as a prerequisite for phloem loading by the SUT sucrose/H+ cotransporters. Using optical sucrose sensors, we identify a subfamily of SWEET sucrose efflux transporters. AtSWEET11 and 12 localize to the plasma membrane of the phloem. Mutant plants carrying insertions in AtSWEET11 and 12 are defective in phloem loading, thus revealing a two-step mechanism of SWEET-mediated export from parenchyma cells feeding H+-coupled import into sieve element companion cells. We discuss how restriction of intercellular transport to the interface of adjacent phloem cells may be an effective mechanism to limit access of pathogens to photosynthetic carbon in the leaf apoplasm.