title: Sucrose efflux mediated by SWEET proteins as a key step for phloem transport.
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(+) (proton) cotransporters. Using optical sucrose sensors, we identified 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 the sieve element-companion cell complex. We discuss how restriction of intercellular transport to the interface of adjacent phloem cells may be an effective mechanism to limit the availability of photosynthetic carbon in the leaf apoplasm in order to prevent pathogen infections.
authors.affiliation: Carnegie Institution for Science, 260 Panama Street, Stanford, CA 94305, USA.
authors.forename: Li-Qing
authors.initials: LQ
authors.lastname: Chen
authors.forename: Xiao-Qing
authors.initials: XQ
authors.lastname: Qu
authors.forename: Bi-Huei
authors.initials: BH
authors.lastname: Hou
authors.forename: Davide
authors.initials: D
authors.lastname: Sosso
authors.forename: Sonia
authors.initials: S
authors.lastname: Osorio
authors.forename: Alisdair R
authors.initials: AR
authors.lastname: Fernie
authors.forename: Wolf B
authors.initials: WB
authors.lastname: Frommer
mesh_terms: D000818:Animals; D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer; D057809:HEK293 Cells; D006801:Humans; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D050505:Mutant Proteins / Q000378:metabolism; D012275:Oryza / Q000378:metabolism; D052585:Phloem / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000254:growth & development; D011401:Promoter Regions, Genetic; D013395:Sucrose / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D026901:Membrane Transport Proteins; D050505:Mutant Proteins; D010940:Plant Proteins; C569466:SWEET11 protein, Arabidopsis; C569467:SWEET12 protein, Arabidopsis; C078729:sucrose transport protein, plant; D013395:Sucrose