title: Evidence for apoplasmic phloem unloading in developing apple fruit.
abstract: The phloem unloading pathway remains unclear in fleshy fruits accumulating a high level of soluble sugars. A structural investigation in apple fruit (Malus domestica Borkh. cv Golden Delicious) showed that the sieve element-companion cell complex of the sepal bundles feeding the fruit flesh is symplasmically isolated over fruit development. 14C-autoradiography indicated that the phloem of the sepal bundles was functional for unloading. Confocal laser scanning microscopy imaging of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands of the sepal bundles from the basal to the apical region of the fruit. A 52-kD putative monosaccharide transporter was immunolocalized predominantly in the plasma membrane of both the sieve elements and parenchyma cells and its amount increased during fruit development. A 90-kD plasma membrane H(+)-ATPase was also localized in the plasma membrane of the sieve element-companion cell complex. Studies of [14C]sorbitol unloading suggested that an energy-driven monosaccharide transporter may be functional in phloem unloading. These data provide clear evidence for an apoplasmic phloem unloading pathway in apple fruit and give information on the structural and molecular features involved in this process.
authors.affiliation: China State Key Laboratory of Plant Physiology and Biochemistry, China Agricultural University, 100094 Beijing, China.
authors.forename: Ling-Yun
authors.initials: LY
authors.lastname: Zhang
authors.forename: Yi-Ben
authors.initials: YB
authors.lastname: Peng
authors.forename: Sandrine
authors.initials: S
authors.lastname: Pelleschi-Travier
authors.forename: Ying
authors.initials: Y
authors.lastname: Fan
authors.forename: Yan-Fen
authors.initials: YF
authors.lastname: Lu
authors.forename: Ying-Min
authors.initials: YM
authors.lastname: Lu
authors.forename: Xiu-Ping
authors.initials: XP
authors.lastname: Gao
authors.forename: Yuan-Yue
authors.initials: YY
authors.lastname: Shen
authors.forename: Serge
authors.initials: S
authors.lastname: Delrot
authors.forename: Da-Peng
authors.initials: DP
authors.lastname: Zhang
mesh_terms: D001692:Biological Transport / Q000502:physiology; D002250:Carbon Radioisotopes; D002462:Cell Membrane / Q000201:enzymology; D005638:Fruit / Q000254:growth & development / Q000378:metabolism* / Q000648:ultrastructure; D027845:Malus / Q000254:growth & development / Q000378:metabolism*; D018613:Microscopy, Confocal; D008854:Microscopy, Electron; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D006180:Proton-Translocating ATPases / Q000378:metabolism; D013012:Sorbitol / Q000378:metabolism
chemical_list: D002250:Carbon Radioisotopes; D009004:Monosaccharide Transport Proteins; D013012:Sorbitol; D006180:Proton-Translocating ATPases