title: Phloem unloading follows an extensive apoplasmic pathway in cucumber (Cucumis sativus L.) fruit from anthesis to marketable maturing stage.
abstract: The phloem unloading pathway remains unclear in fruits of Cucurbitaceae, a classical stachyose-transporting species with bicollateral phloem. Using a combination of electron microscopy, transport of phloem-mobile symplasmic tracer carboxyfluorescein, assays of acid invertase and sucrose transporter, and [(14)C]sugar uptake, the phloem unloading pathway was studied in cucumber (Cucumis sativus) fruit from anthesis to the marketable maturing stage. Structural investigations showed that the sieve element-companion cell (SE-CC) complex of the vascular bundles feeding fruit flesh is apparently symplasmically restricted. Imaging of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands of the vascular bundles in the whole fruit throughout the stages examined. A 37 kDa acid invertase was located predominantly in the cell walls of SE-CC complexes and parenchyma cells. Studies of [(14)C]sugar uptake suggested that energy-driven transporters may be functional in sugar trans-membrane transport within symplasmically restricted SE-CC complex, which was further confirmed by the existence of a functional plasma membrane sucrose transporter (CsSUT4) in cucumber fruit. These data provide a clear evidence for an apoplasmic phloem unloading pathway in cucumber fruit. A presumption that putative raffinose or stachyose transporters may be involved in soluble sugars unloading was discussed.
authors.affiliation: College of Agronomy and Biotechnology, China Agricultural University, Beijing 100193, China.
authors.forename: Liping
authors.initials: L
authors.lastname: Hu
authors.forename: Huihui
authors.initials: H
authors.lastname: Sun
authors.forename: Ruifu
authors.initials: R
authors.lastname: Li
authors.forename: Lingyun
authors.initials: L
authors.lastname: Zhang
authors.forename: Shaohui
authors.initials: S
authors.lastname: Wang
authors.forename: Xiaolei
authors.initials: X
authors.lastname: Sui
authors.forename: Zhenxian
authors.initials: Z
authors.lastname: Zhang
mesh_terms: D002731:4-Chloromercuribenzenesulfonate / Q000494:pharmacology; D002250:Carbon Radioisotopes; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002473:Cell Wall / Q000187:drug effects / Q000201:enzymology; D003001:Cloning, Molecular; D018553:Cucumis sativus / Q000166:cytology / Q000254:growth & development* / Q000378:metabolism* / Q000648:ultrastructure; D035264:Flowers / Q000187:drug effects / Q000254:growth & development*; D005452:Fluoresceins / Q000378:metabolism; D005638:Fruit / Q000166:cytology / Q000201:enzymology / Q000254:growth & development* / Q000648:ultrastructure; D005947:Glucose / Q000378:metabolism; D040541:Marketing; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism; D058503:Mesophyll Cells / Q000166:cytology / Q000187:drug effects / Q000201:enzymology / Q000648:ultrastructure; D018613:Microscopy, Confocal; D008954:Models, Biological; D052585:Phloem / Q000033:anatomy & histology / Q000166:cytology / Q000254:growth & development / Q000378:metabolism*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D031425:Plasmodesmata / Q000187:drug effects / Q000378:metabolism; D013347:Subcellular Fractions / Q000187:drug effects / Q000378:metabolism; D043324:beta-Fructofuranosidase / Q000378:metabolism
chemical_list: D002250:Carbon Radioisotopes; D005452:Fluoresceins; D026901:Membrane Transport Proteins; D010940:Plant Proteins; C078729:sucrose transport protein, plant; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D002731:4-Chloromercuribenzenesulfonate; D043324:beta-Fructofuranosidase; D005947:Glucose