title: Sugar uptake in the Aril of litchi fruit depends on the apoplasmic post-phloem transport and the activity of proton pumps and the putative transporter LcSUT4.
abstract: The post-phloem unloading pathway and the mechanism of sugar accumulation remain unclear in litchi fruit. A combination of electron microscopy, transport of phloem-mobile symplasmic tracer (carboxyfluorescein, CF) and biochemical and molecular assays was used to explore the post-phloem transport pathway and the mechanism of aril sugar accumulation in litchi. In the funicle, where the aril originates, abundant plasmodesmata were observed, and CF introduced from the peduncle diffused to the parenchyma cells. In addition, abundant starch and pentasaccharide were detected and the sugar concentration was positively correlated with activities of sucrose hydrolysis enzymes. These results clearly showed that the phloem unloading and post-phloem transport in the funicle were symplastic. On the other hand, imaging of CF showed that it remained confined to the parenchyma cells in funicle tissues connecting the aril. Infiltration of both an ATPase inhibitor [eosin B (EB)] and a sucrose transporter inhibitor [p-chloromercuribenzene sulfonate (PCMBS)] inhibited sugar accumulation in the aril. These results indicated an apoplasmic post-phloem sugar transport from the funicle to the aril. Although facilitated diffusion might help sucrose uptake from the cytosol to the vacuole in cultivars with high soluble invertase, membrane ATPases in the aril, especially tonoplast ATPase, are crucial for aril sugar accumulation. The expression of a putative aril vacuolar membrane sucrose transporter gene (LcSUT4) was highly correlated with the sugar accumulation in the aril of litchi. These data suggest that apoplasmic transport is critical for sugar accumulation in litchi aril and that LcSUT4 is involved in this step.
authors.affiliation: Physiological Laboratory for South China Fruits, College of Horticulture, South China Agricultural University, Guangzhou, 510642, PR China These authors contributed equally to this work.
authors.forename: Teng-Duan
authors.initials: TD
authors.lastname: Wang
authors.affiliation: Physiological Laboratory for South China Fruits, College of Horticulture, South China Agricultural University, Guangzhou, 510642, PR China These authors contributed equally to this work.
authors.forename: Hui-Fen
authors.initials: HF
authors.lastname: Zhang
authors.affiliation: Physiological Laboratory for South China Fruits, College of Horticulture, South China Agricultural University, Guangzhou, 510642, PR China.
authors.forename: Zi-Chen
authors.initials: ZC
authors.lastname: Wu
authors.affiliation: Physiological Laboratory for South China Fruits, College of Horticulture, South China Agricultural University, Guangzhou, 510642, PR China.
authors.forename: Jian-Guo
authors.initials: JG
authors.lastname: Li
authors.affiliation: Physiological Laboratory for South China Fruits, College of Horticulture, South China Agricultural University, Guangzhou, 510642, PR China wanghc1972@263.net huangxm@scau.edu.cn.
authors.forename: Xu-Ming
authors.initials: XM
authors.lastname: Huang
authors.affiliation: Physiological Laboratory for South China Fruits, College of Horticulture, South China Agricultural University, Guangzhou, 510642, PR China wanghc1972@263.net huangxm@scau.edu.cn.
authors.forename: Hui-Cong
authors.initials: HC
authors.lastname: Wang
mesh_terms: D002731:4-Chloromercuribenzenesulfonate / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D050260:Carbohydrate Metabolism / Q000187:drug effects; D002851:Chromatography, High Pressure Liquid; D010697:Eosine I Bluish / Q000494:pharmacology; D005452:Fluoresceins / Q000378:metabolism; D005638:Fruit / Q000235:genetics / Q000254:growth & development / Q000378:metabolism* / Q000648:ultrastructure; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D017343:Genes, Plant; D032125:Litchi / Q000187:drug effects / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D052585:Phloem / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D031425:Plasmodesmata / Q000378:metabolism / Q000648:ultrastructure; D017494:Proton Pumps / Q000378:metabolism*; D025262:Vacuolar Proton-Translocating ATPases / Q000378:metabolism
chemical_list: D005452:Fluoresceins; D026901:Membrane Transport Proteins; D010940:Plant Proteins; D017494:Proton Pumps; C024098:6-carboxyfluorescein; D002731:4-Chloromercuribenzenesulfonate; D025262:Vacuolar Proton-Translocating ATPases; D010697:Eosine I Bluish
keywords: ATPase; Carboxyfluorescein imaging; Post-phloem transport; Sugar accumulation; Sugar transporters