First published online April 30, 2004; 10.1104/pp.103.036632
Plant Physiology 135:574-586 (2004)
© 2004 American Society of Plant Biologists
WHOLE PLANT AND ECOPHYSIOLOGY
Evidence for Apoplasmic Phloem Unloading in Developing Apple Fruit1
Ling-Yun Zhang2,
Yi-Ben Peng2,
Sandrine Pelleschi-Travier2,3,
Ying Fan,
Yan-Fen Lu,
Ying-Min Lu,
Xiu-Ping Gao,
Yuan-Yue Shen,
Serge Delrot and
Da-Peng Zhang*
China State Key Laboratory of Plant Physiology and Biochemistry, China Agricultural University, 100094 Beijing, China (L.Y.Z., Y.B.P., Y.F., Y.F.L., Y.M.L., X.P.G., Y.Y.S., D.P.Z.); and Unité Mixte de Recherches6161, Centre National de la Recherche Scientifique, 86022 Poitiers, France (S.P.T., S.D.)
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.
1 This work was supported by China National Natural Science Foundation (grant nos. 30070532, 30270919, and 30330420 to D.P.Z.) and by China National Key Basic Research Program (grant nos. G1999011700 and 2003CB114302 to D.P.Z.).
2 These authors contributed equally to the paper.
3 Present address: Unité Mixte de Recherches, Laboratoire Morphogénèse des Ligneux, Université d'Angers, Institut National de la Recherche Agronomique, 2 Boulevard Lavoisier, 49045 Angers, France.
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.036632.
* Corresponding author; e-mail zhangdp{at}sohu.net; fax 861062891899.
Received November 23, 2003;
returned for revision February 10, 2004;
accepted February 25, 2004.
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