Evidence for apoplasmic phloem unloading in developing apple fruit |
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| Authors: | Zhang Ling-Yun Peng Yi-Ben Pelleschi-Travier Sandrine Fan Ying Lu Yan-Fen Lu Ying-Min Gao Xiu-Ping Shen Yuan-Yue Delrot Serge Zhang Da-Peng |
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| Affiliation: | China State Key Laboratory of Plant Physiology and Biochemistry, China Agricultural University, 100094 Beijing, China. |
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| 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. |
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