Evidence for apoplasmic phloem unloading in developing apple fruit.
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.
Zhang
Ling-Yun
LY
China State Key Laboratory of Plant Physiology and Biochemistry, China Agricultural University, 100094 Beijing, China.
Peng
Yi-Ben
YB
Pelleschi-Travier
Sandrine
S
Fan
Ying
Y
Lu
Yan-Fen
YF
Lu
Ying-Min
YM
Gao
Xiu-Ping
XP
Shen
Yuan-Yue
YY
Delrot
Serge
S
Zhang
Da-Peng
DP
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
D002250:Carbon Radioisotopes; D009004:Monosaccharide Transport Proteins; D013012:Sorbitol; D006180:Proton-Translocating ATPases