Journal Article

Phloem Unloading in Developing Walnut Fruit is Symplasmic in the Seed Pericarp and Apoplasmic in the Fleshy Pericarp

Guo-Liang Wu, Xiao-Yan Zhang, Ling-Yun Zhang, Qiu-Hong Pan, Yuan-Yue Shen and Da-Peng Zhang

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 45, issue 10, pages 1461-1470
Published in print October 2004 | ISSN: 0032-0781
Published online October 2004 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pch169
Phloem Unloading in Developing Walnut Fruit is Symplasmic in the Seed Pericarp and Apoplasmic in the Fleshy Pericarp

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The sieve element–companion cell (SE–CC) complex of the sepal bundles feeding the fleshy pericarp of developing walnut (Juglans regia L.) fruit is structurally symplasmically isolated, but the SE–CC complex of the minor ventral carpellary bundles located in the seed pericarp and feeding the seed is structurally symplasmically connected to its adjacent parenchyma cells. 14C-autoradiography indicated that the phloem of both the sepal and carpellary bundles was functional for unloading. Confocal laser scanning microscopy imaging of carboxyfluorescein unloading showed that the dye is confined to the phloem strands of the sepal bundles in the fleshy pericarp, but released from the phloem strands of the minor ventral carpellary bundles into the surrounding parenchyma cells in the seed pericarp. A 60-kDa acid invertase was immunolocalized to the cell wall of SE–CC complex and parenchyma cells in both the fleshy and seed pericarp. These data provide clear evidence for an apoplasmic phloem unloading pathway in the fleshy pericarp and a predominant symplasmic phloem unloading pathway parallel with a possible apoplasmic path as suggested by the presence of the extracellular invertase in the seed pericarp. A model of complex phloem unloading pathways in developing walnut fruit has been proposed.

Keywords: Keywords: Carboxyfluorescein — Cell wall-bound acid invertase — Phloem unloading — Stone fruit — Walnut.; Abbreviations: CC, companion cell; CF, carboxyfluorescein; CFDA, carboxyfluorescein diacetate; CLSM, confocal laser scanning microscopy; FP, sink parenchyma cell of the fleshy pericarp; PP, phloem parenchyma cell; PC, parenchyma cell; SE, sieve element; SE–CC complex, sieve element–companion cell complex; TBST, Tris-buffered saline with Tween 20.

Journal Article.  6204 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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