Journal Article

A xylem sap retrieval pathway in rice leaf blades: evidence of a role for endocytosis?

C. E. J. Botha, N. Aoki, G. N. Scofield, L. Liu, R. T. Furbank and R. G. White

in Journal of Experimental Botany

Published on behalf of Society for Experimental Biology

Volume 59, issue 11, pages 2945-2954
Published in print August 2008 | ISSN: 0022-0957
Published online July 2008 | e-ISSN: 1460-2431 | DOI:

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The structure and transport properties of pit membranes at the interface between the metaxylem and xylem parenchyma cells and the possible role of these pit membranes in solute transfer to the phloem were investigated. Electron microscopy revealed a fibrillar, almost tubular matrix within the pit membrane structure between the xylem vessels and xylem parenchyma of leaf blade bundles in rice (Oryza sativa). These pits are involved primarily with regulating water flux to the surrounding xylem parenchyma cells. Vascular parenchyma cells contain large mitochondrial populations, numerous dictyosomes, endomembrane complexes, and vesicles in close proximity to the pit membrane. Taken collectively, this suggests that endocytosis may occur at this interface. A weak solution of 5,6-carboxyfluorescein diacetate (5,6-CFDA) was applied to cut ends of leaves and, after a minimum of 30 min, the distribution of the fluorescent cleavage product, 5,6-carboxyfluorescein (5,6-CF), was observed using confocal microscopy. Cleavage of 5,6-CFDA occurred within the xylem parenchyma cells, and the non-polar 5,6-CF was then symplasmically transported to other parenchyma elements and ultimately, via numerous pore plasmodesmata, to adjacent thick-walled sieve tubes. Application of Lucifer Yellow, and, separately, Texas Red-labelled dextran (10 kDa) to the transpiration stream, confirmed that these membrane-impermeant probes could only have been offloaded from the xylem via the xylem vessel–xylem parenchyma pit membranes, suggesting endocytotic transmembrane transfer of these membrane-impermeant fluorophores. Accumulation within the thick-walled sieve tubes, but not in thin-walled sieve tubes, confirms the presence of a symplasmic phloem loading pathway, via pore plasmodesmata between xylem parenchyma and thick-walled sieve tubes, but not thin-walled sieve tubes.

Keywords: Apoplasmic and symplasmic transport; companion cell; exo- and endocytosis; Oryza sativa; pit membrane; rice; thick-walled sieve tube; thin-walled sieve tube; xylem parenchyma

Journal Article.  5519 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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