JXB Advance Access originally published online on July 9, 2008
Journal of Experimental Botany 2008 59(11):2945-2954; doi:10.1093/jxb/ern150
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© 2008 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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RESEARCH PAPER |
A xylem sap retrieval pathway in rice leaf blades: evidence of a role for endocytosis?
1Department of Botany, Rhodes University, Grahamstown 6140, South Africa
2Department of Agricultural and Environmental Biology, Graduate School of Agricultural and Life Sciences, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-8657, Japan
3CSIRO Plant Industry, GPO Box 1600, ACT 2601, Canberra, Australia
* To whom correspondence should be addressed. E-mail: t.botha{at}ru.ac.za
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.
Key words: Apoplasmic and symplasmic transport, companion cell, exo- and endocytosis, Oryza sativa, pit membrane, rice, thick-walled sieve tube, thin-walled sieve tube, xylem parenchyma
Received 11 October 2007; Revised 6 March 2008 Accepted 6 May 2008
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