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

• Published in: Journal of experimental botany Vol. 59; no. 11; pp. 2945 - 2954
• Main Authors: Botha, C.E.J, Aoki, N, Scofield, G.N, Liu, L, Furbank, R.T, White, R.G
• Format: Journal Article
• Language: English
• Published: Oxford Oxford University Press 01-08-2008; Oxford Publishing Limited (England)
• Subjects: Agronomy. Soil science and plant productions; Apoplasmic and symplasmic transport; Biological and medical sciences; Bundle sheath cells; Cell membranes; Cell walls; companion cell; Economic plant physiology; Endocytosis; exo- and endocytosis; Fluoresceins - metabolism; Freshwater; Fundamental and applied biological sciences. Psychology; Microscopy, Electron, Transmission; Oryza sativa; Parenchyma; Phloem; Phloem - physiology; Phloem - ultrastructure; pit membrane; Plant Leaves - physiology; Plant Leaves - ultrastructure; Poaceae - physiology; Poaceae - ultrastructure; Research Papers; Rice; Sieve tubes; thick-walled sieve tube; thin-walled sieve tube; Vascular bundles; Water relations, transpiration, stomata; Xylem; Xylem - physiology; Xylem - ultrastructure; xylem parenchyma; Xylem vessels; USA, Texas; thick-walled sieve tube; thin-walled sieve tube; companion cell; pit membrane; exo- and endocytosis; Apoplasmic and symplasmic transport; rice; xylem parenchyma; Monocotyledones; Xylem; Plant leaf; Oryza sativa; Gramineae; Angiospermae; Spermatophyta
• ISSN: 0022-0957; 1460-2431
• DOI: 10.1093/jxb/ern150

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.