Tissue and Cell-Specific Localization of Rice Aquaporins and Their Water Transport Activities

Abstract

Water transport in plants is greatly dependent on the expression and activity of water transport channels, called aquaporins. Here, we have clarified the tissue- and cell-specific localization of aquaporins in rice plants by immunoblotting and immunocytochemistry using seven isoform-specific aquaporin antibodies. We also examined water transport activities of typical aquaporin family members using a yeast expression system in combination with a stopped-flow spectrophotometry assay. OsPIP1 members, OsPIP2;1, OsTIP1;1 and OsTIP2;2 were expressed in both leaf blades and roots, while OsPIP2;3, OsPIP2;5 and OsTIP2;1 were expressed only in roots. In roots, large amounts of aquaporins accumulated in the region adjacent to the root tip (around 1.5-4 mm from the root tip). In this region, cell-specific localization of the various aquaporin members was observed. OsPIP1 members and OsTIP2;2 accumulated predominantly in the endodermis and the central cylinder, respectively. OsTIP1;1 showed specific localization in the rhizodermis and exodermis. OsPIP2;1, OsPIP2;3 and OsPIP2;5 accumulated in all root cells, but they showed higher levels of accumulation in endodermis than other cells. In the region at 35 mm from the root tip, where aerenchyma develops, aquaporins accumulated at low levels. In leaf blades, OsPIP1 members and OsPIP2;1 were localized mainly in mesophyll cells. OsPIP2;1, OsPIP2;3, OsPIP2;5 and OsTIP2;2 expressed in yeast showed high water transport activities. These results suggest that rice aquaporins with various water transport activities may play distinct roles in facilitating water flux and maintaining the water potential in different tissues and cells.

Identification of a Residue in Helix 2 of Rice Plasma Membrane Intrinsic Proteins That Influences Water Permeability

“…4, E-G). In agreement with the previous report, OsPIP1;2 was found to be expressed and trafficked to the yeast membrane with similar efficiency as OsPIP2s, but the water transport activity of OsPIP1;2 was smaller than OsPIP2s (9). Our data further affirmed that OsPIP1s are less permeable to water than OsPIP2s partly due to the Ala/ Ile(Val) difference.…”

“…Size exclusion at the two main constrictions is one of the mechanisms for water transport and substrate selectivity by AQPs (3,4). Extensive investigation of the plant PIPs revealed that PIP2 paralogues induced water permeation when expressing on Xenopus oocyte or yeast membranes, whereas PIP1 paralogues were almost inactive (5)(6)(7)(8)(9)(10). The sequence difference between the two subgroups, however, is quite subtle, as they share the same residues in main pore constrictions and almost the same hydrophobic residues around the aqueous pathway (6,11).…”

Rapid changes in root hydraulic conductivity and aquaporin expression in rice (Oryza sativaL.) in response to shoot removal – xylem tension as a possible signal

“…Previous studies (Sakurai et al, 2005(Sakurai et al, , 2008Guo et al, 2006;Sakurai-Ishikawa et al, 2011) and our own preliminary experiments (not shown) on the Bala rice cultivar analysed showed that all three PIP1 genes and all PIP2 genes except OsPIP2;7 and OsPIP2;8 are expressed in roots at significant, or at least reproducibly detectable levels. Therefore, the latter two rice PIP genes were not included in the present analyses.…”

“…In rice, OsPIP1;3 and OsPIP2;4 are the two PIP genes, besides OsPIP2;5, which show the largest, and significant decrease in expression in response to treatments which reduce shoot transpirational water loss. Similarly to OsPIP2;5, OsPIP1s (including OsPIP1;3) and GmPIP1;6 are localized in most root tissues and in particular in the endodermis; GmPIP1;6 is also localized in stelar tissue (Sakurai et al, 2008;Vandeleur et al, 2014). Together these data point to PIP AQPs, which are localized in the endodermis or stele, as the prime facilitators for mediating the decrease in root Lp in response to shoot removal and changes in shoot transpirational demand.…”

“…† Calculated based on Haagen-Poiseuille's law (Nobel, 1991), using a metaxylem radius of 15 mm, three metaxylem vessels per root, a minimum of five roots per root system and a total length of root system of approx. 20 cm (Meng and Fricke, unpublished; see also Sakurai et al, 2008). ‡ Value derived from leaf water potential measurements; represents a most negative estimate of xylem pressure.…”