| Research Abstract |
Plant aquaporins (water channels) occur in multiple isoforms and are distributed in both plasma and vacuolar membranes. For eight isoforms (plasma-membrane type : PAQ1a, 1b, 1c, 2a, 2b, and 2c ; vacuolar type : γ-VM23 and δ-VM23) of radish, we investigated growth stage specificity and cell-specificity of gene expression and the relationship between the aquaporin content in the membrane and their membrane water permeability.
(1) Investigation of mRNA levels of aquaporins in several tissues of radish showed a marked accumulation of all the mRNAs in hypocotyls and growing taproots. Aquaporin proteins were highly accumulated in the central cylinder of seedling root and hypocotyl, and rich in the vascular tissue of the petiole.
(2) Osmotic water permeabilities of the plasma and vacuolar membranes were determined with a stopped-flow spectrophotometer. The water permeability of the vacuolar membrane was higher than that of the plasma membrane of radish and Graptopetalum, a CAM plant. Also, the
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permeability of both membranes of Graptopetalum was extremely lower than that of radish membranes. The result is consistent with the level of aquaporin in the membrane.
(3) Among eight isoforms of radish aquaporin, the levels of mRNA and protein of members of the PAQ2 group were changed in response to treatment with salt and phytohormones. Furthermore, the protein levels of the PAQ2-group members varied with the growth stage and the tissues.
(4) We developed a new method to specifically isolate a protoplast of interest from plant tissues and to determine the water permeability of the protoplast. We detected a high water permeability (300 μm sec^<-1>) of protoplasts from the cortex and endodermis of radish.
(5) The water channel activity of individual isoforms was determined by functional expression in yeast and stopped-flow spectrophotometrical assay of the membrane preparation. PAQ1 group members did not show the activity. It suggests the other biochemical function of this type aquaporin. Less
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