EXPRESSION OF AN ARABIDOPSIS PLASMA-MEMBRANE AQUAPORIN IN DICTYOSTELIUM RESULTS IN HYPOOSMOTIC SENSITIVITY AND DEVELOPMENTAL ABNORMALITIES

The rd28 gene of Arabidopsis thaliana encodes a water channel protein, or aquaporin, of the plasma membrane. A construct in which transcript ion of the rd28 cDNA is controlled by the Dictyostelium actin15 promot er was transformed into Dictyostelium discoideum cells, Transformants contained RD28 protein in their plasma membranes. When shifted to a lo w-osmotic-strength buffer, cells expressing rd28 swelled rapidly and b urst, indicating that the plant aquaporin allowed rapid water entry in the amoebae, The rate of osmotic lysis was a function of the osmotic pressure of the buffer, We also selected transformants in which the ex pression of the rd28 cDNA is driven by the promoter of the prespore co tB gene, These transformants accumulated rd28 mRNA uniquely in prespor e cells, In low-osmotic-strength buffer, the cotB::rd28 cells aggregat ed and formed normally proportioned slugs but failed to form normal fr uiting bodies, The number of spores was reduced 20-fold, and the stalk s of the fruiting bodies mere abnormally short. The consequences of ex pressing RD28 in prespore cells could be partially overcome by increas ing the osmolarity of the medium, Under these conditions, the cotBr::r d28 cells formed fruiting bodies of more normal appearance, and the nu mber of viable sports increased slightly. Because prespore cells hare to shrink and dehydrate to form spores, it was not unexpected that exp ression of an aquaporin would disrupt this process, but it was surpris ing to find that stalk differentiation was also affected by expression of rd28 in prespore cells, It appears that, osmotic stress on prespor e cells alters their ability to signal terminal differentiation in pre stalk cells, The results provide independent confirmation that plant a quaporins can function in the cells of other organisms, and that D. di scoideum can be used to study the properties of these water channels.