MIP genes are down-regulated under drought stress in Nicotiana glauca

Water flux across cell membranes has been shown to occur not only through t he lipid bilayer, but also through aquaporins, which are:members of the maj or intrinsic protein (MIP) super-family of channel proteins. Aquaporins gre atly increase the membrane permeability for water, but may also be regulate d, allowing cellular control over the rate of water influx/efflux, Water fl ux is crucial for stomatal opening and closing, but little is known about t he role that aquaporins play in stomatal physiology, Our initial goal was t o isolate and characterize the MIP genes expressed in guard cells of the mo del plant, Nicotiana glauca, Degenerate oligonucleotides corresponding to a mino acid sequences conserved in tonoplast intrinsic proteins (TIPs) or pla sma membrane intrinsic proteins (PIPs) were used to amplify portions of MIP genes by RT-PCR, These PCR products were used as probes in screening a N, glauca guard cell cDNA library. We isolated three clones (NgMIP1, NgMIP2 an d NgMIP3) homologous to TIPs and two clones (NgMIP4 and NgMIP5) homologous to PIPs, All of the MIP genes me characterized displayed highest levels of mRNA accumulation in roots or stems, with lower levels of expression in mes ophyll cells and whole leaves, and lowest transcript accumulation in guard cell RNA. Interestingly, the accumulation of transcripts arising from NgMIP 2, NgMIP3 and NgMIP4 diminished dramatically in drought-stressed plants. Th is down-regulation of, MIP gene expression may result in reduced membrane w ater permeability and may encourage cellular water conservation during peri ods of dehydration stress.