Acclimation to low water potential determines changes in membrane fatty acid composition and fluidity in potato cells

Potato (Solanum tuberosum) cells were gradually acclimated to grow at low water potential created by addition of increasing concentrations of polyethylene glycol (PEG) 8000 to the nutrient medium. While growth of cells abruptly transferred into a medium containing 20% PEG was dramatically inhibited, under the same stress intensity gradually acclimated cells were able to sustain active growth similarly to control unstressed cells. Gradual acclimation allowed the maintenance of normal cellular and subcellular membrane structure at an osmotic potential of -2.3 MPa, which caused extended membrane disruption when applied to non-acclimated control cells. Analysis of fatty acids (FAs) from control and acclimated cells revealed that the mol percentage composition of the major extra-plastidial phospholipids (PLs) was basically unchanged, but both phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were more saturated in acclimated cells. The decrease in unsaturation occurred primarily at the expense of linoleic acid (18:2) in the PE fraction. A significant increase in the percentage content of palmitic acid (16:0) of the PE fraction was also detected in acclimated cells. Membrane microviscosity, as determined by diphenyl-hexatriene (DPH) labelling, was higher in PEG-acclimated cells, as expected from the lower degree of unsaturation of membrane FAs found in acclimated cells. The steady-state transcript levels of a Δ⁹-stearoyl-acyl carrier protein-desaturase and a Δ¹²-oleoyl-desaturase gene decreased in acclimated cells, suggesting that the variation in the level of FA unsaturation was at least partially associated with down-regulation of the expression of these genes.