Regulation of desaturase gene expression, changes in membrane lipid composition and freezing tolerance in potato plants

Irreversible damage of the plant plasma membrane is the primary cause of freezing injury. Changes in lipid unsaturation have being largely studied as one of the major components aimed at preserving the integrity and functionality of plasma membranes, and increasing freezing tolerance. In the present study, the potato stearoyl-ACP desaturase (omega-9) gene, encoding the first enzyme involved in plant lipid unsaturation, was cloned and used to monitor its expression during cold acclimation in plants of two Solanum species, known to differ in their ability to cold acclimate. Although up-regulated upon cold acclimation in both potato species, freezing tolerant S. commersonii plants had a constitutive level of the omega-9 desaturase gene transcripts and of other known stress protective proteins (dhn2 and cpn60 beta) remarkably higher than plants of the not freezing tolerant potato species (S. tuberosum). Transcript levels of oleoyl-desaturase (omega-6) and linoleyl CoA desaturase (omega-3), microsomal desaturases involved in further plasma membrane fatty acid ( FA) unsaturation, did not vary appreciably during cold acclimation in both potato species. S. tuberosum potato plants overexpressing the omega-9 desaturase gene were generated, to change FA lipid composition and measure the effect on the basal level freezing tolerance of cultivated potato varieties. Unsaturation level of total leaf polar lipids of one of best omega-9 desaturase overexpressing line, with a high transcript level of the exogenous gene and related protein, was slightly higher compared to potato plants transformed with the empty vector, with the notable appearance of cis-vaccenic acid (C18:1 Delta 11), an unusual monoic FAs in plants. Freezing tolerance, estimated as an increase in LT50 (2 degrees C), derived from the electrolyte leakage test, enhanced in omega-9 desaturase overexpressing transgenic potato plants only upon cold acclimation. These results indicate that modifications in lipid unsaturation account for only a small fraction of the acquired freezing tolerance, while interaction with other protective proteins (dhn2, cpn60 beta) is necessary to fulfil a higher level of freezing tolerance.