adapted and un-adapted cell lines which were classified as saturated and unsaturated fatty acids as shown in the Table S1.
Among these 10 were saturated fatty acids, all these were not detected in both lines. The common saturated fatty acids among both cell lines were Myristic acid (C14:0), Palmitic acid (C16:0), Stearic acid (C18:0) while the common unsaturated fatty acid found were Oleic acid (C18:1c) and Linoleic acid (C18:2c).
Palmitic acid was the most abundant fatty acid detected in all cell lines, followed by Oleic acid, linoleic acid, stearic acid and myristic (Table 1).
3.4. Effect of Temperature Stress on Fatty Acid Profile
In our experiments, the un-adapted line showed higher level of saturated fatty acid as compared to the adapted cell line under optimal conditions. Under cold stress a decrease was observed in both lines while under heat stress the percent level was higher in adapted cell lines than the un-adapted cell line (Figure 3).
Furthermore, our data reveals that the level of fatty acid unsaturation increased at low temperature while at high temperature the level decreased as shown in Figure 4.
It is reported that plasma membrane having a higher proportion of unsaturated
Table 1. Range of most prominent fatty acid detected in control and PEG cell lines under temperature stress using gas chromatography-mass spectrometry (GC-MS).
Figure 3. Total saturated fatty acid profile in rice lines under temperature stress.
Figure 4. Profile of total unsaturated fatty acids in rice cell lines under temperature stress.
fatty acids is usually tolerant to low temperature stress while a higher proportion of saturated fatty acids renders tolerance to higher temperature stress   . Furthermore, the degree of lipid unsaturation is related to cold tolerance due to its effect on PM stability  .
Plant basic physiological response to drought stress overlaps temperature, as well as salt stress; that ultimately leads to dehydration in growing plants  . Thermal adaptation is related to biochemical and physiological responses of the plant which are mainly caused by changes in lipidic fluidity of membranes  . Plants respond to different abiotic stresses (drought, heat and salinity) by the changes in the degree of un-saturation of fatty acids  .
It is interesting to note that PEG adapted cell lines have shown differential response to low and high temperature. Under low temperature it accumulated a high concentration of unsaturated fatty acids while under high temperature it had a high percentage of saturated fatty acids as compared to the un-adapted cell line. Similar results have been reported by  and  that increase in temperature increase the level of saturated fatty acids in membrane lipids and enhance the heat stability of cell membrane. While with decrease in temperature the level of saturated fatty acid decreased. This suggests that the cold temperature activates fatty acid desaturation and the saturated fatty acids were converted to unsaturated fatty acids to stabilize PM under cold environment  . An increase in total saturated and unsaturated fatty acids under high and low temperatures respectively, have obviously affected the fluidity of the plasma membrane  .
Our research findings suggest that adaptation to osmotic stress may result in adjustments due to increased sensitivity or due to enhanced signaling system of the cell that makes cells capable to respond to changing environment effectively than the cells of un-adapted cell lines. Based on the results of the present study it is recommended that a large number of plants need to be regenerated for studying this cross adaptation mechanism at the whole plant level.
Authors are highly thankful to the research supervisor Dr. Safdar Hussain Shah (Director, Institute of Biotechnology and Genetic Engineering) for his guidance and kind support. We are very thankful to Dr. Ali Anwar for critical reading and comments on the manuscript.
Table S1. Most prominent fatty acids detected in PEG adapted and un-adapted cell lines of rice O. sativa L.cv. Swat-1.
“S” denotes saturated and “U” denotes unsaturated fatty acids.
#These authors contributed equally to the paper as first authors.
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