OLEATE DESATURATION AND ACYL TURNOVER IN SUNFLOWER (HELIANTHUS-ANNUUSL.) SEED LIPIDS DURING RAPID TEMPERATURE ADAPTATION

In-vivo experiments with developing sunflower (Helianthus annuus L.) s eeds demonstrated that oleate desaturase activity was stimulated by lo w temperature (10 degrees C), repressed by high temperature (30 degree s C) and rapidly restored by returning the seeds to low temperature. W ithin time periods of 2-4 h, in which the denovo fatty acid synthesis was negligible, the percentages of oleate (18:1) and linoleate (18:2) were modified in the seed lipids as a consequence of temperature adapt ation. When the seeds were transferred to low temperature, the 18:2 co ntent increased in all lipids from both microsomal membranes and oil b odies. After shifting to high temperature, the overall 18:2 content re mained constant, but the 18:2 content decreased in diacylglycerols, ph osphatidylcholine (PC) and other polar lipids of the two fractions and also in triacylglycerols (TAGs) of the microsomes but increased in TA Gs of the oil bodies. The results indicate that the mechanism for the rapid adaptation of sunflower seeds to temperature changes involves (i ) the synthesis or activation of oleate desaturase at low temperature and the reversible inhibition of this enzyme at high temperature and ( ii) the exchange of 18:1 and 18:2 between TAGs and PC. Under both low and high temperature, 18:1 is transferred from reserve TAGs to PC and 18:2 is transferred from PC to reserve TAGs. At low temperature, 18:1 is desaturated to 18:2 thus allowing the enrichment of membrane lipids with 18:2, the excess being stored in reserve TAGs. At high temperatu re, however, and provided that oleate desaturase is repressed, the mem brane lipids become enriched in 18:1 and the oil-body TAGs become enri ched in 18:2.