Oxygen-independent temperature regulation of the microsomal oleate desaturase (FAD2) activity in developing sunflower (Helianthus annuus) seeds

The temperature and oxygen regulation of the microsomal oleate desaturase (FAD2, EC 1.3.1.35) activity has been studied in developing sunflower (Helianthus annuus L.) seeds. In plants cultivated in growth chambers, the linoleic acid content in the seed lipids increased along the 25/15degreesC (day/night) cycle, except during the first hours of the warm period, where it decreased significantly. In contrast, FAD2 activity decreased notably at the beginning of the warm period, showing a small and continuous increase during the rest of the cycle. The temperature effect on the linoleic acid content and the FAD2 activity was also investigated using peeled seeds and detached achenes subjected to temperature changes. In peeled seeds, a change of temperature from 10 to 30degreesC brought about a significant decrease of FAD2 activity. On the contrary, when the temperature shifted from 30 to 10degreesC, FAD2 activity only increased slightly. Unlike peeled seeds, detached achenes showed a fast and dramatic increase or decrease in the level of FAD2 activity in response to a temperature change from 30 to 10degreesC, or from 10 to 30degreesC, respectively. The in vivo and in vitro thermal properties of the FAD2 enzyme were also studied. Optimal temperature and heat-resistance profile showed similar patterns in both conditions. All these data support the hypothesis that temperature regulates FAD2 activity by two different and independent mechanisms: a direct effect, and an indirect effect affecting oxygen availability. Furthermore, these results suggest that the low thermal stability of the enzyme is the main factor responsible for the direct temperature effect on FAD2 activity.