Light-dependent Induction of Polyunsaturated Fatty Acid Biosynthesis in Greening Cucumber Cotyledons

Abstract

Greening cucumber (Cucumis sativus L.) cotyledons exhibited dramatic increases in the ability to desaturate exogenously added [1-¹⁴C]oleic acid and [1-¹⁴C]linoleic acid within 2 to 3 hours of illumination. These increases were effectively inhibited by 10 micrograms per milliliter cycloheximide. Oleate desaturation remained at a high level in constant light for 5 to 6 days after induction and then declined by about 50%; when returned to the dark, the tissue showed a sharp decrease in conversion of [¹⁴C]oleate to [¹⁴C]linoleate. Linoleate desaturation reached a maximum about 15 hours after induction and declined immediately thereafter while the tissue still was in the light; after induction had peaked return of the tissue to the dark showed a dramatic fall of linoleate desaturation. The changes in desaturation were correlated with the conversion of the principal fatty acid in the etiolated cotyledons, linoleate, to α-linolenate, and with the assembly of the chlorophyll-containing photosynthetic membranes. The incorporation of [1-¹⁴C]acetate into lipids showed no significant light stimulation. The role of light in the regulation of certain aspects of plant metabolism during development is discussed.