Spectral quality during pod development affects omega-6 desaturase activity in soybean seed endoplasmic reticulum

Polyunsaturated fatty acids (i.e. linoleic acid 18:2], linolenic acid 18:3]) in triacylglycerols (TAG) of soybean seeds increase more during reproductive development under simulated shadelight: i.e., light with reduced blue and/or increased far-red (Britz and Cavins 1993). Elevation of 18:2 and 18:3 is matched by corresponding reduction of oleic acid (18:1), consistent with observations that total seed oil remains constant. We therefore tested the hypothesis that spectral quality affects the activity of the omega-6 and/or omega-3 desaturases involved in TAG biosynthesis. Membranes were isolated from developing soybean cotyledons 24–31 days after flowering. Separate fractions, enriched for chloroplasts and endoplasmic reticulum (ER) respectively, were obtained by sucrose gradient centrifugation and incubated in an in vitro desaturase assay system containing ¹⁴C-18:1–CoA at room temperature. Omega-6 and omega-3 desaturase activity was calculated from the rate of formation of ¹⁴C-18:2 and ¹⁴C-18:3. Radioactive 18:2 and 18:3 were recovered only from phosphatidylcholine (PC) in both ER and chloroplast membranes, consistent with membrane-bound desaturases with specificity towards PC. The specific activity of omega-6 desaturase was high in ER membranes from seeds matured under light sources that promote a canopy shade response, but was reduced in membranes from seeds matured under lamps (high blue and low far-red emission) previously shown to reduce the level of 18:2 in seed oil by 50%. Chloroplast membranes possessed both omega-6 and omega-3 desaturases. Light appeared to have little or no effect on the activity of chloroplast desaturases.