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 duri ng reproductive development under simulated shadelight: i.e., light wi th reduced blue and/or increased far-red (Britz and Cavins 1993). Elev ation 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 aff ects the activity of the omega-6 and/or omega-3 desaturases involved i n TAG biosynthesis. Membranes were isolated from developing soybean co tyledons 24-31 days after flowering. Separate fractions, enriched for chloroplasts and endoplasmic reticulum (ER) respectively, were obtaine d by sucrose gradient centrifugation and incubated in an in vitro desa turase assay system containing C-14-18:1-CoA at room temperature. Omeg a-6 and omega-3 desaturase activity was calculated from the rate of fo rmation of C-14-18:2 and C-14-18:3. Radioactive 18:2 and 18:3 were rec overed only from phosphatidylcholine (PC) in both ER and chloroplast m embranes, 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 canop y shade response, but was reduced in membranes from seeds matured unde r lamps (high blue and low far-red emission) previously shown to reduc e the level of 18:2 in seed oil by 50%. Chloroplast membranes possesse d both omega-6 and omega-3 desaturases. Light appeared to have little or no effect on the activity of chloroplast desaturases.