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The kinetics of incorporation in vivo of (14C)acetate and (14C)carbon dioxide into the fatty acids of glycerolipids in developing leaves.

ABSTRACT: 1. The patterns of incorporation of (14)C into glycerolipid fatty acids of developing maize leaf lamina from supplied [1-(14)C]acetate and from (14)CO(2) during steady-state photosynthesis were similar. Oleate of phosphatidylcholine and palmitate of phosphatidylglycerol attained linear rates of labelling more rapidly than did other fatty acids, particularly the linoleate and linolenate of monogalactosyl diacylglycerol. 2. After the transfer of lamina from labelled to unlabelled acetate, there was a decrease in labelled oleate and linoleate of phosphatidylcholine and a concomitant increase in the amount of radioactivity in the linoleate and linolenate of monogalactosyl diacylglycerol. 3. The rapidly labelled phospholipids, phosphatidylcholine and phosphatidylglycerol, were shown by differential and sucrose-density-gradient centrifugation to be associated with different organelles, the former being mainly in a low-density membrane fraction, probably microsomal, and the latter mainly in chloroplasts. 4. During a 48h period after supplying spinach leaves with [(14)C]acetate, radioactivity was lost from the oleate of phosphatidylcholine present in fractions sedimented at 12000g and 105000g, and accumulated in the linolenate of monogalactosyl diacylglycerol of the chloroplast. 5. It is proposed that the phosphatidylcholine of some non-plastid membranes is intimately involved in the process of oleate desaturation and that this lipid serves as a donor of unsaturated C(18) fatty acids to other lipids, principally monogalactosyl diacylglycerol, of the chloroplasts.

SUBMITTER: Slack CR

PROVIDER: S-EPMC1172463 | biostudies-other | 1975 Nov

REPOSITORIES: biostudies-other

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Project description:1. 3-sn-Phosphatidylcholine was identified as the major lipid in cotyledons from the developing seeds of soya bean, linseed and safflower when tissue was steamed before lipid extraction. The proportion of oleate in this lipid decreased markedly and that of the polyunsaturated C(18) fatty acids increased when detached developing cotyledons were incubated for up to 3h. Similar but less pronounced changes occurred in diacylglycerol, which had a fatty acid composition resembling that of the 3-sn-phosphatidylcholine from cotyledons of the same species. 2. [1-(14)C]Acetate supplied to detached cotyledons was incorporated into the acyl moieties of mainly 3-sn-phosphatidylcholine, 1,2-diacylglycerol and triacylglycerol. Initially label was predominantly in oleate, but subsequently entered at accelerating rates the linoleoyl moieties of the above lipids in soya-bean and safflower cotyledons and the linoleoyl and linolenyl moieties of these lipids in linseed cotyledons. In pulse-chase experiments label was rapidly lost from the oleate of 3-sn-phosphatidylcholine and accumulated in the linoleoyl and linolenoyl moieties of this phospholipid and of the di- and tri-acylglycerols. 3. [2-(3)H]Glycerol was incorporated into the glycerol moieties of mainly 3-sn-phosphatidylcholine and di- and tri-acylglycerols of developing linseed and soya-bean cotyledons. The label entered the phospholipid and diacylglycerol at rates essentially linear with time from the moment the substrate was supplied, and entered the triacylglycerol at an accelerating rate. With linseed cotyledons the labelled glycerol was incorporated initially mainly into species of 3-sn-phosphatidylcholine and diacylglycerol that contained oleate, but accumulated with time in more highly unsaturated species. In pulse-chase experiments with linseed cotyledons, label was lost from both 3-sn-phosphatidylcholine and diacylglycerol, preferentially from the dioleoyl species, and accumulated in triacylglycerol, mainly in species containing two molecules of linolenate. 4. The results suggest a rapid turnover of 3-sn-phosphatidylcholine during triacylglycerol accumulation in developing oilseeds, and are consistent with the operation of a biosynthetic route whereby oleate initially esterified to the phospholipid is first desaturated, then polyunsaturated fatty acids transferred to triacylglycerol, via diacylglycerol. The possible role of oleoyl phosphatidylcholine as a substrate for oleate desaturation is discussed.

Project description:Studies have indicated that the concentration of carbon dioxide [CO2] during the dark period may influence plant dry matter accumulation. It is often suggested that these effects on growth result from effects of [CO2] on rates of respiration, but responses of respiration to [CO2] remain controversial, and connections between changes in respiration rate and altered growth rate have not always been clear. The present experiments tested whether translocation, a major consumer of energy from respiration in exporting leaves, was sensitive to [CO2]. Nineteen-day-old soybean plants grown initially at a constant [CO2] of 350 micromol mol(-1) were exposed to three consecutive nights with a [CO2] of 220-1400 micromol mol(-1), with a daytime [CO2] of 350 micromol mol(-1). Change in dry mass of the individual second, third and fourth trifoliate leaves over the 3-d period was determined, along with rates of respiration and photosynthesis of second leaves, measured by net CO2 exchange. Translocation was determined from mass balance for second leaves. Additional experiments were conducted where the [CO2] around individual leaves was controlled separately from that of the rest of the plant. Results indicated that low [CO2] at night increased both respiration and translocation and elevated [CO2] decreased both processes, to similar relative extents. The effect of [CO2] during the dark on the change in leaf mass over 3 d was largest in second leaves, where the change in mass was about 50% greater at 1400 micromol mol(-1) CO2 than at 220 micromol mol(-1) CO2. The response of translocation to [CO2] was localized in individual leaves. Results indicated that effects of [CO2] on net carbon dioxide exchange rate in the dark either caused or reflected a change in a physiologically important process which is known to depend on energy supplied by respiration. Thus, it is unlikely that the observed effects of [CO2] on respiration were artefacts of the measurement process in this case.

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The kinetics of incorporation in vivo of (14C)acetate and (14C)carbon dioxide into the fatty acids of glycerolipids in developing leaves.

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