Showing papers by "John L. Harwood published in 1991"

The regulation of triacylglycerol biosynthesis in cocoa (Theobroma cacao) L.

Abstract: Developing cocoa cotyledons accumulate initially an unsaturated oil which is particularly rich in oleate and linoleate. However, as maturation proceeds, the characteristic high stearate levels appear in the storage triacylglycerols. In the early stages of maturation, tissue slices of developing cotyledons (105 days post anthesis, dpa) readily accumulate radioactivity from [14C]acetate into the diacylglycerols and label predominantly palmitate and oleate. In older tissues (130 dpa), by contrast, the triacylglycerols are extensively labelled and, at the same time, there is an increase in the percentage labelling of stearate. Thus, the synthesis of triacylglycerol and the production of stearate are co-ordinated during development. The relative labelling of the phospholipids (particularly phosphatidylcholine) was rather low at both stages of development which contrasts with oil seeds that accumulate a polyunsaturated oil (e.g. safflower). Microsomal membrane preparations from the developing cotyledons readily utilised an equimolar [14C]acyl-CoA substrate (consisting of palmitate, stearate and oleate) and glycerol 3-phosphate to form phosphatidate, diacylglycerol and triacylglycerol. Analysis of the [14C]acyl constituents at the sn-1 and sn-2 positions of phosphatidate and diacylglycerol revealed that the first acylase enzyme (glycerol 3-phosphate acyltransferase) selectively utilised palmitate over stearate and excluded oleate, whereas the second acylase (lysophosphatidate acyltransferase) was highly selective for the unsaturated acyl-CoA. On the other hand, the third acylase (diacylglycerol acyltransferase) exhibited an almost equal selectivity for palmitate and stearate. Thus, stearate is preferentially enriched at position sn-3 of triacylglycerol at 120–130 dpa because of the relatively higher selectivity of the diacylglycerol acyltransferase for this fatty acid compared with those of the other two acylation enzymes.

Temperature‐induced Changes in the Synthesis of Unsaturated Fatty Acids by Acanthamoeba castellanii

Abstract: . Major fatty acid components of Acanthamoeba castellanii lipids extracted after growth at 30°C include myristate, palmitate, stearate and the polyunsaturates linoleate, eicosadienoate, eicosatrienoate and arachidonate, with oleate as the sole major monounsaturated fatty acid. By comparison, growth at 15°C gave increased linoleate, eicosatrienoate and arachidonate, but decreased oleate and palmitate. When the growth temperature was shifted downwards from 30°C to 15°C, increased lipid unsaturation occurred over a period of 24 h; thus decreases of oleate and eicosadienoate were accompanied by increases in linoleate, eicosatrienoate, arachidonate and eicosapentaenoate. An upwards shift from 15°C to 30°C gave negligible alterations in fatty acid composition over a similar period. At 15°C organisms rapidly use [1-14C] acetate for de novo fatty acid synthesis; stearate is converted via oleate to further desaturation and chain elongation products. Similar short term experiments at 30°C indicate only de novo synthesis and Δ9-desaturation; synthesis of polyunsaturates was a much slower process. Rapid incorporation of [1-14C] oleate at 30°C was not accompanied by metabolic conversion over two hours, whereas at 15°C n-6 desaturation to linoleate was observed. Temperature shift of organisms from 15°C to 30°C in the presence of [1-14C] acetate revealed that over half of the fatty acids in newly-synthesised lipids were saturated, but the proportions of unsaturated fatty acids increased with time until the total polyenoate components reached 17% after 22 h. A shift of temperature in the reverse direction gave a corresponding figure of 60% for polyunsaturated fatty acids. These results emphasize the importance of n-6 desaturation in the low temperature adaptation of Acanthamoeba castellanii.

Changes in Lipid Composition during Callus Differentiation in Cultures of Oilseed Rape (Brassica napus L.)

Abstract: The lipid composition of different callus cultures of Brassica napus varied according to their state of differentiation. Photomixotrophic callus was characterized by the ability to synthesize relatively high levels of triacylglycerol (TAG) which was rich in oleate. Glycosyldiacylglycerols were also detected. In contrast, heterotrophic callus was found to possess high proportions of membraneous phospholipids which were rich in palmitate, linoleate, and linolenate. Moreover, the lipid content was considerably less than that of photomixotrophic callus. Caulogenesis was achieved in both types of callus strains and the lipid composition of the regenerated leaves contained a much higher proportion of chloroplast glycosyldiacylglycerols and thus resembled more those of the parent plant. Some calli entered a senescent phase whereby there was considerable degradation of the constituent membrane lipids. Senescent callus also exhibited a high proportion of polyploid nuclei. In this study we have been able to cause large changes in the morphology of callus cultures. These morphological changes were accompanied by significant alterations in the quality and quantity of acyl lipids. In photomixotrophic cells the lipid changes resembled those seen for developing seed tissues where high rates of TAG deposition are accompanied by an altered fatty acid pattern. Thus, the selection of different callus types should be of use for investigations of the regulation of lipid biosynthesis under controlled culture conditions.

Effect of a Safener Towards Thiocarbamates on Plant Lipid Metabolism

Abstract: Abstract It has been found that various thiocarbamate herbicides, known to alter surface wax and cutin synthesis, inhibit the elongation of fatty acids. We have proposed this as a mode of action of such compounds. Because it is believed that the sulphoxide metabolites of thiocarbamates are the active intermediates, we have examined the action of 1-aminobenzotriazole (an inhibitor of sulphoxide formation) on the inhibition of very long-chain fatty acid biosynthesis. In all tissues tested, aminobenzotriazole was able to block the specific inhibitory effect of thiocarbamates on fatty acid elongation. These results add further support to our proposal that fatty acid elongation is a sensitive target site for thiocarbamate herbicides in plants.