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.