Overexpression of 3-ketoacyl-acyl-carrier protein synthase IIIs in plants reduces the rate of lipid synthesis

A cDNA coding for 3-ketoacyl-acyl-carrier protein (ACP) synthase III (KAS I II) from spinach (Spinacia oleracea; So KAS III) was used to isolate two cl osely related KAS III clones (Ch KAS III-1 and Ch KAS III-2) from Cuphea ho okeriana. Both Ch KAS IIIs are expressed constitutively in all tissues exam ined. An increase in the levels of 16:0 was observed in tobacco (Nicotiana tabacum, WT-SR) leaves overexpressing So KAS III when under the control of the cauliflower mosaic virus-35S promoter and in Arabidopsis and rapeseed ( Brassica napus) seeds overexpressing either of the Ch KAS Ills driven by na pin. These data indicate that this enzyme has a universal role in fatty aci d biosynthesis, irrespective of the plant species from which it is derived or the tissue in which it is expressed. The transgenic rapeseed seeds also contained lower levels of oil as compared with the wild-type levels. In add ition, the rate of lipid synthesis in transgenic rapeseed seeds was notably slower than that of the wild-type seeds. The results of the measurements o f the levels of the acyl-ACP intermediates as well as any changes in levels of other fatty acid synthase enzymes suggest that malonyl-ACP, the carbon donor utilized by all the 3- ketoacyl-ACP synthases, is Limiting in the tra nsgenic plants. This further suggests that malonyl-coenzyme A is a potentia l limiting factor impacting the final oil content as well as further extens ion of 16:0.