No Induction of beta-oxidation in leaves of Arabidopsis that over-produce lauric acid

Leaves from transgenic Brassica napus L. plants engineered to produce lauri c acid show increased levels of enzyme activities of the pathways associate d with fatty acid catabolism (V.A. Eccleston and J.B. Ohlrogge, 1998, Plant Cell 10: 613-621). In order to determine if the increases in enzyme activi ty are mirrored by increases in the expression of genes encoding enzymes of beta-oxidation, which is the major pathway of fatty acid catabolism in pla nts, the medium-chain acyl-acyl carrier protein (ACP) thioesterase MCTE fro m California bay (Umbellularia californica) was overexpressed under the con trol of the cauliflower mosaic virus 35S promoter in Arabidopsis thaliana ( L.) Heynh. Arabidopsis was the most suitable choice for these studies since gene expression could be analyzed in a large number of independent MCTE-ex pressing lines using already well-characterized beta-oxidation genes. Level s of MCTE transcripts in leaves varied widely over the population of plants analyzed. Furthermore, active MCTE was produced as determined by enzymatic analysis of leaf extracts of MCTE-expressing plants. These plants incorpor ated laurate into triacylglycerol of seeds, but not into lipids of leaves a s shown by gas-chromatographic analysis of total fatty acid extracts. The e xpression levels of the beta-oxidation and other genes that are highly expr essed during developmental stages involving rapid fatty acid degradation we re measured. No significant difference in gene expression was observed amon g MCTE-expressing plants and transgenic and non-transgenic controls. To eli minate the possibility that post-translational mechanisms are responsible f or the observed increases in enzyme activity acyl-CoA oxidase activity was also measured in leaves of MCTE-expressing plants using medium and long cha in acyl-CoA substrates. No significant increases in either medium- or long- chain acyl-CoA oxidase activities were detected. We conclude that endogenou s beta-oxidation is sufficient to account for the complete degradation of l aurate produced in rosette leaves of Arabidopsis expressing MCTE.