Metabolic engineering of the carotenoid pathway in recent years has successfully
enhanced the carotenoid contents of crop plants, typically through the improvement of biosynthetic capacity. However, it is now clear that only increasing biosynthesis is restrictive, as mechanisms to sequestrate these increased levels in the cell or organelle should be exploited. In the present study, biosynthetic pathway genes have been overexpressed in tomato lines and the effects on carotenoid formation and sequestration revealed. The combination of two bacterial genes, CrtB (phytoene synthase) and CrtI (phytoene desaturase) has a synergistic effect on the production of carotenoids. Transcription of
the pathway genes is perturbed, whereby the tissue specificity of transcripts is altered. Changes in the steady state levels of metabolites in unrelated sectors of metabolism were found. Of particular interest was a concurrent increase of the plastid localized lipid monogalactodiacylglycerol with carotenoids along with membranous subcellular structures. The carotenoids, proteins and lipids in the sub-chromoplast fractions of the transgenic tomato fruit with increased carotenoid content suggest that cellular structures can adapt to facilitate the sequestration of the newly formed products. Moreover, phytoene, the precursor of the pathway, was identified in the plastoglobule, whereas the biosynthetic enzymes were in the membranes. The implications of these findings with respect to new pathway regulation mechanisms and generic engineering strategies are discussed.