ABSTRACT
Secondary metabolites are generally produced by multigene pathways despite their low molecular weight and apparent structural simplicity. At present genetic engineering of plants is limited to the introduction and expression of single genes, although recovery of transformants does depend on introduction of at least a second gene providing a scorable marker or a selectable trait. Some vectors are capable of transferring as many as four genes, but in no case have even two genes of a pathway been transferred and coordinately expressed as yet. Research directed toward the introduction of coordinately expressed genes is in progress. Plants expressing single gene products that have sufficient economic value for commercial development have been confined to antisense mRNA or to protein products such as Bt toxin. There are many secondary metabolites of low molecular weight that have very high economic value in medicine, and others of proven efficacy in crop protection that will eventually be economically attractive targets for genetic engineering once coordinate expression of multigene pathways becomes feasible.
