Abietane diterpenoids, containing a quinone moiety, are synthesized in the roots of several Salvia species. Promising cytotoxicity and antiproliferative activities have been reported for these compounds in various cell and animal models. We have recently shown that aethiopinone, an o-naphto-quinone diterpene, produced in the roots of different Salvia species, is selectively cytotoxic against the A375 melanoma cell line. To enhance the synthesis of this abietane diterpenoid, we have engineered the plastidial 2-C-methyl-D-erythritol 4-phosphate-derived isoprenoid pathway in Salvia sclarea hairy roots by ectopic expression and plastid targeting of cyanobacterial genes encoding the 1-deoxy-D-xylulose 5-phosphate synthase or 1-deoxy-D-xylulose-5-phosphate reductoisomerase gene, the first two enzymatic steps of the plastidial MEP pathway, from which plant diterpenes primarily derive. Plastid-targeted expression of 1-deoxy-D-xylulose 5-phosphate synthase and 1-deoxy-D-xylulose-5-phosphate reductoisomerase proteins significantly enhanced the yield of aethiopinone by a 3-fold and about 6-fold increase, respectively. The accumulation of other abietane-type diterpenes (ferruginol, salvipisone, and carnosic acid), with interesting antiproliferative activity, was also increased. Compared to our previous data obtained by overexpressing the plant orthologous 1-deoxy-D-xylulose 5-phosphate synthase and 1-deoxy-D-xylulose-5-phosphate reductoisomerase genes in S. sclarea hairy roots, the results presented here confirm that the bacterial 1-deoxy-D-xylulose-5-phosphate reductoisomerase enzyme plays a major role than the DXS enzyme in the biosynthetic pathway of this class of compounds and that its ectopic expression does not conflict with active hairy root growth, resulting in a balanced trade-off between the transgenic hairy root final biomass and the increased content of o-naphto-quinone diterpenes, with interesting biological activities.