The effects of methyl jasmonate (MJ) dosage on terpenoid indole alkaloid (TIA) biosynthesis in Catharanthus roseus are correlated with the relative levels of specific MJ-responsive transcription factors. In this study, the expression of transcription factors (Orca, Zct, Gbf, Myc2, At-hook, and Wrky1), TIA pathway genes (G10h, Tdc, Str, and Sgd), and TIA metabolites (secologanin, strictosidine, and tabersonine) were investigated in C. roseus hairy root cultures elicited with a range of MJ dosages (0-1,000 mu M) during mid-exponential growth. The highest production of TIA metabolites occurs at 250 M MJ, increasing by 150-370% compared with untreated controls. At this MJ dosage, the expression of the transcriptional activators (Orca) is dramatically increased (29-40 fold) while the levels of the transcriptional repressors (Zct) remain low (2-7 fold). Simultaneously, the expression of genes coding for key enzymes involved in TIA biosynthesis increases by 8-15 fold. In contrast, high MJ dosages (1,000 mu M) inhibit the production of TIA metabolites. This dosage is correlated with elevated expression levels of Zct (up to 40-fold) relative to Orca (13-19-fold) and minimal induction of the TIA biosynthetic genes (0-6 fold). The significant changes in the expression of Orca and Zct with MJ dosage do not correspond to changes in the expression of the early-response transcription factors (AT-hook, Myc2, and Wrky1) believed to regulate Orca and Zct. In summary, these observations suggest that the dependence of alkaloid production on MJ dosage in C. roseus may be partly mediated through the relative levels of Orca and Zct family transcription factors. (c) 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1367-1376, 2013