Comparative Elicitation Effects of Nickel and Zinc Oxide Nanoparticles on Indole Alkaloid Biosynthesis in in vitro Cultures of Isatis constricta

This study examined the concentration and time-dependent effects of nickel oxide (NiONPs) and zinc oxide (ZnONPs) nanoparticles (0, 10, 25, 50, 75, and 100 mg/L) on the accumulation of indole alkaloids such as indirubin, tryptanthrin, and indigo in Isatis constricta leaf explants cultured in vitro. Exposure to 50 mg/L NiONPs significantly increased the production of tryptanthrin and indirubin during the second week, with roughly 1.6- and 2.3-fold increases compared to the first week. Conversely, indigo levels decreased at all NiONP concentrations, with the greatest reduction observed during prolonged exposure. Similarly, ZnONPs caused notable, concentration-dependent changes in alkaloid production. The highest accumulation of tryptanthrin (2.3-fold) and indirubin (3.4-fold) appeared at 50 mg/L ZnONPs in the second week, while indigo content dropped sharply, especially at 10 mg/L, showing a 1.8-fold decrease compared to the control. Multivariate correlation and chemometric analyses indicated strong links between nanoparticle concentration, exposure time, and metabolite profiles. Overall, these results demonstrate that NiONPs and ZnONPs act as effective abiotic elicitors, influencing indole alkaloid biosynthesis in I. constricta , with ZnONPs exhibiting a stronger elicitation effect at optimal doses.