Modulation of callus growth and secondary metabolites in different Thymus species and Zataria multiflora micropropagated under ZnO nanoparticles stress

Thymus species are aromatic plants with diverse applications in food industries and medicine. This study was conducted to evaluate the potential effect of ZnO nanoparticles (NPs) on callus proliferation and thymol and carvacrol production in three Thymus species, that is, T. vulgaris, T. daenensis, and T. kotschyanus, and Zataria multiflora. For this purpose, callus induction was performed on Murashige and Skoog (MS) medium containing different plant growth regulators (PGRs). After optimization of callus growth, the effects of different concentrations of ZnO NPs (100 and 150 mg L-1 ) were investigated. MS containing 2 mg L-1 of 2, 4-dichlorophenoxy acetic acid (2,4-D) and 1 mg L-1 of kinetin (Kin) revealed significantly highest fresh weight (0.18 g) of callus in T. kotschyanus. Callus growth rate (0.079 mm day-1 ) was found highest in T. vulgaris under similar conditions. Moreover, highest callus induction (92.50%) was achieved by T. kotschyanus in MS containing 2.5 mg L-1 of 2,4-D. Regarding the highest content of thymol (22.8 mg L-1 ) and carvacrol (0.68 mg L-1 ) evaluated by high-performance liquid chromatography, best results were achieved under 150 mg L-1 of ZnO NPs in T. kotschyanus and T. daenesis, respectively. This is simple and cost-effective method to be applied on industrial level for production of enhanced secondary metabolites content.