Are silver nanoparticles the “silver bullet” to promote diterpene production in Stevia rebaudiana ?

S. rebaudiana is a sought after sweetener because of its low-calorie properties. However, the supply of suitable quantities of high quality propagation material is limited by inefficient propagation methods using conventional strategies. In vitro techniques combined with nanotechnology tools offer an attractive alternative not only for improved propagation but also for the stimulation of secondary metabolites which represent the targeted sweetener product for this crop. This report provides an evaluation of silver nanoparticles applied in temporary immersion bioreactors for the abovementioned purpose. Different levels of AgNPs were supplied (0.0—37.5 mg/L) and after 21 d of growth, morphological and biochemical indicators were evaluated. Silver nanoparticles at 25 and 37.5 mg/L decreased shoot multiplication rate, shoot length, and the number of nodes and leaves per shoot compared with the control while no adverse effect was found at the lower tested concentration (12.5 mg/L). Shoot fresh and dry weights also showed statistically significant differences. Regarding the biochemical phenotypes, chlorophyll a, carotenoids and soluble phenolics were increased in plants supplied with 25 mg/L AgNPs, with the latter two indicators suggesting oxidative stress. Interestingly, endogenous levels of diterpenes were significantly increased with the application of 12.5 mg/L AgNPs. It is suggested that AgNPs show potential to act as elicitors to promote the production of diterpenes in stevia but that further work is required to understand the balance between oxidative damage and secondary metabolite production and that optimization of the protocol is required to improve the propagation potential of this strategy.