Metal oxide nanoparticles and plant secondary metabolism: unraveling the game-changer nano-elicitors

Secondary metabolites (SMs) are biologically active compounds that have a very high commercial value due to their invaluable medicinal properties. However, the production level in in-vivo and in-vitro cultures remains low to meet the commercial demands. To overcome this problem, nano-elicitation has emerged as a novel and effective strategy which helps in enhancing the level of SMs production. The principle behind elicitation is that when external stress such as nanoparticles (NPs) is applied to the plant, the accumulation of SMs enhances in plant tissue cultures as a natural defense mechanism. In recent years, research on nano-elicitors using different types of NPs, especially metal oxide NPs (MONPs) has intensified due to enormous increase in medicinally important SMs. MONPs have gained special attention due to their unique physiochemical characteristics. In this review, we have explored the different routes of exposure of MONPs in plants as well as their role as novel elicitors of important classes of SMs (phenols, flavonoids, alkaloids, and terpenes). Moreover, the underlying mechanism of nano-elicitation and their uptake and translocation in plants have also been discussed. The review has been concluded with the knowledge gaps and future research direction, which if addressed will be helpful in solving the challenges of modern times. Graphical abstract