The ability of organisms and organic compounds to reduce metal ions and stabilize them into nanoparticles (NPs) forms the basis of green synthesis. To date, synthesis of NPs from various metal ions using a diverse array of plant extracts has been reported. However, a clear understanding of the mechanism of green synthesis of NPs is lacking. Although most studies have neglected to analyze the green-synthesized NPs (GNPs) for the presence of compounds derived from the extract, several studies have demonstrated the conjugation of sugars, secondary metabolites, and proteins in these biogenic NPs. Despite several reports on the bioactivities (antimicrobial, antioxidant, cytotoxic, catalytic, etc.) of GNPs, only a handful of studies have compared these activities with their chemically synthesized counterparts. These comparisons have demonstrated that GNPs possess better bioactivities than NPs synthesized by other methods, which might be attributed to the presence of plant-derived compounds in these NPs. The ability of NPs to bind with organic compounds to form a stable complex has huge potential in the harvesting of precious molecules and for drug discovery, if harnessed meticulously. A thorough understanding of the mechanisms of green synthesis and high-throughput screening of stabilizing/capping agents on the physico-chemical properties of GNPs is warranted to realize the full potential of green nanotechnology.

Nanoparticles (NPs) are submicron particles with 1- 100 nm lengths in two or three dimensions. NPs released as nanomaterial-containing wastes from household, industrial and medical products are emerging as a new threat to the environment. Plants, being fixed to the two major environmental sinks where NPs accumulate ¬— namely water and soil, cannot escape the impact of nanopollution. Recent studies have shown that plant growth, development and physiology are significantly affected by NPs. But, the effect of NPs on plant secondary metabolism is still obscure. The induction of reactive oxygen species (ROS) following interactions with NPs has been observed consistently across plant species. Taking into account the existing link between ROS and secondary signaling messengers that lead to transcriptional regulation of secondary metabolism, in this perspective we put forward the argument that ROS induced in plants upon their interaction with NPs will likely interfere with plant secondary metabolism. As plant secondary metabolites play vital roles in plant performance, communication and adaptation, a comprehensive understanding of plant secondary metabolism in response to NPs is an utmost priority.

Plant extract is a complex concoction of several phytochemicals, for instance, phenolics, sugars, flavonoids, xanthones, and several others. In general, it is said that hydroxyl-rich phenolics act as reducing agents for metal ions, but little is discussed about the stabilizing ligands of metal nanoparticles (NPs). Thus, despite the popularity of plant extract-mediated synthesis of NPs, the phytochemical basis of the process and the exact mechanism are still unclear. Herein, a systematic study was carried out to unveil the mechanism of St. John’s wort (Hypericum perforatum (H. perforatum) L.) extract-mediated synthesis of silver (Ag) NPs. Among the phytochemicals present in the extract, phenolic acids and flavonoids are shown to be involved in the reduction of Ag⁺ ions, while xanthones and phloroglucinols act as capping agents, and naphthodianthrones were involved in both steps. Analysis of the postreaction residues by ultra-performance liquid chromatography coupled with a photodiode array and high-resolution mass spectrometry showed a sharp decrease in the concentration of various secondary metabolites present in the extract, of which large amounts of xanthones and phloroglucinols were recovered from the NPs. Consequently, the low polar fraction of H. perforatum extract, rich in xanthones and phloroglucinols, and the pure compounds of these classes (mangiferin and hyperforin) did not reduce Ag⁺ ions. In contrast, the polar fraction of the extract, rich in flavonoids and naphthodianthrones, and the pure compounds of these classes (kaempferol-3-glucoside, quercetin, quercetin-3-glucoside, hypericin, pseudohypericin, and protohypericin) were able to reduce Ag⁺ ions. Overall, the present study contributes significantly to the understanding of green synthesis of NPs by plant extracts.

Objectives: In this review, we aim at updating the available information on the improvement of the Hypericum perforatum L. (Hypericaceae) phytochemical profile and pharmacological properties via elicitation. Key findings: Hypericum perforatum seedlings, shoots, roots, calli and cell suspension cultures were treated with diverse elicitors to induce the formation of secondary metabolites. The extracts of the elicitor-treated plant material containing naphthodianthrones, phloroglucinols, xanthones, flavonoids and other new compounds were quantitatively analysed and tested for their bioactivities. While hypericins were mainly produced in H. perforatum cultures containing dark nodules, namely shoots and seedlings, other classes of compounds such as xanthones, phloroglucinols and flavonoids were formed in all types of cultures. The extracts obtained from elicitor-treated samples generally possessed better bioactivities compared to the extract of control biomass. Summary: Although elicitation is an excellent tool for the production of valuable secondary metabolites in H. perforatum cell and tissue cultures, its exploitation is still in its infancy mainly due to the lack of reproducibility and difficulties in scaling up biomass production.

This review details the potential of BDFMs as promising carriers for enzymes for the design of sustainable and robust biocatalysts with improved activity, stability, and reusability.

Silver nanoparticles are widely used in industry, medicine, biotechnology and agriculture. As a consequence, these nanoparticles are reaching the environment as waste products, which might have a negative impact on the environment, especially on plants. This includes the elicitation of various biochemical processes in plants. In this article, we report on the changes in secondary metabolic profile of Arabidopsis thaliana seedlings subjected to silver nanoparticle treatment in vitro. Briefly, various sizes (10 nm, 40 nm and 100 nm in diameter) and concentrations (0.5-5.0 ppm) of silver nanoparticles were tested. Ultraperformance liquid chromatography coupled with ultraviolet and fluorescence detectors as well as hyphenated to a high-resolution mass spectrometer (UPLC-PDA-FLR, UPLC-HESI-HRMS) and HPLC - ion trap mass spectrometer (HPLC-ESI-MS/MS), were applied to identify and quantify secondary metabolites. To understand whether silver ions could induce changes in the secondary metabolite profile, seedlings treated with silver nitrate in concentrations equivalent to these of nanoparticles were also analysed. The results showed significant differences in the accumulation of phenolic and indole compounds between treatments. Silver nanoparticles and silver ions induced the biosynthesis of camalexin, hydroxycamalexin O-hexoside and hydroxycamalexin malonyl-hexoside. These compounds are important phytoalexins for Brassicaceae family (especially for Camelinae clad) and are also synthetized in response to biotic and abiotic stresses. Statistically significant changes have been also observed for five phenolic compounds and 5'Glucosyl-dihydroneoascorbigen in different treatment conditions.