Plants emit an overabundance of volatile compounds, which act in their producers either as appreciated attractants to lure beneficial animals or as repellent toxins to deter pests in a species-specific and concentration-dependent manner. Plants have evolved solutions to provide sufficient volatiles without poisoning themselves. Uridine-diphosphate sugar-dependent glycosyltransferases (UGTs) acting on volatiles is one important part of this sophisticated system, which balances the levels of bioactive metabolites and prepares them for cellular and long-distance transport and storage but enables the remobilization of disarmed toxins for the benefit of plant protection. This review provides an overview of the research history of glycosidically bound volatiles (GBVs), a relatively new group of plant secondary metabolites, and discusses the role of UGTs in the production of GBVs for plant protection.

Summary Enzyme promiscuity, a common property of many uridine diphosphate sugar‐dependent glycosyltransferases (UGTs) that convert small molecules, significantly hinders the identification of natural substrates and therefore the characterization of the physiological role of enzymes. In this paper we present a simple but effective strategy to identify endogenous substrates of plant UGTs using LC‐MS‐guided targeted glycoside analysis of transgenic plants. We successfully identified natural substrates of two promiscuous Nicotiana benthamiana UGTs (NbUGT73A24 and NbUGT73A25), orthologues of pathogen‐induced tobacco UGT (TOGT) from Nicotiana tabacum, which is involved in the hypersensitive reaction. While in N. tabacum, TOGT glucosylated scopoletin after treatment with salicylate, fungal elicitors and the tobacco mosaic virus, NbUGT73A24 and NbUGT73A25 produced glucosides of phytoalexin N‐feruloyl tyramine, which may strengthen cell walls to prevent the intrusion of pathogens, and flavonols after agroinfiltration of the corresponding genes in N. benthamiana. Enzymatic glucosylation of fractions of a physiological aglycone library confirmed the biological substrates of UGTs. In addition, overexpression of both genes in N. benthamiana produced clear lesions on the leaves and led to a significantly reduced content of pathogen‐induced plant metabolites such as phenylalanine and tryptophan. Our results revealed some additional biological functions of TOGT enzymes and indicated a multifunctional role of UGTs in plant resistance.

The spatial size of the catalytic centre and a large hydrophobic pocket in the active site affect the enzymatic activity and substrate preference of uridine diphosphate–sugar-dependent terpenoid glycosyltransferases in plants.

The ability of glycosyltransferases (GTs) to reduce volatility, increase solubility, and thus alter the bioavailability of small molecules through glycosylation has attracted immense attention in pharmaceutical, nutraceutical, and cosmeceutical industries. The lack of GTs known and the scarcity of high-throughput (HTP) available methods, hinders the extrapolation of further novel applications. In this study, the applicability of new GT-assays suitable for HTP screening was tested and compared with regard to harmlessness, robustness, cost-effectiveness and reproducibility. The UDP-Glo GT-assay, Phosphate GT Activity assay, pH-sensitive GT-assay, and UDP2-TR-FRET assay were applied and tailored to plant UDP GTs (UGTs). Vitis vinifera (UGT72B27) GT was subjected to glycosylation reaction with various phenolics. Substrate screening and kinetic parameters were evaluated. The pH-sensitive assay and the UDP2-TR-FRET assay were incomparable and unsuitable for HTP plant GT-1 family UGT screening. Furthermore, the UDP-Glo GT-assay and the Phosphate GT Activity assay yielded closely similar and reproducible KM, vmax, and kcat values. Therefore, with the easy experimental set-up and rapid readout, the two assays are suitable for HTP screening and quantitative kinetic analysis of plant UGTs. This research sheds light on new and emerging HTP assays, which will allow for analysis of novel family-1 plant GTs and will uncover further applications.

Dysfunction of the tumor suppressor p53 occurs in most human cancers. Mdm2 and MdmX are homologous proteins from the Mdm (Murine Double Minute) protein family, which play a critical role in p53 inactivation and degradation. The two proteins interact with one another via the intrinsic RING (Really Interesting New Gene) domains to achieve the negative regulation of p53. The downregulation of p53 is accomplished by Mdm2-mediated p53 ubiquitination and proteasomal degradation through the ubiquitin proteolytic system and by Mdm2 and MdmX mediated inhibition of p53 transactivation. To investigate the role of the RING domain of Mdm2 and MdmX, an analysis of the distinct functionalities of individual RING domains of the Mdm proteins on p53 regulation was conducted in human osteosarcoma (U2OS) cell line. Mdm2 RING domain was observed mainly localized in the cell nucleus, contrasting the localization of MdmX RING domain in the cytoplasm. Mdm2 RING was found to possess an endogenous E3 ligase activity, whereas MdmX RING did not. Both Mdm2 and MdmX RING domains were able to dimerize with endogenous full-length Mdm2 and MdmX protein and affect their cellular function. The results showed that overexpression of the Mdm2 or MdmX RING domains interfered with the endogenous full-length Mdm2 and MdmX activity and resulted in p53 stabilization and p53 target gene activation. However, both Mdm RING domains showed oncogenic activity in a colony formation assay, suggesting that the Mdm RING domains possess p53-independent oncogenic properties. This study highlights the distinct structural and functional traits of the RING domain of Mdm2 and MdmX and characterized their role in cellular responses through interfering with p53 dependent signaling pathway.