Insect pests represent a major global challenge to important agricultural crops. Insecticides are often applied to combat such pests, but their use has caused additional challenges such as environmental contamination and human health issues. Over millions of years, plants have evolved natural defense mechanisms to overcome insect pests and pathogens. One such mechanism is the production of natural repellents or specialized metabolites like glucosinolates. There are three types of glucosinolates produced in the order Brassicales: aliphatic, indole, and benzenic glucosinolates. Upon insect herbivory, a “mustard oil bomb” consisting of glucosinolates and their hydrolyzing enzymes (myrosinases) is triggered to release toxic degradation products that act as insect deterrents. This review aims to provide a comprehensive summary of glucosinolate biosynthesis, the “mustard oil bomb”, and how these metabolites function in plant defense against pathogens and insects. Understanding these defense mechanisms will not only allow us to harness the benefits of this group of natural metabolites for enhancing pest control in Brassicales crops but also to transfer the “mustard oil bomb” to non-glucosinolate producing crops to boost their defense and thereby reduce the use of chemical pesticides.

Melatonin (MT) can protect plants against abiotic stress. In order to explore whether melatonin can improve photosynthetic function under NaCl stress, Solanum lycopersicum L. cv. Liaoyuanduoli were exposed to 150 mmol L⁻¹ NaCl stress with or without pretreatment with 150 μmol L⁻¹ melatonin. The results showed that NaCl stress can lead to reduced chlorophyll content, lower photosynthetic function, increased reaction oxygen species (ROS) levels, and decreased PSII activity. These changes were mainly due to the reduction in oxygen-evolving complex (OEC) activity on the donor side of PSII and the blockage of electron transfer from Q_A to Q_B on receptor side of PSII. The donor side of PSII was more sensitive to NaCl stress relative to the receptor side of PSII. Interestingly, application of MT enhanced tomato NaCl tolerance. MT reduced the production of ROS by balancing the distribution of photosynthetic electron flux, facilitated the repair of PSII by maintaining the abundance of Psb O and D1, and promoting the ability of the donor and acceptor sides of PSII to deliver electrons. MT also enhanced the scavenging ability of ROS by stimulating the activity of enzymes involved in the AsA-GSH cycle.

Bacterial resistance represents one of the emerging obstacles in plants, animals, and humans that impairs treatment with antibacterial agents. Targeting of the bacterial quorum sensing system is one of the strategies to overcome this problem. Recently, research has been focused on natural and food components which can function as quorum sensing inhibitors. In this study, a methanol extract from Salix tetrasperma stem bark was phytochemically profiled by LC-MS analysis. This resulted in the identification of 38 secondary metabolites with (epi)catechin-(epi)catechin, epicatechin, tremulacin, salicortin, and trichocarposide as the major constituents. The extracts of both stem bark and the previously profiled flower of S. tetrasperma were tested for anti-quorum sensing activity in a common and widely distributed pathogen Pseudomonas aeruginosa. The natural products inhibited swimming and swarming motilities, as well as proteolytic and hemolytic activities in a dose-dependent manner. Molecular docking of the constituents from both extracts against the quorum sensing controlling systems Lasl/LasR, rhll/rhlR, and PQS/MvfR showed that epicatechin, (epi)catechin-(epi)catechin, p-hydroxy benzoyl galloyl glucose, p-hydroxy benzoyl protocatechuic acid glucose, and caffeoylmalic acid could be the main active components. This study supports the importance of secondary metabolites, especially polyphenols, as quorum sensing inhibitors.

The persistent evolution of drug-resistant influenza strains represents a global concern. The innovation of new treatment approaches through drug screening strategies and investigating the antiviral potential of bioactive natural-based chemicals may address the issue. Herein, we screened the anti-influenza efficacy of some biologically active indole and β-carboline (βC) indole alkaloids against two different influenza A viruses (IAV) with varied host range ranges; seasonal influenza A/Egypt/NRC098/2019(H1N1) and avian influenza A/chicken/Egypt/N12640A/2016(H5N1). All compounds were first assessed for their half-maximal cytotoxic concentration (CC₅₀) in MDCK cells and half-maximal inhibitory concentrations (IC₅₀) against influenza A/H5N1. Intriguingly, Strychnine sulfate, Harmalol, Harmane, and Harmaline showed robust anti-H5N1 activities with IC₅₀ values of 11.85, 0.02, 0.023, and 3.42 µg/ml, respectively, as compared to zanamivir and amantadine as control drugs (IC₅₀ = 0.079 µg/ml and 17.59 µg/ml, respectively). The efficacy of the predefined phytochemicals was further confirmed against influenza A/H1N1 and they displayed potent anti-H1N1 activities compared to reference drugs. Based on SI values, the highly promising compounds were then evaluated for antiviral efficacy through plaque reduction assay and consistently they revealed high viral inhibition percentages at non-toxic concentrations. By studying the modes of antiviral action, Harmane and Harmalol could suppress viral infection via interfering mainly with the viral replication of the influenza A/H5N1 virus, whilst Harmaline exhibited a viricidal effect against the influenza A/H5N1 virus. Whereas, Strychnine sulfate elucidated its anti-influenza potency by interfering with viral adsorption into MDCK cells. Consistently, chemoinformatic studies showed that all studied phytochemicals illustrated HB formations with essential peptide cleft through the NH of indole moiety. Among active alkaloids, harmalol displayed the best lipophilicity metrics including ligand efficiency (LE) and ligand lipophilic efficiency (LLE) for both viruses. Compounds geometry and their ability to participate in HB formation are very crucial.

Influenza is a contagious infection in humans that is caused frequently by low pathogenic seasonal influenza viruses and occasionally by pathogenic avian influenza viruses (AIV) of H5, H7, and H9 subtypes. Recently, the clinical sector in poultry and humans has been confronted with many challenges, including the limited number of antiviral drugs and the rapid evolution of drug-resistant variants. Herein, the anti-influenza activities of various plant-derived phytochemicals were investigated against highly pathogenic avian influenza A/H5N1 virus (HPAIV H5N1) and seasonal low pathogenic human influenza A/H1N1 virus (LPHIV H1N1). Out of the 22 tested phytochemicals, the steroid compounds β-sitosterol and β-sitosterol-O-glucoside have very potent activity against the predefined influenza A viruses (IAV). Both steroids could induce such activity by affecting multiple stages during IAV replication cycles, including viral adsorption and replication with a major and significant impact on the virus directly in a cell-free status “viricidal effect”. On a molecular level, several molecular docking studies suggested that β-sitosterol and β-sitosterol-O-glucoside exhibited viricidal effects through blocking active binding sites of the hemagglutinin surface protein, as well as showing inhibitory effects against replication through the binding with influenza neuraminidase activity and blocking the active sites of the M2 proton channel activity. The phytoestrogen β-sitosterol has structural similarity with the active form of the female sex hormone estradiol, and this similarity is likely one of the molecular determinants that enables the phytoestrogen β-sitosterol and its derivative to control IAV infection in vitro. This promising anti-influenza activity of β-sitosterol and its O-glycoside derivative, according to both in vitro and cheminformatics studies, recommend both phytochemicals for further studies going through preclinical and clinical phases as efficient anti-influenza drug candidates.