Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play a pivotal role in the dynamic cell signaling systems in plants, even under biotic and abiotic stress conditions. Over the past two decades, various studies have endorsed the notion that these molecules can act as intracellular and intercellular signaling molecules at a very low concentration to control plant growth and development, symbiotic association, and defense mechanisms in response to biotic and abiotic stress conditions. However, the upsurge of ROS and RNS under stressful conditions can lead to cell damage, retarded growth, and delayed development of plants. As signaling molecules, ROS and RNS have gained great attention from plant scientists and have been studied under different developmental stages of plants. However, the role of RNS and RNS signaling in plant–microbe interactions is still unknown. Different organelles of plant cells contain the enzymes necessary for the formation of ROS and RNS as well as their scavengers, and the spatial and temporal positions of these enzymes determine the signaling pathways. In the present review, we aimed to report the production of ROS and RNS, their role as signaling molecules during plant–microbe interactions, and the antioxidant system as a balancing system in the synthesis and elimination of these species.

The bactericidal effects of nanomaterials play an essential role in cytoplasmic leakage, leading to bacterial cell death. In this study, silver nanoparticles (AgNPs) were synthesized using a fungal extract of Aspergillus fumigatus (A. fumigatus). The physicochemical properties of the bare and myco-synthesized AgNPs (MS-AgNPs) were examined by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet–visible (UV–vis) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). XRD revealed the crystalline structure of the prepared NPs. The FTIR spectrum of the MS-AgNPs revealed the presence of the stretching vibrations of hydroxyl (−OH) and carbonyl groups (C=O). The UV results showed absorption from 450 nm to 590 nm, confirming the synthesis of the AgNPs. SEM and TEM showed rough cubic shapes (spheres), 20–60 nm in size, while EDX confirmed the presence of 60% Ag in the sample. The MS-AgNPs revealed the highest antibacterial activity against Staphylococcus aureus, with a zone of inhibition of 18.21 ± 2.1 mm, followed by Shigella dysenteriae and Salmonella typhi. The bimetallic-AgNPs played a vital role in cell membrane damage and the release of cellular contents, specifically nucleic acids and proteins. These results suggest that MS-AgNPs have promising antimicrobial capabilities and might be beneficial for an extensive array of biological applications.

Melatonin was discovered in plants in the late nineties, but its role, signaling, and crosstalk with other phytohormones remain unknown. Research on melatonin in plants has risen dramatically in recent years and the role of this putative plant hormone under biotic and abiotic stress conditions has been reported. In the present review, we discuss the main functions of melatonin in the growth and development of plants, its role under abiotic stresses, such as water stress (waterlogging and drought), extreme temperature (low and high), salinity, heavy metal, and light-induced stress. Similarly, we also discuss the role of melatonin under biotic stresses (antiviral, antibacterial, and antifungal effects). Moreover, the present review meticulously discusses the crosstalk of melatonin with other phytohormones such as auxins, gibberellic acids, cytokinins, ethylene, and salicylic acid under normal and stressful conditions and reports melatonin receptors and signaling in plants. All these aspects of melatonin suggest that phytomelatonin is a key player in crop improvement and biotic and abiotic stress regulation.