Metal toxicity in soils, along with water runoff, are increasing environmental problems that affect agriculture directly and, in turn, human health. In light of finding a suitable and urgent solution, research on plant treatments with specific compounds that can help mitigate these effects has increased, and thus the exogenous application of melatonin (MET) and its role in alleviating the negative effects of metal toxicity in plants, have become more important in the last few years. MET is an important plant-related response molecule involved in growth, development, and reproduction, and in the induction of different stress-related key factors in plants. It has been shown that MET plays a protective role against the toxic effects induced by different metals (Pb, Cd, Cu, Zn, B, Al, V, Ni, La, As, and Cr) by regulating both the enzymatic and non-enzymatic antioxidant plant defense systems. In addition, MET interacts with many other signaling molecules, such as reactive oxygen species (ROS) and nitric oxide (NO) and participates in a wide variety of physiological reactions. Furthermore, MET treatment enhances osmoregulation and photosynthetic efficiency, and increases the concentration of other important antioxidants such as phenolic compounds, flavonoids, polyamines (PAs), and carotenoid compounds. Some recent studies have shown that MET appeared to be involved in the regulation of metal transport in plants, and lastly, various studies have confirmed that MET significantly upregulated stress tolerance-related genes. Despite all the knowledge acquired over the years, there is still more to know about how MET is involved in the metal toxicity tolerance of plants.

Melatonin (MEL), a ubiquitous indolamine molecule, has gained interest in the last few decades due to its regulatory role in plant metabolism. Likewise, nitric oxide (NO), a gasotransmitter, can also affect plant molecular pathways due to its function as a signaling molecule. Both MEL and NO can interact at multiple levels under abiotic stress, starting with their own biosynthetic pathways and inducing a particular signaling response in plants. Moreover, their interaction can result in the formation of NOmela, a very recently discovered nitrosated form of MEL with promising roles in plant physiology. This review summarizes the role of NO and MEL molecules during plant development and fruit ripening, as well as their interactions. Due to the impact of climate-change-related abiotic stresses on agriculture, this review also focuses on the role of these molecules in mediating abiotic stress tolerance and the main mechanisms by which they operate, from the upregulation of the entire antioxidant defense system to the post-translational modifications (PTMs) of important molecules. Their individual interaction and crosstalk with phytohormones and H2S are also discussed. Finally, we introduce and summarize the little information available about NOmela, an emerging and still very unknown molecule, but that seems to have a stronger potential than MEL and NO separately in mediating plant stress response.

The impact of climate change entails a progressive and inexorable modification of the Earth’s climate and events such as salinity, drought, extreme temperatures, high luminous intensity and ultraviolet radiation tend to be more numerous and prolonged in time. Plants face their exposure to these abiotic stresses or their combination through multiple physiological, metabolic and molecular mechanisms, to achieve the long-awaited acclimatization to these extreme conditions, and to thereby increase their survival rate. In recent decades, the increase in the intensity and duration of these climatological events have intensified research into the mechanisms behind plant tolerance to them, with great advances in this field. Among these mechanisms, the overproduction of molecular reactive species stands out, mainly reactive oxygen, nitrogen and sulfur species. These molecules have a dual activity, as they participate in signaling processes under physiological conditions, but, under stress conditions, their production increases, interacting with each other and modifying and-or damaging the main cellular components: lipids, carbohydrates, nucleic acids and proteins. The latter have amino acids in their sequence that are susceptible to post-translational modifications, both reversible and irreversible, through the different reactive species generated by abiotic stresses (redox-based PTMs). Some research suggests that this process does not occur randomly, but that the modification of critical residues in enzymes modulates their biological activity, being able to enhance or inhibit complete metabolic pathways in the process of acclimatization and tolerance to the exposure to the different abiotic stresses. Given the importance of these PTMs-based regulation mechanisms in the acclimatization processes of plants, the present review gathers the knowledge generated in recent years on this subject, delving into the PTMs of the redox-regulated enzymes of plant metabolism, and those that participate in the main stress-related pathways, such as oxidative metabolism, primary metabolism, cell signaling events, and photosynthetic metabolism. The aim is to unify the existing information thus far obtained to shed light on possible fields of future research in the search for the resilience of plants to climate change.