Melatonin-Induced Water Stress Tolerance in Plants: Recent Advances

Looking for Resistance to Soft Rot Disease of Potatoes Facing Environmental Hypoxia

Plants are exposed to various stressors, including pathogens, requiring specific environmental conditions to provoke/induce plant disease. This phenomenon is called the "disease triangle" and is directly connected with a particular plant-pathogen interaction. Only a virulent pathogen interacting with a susceptible plant cultivar will lead to disease under specific environmental conditions. This may seem difficult to accomplish, but soft rot Pectobacteriaceae (SRPs) is a group virulent of pathogenic bacteria with a broad host range. Additionally, waterlogging (and, resulting from it, hypoxia), which is becoming a frequent problem in farming, is a favoring condition for this group of pathogens. Waterlogging by itself is an important source of abiotic stress for plants due to lowered gas exchange. Therefore, plants have evolved an ethylene-based system for hypoxia sensing. Plant response is coordinated by hormonal changes which induce metabolic and physiological adjustment to the environmental conditions. Wetland species such as rice (Oryza sativa L.), and bittersweet nightshade (Solanum dulcamara L.) have developed adaptations enabling them to withstand prolonged periods of decreased oxygen availability. On the other hand, potato (Solanum tuberosum L.), although able to sense and response to hypoxia, is sensitive to this environmental stress. This situation is exploited by SRPs which in response to hypoxia induce the production of virulence factors with the use of cyclic diguanylate (c-di-GMP). Potato tubers in turn reduce their defenses to preserve energy to prevent the negative effects of reactive oxygen species and acidification, making them prone to soft rot disease. To reduce the losses caused by the soft rot disease we need sensitive and reliable methods for the detection of the pathogens, to isolate infected plant material. However, due to the high prevalence of SRPs in the environment, we also need to create new potato varieties more resistant to the disease. To reach that goal, we can look to wild potatoes and other Solanum species for mechanisms of resistance to waterlogging. Potato resistance can also be aided by beneficial microorganisms which can induce the plant's natural defenses to bacterial infections but also waterlogging. However, most of the known plant-beneficial microorganisms suffer from hypoxia and can be outcompeted by plant pathogens. Therefore, it is important to look for microorganisms that can withstand hypoxia or alleviate its effects on the plant, e.g., by improving soil structure. Therefore, this review aims to present crucial elements of potato response to hypoxia and SRP infection and future outlooks for the prevention of soft rot disease considering the influence of environmental conditions.

Plant Adaptation to Flooding Stress under Changing Climate Conditions: Ongoing Breakthroughs and Future Challenges

Climate-change-induced variations in temperature and rainfall patterns are a serious threat across the globe. Flooding is the foremost challenge to agricultural productivity, and it is believed to become more intense under a changing climate. Flooding is a serious form of stress that significantly reduces crop yields, and future climatic anomalies are predicted to make the problem even worse in many areas of the world. To cope with the prevailing flooding stress, plants have developed different morphological and anatomical adaptations in their roots, aerenchyma cells, and leaves. Therefore, researchers are paying more attention to identifying developed and adopted molecular-based plant mechanisms with the objective of obtaining flooding-resistant cultivars. In this review, we discuss the various physiological, anatomical, and morphological adaptations (aerenchyma cells, ROL barriers (redial O2 loss), and adventitious roots) and the phytohormonal regulation in plants under flooding stress. This review comprises ongoing innovations and strategies to mitigate flooding stress, and it also provides new insights into how this knowledge can be used to improve productivity in the scenario of a rapidly changing climate and increasing flood intensity.

Exogenous TiO2 Nanoparticles Alleviate Cd Toxicity by Reducing Cd Uptake and Regulating Plant Physiological Activity and Antioxidant Defense Systems in Rice (Oryza sativa L.)

Cadmium (Cd) is a potentially hazardous element with significant biological toxicity, negatively affecting plant growth and physio-biochemical metabolism. Thus, it is necessary to examine practical and eco-friendly approaches to reduce Cd toxicity. Titanium dioxide nanoparticles (TiO₂-NPs) are growth regulators that help in nutrient uptake and improve plant defense systems against abiotic and biological stress. A pot experiment was performed in the late rice-growing season (July-November) 2022 to explore the role of TiO₂-NPs in relieving Cd toxicity on leaf physiological activity, biochemical attributes, and plant antioxidant defense systems of two different fragrant rice cultivars, i.e., Xiangyaxiangzhan (XGZ) and Meixiangzhan-2 (MXZ-2). Both cultivars were cultivated under normal and Cd-stress conditions. Different doses of TiO₂-NPs with and without Cd-stress conditions were studied. The treatment combinations were: Cd-, 0 mg/kg CdCl₂·2.5 H₂O; Cd+, 50 mg/kg CdCl₂·2.5 H₂O; Cd + NP1, 50 mg/kg Cd + 50 TiO₂-NPs mg/L; Cd + NP2, 50 mg/kg Cd + 100 TiO₂-NPs mg/L; Cd + NP3, 50 mg/kg Cd + 200 TiO₂-NPs mg/L; Cd + NP4, 50 mg/kg Cd + 400 TiO₂-NPs mg/L. Our results showed that the Cd stress significantly (p < 0.05) decreased leaf photosynthetic efficiency, stomatal traits, antioxidant enzyme activities, and the expression of their encoding genes and protein content. Moreover, Cd toxicity destabilized plant metabolism owing to greater accretion of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) levels at vegetative and reproductive stages. However, TiO₂-NPs application improved leaf photosynthetic efficacy, stomatal traits, and protein and antioxidant enzyme activities under Cd toxicity. Application of TiO₂-NPs decreased the uptake and accumulation of Cd in plants and levels of H₂O₂ and MDA, thereby helping to relieve Cd-induced peroxidation damage of leaf membrane lipids by enhancing the activities of different enzymes like ascorbate peroxidase (APX), catalase (CAT), peroxidase (POS), and superoxide dismutase (SOD). Average increases in SOD, APX, CAT, and POS activities of 120.5 and 110.4%, 116.2 and 123.4%, 41.4 and 43.8%, and 36.6 and 34.2% in MXZ-2 and XGZ, respectively, were noted in Cd + NP3 treatment across the growth stages as compared with Cd-stressed plants without NPs. Moreover, the correlation analysis revealed that the leaf net photosynthetic rate is strongly associated with leaf proline and soluble protein content, suggesting that a higher net photosynthetic rate results in higher leaf proline and soluble protein content. Of the treatments, the Cd + NP3 (50 mg/kg Cd + 200 mg/L TiO₂-NPs) performed the best for both fragrant rice cultivars under Cd toxicity. Our results showed that TiO₂-NPs strengthened rice metabolism through an enhanced antioxidant defense system across the growth stages, thereby improving plant physiological activity and biochemical characteristics under Cd toxicity.

Melatonin as a Possible Natural Anti-Viral Compound in Plant Biocontrol

Melatonin is a multifunctional and ubiquitous molecule. In animals, melatonin is a hormone that is involved in a wide range of physiological activities and is also an excellent antioxidant. In plants, it has been considered a master regulator of multiple physiological processes as well as of hormonal homeostasis. Likewise, it is known for its role as a protective biomolecule and activator of tolerance and resistance against biotic and abiotic stress in plants. Since infections by pathogens such as bacteria, fungi and viruses in crops result in large economic losses, interest has been aroused in determining whether melatonin plays a relevant role in plant defense systems against pathogens in general, and against viruses in particular. Currently, several strategies have been applied to combat infection by pathogens, one of them is the use of eco-friendly chemical compounds that induce systemic resistance. Few studies have addressed the use of melatonin as a biocontrol agent for plant diseases caused by viruses. Exogenous melatonin treatments have been used to reduce the incidence of several virus diseases, reducing symptoms, virus titer, and even eradicating the proliferation of viruses such as Tobacco Mosaic Virus, Apple Stem Grooving Virus, Rice Stripe Virus and Alfalfa Mosaic Virus in tomato, apple, rice and eggplant, respectively. The possibilities of using melatonin as a possible natural virus biocontrol agent are discussed.

Human Naa50 Shows Serotonin N-Acetyltransferase Activity, and Its Overexpression Enhances Melatonin Biosynthesis, Resulting in Osmotic Stress Tolerance in Rice

A new clade of serotonin N-acetyltransferase (SNAT), the penultimate enzyme in the melatonin biosynthetic pathway, has been reported in the archaeon Thermoplasma volcanium. The closest homolog of archaea SNAT in human was an N-alpha-acetyltransferase50 (Naa50). To determine whether human Naa50 (hNaa50) shows SNAT enzyme activity, we chemically synthesized and expressed the hNaa50 gene in Escherichia coli, followed by Ni²⁺ affinity purification. Purified recombinant hNaa50 showed SNAT activity (K_m and V_max values of 986 μM and 1800 pmol/min/mg protein, respectively). To assess its in vivo function, hNaa50 was overexpressed in rice (hNaa50-OE). The transgenic rice plants produced more melatonin than nontransgenic wild-type rice, indicating that hNaa50 is functionally coupled with melatonin biosynthesis. Due to its overproduction of melatonin, hNaa50-OE had a higher tolerance against osmotic stress than the wild type. Enhanced expression of the chaperone genes BIP1 and CNX in hNaa50-OE plants was responsible for the increased tolerance. It is concluded that hNaa50 harbors serotonin N-acetyltransferase enzyme activity in addition to its initial N-alpha-acetyltransferase, suggesting the bifunctionality of the hNaa50 enzyme toward serotonin and protein substrates. Consequently, ectopic overexpression of hNaa50 in rice enhanced melatonin synthesis, indicating that hNaa50 is in fact involved in melatonin biosynthesis.

Melatonin from Microorganisms, Algae, and Plants as Possible Alternatives to Synthetic Melatonin

Melatonin dietary supplements are widely consumed worldwide, with developed countries as the largest consumers, with an estimated annual growth rate of approximately 10% until 2027, mainly in developing countries. The wide use of melatonin against sleep disorders and particular problems, such as jet lag, has been added to other applications, such as anti-aging, anti-stress, immune system activation, anticancer, and others, which have triggered its use, normally without a prescription. The chemical industry currently covers 100% of the needs of the melatonin market. Motivated by sectors with more natural consumption habits, a few years ago, the possibility of obtaining melatonin from plants, called phytomelatonin, arose. More recently, the pharmaceutical industry has developed genetically modified microorganisms whose ability to produce biological melatonin in bioreactors has been enhanced. This paper reviews the aspects of the chemical and biological synthesis of melatonin for human consumption, mainly as dietary supplements. The pros and cons of obtaining melatonin from microorganisms and phytomelatonin from plants and algae are analyzed, as well as the advantages of natural melatonin, avoiding unwanted chemical by-products from the chemical synthesis of melatonin. Finally, the economic and quality aspects of these new products, some of which are already marketed, are analyzed.

Melatonin Improves Cotton Salt Tolerance by Regulating ROS Scavenging System and Ca2 + Signal Transduction

As one of the cash crops, cotton is facing the threat of abiotic stress during its growth and development. It has been reported that melatonin is involved in plant defense against salt stress, but whether melatonin can improve cotton salt tolerance and its molecular mechanism remain unclear. We investigated the role of melatonin in cotton salt tolerance by silencing melatonin synthesis gene and exogenous melatonin application in upland cotton. In this study, applicating of melatonin can improve salt tolerance of cotton seedlings. The content of endogenous melatonin was different in cotton varieties with different salt tolerance. The inhibition of melatonin biosynthesis related genes and endogenous melatonin content in cotton resulted in the decrease of antioxidant enzyme activity, Ca²⁺ content and salt tolerance of cotton. To explore the protective mechanism of exogenous melatonin against salt stress by RNA-seq analysis. Melatonin played an important role in the resistance of cotton to salt stress, improved the salt tolerance of cotton by regulating antioxidant enzymes, transcription factors, plant hormones, signal molecules and Ca²⁺ signal transduction. This study proposed a regulatory network for melatonin to regulate cotton's response to salt stress, which provided a theoretical basis for improving cotton's salt tolerance.