5-Methoxyindole, a Chemical Homolog of Melatonin, Adversely Affects the Phytopathogenic Fungus Fusarium graminearum

Exogenous Melatonin Enhances Cold Tolerance by Regulating the Expression of Photosynthetic Performance, Antioxidant System, and Related Genes in Cotton

In China, cotton is a significant cash crop, and cold stress negatively impacts the crop's development, production, and quality formation. Recent studies have shown that melatonin (MT) can alleviate the damage to plants under cold stress and promote good growth and development. In this study, the morphological and physiological changes induced by exogenous melatonin pretreatment on 'Xinluzao 33' cotton seedlings under cold stress were examined to investigate its defensive effects. The results showed that 100 μM MT pretreatment improved the cold resistance of cotton most significantly. It also improved the wilting state of cotton under cold stress, greatly increased the photosynthetic rate (Pn), stomatal conductance (Gs), maximum photochemical efficiency (Fv/Fm), and photosynthetic performance index (PIabs) by 116.92%, 47.16%, 32.30%, and 50.22%, respectively, and mitigated the adverse effects of low-temperature. In addition, MT supplementation substantially reduced the accumulation of superoxide anion (O2•-) and hydrogen peroxide (H2O2) by 14.5% and 45.49%, respectively, in cold-stressed cotton leaves by modulating the antioxidant system, thereby mitigating oxidative damage. Furthermore, MT pretreatment increased the endogenous melatonin content (23.80%) and flavonoid content (21.44%) and considerably induced the expression of biosynthesis enzyme-related genes. The above results indicate that exogenous melatonin improves the low-temperature resistance of cotton seedlings by regulating photosynthetic performance, antioxidant enzyme activity, antioxidant content, endogenous melatonin and flavonoid content, and the expression levels of genes related to their synthesis.

Melatonin: The Multifaceted Molecule in Plant Growth and Defense

Melatonin (MEL), a hormone primarily known for its role in regulating sleep and circadian rhythms in animals, has emerged as a multifaceted molecule in plants. Recent research has shed light on its diverse functions in plant growth and defense mechanisms. This review explores the intricate roles of MEL in plant growth and defense responses. MEL is involved in plant growth owing to its influence on hormone regulation. MEL promotes root elongation and lateral root formation and enhances photosynthesis, thereby promoting overall plant growth and productivity. Additionally, MEL is implicated in regulating the circadian rhythm of plants, affecting key physiological processes that influence plant growth patterns. MEL also exhibits antioxidant properties and scavenges reactive oxygen species, thereby mitigating oxidative stress. Furthermore, it activates defense pathways against various biotic stressors. MEL also enhances the production of secondary metabolites that contribute to plant resistance against environmental changes. MEL's ability to modulate plant response to abiotic stresses has also been extensively studied. It regulates stomatal closure, conserves water, and enhances stress tolerance by activating stress-responsive genes and modulating signaling pathways. Moreover, MEL and nitric oxide cooperate in stress responses, antioxidant defense, and plant growth. Understanding the mechanisms underlying MEL's actions in plants will provide new insights into the development of innovative strategies for enhancing crop productivity, improving stress tolerance, and combating plant diseases. Further research in this area will deepen our knowledge of MEL's intricate functions and its potential applications in sustainable agriculture.

Nitrogen mitigates the negative effects of combined heat and drought stress on winter wheat by improving physiological characteristics

Extreme drought stress is often accompanied by heat stress after anthesis in winter wheat. Whether nitrogen (N) can mitigate the damage caused by combined stress on wheat plants by regulating root physiological characteristics is still unclear. Thus, this study aimed to study the effects of combined heat and drought stress on photosynthesis, leaf water relations, root antioxidant system, osmoregulatory, and yield in wheat to reveal the physiological mechanism of N regulating the adverse impacts of combined stress on wheat. Heat and drought stress markedly reduced photosynthesis, leaf water content, root vitality, and bleeding sap. The combination of heat and drought strengthens these changes. Within a certain stress range, the increase in soluble sugar and proline contents and the activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase under combined stress effectively alleviated the oxidative damage. Compared with those under high N application (N3), wheat plants under low N application (N1) maintained higher yield and yield components under combined stress; the number of grains per spike, 1000-grain weight, and yield increased by 13.65%, 9.07%, and 15.33%, respectively, under N1 compared with those under N3 treatment, which may be attributed to the greater maintenance of photosynthesis, leaf water status, root vitality, and antioxidant and osmoregulation capacities. In summary, reduced N application mitigated the damage caused by combined heat and drought stress in wheat by improving root physiological characteristics and enhanced adaptability to combined stress, which is an appropriate strategy to compensate for yield losses.

Broad-spectrum antagonistic potential of Bacillus spp. volatiles against Rhizoctonia solani and Xanthomonas oryzae pv. oryzae

Rhizoctonia solani and Xanthomonas oryzae pv. oryzae (Xoo) are the two major diseases affecting the quality and quantity of rice production. In the current study, volatile organic compounds (VOCs) of Bacillus spp. were used as green biocontrol agents for plant diseases. In in vitro experiments, Bacillus spp. FZB42, NMTD17, and LLTC93-VOCs displayed strong antimicrobial volatile activity with inhibition rates of 76, 66, and 78% for R. solani and 78, 81, and 76% for Xoo, respectively, compared to control. The synthetic VOCs, namely Pentadecane (PDC), Benzaldehyde (BDH), 1,2-Benz isothiazol-3(2H)-one (1,2-BIT), and mixture (MIX) of VOCs showed high volatile activity with inhibition rates of 86, 86, 89, and 92% against R. solani and 81, 81, 82, and 86%, respectively, against Xoo as compared to control. In addition, the scanning and transmission electron microscopes (SEM and TEM) analyses were performed to examine the effect of Bacillus and synthetic VOC treatments on R. solani and Xoo morphology. The analysis revealed the deformed and irregularized morphology of R. solani mycelia and Xoo cells after VOC treatments. The microscopic analysis showed that the rapid inhibition was due to severe oxidative productions inside the R. solani mycelia and Xoo cells. By using molecular docking, it was determined that the synthetic VOCs entered the active binding site of trehalase and NADH dehydrogenase proteins, causing R. solani and Xoo cells to die prematurely and an accumulation of ROS. In the greenhouse experiment, FZB42, NMTD17, and LLTC93-VOCs significantly reduced the lesions of R. solani 8, 7, and 6 cm, and Xoo 7, 6, and 6 cm, respectively, then control. The synthetic VOCs demonstrated that the PDC, BDH, 1,2-BIT, and MIX-VOCs significantly reduced R. solani lesions on leaves 6, 6, 6, and 5 cm and Xoo 6, 5, 5, and 4 cm, respectively, as compared to control. Furthermore, plant defence-related genes and antioxidant enzymes were upregulated in rice plants. These findings provide novel mechanisms by which Bacillus antimicrobial VOCs control plant diseases.

Exogenous Melatonin Regulates Plant-Disease Interaction by Inducing Maize Resistance and Decreasing the Pathogenicity of Fusarium graminearum

Plants cannot avoid environmental challenges and are constantly threatened by diverse biotic and abiotic stresses. However, plants have developed a unique immune system to defend themselves against the invasion of various pathogens. Melatonin, N-acetyl-5-methoxytryptamine has positive physiological effects in plants that are involved in disease resistance. The processes underlying melatonin-induced pathogen resistance in plants are still unknown. The current study explores how melatonin regulates the plant-disease interaction in maize. The results showed that 400 μM melatonin strongly reduced the disease lesion on maize stalks by 1.5 cm and corn by 4.0 cm caused by Fusarium graminearum PH-1. Furthermore, after treatment with melatonin, the plant defense enzymes like SOD significantly increased, while POD and APX significantly decreased compared to the control. In addition, melatonin can also improve maize's innate immunity, which is mediated by melatonin treatments through the salicylic acid signaling pathway, and up-regulate the defense-associated expression of PR1, LOX1, OXR, serPIN, and WIPI genes in maize. Melatonin not only inhibits the disease in the maize stalks and corn, but also down-regulates the deoxynivalenol (DON) production-related expression of genes Tri1, Tri4, Tri5, and Tri6 in maize. Overall, this study sheds new light on the mechanisms by which melatonin regulates antioxidant enzymes and defense-related genes involved in plant immunity to effectively suppress plant diseases.

Melatonin targets MoIcl1 and works synergistically with fungicide isoprothiolane in rice blast control

Melatonina natural harmless molecule-displays versatile roles in human health and crop disease control such as for rice blast. Rice blast, caused by the filamentous fungus Magnaporthe oryzae, is one devastating disease of rice. Application of fungicides is one of the major measures in the control of various crop diseases. However, fungicide resistance in the pathogen and relevant environmental pollution are becoming serious problems. By screening for possible synergistic combinations, here, we discovered an eco-friendly combination for rice blast control, melatonin, and the fungicide isoprothiolane. These compounds together exhibited significant synergistic inhibitory effects on vegetative growth, conidial germination, appressorium formation, penetration, and plant infection by M. oryzae. The combination of melatonin and isoprothiolane reduced the effective concentration of isoprothiolane by over 10-fold as well as residual levels of isoprothiolane. Transcriptomics and lipidomics revealed that melatonin and isoprothiolane synergistically interfered with lipid metabolism by regulating many common targets, including the predicted isocitrate lyase-encoding gene MoICL1. Furthermore, using different techniques, we show that melatonin and isoprothiolane interact with MoIcl1. This study demonstrates that melatonin and isoprothiolane function synergistically and can be used to reduce the dosage and residual level of isoprothiolane, potentially contributing to the environment-friendly and sustainable control of crop diseases.

Phytomelatonin: A key regulator of redox and phytohormones signaling against biotic/abiotic stresses

Plants being sessile in nature, are exposed to unwarranted threats as a result of constantly changing environmental conditions. These adverse factors can have negative impacts on their growth, development, and yield. Hormones are key signaling molecules enabling cells to respond rapidly to different external and internal stimuli. In plants, melatonin (MT) plays a critical role in the integration of various environmental signals and activation of stress-response networks to develop defense mechanisms and plant resilience. Additionally, melatonin can tackle the stress-induced alteration of cellular redox equilibrium by regulating the expression of redox hemostasis-related genes and proteins. The purpose of this article is to compile and summarize the scientific research pertaining to MT's effects on plants' resilience to biotic and abiotic stresses. Here, we have summarized that MT exerts a synergistic effect with other phytohormones, for instance, ethylene, jasmonic acid, and salicylic acid, and activates plant defense-related genes against phytopathogens. Furthermore, MT interacts with secondary messengers like Ca²⁺, nitric oxide, and reactive oxygen species to regulate the redox network. This interaction triggers different transcription factors to alleviate stress-related responses in plants. Hence, the critical synergic role of MT with diverse plant hormones and secondary messengers demonstrates phytomelatonin's importance in influencing multiple mechanisms to contribute to plant resilience against harsh environmental factors.

Application of silicon and sodium hydrosulfide alleviates arsenic toxicity by regulating the physio-biochemical and molecular mechanisms of Zea mays

Soil contamination with toxic heavy metals (such as arsenic (As)) is becoming a serious global problem due to rapid development of social economy, although the use of silicon (Si) and sodium hydrosulfide (NaHS) has been found effective in enhancing plant tolerance against biotic and abiotic stresses including the As toxicity. For this purpose, a pot experiment was conducted using the different levels of As toxicity in the soil, i.e., (0 mM (no As), 50, and 100 µM) which were also supplied with the different exogenous levels of Si, i.e., (0 (no Si), 1.5, and 3 mM) and also with the NaHS, i.e., (0 (no NaHS), 1, and 2 mM) on growth, photosynthetic pigments, gas exchange characteristics, oxidative stress biomarkers, antioxidant machinery (enzymatic and non-enzymatic antioxidants), and their gene expression, ion uptake, organic acid exudation, and As uptake of maize (Zea mays L.). Results from the present study showed that the increasing levels of As in the soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugars, and nutritional contents from the roots and shoots of the plants. In contrast, increasing levels of As in the soil significantly (P < 0.05) increased oxidative stress indicators in terms of malondialdehyde, hydrogen peroxide, and electrolyte leakage and also increased organic acid exudation patter in the roots of Z. mays, although the activities of enzymatic antioxidants and the response of their gene expressions in the roots and shoots of the plants and non-enzymatic such as phenolic, flavonoid, ascorbic acid, and anthocyanin contents were initially increased with the exposure of 50 µM As, but decreased by the increasing the As concentration 100 µM in the soil. The negative impact of As toxicity can overcome the application of Si and NaHS, which ultimately increased plant growth and biomass by capturing the reactive oxygen species and decreased oxidative stress in Z. mays by decreasing the As contents in the roots and shoots of the plants. Our results also showed that the Si was more sever and showed better results when we compared with NaHS under the same treatment of As in the soil. Research findings, therefore, suggest that the combined application of Si and NaHS can ameliorate As toxicity in Z. mays, resulting in improved plant growth and composition under metal stress, as depicted by balanced exudation of organic acids.

Bacillus mycoides PM35 in combination with titanium dioxide (TiO2)⎯nanoparticles enhanced morpho-physio-biochemical attributes in Barley (Hordeum vulgare L.) under cadmium stress

Plant growth-promoting rhizobacteria (PGPR) are naturally occurring soil bacteria and are known to induce plant growth promotion and titanium dioxide (TiO₂)⎯nanoparticles (NPs) used in a range of applications that need increased whiteness, improved corrosion resistance and photocatalytic activity. Keeping in view the stress mitigation potential of TiO₂⎯NPS and B. mycoides PM35, the existing research work was premeditated to inspect the beneficial role of seed priming with using different levels of TiO₂⎯NPs i.e., [(0 no TiO₂⎯NPs), 25 and 50 μg/ml] and soil incubation plant growth promoting rhizobacteria (B. mycoides PM35) i.e., [(0 no B. mycoides PM35), 10 and 20 μL] on biochemical, morphological and physiological characteristics of Barley (Hordeum vulgare L.) plants under different levels of Cd in the soil i.e., [(0 Cd), 50 and 100 mg kg^-1]. Results from the present study showed that the increasing levels of Cd in the soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugars, and nutritional contents from the roots and shoots of the plants. In contrast, increasing levels of Cd in the soil significantly (P < 0.05) increased oxidative stress indicators in term of malondialdehyde, hydrogen peroxide, and electrolyte leakage, and also increased organic acid exudation patter in the roots of H. vulgare. Although, the activities of enzymatic antioxidants and the response of their gene expressions in the roots and shoots of the plants and non-enzymatic such as phenolic, flavonoid, ascorbic acid, and anthocyanin contents were initially increased with the exposure of 50 mg kg^-1 Cd, but decreased by the increasing the Cd concentration 100 mg kg^-1 in the soil. The negative impact of Cd toxicity can overcome the application of PGPR (B. mycoides PM35) and TiO₂⎯NPs, which ultimately increased plant growth and biomass by capturing the reactive oxygen species, and decreased oxidative stress in H. vulgare by decreasing the Cd contents in the roots and shoots of the plants. Our results also showed that the TiO₂⎯NPs were more sever and showed better results when we compared with PGPR (B. mycoides PM35) under the same treatment of Cd in the soil. Research findings, therefore, suggest that the combined application of PGPR (B. mycoides PM35) and TiO₂⎯NPs can ameliorate Cd toxicity in H. vulgare, resulting in improved plant growth and composition under metal stress, as depicted by balanced exudation of organic acids.

High killing rate of nematode and promotion of rice growth by synthetic volatiles from Bacillus strains due to enhanced oxidative stress response

The plant parasitic nematode Aphelenchoides besseyi is a major pest that poses serious threats to different vegetables and crop plants. In the present study, volatiles isolated from Bacillus spp. were utilized as green biocontrol agents to overcome nematodes. In in vitro experiment, Bacillus spp. GBSC56, SYST2, and FZB42 showed the strongest nematicidal activity with killing rates of 80.78%, 75.69%, and 60.45%, respectively, as compared with control. The selected synthetic volatile organic compounds (VOCs), namely albuterol, benzaldehyde (BDH), 1,2-benzisothiazol-3(2H)-one (1,2-HIT), dimethyl disulfide (DMDS), 2-undecanone (2-UD), and 1,3-propanediole (1,3-PD), exhibited strong nematicidal activity, with A. besseyi killing rate of 85.58%, 82.65%, 81.75%, 80.36%, 84.45%, and 82.36%, respectively, at 400 μg/mL. Microscopic analysis proved that the rapid mortality was due to the production of reactive oxygen species (ROS). Molecular docking attributed this ROS production to the nematicidal effect of synthetic VOCs on NADH DEHYDROGENASE SUBUNIT 2, which is known to play a critical role in the suppression of ROS in nematode models. In a greenhouse experiment, the Bacillus strains GBSC56, SYST2, and FZB42 and their synthetic VOCs significantly improved the physiological parameters in terms of growth promotion traits. In addition, selected genes related to growth promotion and defense genes showed a significant upregulation of their expression in rice seedlings treated with those synthetic VOCs. Overall, these findings revealed that the selected Bacillus strains and their synthetic VOCs possess high potential against A. besseyi. Moreover, this study also sheds new light on the mechanisms by which specific Bacillus nematicidal VOCs influence important genes involved in rice plant growth promotion and could effectively be used to suppress plant parasitic nematodes.

Bacillus spp.-Mediated Growth Promotion of Rice Seedlings and Suppression of Bacterial Blight Disease under Greenhouse Conditions

Rice (Oryza sativa L.) is a major cereal and staple food crop worldwide, and its growth and production are affected by several fungal and bacterial phytopathogens. Bacterial blight (BB) is one of the world's most devastating rice diseases, caused by Xanthomonas oryzae pv. oryzae (Xoo). In the current study, Bacillus atrophaeus FA12 and B. cabrialesii FA26 were isolated from the rice rhizosphere and characterized as having broad-range antifungal and antibacterial activities against various phytopathogens, including Xoo. In addition, the selected strains were further evaluated for their potent rice growth promotion and suppression efficacy against BB under greenhouse conditions. The result shows that FA12 and FA26, applied as seed inoculants, significantly enhanced the vigor index of rice seedlings by 78.89% and 108.70%, respectively. Suppression efficacy against BB disease by FA12 and FA26 reached up to 59.74% and 54.70%, respectively, in pot experiments. Furthermore, MALDI-TOF MS analysis of selected strains revealed the masses ranged from m/z 1040 to 1540, representing that iturins and fengycin are the major antimicrobial compounds in the crude extracts, which might have beneficial roles in rice defence responses against BB. In conclusion, FA12 and FA26 possess broad-range antagonistic activity and have the capability to promote plant growth traits. More importantly, applying these strains has a high potential for implementing eco-friendly, cost-effective, and sustainable management practices for BB disease.

Enhancement of nitrogen use efficiency through agronomic and molecular based approaches in cotton

Cotton is a major fiber crop grown worldwide. Nitrogen (N) is an essential nutrient for cotton production and supports efficient crop production. It is a crucial nutrient that is required more than any other. Nitrogen management is a daunting task for plants; thus, various strategies, individually and collectively, have been adopted to improve its efficacy. The negative environmental impacts of excessive N application on cotton production have become harmful to consumers and growers. The 4R's of nutrient stewardship (right product, right rate, right time, and right place) is a newly developed agronomic practice that provides a solid foundation for achieving nitrogen use efficiency (NUE) in cotton production. Cropping systems are equally crucial for increasing production, profitability, environmental growth protection, and sustainability. This concept incorporates the right fertilizer source at the right rate, time, and place. In addition to agronomic practices, molecular approaches are equally important for improving cotton NUE. This could be achieved by increasing the efficacy of metabolic pathways at the cellular, organ, and structural levels and NUE-regulating enzymes and genes. This is a potential method to improve the role of N transporters in plants, resulting in better utilization and remobilization of N in cotton plants. Therefore, we suggest effective methods for accelerating NUE in cotton. This review aims to provide a detailed overview of agronomic and molecular approaches for improving NUE in cotton production, which benefits both the environment and growers.

Melatonin-Induced Inhibition of Shiraia Hypocrellin A Biosynthesis Is Mediated by Hydrogen Peroxide and Nitric Oxide

Melatonin (MLT), an evolutionarily conserved pleiotropic molecule, is implicated in numerous physiological processes in plants and animals. However, the effects of MLT on microbes have seldom been reported. In this study, we examined the influence of exogenous MLT on the growth and hypocrellin biosynthesis of bambusicolous fungus Shiraia sp. S9. Hypocrellin A (HA) is a photoactivated and photoinduced perylenequinone (PQ) toxin in Shiraia. Exogenous MLT at 100.00 μM not only decreased fungal conidiation and spore germination but inhibited HA contents significantly in fungal cultures under a light/dark (24 h:24 h) shift. MLT treatment was associated with higher activity of antioxidant enzymes (superoxide dismutase, catalase and peroxidase) and a marked decline in reactive oxygen species (ROS) production in the mycelia. Moreover, MLT induced endogenous nitric oxide (NO) production during the culture. The NO donor sodium nitroprusside (SNP) potentiated MLT-induced inhibition of O₂⁻ production, but NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) enhanced O₂⁻ production, whereas MLT-induced NO level was increased by the ROS scavenger vitamin C (Vc). The changes in NO and H₂O₂ were proved to be involved in the MLT-induced downregulation of the expressions of HA biosynthetic genes, leading to the suppression of HA production. This study provides new insight into the regulatory roles of MLT on fungal secondary metabolism activities and a basis for understanding self-resistance in phototoxin-producing fungi.

Toxicity of Bacillus thuringiensis Strains Derived from the Novel Crystal Protein Cry31Aa with High Nematicidal Activity against Rice Parasitic Nematode Aphelenchoides besseyi

The plant parasitic nematode, Aphelenchoides besseyi, is a serious pest causing severe damage to various crop plants and vegetables. The Bacillus thuringiensis (Bt) strains, GBAC46 and NMTD81, and the biological strain, FZB42, showed higher nematicidal activity against A. besseyi, by up to 88.80, 82.65, and 75.87%, respectively, in a 96-well plate experiment. We screened the whole genomes of the selected strains by protein-nucleic acid alignment. It was found that the Bt strain GBAC46 showed three novel crystal proteins, namely, Cry31Aa, Cry73Aa, and Cry40ORF, which likely provide for the safe control of nematodes. The Cry31Aa protein was composed of 802 amino acids with a molecular weight of 90.257 kDa and contained a conserved delta-endotoxin insecticidal domain. The Cry31Aa exhibited significant nematicidal activity against A. besseyi with a lethal concentration (LC₅₀) value of 131.80 μg/mL. Furthermore, the results of in vitro experiments (i.e., rhodamine and propidium iodide (PI) experiments) revealed that the Cry31Aa protein was taken up by A. besseyi, which caused damage to the nematode's intestinal cell membrane, indicating that the Cry31Aa produced a pore-formation toxin. In pot experiments, the selected strains GBAC46, NMTD81, and FZB42 significantly reduced the lesions on leaves by up to 33.56%, 45.66, and 30.34% and also enhanced physiological growth parameters such as root length (65.10, 50.65, and 55.60%), shoot length (68.10, 55.60, and 59.45%), and plant fresh weight (60.71, 56.45, and 55.65%), respectively. The number of nematodes obtained from the plants treated with the selected strains (i.e., GBAC46, NMTD81, and FZB42) and A. besseyi was significantly reduced, with 0.56, 0.83., 1.11, and 5.04 seedling mL^-1 nematodes were achieved, respectively. Moreover, the qRT-PCR analysis showed that the defense-related genes were upregulated, and the activity of hydrogen peroxide (H₂O₂) increased while malondialdehyde (MDA) decreased in rice leaves compared to the control. Therefore, it was concluded that the Bt strains GBAC46 and NMTD81 can promote rice growth, induce high expression of rice defense-related genes, and activate systemic resistance in rice. More importantly, the application of the novel Cry31Aa protein has high potential for the efficient and safe prevention and green control of plant parasitic nematodes.