Melatonin can be a defense signaling molecule in plant-pathogen interactions. Application of melatonin (MEL, N-acetyl-5-methoxytrptamine) an indole amine molecule, protect plant against biotic and abiotic stress (Savvides et al., 2016; Zhao et al., 2019). Recent studies suggest that melatonin regulates the expression of multiple elements including enzymes, receptors and transcription factors involved in auxin, gibberellic acid, salicylic acid (SA), ethylene (ET), abscisic acid signaling, nitric oxide (NO) (signaling molecule), strigolactones and brassinosteroids (Sun et al., 2020). Many strategies have been applied to control infection by pathogens and one of them is the use of melatonin application, an eco-friendly immune inducer that may enhance plant immunity and suppress pathogen growth (Mandal et al., 2018).
Application of melatonin on Arabidopsis and tobacco leaves induced various pathogenesis related genes including series of defense genes activated by SA, and ET (Lee et al., 2014). SA activates plant defense responses after pathogen attack. Nitric oxide is another signal that activates defense response after pathogen attack (Klessig et al., 2000). Exogenously applied melatonin works as a signal molecule and enhances synthesis of NO and SA. The NO triggers the induction of SA accumulation which further activates pathogenesis-related (PR) gene expression, improving plant resistance to pathogen infection (Ali et al., 2021). Exogenous melatonin can activate complex set of defense mechanism against Botrytis cinerea attacking tomato plant. Melatonin treatment enhanced disease resistance to gray mold rot caused by B. cinerea in cherry tomato (Li et al., 2019b). The enhanced disease resistance is attributed to the increased endogenous melatonin, inducing SA signaling pathway associated with reactive oxygen species (ROS) burst, as well as the PRs and moreover activating the phenylpropanoid pathway along with increase in lignin, phenolic compound and flavonoid content in cherry tomato fruit (Li et al., 2019b). Melatonin is known for its antioxidant ability by not only directly scavenging ROS and reactive nitrogen species but also influence antioxidant enzyme activity, thereby improving its ability to protect cells from oxidative stress (Reiter et al., 2001; Rodriguez et al., 2004; Galano et al., 2013; Arnao and Hernandez-Ruiz 2019).
Exogenous melatonin directly inhibits fungal growth and indirectly increase the plant defense activities against fungal infection (Thangaraj et al., 2022). Melatonin plays a role in invertase related carbohydrate metabolism (Zhao et al., 2015). Exogenous melatonin treatment cause an increase in the level of cellulose, xylose and galactose of cell wall in Arabidopsis thaliana during Pseudomonas syringae pv. tomato DC3000 infection which implies for cell wall reinforcement and callose deposition providing resistance to plant against pathogen invasion (Zhao et al., 2015; Hernandez-Ruiz et al., 2023). Melatonin confers resistance to banana (Musa acuminata) against Fusarium oxysporum f. sp. cubense (Wei et al., 2017). The use of melatonin as a biocontrol agent for plant disease caused by viruses has been addressed. Melatonin supplement induces an increase in levels of defense hormones SA, jasmonic acid and ET as well as NO which may improve resistance to virus attack (Hernandez-Ruiz et al., 2023).
Exogenous application of melatonin before inoculationwith Colletotrichum gloesporioides protected pepper plant from the impact of pathogen infestation. In addition to inducing plant immunity against the pathogen attack, melatonin application reduced C. gloesporioides by inhibiting mycelial growth (Ali et al., 2021).
Exogenous melatonin application:
- Improves Malus prunifolia (apple plant) resistance to Marssonina apple blotch caused by Diplocarpon mali (Yin et al., 2013).
- Enhances Cotton immunity to soil-borne fungal pathogen Verticillium dahlia by increase in lignin and gossypol biosynthesis (Li et al., 2019a).
- Reduced tobacco mosaic virus (TMV) viral RNA and virus concentration in infected Nicotiana glutinosa and Solanum lycopersicum seedlings (Moustafa-Farag et al., 2020).
- Leads to reduced decay of strawberry (Fragaria anannasa cv. Selva) fruit which is prone to post harvest fungal decay caused by Botrytis cinerea and Rhizopus stolonifera (Aghdam and Fard 2017).
Different concentration of melatonin display growth inhibitory activity against several plant fungal pathogens such as Alternaria sp., Botrytis sp. and Fusarium sp. growing in standard media(Arnao and Hernandez-Ruiz 2015). Melatonin application impaired potato late blight by inhibiting mycelial growth of Phytophthora infestans the causal organism (Zhang et al., 2017). Melatonin may be used as a biocontrol agent.
References:
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