Melatonin regulates plant stress resistance and immune system
Melatonin (N-acetyl-5-methoxytryptamine) is a kind of hydrophilic and lipophilic hormone with various regulation of circadian rhythm (Behram, Aydin, & Gorgisen, 2017), which regulates sleep and circadian functions in animals (Radogna, Diederich, & Ghibelli, 2010). Melatonin also plays crucial role as an anti-oxidant, anti-inflammatory, immune-modulatory effects on human tissues (Nayak et al., 2019).
Notably, endogenous melatonin has also been found in plants as an effective antioxidant and free radical scavenger (Kang, Lee, Park, Byeon, & Back, 2013; Murch, Alan, Cao, & Saxena, 2009). Studies have shown that melatonin can modify the structure of plant hormone carriers and the development pattern of stems and leaves (Arnao, & Hernández-Ruiz, 2015). Physiological processes including seed and root growth (Arnao, & Hernández-Ruiz, 2014; Sarropoulou, Dimassi-Theriou, Therios, & Koukourikou-Petridou,2012; Byeon, & Back, 2014; Park, Le, Byeon, Kim, & Back, 2013), flowering (Huang et al., 2017; Kolář, Johnson, & Macháčková, 2003), photosynthetic system (Zhao et al., 2015; Arnao, & Hernández‐Ruiz, 2009) and reproductive system could be regulated by melatonin. As antioxidant in plant, previous research has found that melatonin decreased significantly free radicals in barley tissues, which protect chlorophyll from aging (Arnao & Hernández‐Ruiz, 2009; Wang et al., 2013). Furthermore, melatonin can keep leaf from senescence by regulating the activity of antioxidant enzymes (Zhao et al., 2017). It improves photosynthesis, CO2 uptake and biomass. Accordingly, it can be used to protect plants exposed to strong light from burns and enhance the resistance to strong light (Zhao et al., 2017). Additionally, exogenous melatonin can influence the early stages of flower induction and development.
In addition, melatonin can resist biotic and abiotic stress (Fu et al., 2017; Lee, Byeon, & Back, 2014; Weeda et al., 2014) and regulate plant physiological ion balance against stress (Wei et al., 2015; Zeng et al., 2018) Plant damage caused by environmental stress including cold, drought, UV irradiation, and chemical stressors (Arnao, & Hernández-Ruiz, 2015; Weeda et al., 2014; Mandal, Suren, Ward, Boroujerdi, & Kousik, 2018) can be mitigated by melatonin. Under bacterial pathogen infection, increased endogenous can induce the transcription of stress-related genes, such as CBF/DREB1s inArabidopsis thaliana (Shi, Qian, Tan, Reiter, & He, 2015). Moreover, the rise of soluble sugar caused by melatonin is also responsible for its biological resistance (Yang et al., 2010; Qian, Tan, Reiter, & Shi, 2015). The overexpression of melatonin andCBF/DREB1s results in the increase of transcription levels of multiple stress response genes, and the accumulation of soluble sugars, such as sucrose (Qian, Tan, Reiter, & Shi, 2015). It has great potential for melatonin to regulate plant against biotic and abiotic stress. Ideas about melatonin as new plant growth regulator could provide insights into the solution for food security issues (Lee, Byeon, & Back, 2014).
Furthermore, melatonin can interact with the endogenous plant hormones. Previous research showed that melatonin can stimulate plant to produce ABA and ethylene, which lead to berry ripening (Fu et al., 2017; Qian, Tan, Reiter, & Shi, 2015). In addition, it could regulate nitrogen oxides and salicylic acid levels by activating plant defense-related genes to relieve the biological pressure (Yin et al., 2013; Li et al., 2018). The effects of melatonin on plants can be used as a plant regulator with potential applications in crop improvement and protection (Arnao, & Hernández-Ruiz, 2015).