Melatonin-Mediated Signalling and Regulation of Viral and Bacterial Diseases

The steadily increasing stricter legislation regarding the accessibility and application of chemically synthesized pesticides in crop protection from the devastating effects of phytopathogens has redirected the interest to nontoxic, biocompatible, and biodegradable alternatives. Accordingly, the strengthening of plant's innate immune system with naturally occurring organisms or derivatives has gained widespread acceptance within the current context of climate disorders. The so-called biopriming comprises the use of oligomers, like polysaccharides, proteins, chitosan secondary metabolites (phenolics, flavonoids, alkaloids, etc.) and of relatively recent, melatonin. Melatonin also called phytomelatonin is a ubiquitous multifaceted biomolecule found in both animal and plant kingdoms. Research on melatonin in plants has risen exponentially in recent years and its role as master regulator of multiple physiological processes including but not limited to mitigation of biotic and abiotic stress in plants has been widely reported. Hence, exogenous melatonin application has proven its ability to dwarf-off several plants' diseases including fungal, bacterial, and viral diseases. However, very few studies have focused on how melatonin helps prevent crops against viral infections. In addition, the role of Mel in signalling events leading to induced tolerance to bacterial and viral diseases is still quite speculative. Clear elucidation of such pathways could shift the paradigm of plant disease control by providing paths to plant scientists for efficient use of phytomelatonin in crop protection against major plant diseases. In this chapter, we discussed the destructive effects of plants bacterial and viral diseases in agriculture; we also provided recent findings on physiological roles of melatonin in plants. And we meticulously described the components of Mel-mediated plant-triggered immune response towards bacterial and viral infections and the interplay with other signal compounds.