Waking up for defense! Melatonin as a regulator of stomatal immunity in plants

Abstract

Melatonin is a tryptophan-derived compound discovered in the late 1950s as a molecule produced by the pineal gland of most vertebrates. Melatonin gained notoriety in the 1970s when it was demonstrated that its production increased in humans during the night in a rhythmic fashion associated with sleep synchronization (Xie et al., 2017). Melatonin is a ubiquitous molecule produced by bacteria, fungi, plants, and animals. In plants, melatonin regulates a wide range of processes, such as seed germination, root and shoot growth, and production of secondary metabolites, and is involved in circadian cycle regulation (Arnao and Hernández-Ruiz, 2019). The recent identification of a melatonin receptor named PHYTOMELATONIN RECEPTOR1 (AtPMTR1) in the plant model Arabidopsis (Arabidopsis thaliana) demonstrated that melatonin also governs an AtPMTR1-dependent mechanism of stomatal closure (Wei et al., 2018). As stomatal closure is a well-known process utilized by plants to restrain the invasion of pathogens, usually referred to as stomatal immunity (Melotto et al., 2006), melatonin is now gaining attention for its capacity to mediate responses to biotic stress in plants (Moustafa-Farag et al., 2020). Unfortunately, the underlying molecular mechanisms of melatonin regulation of plant immunity remain largely unknown.