Acetylcholine enables the exchange of information between different plant tissues
Acetylcholine (ACh) is a well-known neurotransmitter in the central and peripheral nervous system (Picciotto, Higley, & Mineur, 2012), which regulates neuronal excitability, presynaptic release of neurotransmitters, synaptic transmission, and coordinates firing of groups of neurons (Lazar, Metherate, & Kawai, 2007). The ability of acetylcholine to coordinate the neuronal networks makes it an essential mechanism for controlling complex behaviors in animals (Picciotto, Higley, & Mineur, 2012). We consider acetylcholine here as an animal hormone because of its activity of cholinesterases. This meets all the criteria for an autocrine/paracrine hormone in lung bronchial epithelium (Proskocil et al., 2004).
ACh has been detected in many taxonomic groups throughout plant kingdom (Brenner et al., 2006). Both young leaves and root can synthesis ACh (Tretyn, & Kendrick, 1991; Bamel, Gupta, & Gupta, 2015) which regulates various physiological processes in plants. In terms of plant growth and development, previous research showed an increase in the number of secondary roots in tomato and radish caused by exogenous ACh (Bamel, Gupta, & Gupta, 2015; Sugiyama, & Tezuka, 2011). Existing evidence has shown ACh can also accelerate the germination of seeds in the control of light (Tretyn, & Kendrick, 1991) and cause the merging of vacuolar fluorescent reporters to result in an indirect stimulation of cell expansion in vacuolar (Di , Fornaciari, Barozzi, Piro, & Arru, 2014). Additionally, there have been interactions between Ach and plant hormone, such as auxin to regulate root and interact with phytochrome (Di , Fornaciari, Barozzi, Piro, & Arru, 2014). From the perspective of reproductive system, ACh could participate in the phytochrome-controlled flowering in plants. ACh affects photoperiodic induction of flowering by altering the bioelectric potentials of leaves (Tretyn, & Kendrick, 1991).
Moreover, ACh can help plants to cope with abiotic stress. Previous studies suggested that a positive trend on germination traits in soybean under osmotic stress (Braga, Pissolato, & Souza, 2017). ACh enhances activities of antioxidant enzymes, such as peroxidase and superoxide dismutase, meaning a potential application to alleviate salt-stress damage (Kang, Kim, Park, & Back, 2009). The control of stomatal movement may also be one of the reasons for ACh against environmental stress. Previous studies showed reduced amount of ATP synthesis in chloroplasts to regulate stomatal movement (Tretyn, & Kendrick, 1991; Bamel, Gupta, & Gupta, 2015). The reason stomata are regulated is probably to maximize water use under stress.