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.