Neurotransmitters, Neuroregulators and Neurotoxins in Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 137

10 Neurotransmitters, Neuroregulators

and Neurotoxins in Plants
Susan J. Murch

Abstract The transmission of signals between living cells is essential for the life of an organ-
ism as it provides the mechanism by which cells respond to external stimuli. Plants produce
a wide range of phytochemicals that mediate cell function and translate environmental
cues for survival and many of these are human neuroregulatory molecules. For example,
the human neurotransmitter melatonin ( N -acetyl-5-methoxytryptamine) is a ubiquitous,
highly conserved molecule associated with timing of circadian rhythms in many organisms,
including higher plants. Other compounds such as hyperforin, now isolated from several
plant species, may function as serotonin transport or nonspecific cation channel activators
in human brains and potentially in higher plants. A different group of neuroregulatory
molecules produced by plants overstimulate human neurons, resulting in neuronal cell
damage and death. Excitotoxins such as β -methylamino-L-alanine not only affect human

health but are also regulatory molecules redirecting plant growth. Many fascinating ques-
tionsinfutureresearchwillbedefiningtheroleofneurotransmitters,neuroregulatorsand
neurotoxins in the growth and development of plants. As newer technologies emerge, it
will become possible to understand more about the role of neurological compounds in the
inner workings of plant metabolism, plant environment interactions and the impact of plant
neurosystems on human neurology.

10.1
Neurotransmitters: Signaling Molecule in Plants?

Neurotransmitters found in plants to date include acetylcholine, epineph-
rine, dopamine, levodopa,

γ -aminobutyrate (GABA), glutamate, indole-3-

acetic acid, 5-hydroxyindole acetic acid, melatonin and serotonin (Fig. 10.1).
Thehuman“fightorflight”neurohormones,catecholamines,promote
flowering in short day plants (Khurana et al. 1987) and accumulate when
plants are stressed (Swiedrych et al. 2004). Other human neurohormones,
such as GABA, function as mechanisms to conserve nutrients under stress
conditions in plants or as antiherbivory compounds (Shelp et al. 2003).
Another class of human neurohormones, indoleamines, can also influ-
ence plant growth and development (Murch and Saxena 2002a). Melatonin
( N -acetyl-5-methoxytryptamine) was first isolated from the bovine pineal
gland in 1958 (Lerner et al. 1958). In the human brain, the highest levels
of melatonin are found during deep sleep and melatonin levels decline
with sunrise. The amplitude of the fluctuation of melatonin concentration
declines with age, a phenomenon associated with sleep rhythm disorders,