References

142 S.J. Murch
However, over the last 45 years, there have been numerous reports of
other compounds that modulate the auxin:cytokinin responses or of synthetic
compounds that induce regenerative responses in the absence of
exogenous auxin or cytokinin. As well, one of the problems inherent in
attributing plant responses to the exogenous application of a plant growth
regulatormaybethefailuretoincorporatethemetabolismofthemolecule
into the model. Therefore, the various plant responses to auxin may actually
represent the responses of plant tissue to a range of auxin-derived
or auxin-stimulated metabolites. An alternate potential role for melatonin
may be in the photoregulation of plant morphogenic processes, specifically
those associated with auxin. A classic bioassay of auxin activity in etiolated
lupin hypocotyls indicated that melatonin can play an auxenic role
in plant physiology (Hernandez-Ruiz et al. 2004). Further reexamination
of the classic systems of plant physiology may provide new clues to the
role of melatonin that may be independent of other growth regulators, may
mediate other growth regulators or may be involved in the expression of
responses attributed to other compounds.
10.2
Neuroregulators in Plants
Plantsareamongthemostdiverseofthebiologicalkingdomsandthesheer
number of species provides for an almost endless potential for discovery.
Plants are also limited by an inability to flee from hostile environments
andhaveevolvedaseriesofstrategiesforsurvivalthatincludesecondary
metabolism for the synthesis of numerous interesting molecules. Many of
thesemoleculesmediateneurologicalprocessesandthereforeareusedas
pain medications, hallucinogens, antidepressants and poisons. It has been
estimated that plant species produce more than 35,000 unique biochemicals
that are not required for normal growth or development. Since insects
lack a blood brain barrier, neurologically targeted plant compounds can
have an immediate and often disruptive or fatal effect. The production
of secondary metabolites that are neuroregulating compounds provides
plants with a defense system against predators as these compounds affect
receptorsasagonistsorantagonistsortheymodulatevariousstepsin
neuronal signal transduction such as ion channels or enzymes that take up
or degrade neurotransmitters.
One of the essential shared experiences in virtually every human culture
is a gathering to consume a plant preparation that alters mood, experience
or feeling. We meet for coffee (Coffea arabica), share a cup of tea (Camellia
sinensis), or gather to smoke khat (Celastrus edulis) oracigarette(Nicotiana
tabacum). Traditional cultures in many parts of the world also use