References

140 S.J. Murch
edible plants (Manchester et al. 2000). Exogenous application of melatonin
to plant tissues was shown to increase the birefringence and number of
mitotic spindles in lily cells (Jackson 1969) and to disrupt mitosis in onion
root cells (Banerjee and Margulis 1973). Melatonin also promoted vegetative
growth of etiolated hypocotyls in lupins (Lupinus albus)(Hernandez-
Ruiz et al. 2004). Exogenously applied melatonin also affected the early
steps in the transition to flowering, thereby reducing the number of flowers
initiated in Chenopodium rubrum (Kolar et al. 2003). These results indicate
an association between melatonin and photoregulated metabolic processes
in plants such as flowering, seed and root development and is indicative of
how much there is left to discover about the inner mechanisms of plant life,
plant signaling and plant behavior. However, signaling molecules, by their
nature, are short-lived, unstable, difficult to detect and difficult to quantify.
To further investigate the role of melatonin in plant metabolism, a series
of inhibitors of the melatonin synthesis pathway were evaluated. There
are a wide range of compounds commercially available that mediate the
serotonin–melatonin pathway as these are common pharmaceutical targets
for medications for depression, migraine and attention-deficit disorders.
Four inhibitors of auxin and indoleamine metabolism in plants were identified
from a group of more than 20 pharmaceutical compounds by exposure
of axenic cultures and quantification of auxin, serotonin, melatonin and related
metabolites (Murch 2000). p-Chlorophenylalanine, D-amphetamine,
fluoxetine (Prozac) and methylphenidate (Ritalin) were shown to alter
the rooting potential of stem and hypocotyl explants of St. John’s wort
(Murch et al. 2001, 2004). In general, significant reductions in de novo
root regeneration were found to correspond with decreases in the pool
of both indole acetic acid and melatonin. In one instance, root organogenesis
was impaired when melatonin concentration decreased but auxin
concentration remained unchanged, indicating that the compounds are
interdependent (Murch et al. 2001). Interestingly, p-chlorophenylalanine
significantly decreased the endogenous concentration of melatonin to below
detection limits with concurrent increases in serotonin concentration.
p-Chlorophenylalanine is produced as a pharmaceutical compound that
depletes mammalian serum serotonin concentrations (Yamada et al. 1999).
In plants, p-chlorophenylalanine mediated the same the biochemical pathway
and was manifested as a morphological response characterized by
the total inhibition of root organogenesis and the development of shoots
(Fig. 10.3; Murch et al. 2001).
In 1957, Skoog and Miller hypothesized that the redirection of plant
growth is initiated by changes in the relative ratio of plant growth regulators,
viz., auxins and cytokinins. Auxin, often refereed to as “master controller”
(Trewavas 1997), is among the best characterized metabolites of
tryptophan (Fig. 10.4). Auxins induce a polarized growth, while cytokinins