References

10 Neurotransmitters, Neuroregulators and Neurotoxins in Plants 139
high levels in 108 plant species used in traditional Chinese medicine (Chen
et al. 2003). The fundamental questions underlying the discovery of melatonin
continue to be centered on the role of melatonin in the metabolism of
higher plants. It has been hypothesized that the role of melatonin in plants
may be analogous to its function in mammals as a chemical messenger of
light and dark, calmodulin binding factor or an antioxidant (Balzer and
Hardeland 1996).
To investigate these questions, in vitro cultures of several medicinal
plants were established to ensure that the quantified melatonin did not originate
from a bacterial or fungal contaminant. Melatonin has been quantified
in axenic cultures of Hypericum perforatum, Arabidopsis thaliana, Scutellaria
baicalensis, Artemisia judaica and many others (Murch et al. 1977;
Murch and Saxena, unpublished results). The recovery of radiolabel from
14 C-tryptophan as 14 C-melatonin and 14 C-serotonininsterileSt.John’swort
plantlets provided the first evidence of endogenous biosynthesis (Murch
et al. 2000). Circadian rhythms in melatonin concentration were observed
in Chenopodium rubrum (Kolar et al. 1997) and melatonin concentrations
werehighestatspecificstagesofflowerbuddevelopment(MurchandSaxena
2002b). In A. thaliana, melatonin levels were significantly higher in
sterile etiolated seedlings than in those seedlings that had been grown in
light (Fig. 10.2). Additionally, significantly higher levels of melatonin were
found in mature ripe tomatoes than in green tomatoes (Van Tassel et al.
2001) and melatonin concentrations varied from 2 to 190 ng g in the seeds of
Mel
Concentration of Melatonin
250
200
150
100
50
0
Light Dark
Treatment
Fig.10.2. Melatonin detection and quantification in Arabidopsis thaliana. a Melatonin was
detected by high-performance liquid chromatography separation with electrochemical detection
as described previously (Murch et al. 2000). b Significantly more melatonin was
found in sterile etiolated seedlings germinated and grown in complete darkness than was
quantifiedinseedlingsgrownwitha16-hphotoperiod