Abstract
This review summarizes the advances that have been made in terms of the identified functions of melatonin in plants. Melatonin is an endogenously-produced molecule in all plant species that have been investigated. Its concentration in plant organs varies in different tissues, e.g., roots versus leaves, and with their developmental stage. As in animals, the pathway of melatonin synthesis in plants utilizes tryptophan as an essential precursor molecule. Melatonin synthesis is inducible in plants when they are exposed to abiotic stresses (extremes of temperature, toxins, increased soil salinity, drought, etc.) as well as to biotic stresses (fungal infection). Melatonin aids plants in terms of root growth, leaf morphology, chlorophyll preservation and fruit development. There is also evidence that exogenously-applied melatonin improves seed germination, plant growth and crop yield and its application to plant products post-harvest shows that melatonin advances fruit ripening and may improve food quality. Since melatonin was only discovered in plants two decades ago, there is still a great deal to learn about the functional significance of melatonin in plants. It is the hope of the authors that the current review will serve as a stimulus for scientists to join the endeavor of clarifying the function of this phylogenetically-ancient molecule in plants and particularly in reference to the mechanisms by which melatonin mediates its multiple actions.
Highlights
The identification of the tryptophan derivative, N-acetyl-5-methoxytryptamine, in bovine pineal tissue in the late 1950s was a major discovery [1,2]
Since melatonin synthesis in the animal pineal gland typically depletes serotonin levels in contrast to the present study where they were elevated under salt stress, the increase in this constituent obviously required an independent process mediated by NaCl
UVB light, melatonin levels increased in the roots several-fold; the authors speculated that the rise in melatonin was for the purpose of protecting plant tissues from oxidative stress caused by the free radicals induced by UVB exposure
Summary
The identification of the tryptophan derivative, N-acetyl-5-methoxytryptamine, in bovine pineal tissue in the late 1950s was a major discovery [1,2]. The second seminal report that identified melatonin in higher plants is that of Hattori et al [39] Their rationale for initiating the studies differed from that of Dubbels and co-workers [32]. As in the first study, the concentrations of melatonin in plant products measured varied widely from 5 pg/g to 5 μg/g tissue To further verify their findings, Hattori et al [39] generated concentration displacement curves using authentic melatonin and extracts of several plants. Following the publication of these reports [32,39], major questions obviously remained unanswered These included an explanation for the wide range of melatonin concentrations in plants and the functional significance of the indoleamine in plant cells. It was unknown whether plants synthesized the melatonin they contained or whether it was merely taken up from the growth medium
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