Occurrence of provitamins and vitamins D in higher plants
Plants produce provitamins D, vitamins D and related compounds in their leaves (Napoli et al. 1977;
Wasserman 1975; Prema & Raghuramulu 1994, 1996; Zucker et al. 1980; Rambeck et al. 1981; Horst et al.
1984).
In the case of ergosterol and vitamin D2 one has to be cautious in assigning substances found in the analysis
of plants to synthesis by the plants themselves. Many plants, among them many grasses, harbour en-
dophytic fungi (Clay 1990; Redlin & Carris 1996; Siegel et al. 1987), and fungi regularly produce ergos-
terol as their major sterol. The content of ergosterol in plant tissue has been used as a measure of fungal
contamination (Gessner & Schmitt 1996).
It is a widespread misconception in the literature that plants produce only provitamin D2 and vitamin D2
(e.g., Buddecke 1980). Often as much provitamin D3 and vitamin D3 are produced (Zucker et al. 1980,
Prema & Raghuramulu 1996). Even 1,25-dihydroxy vitamin D3 has been found in plants (Napoli et al.
1977), as well as a glycoside of this compound, sometimes at concentrations high enough to poison grazing
animals (Wasserman et al. 1976).
We have confirmed a UV-B dependent synthesis of vitamins D2 and D3 in the leaves of the tomato plant.
An interesting observation is that the provitamin D3 content of tomato leaves is not reduced by growing
plants under UV-B radiation, although a substantial amount of vitamin D3 is formed (Table 2). This points
to a feedback mechanism regulating the amount of the provitamin.
Table 2. Contents of provitamins and vitamins D2 and D3 in tomato (Lycopersicon esculentum Mill).
Tomato plants were grown in a greenhouse with or without UV-B radiation (0.85 kJ plant weighted UV-B
radiation per m2 and day).
Organism Micrograms per gram dry weight
Provit.D2 Provit. D3 Vitamin D2 Vitamin D3
Tomato (-UV-B) 1.83 0.61 0 0
Tomato (+UV-B) 2.23 0.76 0.087 0.28
Recently Curino et al. (1998) made the startling discovery that Solanum glaucophyllum cells are able to
synthesise vitamin D3 and its derivatives in darkness. This is the only documented case of vitamin D3
synthesis in the absence of UV-B. Solanum glaucophyllum is a very special plant which accumulates large
amounts of dihydroxy vitamin D3 as a protection against grazing mammals. Mechanisms for nonphoto-
chemical formation of vitamin D have been proposed by Norman & Norman (1993).
[...]
Conclusion
Plants and some algae are able to perceive ultraviolet-B radiation and regulate chemical processes and morphogenesis in a radiation-dependent manner, and are thought to have an ultraviolet-B specific photorecep-
tor. Provitamins D2 and D3 are present in leaves of land plants and in some algae, and are converted, with a high quantum yield, to previtamins D and vitamins D upon exposure to ultraviolet-B radiation. The action
spectrum for several ultraviolet-induced phenomena in plants (Björn 1999) and for provitamin D conversion in human skin peak at the same wavelength. Photodestruction of provitamin D is known to
change the activity of membrane-bound enzymes in yeast cells. We have pointed to the possibility that provitamins D act as ultraviolet-B photoreceptors for UV-B induced regulatory reactions in plants, and
proposed that the proportions between provitamins, previtamins, and vitamins D can be used to evaluate the UV-B exposure of phytoplankton and plants.
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