Vitamin D compounds in plants

An appreciable number (15) of flowering plants. specially within the Solanaceae family, have been shown to contain vitamin D-3 and its hydroxylated derivatives, including 1alpha,25(OH)(2)-vitamin D-3 [1alpha,25(OH)D-3], a pluripotent hormone in animals. These secosteroids have also been detected in members of the Cucurbitaceae, Fabaccae and Poaceae families. On the basis of recent cladistic analysis it is possible to predict that the synthesis of these compounds is a characteristic of the Angiosperms. Highly specific and sensitive bioassays and analytical procedures of high resolution are now available which may allow confirmation of this hypothesis. Solanum glaucophyllum is the species which accumulates 1alpha,25(OH)D-3 to the greatest extent. The metabolite is distributed among different tissues of the plant as a free steroid or glycoside derivatives. The presence of 7-dehydrocholesterol., vitamin D-3, 25(OH)D-3 and 1alpha,25(OH)(2)D-3 in S. glaucophyllum and other species has been unequivocally demonstrated. Moreover, recent studies with radioactive precursors. protein immunoblot and RNA hybridization analysis suggest the presence of specific hydroxylases for vitamin D-3 and 25(OH)D-3 structurally related to the corresponding enzymes in vertebrates. It appears then that plants possess a similar synthetic route to 1alpha,25(OH)(2)D-3 as in animals. However, of unique biological importance, evidence obtained with S. glaucophyllum and Nicotiana glauca supports the operation of a non-photolytic reaction of vitamin D-3 synthesis. Both intermediates and enzymes of the vitamin D-3 pathway can be detected in cell and tissue cultures thus affording a convenient experimental model for studies on its molecular characterization and regulation. This is also endowed with biotechnological significance. Vitamin D-3 compounds may play a function in Angiosperms. Studies in vitro have shown that, like mitogenic plant hormones, they stimulate root growth and differentiation through activation of the Ca2+ messenger system. There are vitamin D-3 and 1alpha,25(OH)(2)D-3 binding proteins in plants which exhibit characteristics suggesting a regulatory function. It remains to be established whether these binders act Lis true steroid hormone receptors. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.