Furocoumarin production by pastinaca sativa and apium graveolens
Date
1988
Authors
Woodcock, Paul Vincent
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Abstract
The furocournarins are a group of secondary metabolites, produced from the same precursors as the flavonoids in Pastinaca sativa (parsnip), and Apium graveolens (celery). The compounds help protect plants from disease and herbivory. The furocournarins produced by these plants are capable of intercalating into the double helix of DNA. If excited by ultra violet (UV) irradiation they are capable of forming mono- and di-adducts with pyrimidine bases, especially thymine. The photo-adducts have been shown to be toxic and mutagenic in both prokaryotic and eukaryotic cells. This property makes the furocournarins, in the presence of UV light, an effective deterrent against fungal, bacterial, and insect attack. The compounds also pose some problems for humans handling plant products containing furocournarins.
The aims of this work are to look at the patterns of furocournarin production during the first year of growth of P. sativa and~. graveolens. Changes in the levels of production of these molecules will affect the ability of the plants to defend themselves against opportunistic attack.
The furocournarins were extracted from fresh plant tissue with ethyl acetate. The compounds were qualitatively detected by biological photo-assay using a DNA repair deficient Escherichia coli, and quantification was done by high pressure liquid chromatography.
P. sativa produces seven furocoumarins during the first year of growth. Seedlings produce the highest levels of furocoumarin. Seedlings require the most protection, as any damage to the small plants will probably kill them. Between 19 and 55 days after planting there is a reduction in furocoumarin production, which remains depressed until after 132 days after planting. At this stage the plants are growing most actively. Plants may reduce furocoumarin production to prevent damage to the mitotically active cells. After 132 days after planting furocoumarin production in leaves and petioles rises, but falls in roots. Protection of the established photosynthetic tissue by the production of substantial amounts of furocoumarin will ensure the overwintering roots have sufficient nutrients to grow in the next year. Roots at this stage of development are unlikely to benefit from the presence of furocoumarins. The roots are firmly established underground and, in the absence of UV irradiation, the furocoumarins would have little or no inhibitory effect on invading pathogens.
A. graveolens produces much lower concentrations of furocoumarin than P. sativa. Only three furocoumarins were detected in A. graveolens, 5-methoxypsoralen and 8-methoxypsoralen which are photoactive, and isopimpinellin which is not. No clear patterns of furocoumarin production emerged either between tissues or as the plants age. Levels are not high enough to clearly deter insect feeding or inhibit pathogen attack. It seems likely that A. graveolens would have to rely on the defence mechanisms, which would increase furocoumarin production, to prevent successful microbial attack.