Thursday, June 24, 2010

Vampire Plants?

Common Broomrape, Orobranche minor

Broomrapes (Genus: Orobranche, Family Orobanchaceae) are holoparasitic plants that depend wholly on their hosts for nutrition (Young et al., 1999 Annals of the Missouri Botanical Garden 86 pp. 876–893). Agriculturally they are a major pest of fava bean, chickpea, lentil, tomato, potato, tobacco and sunflower, to name but a few, in East Africa, the Mediterranean region and the Middle East (Takeuchi et al., 1998 Phytochemistry 49 pp. 1967-1973).

They lack chlorophyll and germinate in response to stimulants present in the root exudates of the host plant (Joel et al., Physiologia Plantarum 120 pp. 328 - 337). The fact that broomrapes are totally dependent on their hosts for water, nutrients and reduced carbon, and the fact that they have lost photsynthetic function, indicates eerie parallels with the sun-hating vampire of Stoker's Dracula.

So, is it true that the broomrapes need no light? It has been shown that the photosynthetic genes have been eliminated from broomrape plastid genomes, meaning these plastids can never develop to mature chloroplasts and the plants can never photosynthesise (dePamphilis and Palmer, 1990 Nature 348 pp. 337–339). However, Kobayashi et al. (2005, Plant Physiology and Biochemistry 43 pp. 499-502) showed that a homologue of a cryptochrome gene (OmCRY1) in Common Broomrape (Orobranche minor) was expressed in higher amounts in plants grown in the dark than in the plants grown under natural daylight. Cryptochrome are blue-light photoreceptors that regulate factors such as the circadian clock in higher plants. This work shows that, although they are non-photosynthetic, broomrapes still use light to control non-photosynthetic functions.

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