Parasitic Plants

The parasitic mode of existence is frequently encountered among all life forms, including flowering plants. In this discussion a plant will be considered parasitic only if it produces a haustorium, the modified root that forms the morphological and physiological link to another plant (the host). Some plants, such as the ghostly white Indian Pipe (Monotropa ) are often called parasites, but are more properly termed mycotrophs (Greek mykes, meaning "fungus," and trophos, meaning "feeder"). Mycotrophs, which occur in many plant families, lack chlorophyll and are nonphotosynthetic, and their

roots are associated with mycorrhizal fungi, which surround tree roots. Bromeliads such as Spanish moss (Tillandsia ) and some orchids are also sometimes mistaken for parasites, but these plants are actually epiphytes (Greek epi, meaning "upon," and phyton, meaning "plant"). Epiphytes use the other plant simply as a support and do not derive water or nutrients directly from their tissues. In true parasitic plants, the haustorium physically penetrates the host stem or root thus connecting to the water-conducting and/or sugar-conducting tissues (xylem and phloem, respectively).

The degree of nutritional dependence on the host varies widely among parasitic plants. Some parasites are photosynthetic and can therefore produce their own food from sunlight as is done by other green plants. Such hemi-parasites include root parasites such as Indian paintbrush (Castilleja, Scrophulariaceae) and stem parasites such as mistletoes (Loranthaceae, Viscaceae; see accompanying table). Some root hemiparasites can actually grow to maturity in the absence of a host plant, and hence are termed facultative hemi-parasites. Others, such as the mistletoes, must attach to a host in order to complete their life cycle and are thus referred to as obligate hemiparasites. Hemiparasites can be considered xylem parasites in that they derive only water and dissolved minerals from their hosts. In contrast, holoparasites, being nonphotosynthetic, must also obtain the carbohydrates found in host phloem.

Parasitism has evolved in angiosperms at least nine independent times, but, interestingly, not in monocots (grasses, palms, lilies, etc.). There exists more than 270 genera and 4,000 species of parasitic plants worldwide. Holoparasitism has evolved at least six times independently. In two families (Cuscutaceae and Scrophulariaceae) both hemi- and holoparasites can be found. Members of these families represent important organisms for studying the genetic changes that occur when photosynthesis is lost. For example, a root parasite of beech trees (Fagus ) found in Eastern North America is called beechdrops (Epifagus ). The complete deoxyribonucleic acid (DNA) sequence of the beechdrops chloroplast genome has been obtained and is less than half the size of a typical photosynthetic plant, mainly owing to the loss of genes specifically involved in photosynthesis.

Other members of Scrophulariaceae represent some of the most economically damaging pathogens of crop plants in Africa, the Middle East, and Asia. Witchweed (Striga ) is a devastating pest on maize (corn), sorghum, and other grasses, while broomrape (Orobanche ) parasitizes sunflowers, tomatoes, and beans. Similarly, the spaghetti-like dodder (Cuscuta ) can become a problem weed on crops such as alfalfa. These parasites are difficult to eradicate because they produce thousands of tiny, dustlike seeds that persist in the soil and are easily moved from site to site. In North America, the genus Arceuthobium (dwarf mistletoe) destroys commercially important trees in the pine family (Douglas-fir, hemlock, pine, etc.). Unlike other members of its family (Viscaceae) whose seeds are bird-dispersed, dwarf mistletoes have evolved a fruit that explosively expels the sticky seed, which can reach a velocity of 27 meters per second and can travel up to 16 meters.

Although some parasitic plants are weeds, the vast majority are benign and often go unnoticed by the casual observer. Some of the most spectacularly beautiful flowers that exist in nature can be found in the showy mistletoe family (Loranthaceae). Many species produce long, tubular red flowers that are bird-pollinated. Indeed, the mistletoe bird (Dicaeum ) effects polli-nation when feeding upon the nectar and aids in seed dispersal when feeding on the fruits, a good example of coevolution.

Certainly, no treatment of parasitic plants would be complete without mention of Rafflesia, the queen of the parasites. This holoparasite has no stems, leaves, or roots but exists within the host vine (Tetrastigma, Vitaceae) as a fungal-like mycelium until flowering. At that time, the flower emerges from the host as a small, golf-ball sized bud and continues to grow until it is the size of a cabbage. Eventually it opens as a flower that may exceed 1 meter in diameter—the largest flower in the world. The spotted red flower has five leathery petals surrounding a deep cup that exudes a stench like that of rotting flesh, thus attracting flies (the pollinators). All species of Rafflesia are endangered owing to habitat loss in Malaysia, Indonesia, and the Philippines.

see also Endangered Species; Epiphytes; Fungi; Interactions, Plant-Plant; Mycorrhizae; Record-Holding Plants.

Daniel L. Nickrent

Bibliography

Calder, Malcolm, and Peter Bernhardt. The Biology of Mistletoes. New York: Academic Press, 1983.

Parasitic Plant Connection. [Online] Available at http://www.science.siu.edu/parasiticplants/index.html

Kuijt, Job. The Biology of Parasitic Flowering Plants. Berkeley, CA: University of California Press, 1969.

Press, Malcolm C., and Jonathan D. Graves. Parasitic Plants. London: Chapman &Hall, 1995.