|This article is missing information about the mechanism of parasitic action. (October 2011)|
A parasitic plant is one that derives some or all of its nutritional requirements from another living plant. All parasitic plants have special organs, named haustoria (singular: haustorium), which connect them to the conductive system of their host and provide them with the ability to extract water and nutrient from the hosts. About 4,100 species in approximately 19 families of flowering plants are known. Parasitic plants have a modified root, the haustorium, that penetrates the host plant and connects to the xylem, phloem, or both.
Parasitic plants are characterized as follows:
- 1a. Obligate parasite – a parasite that cannot complete its life cycle without a host.
- 1b. Facultative parasite – a parasite that can complete its life cycle independent of a host.
- 2a. Stem parasite – a parasite that attaches to the host stem.
- 2b. Root parasite – a parasite that attaches to the host root.
- 3a. Holoparasite – a plant that is completely parasitic on other plants and has virtually no chlorophyll.
- 3b. Hemiparasite – a plant that is parasitic under natural conditions and is also photosynthetic to some degree. Hemiparasites may just obtain water and mineral nutrients from the host plant. Many obtain at least part of their organic nutrients from the host as well.
For hemiparasites, one from each of the three sets of terms can be applied to the same species, e.g.
- Nuytsia floribunda (Western Australian Christmas tree) is an obligate root hemiparasite.
- Rhinanthus (e.g. Yellow rattle) is a facultative root hemiparasite.
- Mistletoe is an obligate stem hemiparasite.
Holoparasites are always obligate so only two terms are needed, e.g.
- Dodder is a stem holoparasite.
- Hydnora spp. are root holoparasites.
Plants usually considered holoparasites include broomrape, dodder, Rafflesia, and Hydnoraceae. Plants usually considered hemiparasites include Castilleja, mistletoe, Western Australian Christmas tree and yellow rattle.
Seed germination of parasitic plants occurs in a variety of ways. These means can either be chemical or mechanical and the means used by seeds often depends on whether or not the parasites are root parasites or stem parasites. Both root and stem parasitic plants have evolved to use one or more means of finding their hosts in order to germinate. Most parasitic plants need to germinate in close proximity to their host plants because their seeds are limited in the amount of resources necessary to survive without nutrients from their host plants. Resources are limited due in part to the fact that most parasitic plants are not able to use autotrophic nutrition to establish the early stages of seeding.
Root parasitic plant seeds tend to use chemical cues for germination. In order for germination to occur, seeds need to be fairly close to their host plant. For example, the seeds of the parasitic plant Witchweed (Striga asiatica) need to be within 3 to 4 millimeters (mm) of its host in order to pick up chemical signals in the soil to signal germination. This range is important because Striga asiatica will only grow about 4 mm after germination. Chemical compound cues sensed by parasitic plant seeds are from host plant root exudates that are leached in close proximity from the host’s root system into the surrounding soil. These chemical cues are a variety of compounds that are unstable and rapidly degraded in soil and are present within a radius of a few meters of the plant exuding them. Parasitic plants germinate and follow a concentration gradient of these compounds in the soil toward the host plants if close enough. These compounds are called strigolactones. Strigolactone stimulates ethylene biosynthesis in seeds causing them to germinate.
There are a variety of chemical germination stimulants. Strigol was the first of the germination stimulants to be isolated. It was isolated from a non-host cotton plant and has been found in true host plants such as corn and millets. The stimulants are usually plant specific, examples of other germination stimulants include sorgolactone from sorghum, orobanchol and alectrol from red clover, and 5-deoxystrigol from Lotus japonicas. Strigolactones are apocarotenoids that are produced via the carotenoid pathway of plants. Strigolactones and mycorrhizal fungi have a relationship in which Strigolactone also cues the growth of mycorrhizal fungus.
Stem parasitic plants unlike most root plants germinate using the resources inside its endosperm and are able to survive for a small amount of time. An example, Dodder (Cuscuta spp.) is a parasitic plant whose seed falls to the ground and may remain dormant for up to five years before it is able to sense a host plants nearby. Using the resources in the seed endosperm, Dodder is able to germinate. Once germinated, the plant has 6 days to find and establish a connection with its host plant before its resources run out.
Dodder seeds germinate above ground and then the plant sends out stems in search of its host plant reaching up to 6 cm before it dies. It is believed that the plant uses two methods of finding a host. The stem is able to pick up its host plant’s scent whereby it then is able to orient itself in the direction of its host. Scientists used volatiles from tomato plants (α-pinene, β-myrcene, and β-phellandrene) to test the reaction of C. pentagona and found that the stem will oriented itself in the direction of the odor. Some studies suggest that by using light reflecting from nearby plants dodders are able to select host with higher sugar because of the levels of chlorophyll in the leaves. Once Dodder finds its host, it wraps itself around the host plants stem. Using adventitious roots, Dodder taps into the host plant’s stem and creates a haustorium, which is a special connection into the host plant vascular tissue. Dodder makes several of these connections with the host as it moves up the plant.
Some parasitic plants are generalists and parasitize many different species, even several different species at once. Dodder (Cassytha spp., Cuscuta spp.) and red rattle (Odontites verna) are generalist parasites. Other parasitic plants are specialists that parasitize a few or even just one species. Beech drops (Epifagus virginiana) is a root holoparasite only on American beech (Fagus grandifolia). Rafflesia is a holoparasite on the vine Tetrastigma.
- Witchweed, broomrape and dodder cause huge economic losses in a variety of herbaceous crops. Mistletoes cause economic damage to forest and ornamental trees.
- Rafflesia arnoldii produces the world's largest flowers at about one meter in diameter. It is a tourist attraction in its native habitat.
- Indian paintbrush (Castilleja linariaefolia) is the state flower of Wyoming.
- The Oak Mistletoe (Phoradendron serotinum) is the floral emblem of Oklahoma.
- A few other parasitic plants are occasionally cultivated for their attractive flowers, such as Nutysia and broomrape.
- Parasitic plants are important in research, especially on the loss of photosynthesis during evolution.
- A few dozen parasitic plants have occasionally been used as food by people.
- Western Australian Christmas tree (Nuytsia floribunda) sometimes damages underground cables. It mistakes the cables for host roots and tries to parasitize them using its sclerenchymatic guillotine.
Plants parasitic on fungi
About 400 species of flowering plants and one gymnosperm (Parasitaxus usta), are parasitic on mycorrhizal fungi. They are termed myco-heterotrophs rather than parasitic plants. Some myco-heterotrophs are Indian pipe (Monotropa uniflora), snow plant (Sarcodes sanguinea), underground orchid (Rhizanthella gardneri), bird's nest orchid (Neottia nidus-avis) and sugarstick (Allotropa virgata).
- Nickrent, D. L. and Musselman, L. J. 2004. Introduction to Parasitic Flowering Plants. The Plant Health Instructor. doi:10.1094/PHI-I-2004-0330-01 
- Scott, P. 2008. Physiology and behavior of plants: parasitic plants. John Wiley & sons pp. 103–112.
- Runyon, J. Tooker, J. Mescher, M. De Moraes, C. 2009. Parasitic plants in agriculture: Chemical ecology of germination and host-plant location as targets for sustainable control: A review. Sustainable Agriculture Reviews 1. pp. 123-136.
- Schneeweiss, G. 2007. Corelated evolution of life history and host range in the nonphotosynthetic parasitic flowering plants Orobanche and Phelipanche (Orobanchaceae). Journal Compilation. European Society for Evolutionary Biology. 20 471-478.
- Lesica, P. 2010. Dodder: Hardly Doddering. Kelseya Newsletter of Montana Native Plant Society. Vol 23. 2, 6
- Parasitic Angiosperms Used for Food?
- Sclerenchymatic guillotine in the haustorium of Nuytsia floribunda
- Joel D.M. et al. Eds.(2013) Parasitic Orobanchaceae: Parasitic Mechanisms and Control Strategies. Springer, Heidelberg
- Digital Atlas of Cuscuta (Convolvulaceae)
- The Parasitic Plant Connection
- The Strange and Wonderful Myco-heterotrophs
- Parasitic Flowering Plants
- The Mistletoe Center
- Parasitic Plants Biology Study Guide
- Nickrent, Daniel L. 2002. Parasitic plants of the world.
- Calladine, Ainsley and Pate, John S. 2000. Haustorial structure and functioning of the root hemiparastic tree Nuytsia floribunda (Labill.) R.Br. and water relationships with its hosts. Annals of Botany 85: 723-731.
- Milius, Susan. 2000. Botany under the mistletoe: Twisters, spitters, and other flowery thoughts for romantic moments. Science News 158: 411.
- Hibberd, Julian M. and Jeschke, W. Dieter. 2001. Solute flux into parasitic plants. Journal of Experimental Botany 52: 2043-2049.