Parasitic Plant
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A parasitic plant is a
plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every
biome A biome () is a distinct geographical region with specific climate, vegetation, and animal life. It consists of a biological community that has formed in response to its physical environment and regional climate. In 1935, Tansley added the ...
. All parasitic plants develop a specialized organ called the haustorium, which penetrates the host plant, connecting them to the host vasculature—either the
xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...
, phloem, or both. For example, plants like '' Striga'' or '' Rhinanthus'' connect only to the xylem, via xylem bridges (xylem-feeding). Alternately,
plants Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars f ...
like '' Cuscuta'' and some members of '' Orobanche'' connect to both the xylem and phloem of the host. This provides them with the ability to extract resources from the host. These resources can include water, nitrogen, carbon and/or sugars. Parasitic plants are classified depending on the location where the parasitic plant latches onto the host (root or stem), the amount of nutrients it requires, and their photosynthetic capability. Some parasitic plants can locate their host plants by detecting volatile chemicals in the air or soil given off by host shoots or
root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...
s, respectively. About 4,500
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of parasitic plants in approximately 20 families of
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...
s are known. There is a wide range of effects that may occur to a host plant due to the presence of a parasitic plant. Often there is a pattern of stunted growth in hosts especially in hemi-parasitic cases, but may also result in higher mortality rates in host plant species following introduction of larger parasitic plant populations.


Classification

Parasitic plants occur in multiple plant families, indicating that the evolution is
polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...
. Some families consist mostly of parasitic representatives such as
Balanophoraceae The Balanophoraceae are a subtropical to tropical family of obligate parasitic flowering plants, notable for their unusual development and formerly obscure affinities. In the broadest circumscription, the family consists of 16 genera. Alterna ...
, while other families have only a few representatives. One example is the North American '' Monotropa uniflora'' (Indian pipe or corpse plant) which is a member of the heath family,
Ericaceae The Ericaceae () are a Family (biology), family of flowering plants, commonly known as the heath or heather family, found most commonly in acidic and infertile growing conditions. The family is large, with about 4,250 known species spread acros ...
, better known for its member blueberries, cranberries, and rhododendrons. Parasitic plants are characterized as follows: 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 the Hydnoraceae. Plants usually considered hemiparasites include '' Castilleja'', mistletoe, Western Australian Christmas tree, and yellow rattle.


Evolution of parasitism

Parasitic behavior evolved in angiosperms roughly 12-13 times independently, a classic example of
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
. Roughly 1% of all angiosperm species are parasitic, with a large degree of host dependence. The taxonomic family ''
Orobanchaceae Orobanchaceae, the broomrapes, is a family (biology), family of mostly parasitic plants of the order (biology), order Lamiales, with about 90 genus, genera and more than 2000 species. Many of these genera (e.g., ''Pedicularis'', ''Rhinanthus'', ...
'' (encompassing the genera '' Triphysaria'', '' Striga'', and '' Orobanche'') is the only family that contains both holoparasitic and hemiparasitic species, making it a model group for studying the evolutionary rise of parasitism. The remaining groups contain only hemiparasites or holoparasites. The evolutionary event which gave rise to parasitism in plants was the development of haustoria. The first, most ancestral, haustoria are thought to be similar to that of the facultative hemiparasites within '' Triphysaria'', lateral haustoria develop along the surface of the roots in these species. Later evolution led to the development of terminal or primary haustoria at the tip of the juvenile radicle, seen in obligate hemiparasitic species within '' Striga''. Lastly, holoparasitic plants, always forms of obligate parasites, evolved over the loss of photosynthesis, seen in the genus '' Orobanche''. The most specialized forms of holoparasitic plants are the four families Rafflesiaceae, Cytinaceae, Mitrastemonaceae and Apodanthaceae, lineages which independently has evolved further into endoparasites that, except for the flowers, spend their entire life cycle within the tissue of their host. To maximize resources, many parasitic plants have evolved 'self-incompatibility', to avoid parasitizing themselves. Others such as '' Triphysaria'' usually avoid parasitizing other members of their species, but some parasitic plants have no such limits. The albino redwood is a mutant ''
Sequoia sempervirens ''Sequoia sempervirens'' ()''Sunset Western Garden Book,'' 1995: 606–607 is the sole living species of the genus ''Sequoia (genus), Sequoia'' in the cypress family Cupressaceae (formerly treated in Taxodiaceae). Common names include coast ...
'' that produces no chlorophyll; they live on sugars from neighbouring trees, usually the parent tree from which they have grown (via a somatic mutation).


Seed germination

Parasitic plants germinate in several methods. These 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. Most parasitic plants need to germinate near their host plants because their seeds are limited in the number 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.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. Root parasitic plant seeds tend to use chemical cues for germination. For germination to occur, seeds need to be quite close to the host plant. For example, the seeds of witchweed ('' Striga asiatica'') need to be within 3 to 4 millimeters (mm) of its host to receive chemical signals in the soil to trigger 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 nearby from the host's
root system In mathematics, a root system is a configuration of vector space, vectors in a Euclidean space satisfying certain geometrical properties. The concept is fundamental in the theory of Lie groups and Lie algebras, especially the classification and ...
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, Orobanche and electoral from red clover, and 5-deoxystrigol from '' Lotus japonicus''. 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 parasites, germinate using the resources inside their endosperms and can survive for some time. For example, the dodders (''Cuscuta'' spp.) drop their seeds to the ground. These may remain dormant for up to five years before they find a host plant. Using the resources in the seed
endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the Embryo#Pla ...
, the dodder can germinate. Once germinated, the plant has six days to find and establish a connection with its host plant before its resources are exhausted. Dodder seeds germinate above ground, 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 detects its host plant's scent and orients itself in that direction. Scientists used volatiles from
tomato The tomato (, ), ''Solanum lycopersicum'', is a plant whose fruit is an edible Berry (botany), berry that is eaten as a vegetable. The tomato is a member of the nightshade family that includes tobacco, potato, and chili peppers. It originate ...
plants ( α-pinene, β-myrcene, and β-phellandrene) to test the reaction of '' C. pentagona'' and found that the stem orients itself in the direction of the odor. Some studies suggest that by using light reflecting from nearby plants dodders can select hosts with higher sugar because of the levels of
chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
in the leaves.Lesica, P. 2010. Dodder: Hardly Doddering. Kelsey Newsletter of Montana Native Plant Society. Vol 23. 2, 6 Once the dodder finds its host, it wraps itself around the host plant's stem. Using adventitious roots, the dodder taps into the host plant's stem with a haustorium, an absorptive organ within the host plant
vascular tissue Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. T ...
. Dodder makes several of these connections with the host as it moves up the plant.


Seed dispersal

There are several methods of seed dispersal, but all the strategies aim to put the seed in direct contact with, or within a critical distance of, the host. # The ''Cuscuta'' seedling can live for 3–7 days and extend out 35  cm in search of the host before it dies. This is because the ''Cuscuta'' seed is large and has stored nutrients to sustain its life. This is also useful for seeds that get digested by animals and are excreted. # Mistletoe use a sticky seed for dispersal. The seed sticks to nearby animals and birds and then comes into direct contact with the host. # '' Arceuthobium'' seeds have a similarly sticky seed as the mistletoe but they do not rely on animals and birds, they mainly disperse by fruit explosiveness. Once the seed makes contact with the host, rainwater can help position the seed in a suitable position. # Some seeds detect and respond to chemical stimulations produced in the host's roots and start to grow towards the host.


Obstacles to host attachment

A parasitic plant has many obstacles to overcome to attach to a host. Distance from the host and stored nutrients are some of the problems, and the host's defenses are an obstacle to overcome. The first hurdle is penetrating the host since the host has systems to reinforce the cell wall by protein cross-linking so that it stops the parasitic progress at the cortex of the host's roots. The second hurdle is the host's ability to secrete germination inhibitors. This prevents germination of the parasitic seed. The third hurdle is the host's ability to create a toxic environment at the location where the parasitic plant attaches. The host secretes phenolic compounds into the apoplast. This creates a toxic environment for the parasitic plant, eventually killing it. The fourth hurdle is the host's ability to ruin the tubercle using gums and gels or injecting toxins into the tubercle.


Host range

Some parasitic plants are generalists and parasitize many different species, even several different species at once. Dodder ('' Cuscuta'' spp.) and red rattle ('' Odontites vernus'') are generalist parasites. Other parasitic plants are specialists that parasitize a few or 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''. Plants such as '' Pterospora'' become parasites of mycorrhizal fungi. There is evidence that parasites also practice self-discrimination, species of '' Triphysaria'' experience reduced haustorium development in the presence of other ''Triphysaria''. The mechanism for self-discrimination in parasites is not yet known.


Aquatic parasitic plants

Parasitism also evolved within aquatic species of plants and algae. Parasitic marine plants are described as benthic, meaning that they are sedentary or attached to another structure. Plants and algae that grow on the host plant, using it as an attachment point are given the designation
epiphytic An epiphyte is a plant or plant-like organism that grows on the surface of another plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphyt ...
( epilithic is the name given to plants/algae that use rocks or boulders for attachment), while not necessarily parasitic, some species occur in high correlation with a certain host species, suggesting that they rely on the host plant in some way or another. In contrast, endophytic plants and algae grow inside their host plant, these have a wide range of host dependence from obligate holoparasites to facultative hemiparasites. Marine parasites occur as a higher proportion of marine flora in temperate rather than tropical waters. While no full explanation for this is available, many of the potential host plants such as kelp and other macroscopic
brown algae Brown algae (: alga) are a large group of multicellular algae comprising the class (biology), class Phaeophyceae. They include many seaweeds located in colder waters of the Northern Hemisphere. Brown algae are the major seaweeds of the temperate ...
are generally restricted to temperate areas. Roughly 75% of parasitic
red algae Red algae, or Rhodophyta (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest Phylum, phyla of algae, containing over 7,000 recognized species within over 900 Genus, genera amidst ongoing taxon ...
infect hosts in the same taxonomic family as themselves, these are given the designation adelphoparasites. Other marine parasites, deemed endozoic, are parasites of marine invertebrates ( mollusks, flatworms, sponges) and can be either holoparasitic or hemiparasitic, some retaining the ability to photosynthesize after infection. These are the only parasitic plants that parasitize animal hosts.


Importance

Species within ''
Orobanchaceae Orobanchaceae, the broomrapes, is a family (biology), family of mostly parasitic plants of the order (biology), order Lamiales, with about 90 genus, genera and more than 2000 species. Many of these genera (e.g., ''Pedicularis'', ''Rhinanthus'', ...
'' are some of the most economically destructive species on Earth. Species of '' Striga'' alone are estimated to cost billions of dollars a year in crop yield loss annually, infesting over 50 million hectares of cultivated land within
sub-Saharan Africa Sub-Saharan Africa is the area and regions of the continent of Africa that lie south of the Sahara. These include Central Africa, East Africa, Southern Africa, and West Africa. Geopolitically, in addition to the list of sovereign states and ...
alone. '' Striga'' can infest both grasses and grains, including corn,
rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...
and
sorghum ''Sorghum bicolor'', commonly called sorghum () and also known as great millet, broomcorn, guinea corn, durra, imphee, jowar, or milo, is a species in the Poaceae, grass genus ''Sorghum (genus), Sorghum'' cultivated for its grain. The grain i ...
, some of the most important food crops. '' Orobanche'' also threatens a wide range of important crops, including peas, chickpeas, tomatoes, carrots, lettuce, and varieties of the genus '' Brassica'' (e.g. cabbage and broccoli). Yield loss from '' Orobanche'' can reach 100% and has caused farmers in some regions of the world to abandon certain staple crops and begin importing others as an alternative. Much research has been devoted to the control of ''Orobanche'' and ''Striga'' species, which are even more devastating in developing areas of the world, though no method has been found to be entirely successful. *Mistletoes cause economic damage to forests 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. *
Sandalwood Sandalwood is a class of woods from trees in the genus ''Santalum''. The woods are heavy, yellow, and fine-grained, and, unlike many other aromatic woods, they retain their fragrance for decades. Sandalwood oil is extracted from the woods. Sanda ...
trees (''Santalum'' species) have many important cultural uses and their fragrant oils have high commercial value. *Indian paintbrush ('' Castilleja linariaefolia'') is the state flower of
Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...
. *The oak mistletoe ('' Phoradendron serotinum'') is the floral emblem of
Oklahoma Oklahoma ( ; Choctaw language, Choctaw: , ) is a landlocked U.S. state, state in the South Central United States, South Central region of the United States. It borders Texas to the south and west, Kansas to the north, Missouri to the northea ...
. *A few other parasitic plants are occasionally cultivated for their attractive flowers, such as '' Nuytsia'' and broomrape. *Parasitic plants are important in research, especially on the loss of photosynthesis and the co-dependency of functional, genetic and lifestyle changes. *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. Some parasitic plants are destructive while some have positive influences in their
communities A community is a Level of analysis, social unit (a group of people) with a shared socially-significant characteristic, such as place (geography), place, set of Norm (social), norms, culture, religion, values, Convention (norm), customs, or Ide ...
. Some parasitic plants damage
invasive species An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native spec ...
more than
native species In biogeography, a native species is indigenous to a given region or ecosystem if its presence in that region is the result of only local natural evolution (though often popularised as "with no human intervention") during history. The term is equi ...
. This results in the reduced damage of
invasive species An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native spec ...
in the community. Parasitic plants are major shapers of their community, affecting not just the host species but indirectly affecting others. Competition amongst host species will change due to the parasitic plant. Plant parasitism have been shown to keep invasive species under control and become keystone species in an ecosystem. In many regions, including the
Nepal Nepal, officially the Federal Democratic Republic of Nepal, is a landlocked country in South Asia. It is mainly situated in the Himalayas, but also includes parts of the Indo-Gangetic Plain. It borders the Tibet Autonomous Region of China Ch ...
Eastern Himalayas, parasitic plants are used for medicinal and ritual purposes.


Plants parasitic on fungi

About 400 species of flowering plants, plus one gymnosperm ('' Parasitaxus usta'') and one
bryophyte Bryophytes () are a group of embryophyte, land plants (embryophytes), sometimes treated as a taxonomic Division (taxonomy), division referred to as Bryophyta ''Sensu#Common qualifiers, sensu lato'', that contains three groups of non-vascular pla ...
(the
liverwort Liverworts are a group of non-vascular land plants forming the division Marchantiophyta (). They may also be referred to as hepatics. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry ...
'' Aneura mirabilis''), are parasitic on mycorrhizal fungi. This effectively gives these plants the ability to become associated with many of the other plants around them. They are termed myco-heterotrophs. 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''). Within the taxonomic family ''
Ericaceae The Ericaceae () are a Family (biology), family of flowering plants, commonly known as the heath or heather family, found most commonly in acidic and infertile growing conditions. The family is large, with about 4,250 known species spread acros ...
'', known for extensive mycorrhizal relationships, there are the Monotropoids. The Monotropoids include the genera '' Monotropa'', '' Monotropsis'', and '' Pterospora'' among others. Myco-heterotrophic behavior is commonly accompanied by the loss of chlorophyll.Judd, Walter S., Christopher Campbell, and Elizabeth A. Kellogg. ''Plant Systematics: A Phylogenetic Approach''. Sunderland, MA: Sinauer Associates, 2008. Print.


See also

* Living stump


References


External links

*
The International Parasitic Plant Society
{{Authority control Plant physiology