It doesn’t sound rationale that there are plants that do not make their own foods and living as parasites on other plants. As autotrophs, plants are supposed to live on their own because plants, as most people would assume, are entitled to their abilities to photosynthesis, which is the process of making foods with light and air. However, nature never fails to challenge people’s perceptions with exceptions. In fact, there are about 4500 species of parasitic plants that account for about 1% of the whole angiosperms in the world. Parasitic plants have a higher mutation rate compared to nonparasitic plants, so they tend to have greater diversity.
Parasitism is one of the symbiotic relationships in which the symbionts get the advantages, mainly nutrients, and shelters, while the host gets damaged or hurts which may eventually lead to death. By its nature, parasitism is a common relationship across all kinds of organisms. Parasites are living with humans, such as internal parasites like tapeworms in intestines and external parasites like lice that lust after blood. Parasitic plants have developed a specially modified structure that is capable to penetrate the tissue of another plant, the host, and retrieve nutrients from the living tissues, mostly from the vesicular tissue of the host. They also steal genomic DNA trunks from their host by horizontal gene transformation, which is a unique mechanism that probably helpful for better adaptation to metabolite takeups, such as nutrients absorbance and detection of volatile compounds of the host. They even inject RNA molecules that encode proteins impair the defense system of the host to facilitate the invasion process.
However, the definition of parasitic plants should not be confused with other symbioses. Some plants look like parasitic plants but they just merely growing on the branches of a tree. Epiphytes such as lichens, mosses, ferns, orchids, air plants, bromeliads are sometimes mistaken as parasitic plants. Some plants like a fig with roots developed into stranglers that sometimes kill a tree by competing for light, space, and nutrients. But they kill other plants as an outside force, not taking nutrients from the inside tissue. Precisely speaking, a parasitic plant must have a physiological structure grown inside the host through which the water and nutrients are transported from the host to the parasite. They are heterotrophic that thrive on the essential nutrients like carbon, nitrogen, and phosphate from the host.
How a parasitic plant becomes established with its host? First of all, the parasitic plant locates its specific host through various seed dispersal strategies. Their fruits or seeds could be brought in contact with the host directly or indirectly by animals and other agents. The seeds are relatively large and stored with plenty of nutrients such as starch, fat, and protein in the endosperm, which attracts volunteers of seeds disperser such as fruit-lover rodents and birds. The germination rate is also increased after passing through the digestive canals of the animals which serve as stratification treatment. Besides, some seeds are sticky and tend to stick onto some animals, especially onto the feather of birds. Or through the help of the wind and rainwater, or by the explosive of the capsule containing the seeds. Furthermore, the radicle of the parasite has a special sense which is the chemotaxis towards the chemical germination stimulants produced by the host roots. Another strategy that yet to be confirmed is that the root tip of the host penetrates and grows in the seeds of a parasitic plant. From there, the primary haustorium is initiated.
When haustoria reaching maturation they become the physiological bridge between the parasitic plant and the host. Haustorium (singular) comes from the Latin words, “haustor” means to drink and “orium” means a device used for. Haustoria (plural) is the modified stem or root that used by the symbionts to penetrate the host plant then connecting to the conductive system like xylem, phloem, or both. After the vesicular tissue fused together, stealing nutrients is easy and efficient that the transpiration rate towards the parasitic plant is even faster than that of the host plants. Similar modification for invasion has also been observed from exophytic fungus whereby the hairlike filament, known as hypha to penetrate the host organ such as leaves to thrive the nutrients through vascular bundles. The intrusion mechanism is straightforward as the haustoria consist of two parts, the external holdfast, which prepares the site for penetration, and the intrusive organ that penetrates the living tissues. The holdfast counteracts the mechanical aspects but may not always develop. The intrusive organ forms a conduit with conductive tissue to the other parts of the hosts. For instance, a xylem bridge is the most typical anatomical feature of a haustorium.
There are several types of parasitic plants. A stem parasitic plant targeted and grew on the stems of the host plants, whereas a root parasitic plant relies on the nutrients from the roots of the host plants. Facultative parasitic means they are able to grow without a host. Whereas, the obligate parasitic plant is a parasitic plant that cannot survive without a host. Hemiparasite plant partially produces its own food by photosynthesis and is capable of surviving with limited growth in the absence of the host plant. Some of them lack a fully developed root system and form connections with another plant, from which it obtains some or all of its water and minerals. They tap into the sap-conducting tissue of the host by means of specialized structures called haustoria. Some plants such as eyebright (Euphrasia spp.), attach themselves to the roots of their host and appear like normal plants growing in the soil. Some plants grow on the aerial parts of their host such as mistletoes which are well-known examples that colonize the branches of trees. Whereas, holoparasite plants have lost chlorophyll thus not able to obtain food via photosynthesis. They solely rely on the host plant for survival.
Generally, holoparasites are more host-specific as compared to hemiparasites. The plant itself might be attacked by a parasitic plant or a parasite (animal) such as nematodes, which could be deadly as their feeding process cause injuries that lead to various diseases. The phenomenon of one parasitic plant attacks another parasitic plant is known as epiparasitism, which also referred to as hyperparasitism. If the two plants are from the same species, it’s known as autoparasitism, which also termed self-parasitism. Some parasites have synchronized their host with morphological similarities, which are phylogenetically unrelated. The flower structures are similar, even a comparable blooming period. This phenomenon is known as mimicry.
However, the host is not stupid or indifferent to these crafty attacks, which lead to the triggering of a series of self-defense reactions to fight off the harmful relationship. At the biochemical level, the host will release secondary metabolites such as alkaloids, which are toxic to cells and will accelerate the senescence and reduces herbivory of the parasite. At the attacking site, the hypertrophy and hormonal disorder will induce excessive growth in the thickness of the host branch. For instance, adventitious roots formation will be observed around the haustorium which helps to absorb additional soil water and nutrients that are beneficial to both partners. Furthermore, the host may reject this relationship through mechanical defense via the production of excessive amounts of woody structure, which is lignified sclerenchyma or cork tissue.
Parasitic plants grow flowers that are co-evolved with their pollinator, such as birds. They had evolved to have some sophisticated pollination mechanism like rewarding nectar to the birds and the flowers are usually red with yellow and green with no scent as the birds have little sense of smell. Their flowers and leaves are spatially separated. Also, the flowers had evolved so that the corolla is tubular or the perianth members have longspurs. Most of their flowers are epigynous for better protection in the ovules. In conclusion, the characteristics of successful development of parasitism will require tissue compatibility, the presence of growth-stimulating root exudates from potential host roots, and a greater rate of transpiration in the potential parasite than in the host.
In spite of the complicated mechanisms and high price of being parasitic, it’s not clear how they lost the ability of photosynthesis and what their roles in the plant ecosystem. Parasitic plants could be a positive contributor to other creatures as they are great food sources for herbivores from insects to large mammals. However, most parasitic plants are noxious weeds in agriculture as they attack almost all kinds of crops and may lead to a severe loss of yield. For small scale planting, control methods including deep plowing the field with seed banks of parasitic plants, hand-pulling of existing parasitic plants, and prescribed burning on heavily infected fields. For large plantations, the cleaning of the tools, machines, and shoes of the workers is important as they may serve as vehicles for parasitic plant seeds. Besides, solarization, incrementing fertilization, intercropping and crop rotation, surface sterilizing of seeds, usage of herbicides, and biological control are also useful measures. Genetically modified organism (GMO) that is resistant to parasitic plants also show promising results.
Because of their superb skills in stealing things, which are not limited to nutrients but also genes from multiple hosts, parasitic plants may serve as a bridge between reproduction isolated plant species for genetic information exchange. Initially, they steal host genes for the purpose of enhancing nutrient absorption. But the stolen material including genomic DNA/RNA may be transferred from a plant to another plant, which may benefit plant evolution in the big picture. Genetic information is generally passed from parent to offspring, known as vertical gene transfer through chromosomal recombination. For different plant species that are reproduction incompatible, there is no direct genetic information exchange. The genetic information transferred laterally or horizontally from one species to another species, for example, between plants and sucking insects like aphids, or pollination insects are observed in nature. But the examples of horizontal gene transfer between higher plants are rare. However, parasitic plants are likely to be involved in the horizontal transfer of genetic information.
Further reading:
Heide-Jørgensen, H. S. (2008). Parasitic flowering plants. Koninklijke Brill NV, Leiden, The Nertherlands.
