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Parasitic Plant

A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. All parasitic plants have modified roots, named haustoria (singular: haustorium), which penetrate the host plants, connecting them to the conductive system – either the xylem, the phloem, or both. This provides them with the ability to extract water and nutrients from the host. The organisms which support the parasite are called hosts. Some parasitic plants are able to locate their host plants by detecting chemicals in the air or soil given off by host shoots or roots, respectively. About 4,100 species of parasitic plant in approximately 19 families of flowering plants are known.  

Parasitic plants are not classified by their morphological characteristics because the extreme changes of vegetative and floral morphology make it difficult. One way these plants are classified is by their photosynthetic status. Parasitic plants can be hemiparasites, which means they are photosynthetically competent. They could also be holoparasites, where they lack photosynthetic activity and obtain reduced carbon through their haustorial connection with host.  While both hemiparasites and holoparasites mainly grow largely exterior to the host, some holoparasites grow completely within the host plant tissues as endoparasites, only presenting itself during sexual reproduction. Parasitic plants can either be facultative or obligate. Being a facultative plant means they live autotrophically and reproduce without host contact while being an obligate plant requires them to parasitize a host in order to complete their life cycle.

Evolution of Parasitic Behaviour

Parasitic behavior evolved in angiosperms roughly 12-13 times independently, a classic example of convergent evolution. Roughly 1% of all angiosperm species are parasitic, with a large degree of host dependence. These plants directly feed from other plants by invading their roots through haustoria. A haustorium is a projection from the root of the parasitic plant that allows for parasites to get into the tissue of the host and absorb nutrients from it. This is a key evolutionary event that led to the non-parasitic plant becoming parasitic.

Due to the intimate contact parasitic plants have with others, DNA can be easily transferred between unrelated species. This is known as horizontal gene transfer (HGT). It has been identified in many different plant clades such as bryophytes, grasses, basal angiosperms and ferns. It has been shown that gene transfer involving mitochondrial genome is significantly higher than nuclear genome. Despite the transfer of nuclear genome being lower, this event could have a major influence on the evolution of future lineages. Phylogenetic studies from different clades indicate that parasitic plants are an excellent system for studying this gene transfer phenomenon.

There has been more than 50 expressed and functional HGT events in one family of parasitic plants. HGT shows parasite preferences for different host plants and was much more frequent in plants with large parasitic dependency. Three main mitochondrial genes prove the evolution of parasitic plants through horizontal gene transfer: atp1, coxI, and matR. Among all parasitic lineages, the Orobanchaceae family is the only one that has a complete spectrum of parasitic capabilities, thus the best for proving HGT. Four nuclear genes from this family have been transferred to date.

HGT inferences with gene trees can have high error rates due to factors such as misidentification of sequences, insufficient taxon sampling, complex gene birth and death processes, gene tree errors, inappropriate rooting, and frame-shift errors. Rafflesiaceae sensu stricto, which belong to the order Malpighiales is a useful clade to study because it overcomes some of these errors. Sensu stricto, is a class of parasitic plants that are prone to HGT. Its complete genome is sequenced for all its close relatives. The hosts and parasites are separated by at least 100 million years and they have a narrow host specialization range.

Researchers suggest that this HGT could potentially boost the parasitic plant's ability to invade hosts and combat the defences that the host plant creates to ward off attacks. The chance of the parasite becoming infected is also lessened by this gene transfer. The mechanism of this HGT is still unknown.

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