Living things have different interactions with each other. Some classifications refer to the possible benefits or harms that organisms obtain from said interaction. Thus, when the two organisms benefit from the interaction, we speak of mutualism —or symbiosis , if that interaction is required—. When one benefits and is neutral to the other, it is called commensalism , while amensalism is detrimental to one and neutral to the other. If neither of the two is benefited or harmed, it is called neutralism , and when both organisms harm each other, they are said to be in competition .
The best known relationships are those that benefit some to the detriment of others; depending on how it happens, we speak of predation or herbivory, and also of parasitism.
When talking about parasitic organisms, it is usual to think of bacteria, fungi, protozoa or animals . Creatures that, either living inside others or anchored to the outside, feed themselves and complete their life cycle at the expense of their host; be it a tick that sucks the blood of its host, a tapeworm that feeds on its food, or a graphiosis fungus that lives under the bark of an elm tree. But also among plants there are parasitic species that depend on other plants to stay alive, at their expense.
Dodder, rooting on stems
The dodder ( Cuscuta spp.) is a holoparasitic plant, totally dependent on its host, and does not photosynthesize . There are more than 200 species of the genus Cuscuta . They parasitize the stem of other plants, they do not have leaves, and their roots are modified into a structure called a haustorium , which anchors and penetrates the vascular tissues of the host plant.
In general, parasitic organisms use certain chemical signals to recognize their host and attach to it. But the dodders do not; they do this by recognizing reflected light. In sunlight, there is a certain ratio between the wavelength of near red light and far red light—near infrared. That ratio is much lower when plants are close by, since nearby red light is absorbed more efficiently than distant red light. The dodder detects this variation in the proportions of light to recognize the presence of a potential host.
The connection between the dodder and its host is very intimate; not only do they share vascular tissues through the haustorium, but the dodder is capable of translocating proteins and messenger RNA between itself and its host.
Epiphagus , parasitizing beeches
As its name suggests, the Epifagus virginiana species is a plant that grows on beeches : the prefix epi , in Latin, means “on”, and Fagus is the generic name of such an emblematic tree.
As in the case of the dodder, we are dealing with a plant that lacks chlorophyll and, therefore, is unable to carry out photosynthesis . All the vital functions of Epiphagus depend, therefore, on what its host provides.
Genetic studies show that the plants of this genus have lost the genes responsible for coding for photosynthesis, including those that allow chloroplasts to carry out their own metabolism. It is one more example of how the evolutionary process can benefit, not always the strongest, but sometimes the simplest, to the point of simplifying the genetic material of living beings; a fairly common phenomenon among parasitic organisms.
Hydnora , the strangest plant
Described by some as “ the strangest plants in the world ”, the eight known species of the genus Hydnora are little-known plants, native to Africa and the southern Arabian peninsula.
They lack leaves , and like good holoparasites, also chlorophyll; therefore, they do not perform any photosynthesis , depending on the plant they parasitize. It lives by spreading its rhizomes underground, and infiltrating its haustoria through the root tissues of other plants, only emerging from the ground to flower. This emergency is carried out with such force that it can lift paved or paved floors. Its flowers have a rather strange appearance, attracting pollinators thanks to bait structures that emit a foul odor, a slight heat production and a strong resemblance to the insect’s breeding site. In addition, the plant closes the flower when insects enter, and they are temporarily trapped to promote contact and pollination. Hydnora is therefore said to be pollinated through incarceration mimicry .
These are very elusive species and difficult to see. Until now, it is not possible to predict the moment of its flowering, the only moment in which the plant is visible, and it would not be uncommon for new species of Hydnora to be discovered in the future. In fact, the last one, H. arabica , was described in 2018.
Ethnobotanical studies can be of great help in the identification and discovery of new species of Hydnora . After all, they are the inhabitants of the regions where it lives, those who are in continuous contact with the plants it parasitizes and, therefore, who have the greatest opportunities to discover new flowers or pieces of plants, or even to obtain them. and extract them for other different purposes. In fact, some species such as H. abyssinica were discovered thanks to the fact that their rhizomes were found as supposed traditional medicine remedies, in a market in Mozambique.
References:
Furuhashi, T. et al. 2011. The parasitic mechanism of the holostemparasitic plant Cuscuta. Journal of Plant Interactions, 6(4), 207-219. DOI: 10.1080/17429145.2010.541945
Glatzel, G. et al. 2009. Mistletoe ecophysiology: host–parasite interactions This review is one of a collection of papers based on a presentation from the Stem and Shoot Fungal Pathogens and Parasitic Plants: the Values of Biological Diversity session of the XXII International Union of Forestry Research Organization World Congress meeting held in Brisbane, Queensland, Australia, in 2005. Botany, 87(1), 10-15. DOI: 10.1139/B08-096
Thorogood, C. 2019. Hydnora: The strangest plant in the world? PLANTS, PEOPLE, PLANET, 1(1), 5-7. DOI: 10.1002/ppp3.9
