Plant Parasitism

The dodder, the mistletoe and the stinking corpse lily

In order to survive the relentless onslaught of nature, plants have developed a diversity of robust survival strategies. Some plants form mutual relationships with soil microbiota (bacteria, fungi and insect) and by secreting energy rich mucilage to strengthen their uptake of resources needed for growth. Other plants use adverse environmental conditions to their advantage. Several plant species use fire to vernalise their seeds so that they can germinate in a forest without much competition. Some plants even form mutualistic relationships with invertebrates and mammals to defend themselves against predators, in return for food and shelter.

Contrary to beneficial relationships, there are a suite of plants that have turn to parasitism to meet their resource demands. A particular plant, the dodder (Cuscuta sp.) has emerged to become a formidable parasite that severely diminishes crop yield in many parts of the world. This genus of parasite contains over 200 species of yellowy-orange (sometimes green) vine-like plants. The dodder develops from seeds that have landed on soil by the wind and can remain dormant for up to 10 years. Once germinated, they rapidly grow to seek out the nearest host by detecting a cocktail of volatiles released from the host. Once detected, the dodder latches onto the stem of its host and twists around to tighten its grip. Next a haustoria, a specialised feeding organ develops a pincer-like appendage to penetrate through the host’s cell walls to extract all resources needed to fuel its prolific growth. Prior to infection the dodder secretes a thick gel-like matrix to help stick its haustoria to its host stem. This makes removing it highly challenging.

The haustoria, as an organ is fascinating as it must penetrate through the host’s thick, rigid epidermal cell walls but they must also convert to resource extraction, similar to that of an oil rigg’s exploratory pipeline. They break through cell walls through a pincer-like structure and then develop large complex vascular bundles (xylem and phloem) to extract water, sugar and nutrients. Once they mature, they bulge and harden. Dodders are rapid growers in ideal conditions and can produce several haustoria a day. Even if the dodder stem breaks and has at least one operational haustoria they can continue their assault.

The dodder is an usual plant whereby it is formed of a tentacle-like vine (image above) that does not contain sufficient chlorophyll needed to generate energy. Early in its development, the parasite produces basic roots to support its initial growth prior to feeding off its host. After the initial feeding organs develops, these roots quickly die off. Once enough resources have been sequestered and at a sustainable rate, the dodder flowers to produce seed. The flowers of the dodder are highly inconspicuous, have no scent and have tiny unremarkable white petals.

The most effective method of controlling an outbreak of this parasite is to burn infected plants, and to clear a large area surrounding the initial outbreak to buffer against further spread. Dodders are able to affect most crops excluding grasses and monocotyledons such as wheat, barley and rice that have a radically different cell wall structure compared to the dodder’s preferred hosts including tomatoes. This has led many plant scientists to theorise ways of combating this agricultural nuisance. However, any solution that may be developed will only be available in the far-future. This nuisance when spotted in fields causes fear amongst growers who may face major losses in revenue and crucial production. Unfortunately, this parasite mainly effects less economically developed countries with warm climates such as Afghanistan, Armenia, China, India and Pakistan.

The dodder is not widely known apart from those who are directly affected by them. Other plant parasites exist and are in fact well-known such as the Yuletide decoration, mistletoe (Viscum sp.). Within Europe, Southern England produces the most amount. The mistletoe infects broad leaf trees whereby, they are spread by unknowing birds who feast on their white glutinous berries. Cunningly, these berries are only produced during the winter where other food resources for birds are limited. Although a parasite, it is not fully parasitic (hemi-parasitic) as it develops its own photosynthetic leaves. The mistletoe like that of the dodder, sends out haustoria to extract water and nutrients from its host tree. These examples of plant parasites demonstrate the importance of this tiny plant organ, which effects many growers and thus our food prices at the supermarket.

The final example of a plant parasite is the stinking corpse lily (Rafflesia arnoldii), which infects its host by the same measures at the other parasite outlined in this article. The stinking corpse lily has the largest flower in the world, growing up to one metre wide and weighing up to 11 kg. The flower emits the smell of rotten flesh to attract flies for pollination. Interestingly, the large bowl-shaped flower can generate its own heat to approximately body temperature (36.7°C) helping to spread its smell, and make it more appealing to files. The corpse lily infects its hosts usually the closest tree, and particularly coffee plants through haustoria that infect roots.

Collectively, plant parasitism forms a small share in the greater kingdom of plant life. These unique plants employ a specialised strategy in order to survive in nature. The haustoria makes it possible for them to penetrate through the cell walls of their host and to drain them of resources. Some plant parasites are fully reliant on their hosts such as the dodder whereas, others are partial reliant such as the mistletoe, extracting only water and nutrients. These plants are a major agricultural nuisance, and can result in major losses for growers. The only known cure is to isolate infected plants and to burn them. Much more research is needed to further understand these plants and how to prevent their hosts from succumbing to their predator.

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