The strange and sinister world of parasitic plants

In the world of plants, most rely on the process of photosynthesis to produce their food. However, some plants have found a way to survive by taking the easy route — stealing from other plants. These plants are known as parasitic plants. Around 1% of flowering plants, or roughly 4,000 species, are parasitic. These plants have evolved unique ways of extracting nutrients from their hosts, creating an unusual and sometimes sinister relationship that resembles a “zombie” effect. Let’s take a deeper look at this fascinating phenomenon.

What makes a plant parasitic?

Parasitic plants have developed specialized structures called haustoria, which penetrate the host plant and allow the parasite to extract water, minerals, and nutrients. Some parasitic plants, like Rafflesia and Thurber’s stemsucker, grow entirely within their host, only emerging to flower. Others, like mistletoe, attach their haustoria to the outside of their host plants.

There are two main types of parasitic plants:

1. Holoparasites: These plants cannot photosynthesize and are entirely dependent on their host for nutrition.
2. Hemiparasites: These plants can photosynthesize but still rely on their host for water and minerals.

Some parasitic plants are obligate — meaning they cannot live without a host. Others, known as facultative parasites, can survive independently if needed.

Please also read: Paleobotany: Decoding the earth’s climate and evolution through plants

The most interesting parasitic plants

1. Cuscuta (Dodder)
   Dodder is an aggressive, obligate parasite that has no roots or leaves. It grows long, yellow-orange stems that wind around its host, draining nutrients until the host plant dies. Dodder seedlings must find a host within a few days or they will die. They have an impressive ability to “sense” nearby plants that would make good hosts.
2. Mistletoe
   Mistletoe is famous for its association with Christmas, but its parasitic nature is often overlooked. The mistletoe plant spreads through birds and explodes its seeds across treetops at incredible speeds. Some mistletoe species, like Viscum album, parasitize trees by attaching to their branches and stealing water and minerals.
3. Australian Christmas Tree
   The Australian Christmas tree, Nuytsia floribunda, photosynthesizes but also steals water from neighboring plants. Using a special “guillotine” structure in its haustoria, it can cut through the xylem vessels of other plants, ensuring a constant water supply.
4. Ghost Plant (Monotropa uniflora)
   The ghost plant, also known as Indian pipe, lacks chlorophyll and cannot photosynthesize. It parasitizes fungi, which in turn get their nutrients from trees. The ghost plant tricks the fungi into thinking it will exchange sugars, while it secretly steals them.
5. Indian Paintbrush (Castilleja)
   The Indian paintbrush is a hemiparasite that can photosynthesize but also parasitizes the roots of nearby plants. It’s known for its striking, colorful flowers and is a favorite among hummingbirds.
6. Giant Padma (Rafflesia arnoldii)
   Rafflesia is infamous for having the world’s largest flower, which can grow up to one meter in diameter. This parasitic plant does not photosynthesize. Instead, it steals nutrients from nearby Tetrastigma vines. The flower also produces a terrible odor to attract pollinators, mainly flies.
7. Thurber’s Stemsucker (Pilostyles thurberi)
   This tiny parasite lives inside the stems of shrubs, particularly those in the pea family. Measuring only six millimeters in length, it lacks leaves, roots, and chlorophyll. It only emerges to produce flowers, and no one is sure what pollinates them.
8. Yellow Rattle (Rhinanthus minor)
   Yellow rattle is a hemiparasite that steals nutrients from grasses, which slows their growth and allows other plants to thrive. This little wildflower plays an essential role in promoting biodiversity in meadows by preventing the dominance of grasses.
9. Striga (Witchweed)
   Striga, or witchweed, is a significant agricultural pest in Africa, Asia, and Australia. It attaches to the roots of crops like maize, millet, and rice, draining them of vital nutrients. The presence of witchweed can devastate crop yields and poses a serious challenge to farmers.
10. Broomrapes (Orobanche)
    Broomrapes target legumes, such as tomatoes and sunflowers, and can severely damage crops. These parasites attach themselves to the roots of their host plants and, like Striga, delay growth and reduce crop yield.

Plants that aren’t parasites

It’s important to note that not all plants that use other plants for support are parasites. Vines, lianas, and aerophytes, for example, may grow on other plants but don’t extract nutrients from them. These plants are considered epiphytes rather than parasites, as they only use the host plant for physical support.

The evolution of parasitic plants

The evolution of parasitic plants is still a mystery in many ways. These plants evolved from non-parasitic ancestors and developed the ability to tap into the resources of other plants. Research shows that parasitic plants often undergo horizontal gene transfer (HGT), a process where they exchange genetic material with their hosts. This can lead to the development of new traits, such as the ability to camouflage or manipulate their host’s immune system.

Scientists at Oxford Botanic Garden are studying these plants and exploring how the shift to parasitism may have helped form new plant species. This research includes understanding how parasitic plants, like Hydnora, evolved and adapted over time.

Conclusion

Parasitic plants are an extraordinary part of the natural world. They may appear to be vampires of the plant kingdom, but their survival tactics are a testament to the remarkable adaptability of life. These plants have evolved incredible strategies to thrive in their environments, whether by hijacking other plants’ nutrients or exploiting fungi for survival. While many parasitic plants can be destructive, especially to crops, they also provide valuable insight into plant evolution and the complex relationships between organisms.

Understanding these “zombie” plants helps us appreciate the diversity of life and the often surprising ways nature solves problems. As scientists continue to study these fascinating organisms, we may uncover even more secrets about their survival and the intricate web of life they are a part of.