The Eavesdropping Parasite

ILLUSTRATION BY HANNAH ABRAHAM

SQ FALL INSIDER 2020

THE EAVESDROPPING PARASITE

written by Ananya Prasad

illustrated by Yichen Wang

Upon hearing the word “parasite,” I am reminded of the extraordinary Korean social satire film of 2019, Parasite. Although the movie was received with warm reception and praised greatly, parasitic organisms are quite the opposite. You may imagine parasites as invasive and exploitative microscopic organisms that live beyond the realm of human detection; however, parasites come in all shapes and sizes. One such particularly interesting parasite is the dodder (scientific name: Cuscuta)—the most widespread parasitic plant species in the world!

Dodders are a vine-like genus of parasitic plants with over 200 species. They are widely distributed throughout the tropical and temperate zones; however, many species have piggybacked on their hosts, entering other regions. The dodders have crossed country and climate lines with the help of humans. These parasites prolifically produce seeds in the soil surrounding their host. When humans relocate the host, the dodders get a free ride to previously unexplored regions.

Dodders begin their life cycle looking akin to a piece of string. In contrast to many autotrophic plant species (plants capable of self-nourishment), dodders are rootless, leafless, and cannot efficiently photosynthesize. Hence, these plants need a host to survive. Dodders are parasitic on a wide range of domestic, wild, and agricultural plant species, so finding a host is rarely difficult. The dodders elongate in a spiral fashion, searching for a host to infiltrate. When it identifies a suitable host, dodders produce haustoria (root- like tubes) that penetrate the stems of the hosts to draw water and nutrition. With time, the dodder covers the host plant in a woven cocoon of yellow stems, thriving at the expense of their victim.

The dodders have siphoned nutrients and water, but how do they ensure that their own parasitic plant species lives on to breed future generations? Autotrophic plants can rely on their leaves to sense optimal environmental conditions to initiate flowering, pollination, and reproduction. Unfortunately, dodders can neither grow leaves, nor do they possess the adequate gene sequences to control their flowering mechanism. This raises the question of how the dodder knows when to flower.

Dodders solve this problem by listening in on their hosts. Dodder cell tissues absorb Flowering Locus T (FT), an air-borne protein produced by their hosts, and use this chemical signal to flower synchronously with their host plant. The FT gene, which codes for the FT protein, plays a highly important role in reproduction; it triggers flowering. In dodders, this gene is a pseudogene (non-functional), so they need a host to know when to flower. A recent study on Australian dodders (Cuscutta australis), published in the Proceedings of the National Academy of Sciences, involved an enclosed experiment in which researchers infested three photoperiod-insensitive species with dodders. Naturally, each dodder cluster synchronized its flowering period with that of its host. Furthermore, when the FT gene in the host plants were disabled, the dodders were unable to flower. The researchers also attached a fluorescent protein to the hosts’ FT protein which was found glowing in dodder tissue prior to their flowering. The overwhelming evidence in this study led scientists to the conclusion that dodders do in fact eavesdrop on their hosts!

Synchronous flowering is a unique behavioral characteristic of dodders. If the dodders flowered too soon, their growth would end prematurely and result in dramatically fewer seeds. Conversely, flowering after their hosts would leave the dodders with less nutrients as the host may have already died.

Dodders are also a great example for the evolution of plant parasitism. Scientists have found significant similarities between the genes that code for haustorium in parasitic plants and roots of autotrophic plants. Other studies have shown that host plants use dodders as a channel of communication to warn other plants about impeding insect attacks and other threats. Digging deeper into the physiology and ecology of dodders can be an instrumental tool for developing new strategies in agricultural crop optimization and forest conservation. Due to evolution, dodders are now the prime thieving masterminds of the plant world.

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