A plant by any other name...

The odd, spike-like 'inflorescence' (or flowers) of parasitic Squawroot pierces through the leaf litter in an Appalachian hardwood forest.

One of life’s biggest challenges – if not its biggest single challenge – is obtaining the energy required to power the processes of life. Many of a cell’s chemical reactions, the anabolic reactions that make up part of the cell’s metabolism, are centered around the construction of products within the cell such as proteins and nucleic acids that are necessary for life. And since these products cannot be assembled at random, the reactions building these products require an energy input that is often in short supply.

When looking across the breadth of biodiversity from bacteria to plants, animals, and fungi, one of life’s most fascinating evolutionary consequences can be found in the variety of strategies organisms have developed to harvest the energy required to power life. Animals like ourselves, for example, obtain energy through a process known as heterotrophy: ingesting living matter from other organisms and harnessing the energy stored in the chemical bonds of those organisms’ tissues. Other organisms, including plants and many bacteria, use chemical pigments to capture solar energy and shuttle it to reactions in the cell that build energy-storing molecules like glucose (sugar).

Members of the plant kingdom are the classic example of these solar-powered organisms. Nearly all plants contain a cocktail of pigments within their cells  - most famously green-colored chlorophyll – that serve to capture incoming light energy from the sun. This energy is then used to build glucose within the cell, which serves to power the necessary reactions the plant needs to sustain itself and to reproduce. This strategy, called photosynthesis, is one of the most successful processes found in living organisms and forms an energy base for most of the food webs found in the world’s ecosystems.

However, not all plants follow this rule. A much smaller number of plants siphon energy from other living plants as heterotrophs. Some of these plants are direct parasites, obtaining energy  from other living plant species through specialized structures. The remainder of these plants, called mycotrophic plants, actually harness energy by tapping into fungi that live in association with another plant’s roots.

Here in Appalachia, two plants in particular stand out as commonly-seen organisms that nonetheless belong to this more uncommon group of plant strategies. The first, Squawroot (also called Bear Corn or Cancer Root), follows a parasitic strategy. This plant attaches to the roots of oaks and beech trees, siphoning off energy and nutrients from host trees’ tissues. Squawroot is easily distinguishable by its flowers, which appear as corn cob-like spikes projecting from the soil. Black bears infamously flock to Squawroot in the spring as a high-energy food source (hence the colloquial name “Bear Corn”), but most of squawroot itself remains hidden belowground, as its spiked flowers only appear during the reproductive season.

Unlike most other plants, which use leaves and photosynthetic pigments as solar panels, Squawroot is both leafless and lacks chlorophyll altogether, a fact that produces its odd shape and yellow-brown coloration. The reason behind these differences is simple enough: why waste energy producing chemical pigments and leaf tissue when you can obtain nutrition from another organism? Biologists have further delved into this odd strategy and found that not only does Squawroot lack chlorophyll – it also lacks the genes that code for the production of photosynthetic pigments altogether! This information suggests that Squawroot’s odd lifestyle is not a one-time fluke but rather an evolutionary adaptation that has been produced over time and is passed down across generations. Genetic comparisons across several parasitic species, in fact, suggest that the type of parasitic strategy employed by Squawroot may have evolved several separate times throughout the plant world.

Ghost-like Indian Pipe grows out of fungal association with a host plant on the forest floor.
Photo Attribution:Tim Pierce [Public domain], via Wikimedia Commons

Squawroot, however, is far from the only heterotrophic plant in Appalachia. Another species, called Indian Pipe (Monotropa uniflora) also lacks chlorophyll and is commonly mistaken for a fungus by many at first glance. Instead of simply attaching to a host plant’s roots, though, Indian Pipe takes a more roundabout route to accessing nutrients and energy from another plant’s tissues. To understand this approach, it is best to first consider what occurs with many plants belowground.

           

Found across most members of the plant kingdom are a number of mutualistic relationships in which individual plants pair with fungi, called mycorrhizae, which typically grow in association with a plant’s roots. Being a mutualism, both members of this pair benefit from their partnership: plants provide the mycorrhizae with sugars, while the mycorrhizae return the favor by helping the plant obtain nutrients, such as phosphorus. Indian Pipe has found a way to game this system by linking its roots with the mycorrhizae growing in association with a host plant. Indian Pipe then siphons off energy from the fungi, which in turn are pulling energy from the other plant, a chain-reaction type of parasitic relationship called mycotrophy.

Just like Squawroot, Indian Pipe grows close to the ground layer and looks very little like other land plants, appearing as a narrow stalk topped with a single flower, colored white or (more rarely) a pink or red hue. And also like Squawroot, studies of Indian Pipe’s genetic material have uncovered a fascinating evolutionary past. Researchers have specifically sequenced the DNA of Indian Pipe and other close relatives and compared these sequences with the DNA of many different fungal mycorrhizae, finding that most species in Indian Pipe’s genus prefer very specific groups of fungi when siphoning off energy. This information suggests that mycotrophy is not a “one size fits all” relationship; rather, the evolution of this unique energetic strategy appears to occur in close tandem with belowground fungal hosts.

Both Squawroot and Indian Pipe can be incredibly common in Appalachia....if you know when and where to look. Squawroot tends to appear in spring, while Indian Pipe is most common in summer months, both growing close to the ground layer in mature forests.

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See it for yourself

(Note: Although both Indian Pipe and Squawroot are common in Appalachia, it can be next to impossible to pinpoint any single spot where these plants may be visible, and either plant may be present in almost any healthy, moist hardwood forest. However, the below destinations all encompass hikes through hardwood forests where either plant may appear during the flowering season.)

Standing Indian Basin, North Carolina (Appalachian Trail)

High Knob, Virginia (In particular the Chief Benge Trail below the mountain's summit)

Blood Mountain Wilderness, Georgia (Appalachian Trail)

Cucumber Gap Loop, Great Smoky Mountains National Park

Pipestem State Resort Park, West Virginia

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Relevant Journal Articles

Cullings, K. W., T. M. Szaro, and T. D. Bruns. 1996. Evolution of extreme specialization within a lineage of ectomycorrhizal epiparasites. Nature 379:63-66.

dePamphilis, C.W., N.D. Young, and A.D. Wolfe. 1997. Evolution of plastid gene rps2 in a lineage of hemiparasitic and holoparasitic plants: Many losses of photosynthesis and complex patterns of ratevariation. PNAS. 94:7367-7372.

McNeal, J.R., J.R. Bennett, A.D. Wolfe, and S. Matthews. 2013. Phylogeny and origins of holoparasitism in Orobanchaceae. American Journal of Botany. 100:971-983.