Some orchids turn parasitic when conditions allow

Orchids are among the most diverse and fascinating plants in the world. Their ability to adapt to different environments has allowed them to thrive in forests, grasslands, and even rocky terrains.

While most orchids rely on photosynthesis to produce their own food, a few species have taken a different path. Instead of creating their own energy, they survive by feeding on fungi.

This parasitic shift raises an intriguing question: why would a plant abandon its ability to produce food and depend entirely on another organism?

Kobe University researchers explored this mystery by studying Oreorchis patens, an orchid that exists in both photosynthetic and parasitic forms. The findings suggest that this transition may not be a matter of necessity but rather an opportunity that arises under specific conditions.

Balancing photosynthesis and parasitism

Most orchids maintain a symbiotic relationship with fungi. The plants supply sugars produced through photosynthesis, while fungi provide water and essential nutrients.

This relationship benefits both organisms and is a key survival strategy for orchids in nutrient-poor environments. However, some orchids no longer contribute to this exchange. Instead of photosynthesizing, they take all their nutrients from fungi, effectively becoming parasites.

“I’ve always been intrigued by how orchids turn parasitic. Why would a plant give up its reliance on photosynthesis and instead ‘steal’ from fungi?” noted Kobe University botanist Suetsugu Kenji.

This question led researchers to examine Oreorchis patens, a species that offers a rare glimpse into the transition from mutualism to parasitism.

An orchid’s dual strategy

Unlike fully parasitic orchids, Oreorchis patens still retains its ability to photosynthesize. However, it can also absorb up to half of its nutrients from fungi. This makes it an excellent model for studying why some orchids partially abandon photosynthesis.

Suetsugu observed that some Oreorchis patens individuals develop unusual coral-shaped rootstalks. These structures resemble those found in orchids that rely entirely on fungi.

“I thought that this would allow me to compare plants with these organs to those with normal roots, quantify how much extra nutrients they might be gaining, and determine whether that extra translates into enhanced growth or reproductive success,” said Suetsugu.

Wood helps orchids absorb fungi nutrients

To uncover the role of fungi in this process, researchers examined orchids growing in different environments. Their study, published in The Plant Journal, revealed a fascinating pattern.

When Oreorchis patens grows near decomposing wood, it shifts its fungal partners to species that break down rotting material. This shift allows the orchid to extract significantly more nutrients from its fungal hosts.

Surprisingly, despite its increased reliance on fungi, the orchid does not stop photosynthesizing. Instead, the extra nutrients enhance its growth, making it larger and more robust. The orchids in these conditions also produce more flowers, indicating a clear reproductive advantage.

“In short, these orchids aren’t merely substituting for diminished photosynthesis, they’re boosting their overall nutrient budget,“ noted Suetsugu.

“This clear, adaptive link between fungal parasitism and improved plant vigor is, to me, the most thrilling aspect of our discovery, as it provides a concrete ecological explanation for why a photosynthetic plant might choose this path.”

Why don’t all orchids become parasites?

Despite the apparent benefits, fewer than 10% of Oreorchis patens individuals exhibit parasitic behavior. The researchers noticed that parasitic orchids were only found near fallen and rotting tree trunks.

This suggests that switching to parasitism requires a specific set of conditions, particularly the presence of fungi that can handle an increased nutritional load.

These specialized fungi thrive in decaying wood but only at certain stages of decomposition. Since such conditions are rare and unpredictable, most Oreorchis patens individuals continue relying primarily on photosynthesis.

The transition to parasitism happens only when the right fungi are available, not necessarily when the orchid needs extra nutrients.

Future research directions

Although this study sheds light on why some orchids turn parasitic, many questions remain. Scientists still do not know what triggers the development of the coral-like rootstalks in Oreorchis patens.

Environmental factors likely play a role, but the exact mechanisms remain unclear. Another question is how flexible the orchid is in choosing its fungal partners and whether other species exhibit similar behavior.

Suetsugu sees this research as part of a larger effort to understand the full range of plant lifestyles.

“This work is part of a broader effort to unravel the continuum from photosynthesis to complete parasitism,” said Suetsugu.

“Ultimately, I hope such discoveries will deepen our understanding of the diverse strategies orchids employ to balance different lifestyles, thereby aiding in the preservation of the incredible diversity of these plants in our forests.”

Conservation efforts for orchids

Understanding how orchids interact with fungi is essential for conservation efforts. Many parasitic orchids depend on specific fungal species, which means changes in forest ecosystems could threaten their survival.

By studying these relationships, researchers can better predict how orchids will respond to environmental changes and how best to protect them.

This research was supported by the Japan Society for the Promotion of Science, the Japan Science and Technology Agency (JST), and the Research Institute for Humanity and Nature.

The study is published in The Plant Journal.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–