Parasitic orchids are healthier

Most orchids live in a symbiotic relationship with fungi in their roots: The plants provide sugar they produce through photosynthesis and in return receive water and minerals from the fungi. However, some orchids have stopped producing their own food and completely feed on fungi. The Kobe University botanist SUETSUGU Kenji says: "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?"

The orchid Oreorchis patens offers a prime opportunity to study this question, as it is a partial parasite, meaning that it can produce its own food but also takes up to half of its budget from fungi. The key question in the field was whether the orchids do so to top up what they can't get enough of through photosynthesis, or whether they actually derive an additional benefit from their parasitism. Suetsugu explains: "I noticed that Oreorchis patens sometimes grows unusual coral-shaped rootstalks, a trait reminiscent of orchids fully relying 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."

In a paper now published in The Plant Journal, the Kobe University team shows that when the orchid happens to grow close to rotten wood, it shifts its fungal symbionts to those that decompose the wood and significantly increases the amount of nutrients it takes from them -- without ceasing to employ photosynthesis. As a result, the plants are bigger and produce more flowers. "In short, these orchids aren't merely substituting for diminished photosynthesis, they're boosting their overall nutrient budget. 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," says Suetsugu.

But then, why do only less than 10% of these orchids exhibit this behavior? The answer might be found in the fact that the researchers could only see parasitic individuals near fallen and rotting tree trunks. Becoming a parasite means that the orchids need to switch from their usual symbionts to different fungi that can handle the increased nutritional load. But appropriate fungi only occur when there is fallen wood and only in certain stages of the decomposition process. In other words, the orchids become parasitic only when they can, not whenever they need to, and this opportunity does not present itself often.

Many questions are still left open, such as what triggers the orchids to develop the coral-like rootstalks and whether environmental factors influence the amount of nutrients the plants take from the fungi. Suetsugu explains his wider outlook: "This work is part of a broader effort to unravel the continuum from photosynthesis to complete parasitism. 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."

This research was funded by the Japan Society for the Promotion of Science (grant 17H05016), the Japan Science and Technology Agency (grant JPMJPR21D6) and the Research Institute for Humanity and Nature.