How a Giant Parasitic Orchid Makes a Living

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Imagine a giant vine with no leaves and no chlorophyll scrambling over decaying wood and branches of a warm tropical forest. As remarkable as that may seem, that is exactly what Erythrorchis altissima is. With stems that can grow to upwards of 10 meters in length, this bizarre orchid from tropical Asia is the largest mycoheterotrophic plant known to science.

Mycoheterotrophs are plants that obtain all of their energy needs by parasitizing fungi. As you can probably imagine, this is an extremely indirect way for a plant to make a living. In most instances, this means the parasitic plants are stealing nutrients from the fungi that were obtained via a partnership with photosynthetic plants in the area. In other words, mycoheterotrophic plants are indirectly stealing from photosynthetic plants.

In the case of E. altissima, this begs the question of where does all of the carbon needed to build a surprising amount of plant come from? Is it parasitizing the mycorrhizal network associated with its photosynthetic neighbors or is it up to something else? These are exactly the sorts of questions a team from Saga University in Japan wanted to answer.

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All orchids require fungal partners for germination and survival. That is one of the main reasons why orchids can be so finicky about where they will grow. Without the fungi, especially in the early years of growth, you simply don't have orchids. The first step in figuring out how this massive parasitic orchid makes its living was to identify what types of fungi it partners with. To do this, the team took root samples and isolated the fungi living within.

By looking at their DNA, the team was able to identify 37 unique fungal taxa associated with this species. Most surprising was that a majority of those fungi were not considered mycorrhizal (though at least one mycorrhizal species was identified). Instead, the vast majority of the fungi associated with with this orchid are involved in wood decay.

Stems climbing on fallen dead wood (a) or on standing living trees (b). A thick and densely branched root clump (c) and thin and elongate roots (d) [Source]

Stems climbing on fallen dead wood (a) or on standing living trees (b). A thick and densely branched root clump (c) and thin and elongate roots (d) [Source]

To ensure that these wood decay fungi weren't simply partnering with adult plants, the team decided to test whether or not the wood decay fungi were able to induce germination of E. altissima seeds. In vitro germination trials revealed that not only do these fungi induce seed germination in this orchid, they also fuel the early growth stages of the plant. Further tests also revealed that all of the carbon and nitrogen needs of E. altissima are met by these wood decay fungi.

These results are amazing. It shows that the largest mycoheterotrophic plant we know of lives entirely off of a generalized group of fungi responsible for the breakdown of wood. By parasitizing these fungi, the orchid has gained access to one of the largest pools of carbon (and other nutrients) without having to give anything back in return. It is no wonder then that this orchid is able to reach such epic proportions without having to do any photosynthesizing of its own. What an incredible world we live in!

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Photo Credits: [1] [2]

Further Reading: [1]

Host Coercion

Moving from one host to another can be difficult for parasites, especially for those specializing on plants. Because they rely on other organisms for their survival, they have evolved some amazing strategies at getting what they need. A recent study published in PLOS Biology has shed some light on one interesting strategy.

Phytoplasma are bacterial parasites of a variety of plant species. In order to get from one host to another, these bacteria utilize insect hosts. How they do this is quite incredible. These bacteria produce specialized proteins that have some strange effects on plant tissues.

The proteins actually sterilize the host plant. They do this by interfering with the proteins responsible for flower development. Instead of producing normal flowers, the plants produce mutated leaf-like structures. You can see an example of a healthy plant on the left and an infected one on the right. So, why does the bacteria do cause such mutations?

This is where the insects enter the picture. Researchers found that infected plants that produced these mutated leaf-like structures were more attractive to leaf hoppers. The leaf hoppers readily feed and reproduce on these infected plants at a higher rate than they do healthy plants. In feeding, the leaf hoppers inevitably suck up bacteria in the sap.

When the leaf hoppers go on to feed on healthy plants, some of the bacteria get transferred in their saliva, thus completing the parasitic lifecycle. This is what parasitologists call "host coercion." The parasite, in this case phytoplasma bacteria, alter their host in some manner that increases the fitness of the parasite. This is one of the first examples in which researchers have been able to identify the exact mechanism by which a parasite makes this happen.

Photo Credit: John Innes Centre (https://www.jic.ac.uk/)

Further Reading:
http://journals.plos.org/plosbiology/article?id=10.1371%2Fjournal.pbio.1001835