Sometimes parasites get their own parasites too, and if you think that "enemy of my enemy is my friend", then you'd think this would be good news for the host. But that depends on the host-parasite pairings in question. This post is about a study on mistletoes, a plant that many people associate with Christmas celebrations, but they are also parasitic plants, specifically, they are "hemiparasites" - which are plants that can do their own photosynthesis, but they draw water and other nutrients from a host plant.
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| Left: A harlequin mistletoe attached to a box mistletoe (red arrow indicating attachment point), Right: Close-up of the attachment point (indicated by red arrow) between a harlequin mistletoe and box mistletoe. From Fig. 1 of the paper |
Mistletoes have varying degrees of host specificity, with some of them parasitising only a selected handful of trees and shrubs species, while others can infect a wide range of different plants. They parasitise their host using a modified root called haustorium, which bores into the host plant's stem, tapping into its flow of water and nutrients. But sometimes, mistletoes find themselves on the receiving end of a haustorium from another mistletoe. After all, mistletoes are just another type of plant. Parasitic plants that engage in such a lifestyle are called "epiparasites" by botanists, though they also fall under the larger umbrella of hyperparasites - parasites of parasites.
The Australian harlequin mistletoe (Lysiana exocarpi) is a very versatile hemiparasite - it can infect over a hundred different plant species and when the opportunity arises, it parasitises a fellow mistletoe, namely the box mistletoe (Amyema miquelii). One of the challenges for an epiparasite is maintaining a lower water potential than its host. Water has a tendency to move from areas of high concentration to lower concentration, and in plants, this is how water is transported from the roots to the shoots/leaves because the atmosphere (where the shoots/leaves are) have lower water concentration than the soil (where the roots are). As water diffuses into the atmosphere from the leaves, it draws more water from the roots to the shoots.
So in order to suck up water from its host, a mistletoe would need to maintain a lower water potential than the shoots of the host tree - this is why mistletoes are very thirsty plants. And an epiparasite parasitising another mistletoe would need to maintain an even lower water potential to ensure water would flow to it through both its host mistletoe as well as the tree that its host mistletoe is parasitising. So when a mistletoe is parasitising another parasitic plant, it would need to change certain aspects of its physiology.
This study took place at the Onkaparinga River National Park in South Australia, in a woodland composed mostly of pink gum (Eucalyptus fasciculosa). The researchers conducted a variety of measurements on both host trees and mistletoes, and collected samples of their leaves. What they found was that when the harlequin mistletoe is parasitising another mistletoe, it opened up more of the stomata on its leaves, so water is released into the atmosphere at a higher rate. At the same time, it also grew leaves with larger surface area, and had higher concentration of potassium and magnesium in them. All this decreases the mistletoe's water potential, which means the harlequin mistletoe gets more thirsty when it's parasitising another mistletoe.
But what happens to its host mistletoe? Well, surprisingly enough, it seems that the box mistletoe doesn't suffer from being parasitised. It compensates for the cost of its thirsty epiparasite by simply drawing even more resources from its eucalyptus host, essentially outsourcing the cost of hosting a harlequin mistletoe to the tree. All this means that the host tree ends up taking the full brunt of BOTH parasites. Eucalyptus trees which are host to a parasitised box mistletoe have stiffer leaves than if it is parasitised by the box mistletoe alone. Among eucalyptus, growing stiffer leaves is often a symptom of nutrient and water deprivation, which is perhaps not surprising since the tree is hosting a pair of very thirsty plants, and this can have long term impacts on its growth and reproduction.
So at least when it comes to parasitic plants, the enemy of your enemy is not necessarily your friend, in fact, you might end up paying the price for their antagonistic relationship.
Reference:
Scalon, M. C., & Rossatto, D. R. (2024). Challenging the 'Immunity Hypothesis': Primary or Secondary Parasitism as Different Survival Strategies for the Harlequin Mistletoe Lysiana exocarpi (Behr) Tiegh. Flora 323:152662.
