What happens when a vampire plant ‘bites’ two victims at once?

Just in time for Hallowe’en, Shalan Li and colleagues have a paper investigating the closest thing to a real vampire in the plant world. Dodder is a parasite that attacks other plants and, while it can’t aim for the neck, it can aim for a shoot. Once the haustoria strike then the dodder can use these to draw nutrients from its victim’s vascular system. It feeds on the nutrients drawn in from its victim’s xylem and phloem. But life is not always that simple.

The dodder finds victims by twining a shoot around and around till it finds something. But as it grows, it’s likely to find far more that one plant. The authors describe the result as a ‘plant cluster’ connected by dodder. The dodder is drawing the nutrients from all these plants and then passing it back out. Li and colleagues wondered if along with the nutrients, there could also be signalling molecules passing between plants.

To find out they set up a dodder plant to parasitise two cucumber plants. They then subjected one cucumber plant to salt stress.

What they found was that, after a day, the transcriptomes in both cucumber plants were similar. The unstressed (apart from the parasite) plant was acting as though it was primed for salt stress. When the second cucumber was then subjected to salt stress, it dealt with it better as the coping mechanisms were already up and running.

Some of the authors on this paper were primed for this result, as this JXB paper shares a few authors from an earlier article in PNAS. It’s not just salt stress that dodder can pass along.

In 2017 Christian Hettenhausen and colleagues connected two soybean plants through a parasitic dodder plant. For one of the plants, a bad day became worse, as they then released Spodoptera litura caterpillars onto it. While S. litura might be called tobacco cutworm, it’s happy to eat soybean leaves too. The soybean plant was not at all happy about this, so started releasing chemicals to prime defences in its unattacked leaves. That way, it would be ready when the caterpillars moved on.

But the dodder was drawing out some of these signals through its connection to the soybean plant.

They found that the unattacked plant was receiving these signals from its dodder connection, and had started defending its leaves. Further experiments without a dodder connection didn’t show this response. So the two plants had to be connected through a shared parasite for both of them to react to one plant’s problem.

They also found that the plants don’t even need to be the same species to react to caterpillars. Connecting Arabidopsis thaliana to tobacco, and tomato to soybean also produced similar results.

The findings suggest that the plants are all using the same kind of cellular equipment to signal stress to other parts of the plant, and that dodder can tap into this signalling system, just as it taps into the nutrients in its hosts.

If dodder indeed were a real-life vampire, this would suggest an effective way to kill it – connect it to a garlic plant, and all plants in the network would be protected. Alas, nature is cruel. Dodder is perfectly happy parasitising garlic as it is other plants. It’s not quite a vampire – it’s a bit more merciless.