In late July of 2023, I hiked to one of my favorite high-desert Tiger Salamander ponds here in Southwestern Colorado. The pond had dried out – there was nothing but cracked mud and dust. No water, no Salamanders. But I did see something strange on the bank of the stock-tank: a bright yellow-orange splash of color. From a distance, it looked like wiry spaghetti strands draped over the plants, as if somebody had sprayed the area with bright yellow “Silly String“.
Ah – this was a Dodder (Cuscuta sp.) plant.
Dodder is fascinating from a number of perspectives. It has only small amounts of chlorophyll, not enough to support itself through photosynthesis. It’s also got no leaves to speak of, it is mostly just comprised of a stem. But that stem is a hungry, searching stem – Dodder is a parasite. It must steal its nutrients and water from a host plant.
In this particular case, the Dodder was growing on a Cocklebur (Xanthium sp.)
Dodder initially grows from a seed in the soil. When a Dodder seed germinates, the seedling grows as quickly as it can towards a potential host plant. The Dodder seedling has got to find a host within 10 or 15 days, or it will starve to death. If you watch a time-lapse video of Dodder seedlings, you can see them waving and sweeping and probing the area around them, grasping for a host plant.
But the shoot of the Dodder seedling doesn’t just randomly sweep the area as it searches. Rather, it tends to grow towards potential host plants. How does it do this? Turns out that the Dodder seedlings can detect volatile compounds released by nearby plants. It ‘smells’ them, then it tracks them down.
The Dodder seedlings can even differentiate between nearby plants based on the volatiles that the plants release…and then the Dodder grows toward whichever one it would ‘prefer’ to parasitize.
Additionally, in her book The Light Eaters, author Zoë Schlanger cites research indicating that Dodder vines can ‘see’ red-shifted light – light which has been altered by passing through a chlorophyll-containing host. It can seemingly ‘discern’ the general growth form of the plant that it’s seeing, at least so far as whether it is a linear shape, such as grass, or a branching shape, such as the flowering plants Dodder requires as hosts.
So…the Dodder seedlings can detect and choose their prospective host, before they come into contact with it.
Dodder seedling ‘roots’ don’t seem to function well, or at all, for absorbing nutrients or water from the soil. Sherman etal describe the Dodder seedling’s root as more like a specialized stem base than a functional root. This stem-base/root degenerates within a few days, often even before the Dodder seedling has successfully found a host.
The nutrients released from the degeneration of the ‘root’ help to fuel the rapid growth of the Dodder seedling’s shoot. The ‘root’ seem to be used mostly as temporary storage for the nutrients that were in the original seed.
When (if!) the Dodder seedling reaches a host plant, the Dodder tendrils grow around the host, climbing, twisting and draping over the host. But Dodder does more than just drape itself over its hosts..
The little bumps I’ve circled in the image below are structures called haustoria. They seem to be similar to adventitious roots in ivies and other vines. As the Dodder grows over its host, the haustoria grow into the host. The Dodder pulls carbohydrates, nutrients and water from its victim.
But it’s not just taking sustenance from its victims. It’s also taking proteins and hormones from its host. For example, if Dodder is feeding on a plant, such as a Soybean, that has been genetically modified to increase herbicide resistance, Dodder’s uptake of the host’s circulating ‘herbicide resistance’ proteins grant herbicide resistance to the Dodder, as well.
Another benefit to Dodder of imbibing the host’s proteins is that they can tell Dodder when the host is going to flower. Imagine if Dodder had gone to all of this trouble finding, then attaching to its host, only to have the host set seed and die before the Dodder flowered.
That’s an especially difficult problem for Dodder, given that it parasitizes many different species of plant, all of whom may time their flowering differently.
Dodder gets around this problem by detecting a protein that the host plants circulate when they are getting ready to flower – “Flowering Locus T (FT)” protein. When Dodder detects circulating FT protein, the Dodder starts getting ready to flower as well.. Researchers also note that Dodder doesn’t seem to make functional FT protein of its own, it is actually reliant upon the host’s.
Researchers suspect that the loss of functional FT protein happened fairly early in the diversification of Dodder species, since many Dodder species don’t have a functional FT protein of their own. Think how useful tapping into the host’s flowering schedule would be for a plant parasite.
Revisit – a year later
I came back to the same location about a year later – in mid-August of 2024. Both the Dodder and its host Cocklebur had died. That was expected, since both were annuals. Interestingly, both plants had managed to produce seeds.
In fact, the Dodder seemed to have produced a lot of seeds.
I don’t know of any long-distance seed dispersers for Dodder, other than humans moving contaminated soil or seed from one area to another.
Another interesting thing about Dodder – the seeds will remain viable in the soil for 10-20 years, and even under favorable conditions, all of them won’t germinate in any given year. Some will wait for future years.
So…maybe next year a couple of seeds might germinate, and then a few more the year after that, until perhaps one year the seeds happen upon just the right conditions, and they also happen to have just the right host nearby.
And then the cycle begins again.
A Closing Note
I had a hard time writing this blog post.
I thought that I’d just be writing about one attractive but noxious weed (Dodder) that was parasitizing another noxious weed (Cocklebur). Perhaps it would take an hour or two to write.
Hah! What actually happened was that I fell down a rabbit hole of stunning facts, only a few of which I’ve managed to discuss here. It all started when I read Joe Boggs’ 2024 fascinating “Untangle Dodder’s Tale” over on the Buckeye Yard & Garden onLine. If you want to read even more fascinating stuff about Dodder, I highly recommend that you take a look at his great gateway drug article.
Sources
Boggs, Joe. September 9, 2024. “Untangling Dodder’s Tale“. Url: https://bygl.osu.edu/node/2421 Buckeye Yard & Garden onLine. Ohio State University. I came across Dr. Boggs’ article while researching this post. He has written a fascinating article – I recommend it.
Runyan, Justin B., Mark C. Mescher, and Consuelo M. De Moraes. 2006. “Volatile Chemical Cues Guide Host Location and Host Selection by Parasitic Plants”. Science 313. 29 September, 2006. DOI 10.1126/science.1131371.
Shen, Guojing, Lian liu, Jingxiong Zhang and Jianqlang Wu. 2020. Cuscuta australis (dodder) parasite eavesdrops on the host plants’ FT signals to flower. PNAS vol 117, no 37. pp 23125-23130. www.pnas.org/cgi/doi/10.1073/pnas.2009445117. This is yet another mind-blowing paper.
Schlanger, Zoë. 2024. The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth. Harper, May 2024. 298pp. ISBN 0008445346.
Sherman, Timothy D, Andrew J. Bowling, T. Wayne Barger and Kevin C. Vaughn. 2008. The Vestigial Root of Dodder (Cuscuta pentagona) Seedlings. International Journal of Plant Sciences. Vol 169, Number 8. The University of Chicago Press: Journals. DOI: 10.1086/590442.
