Cuscuta australis, Australian Dodder, is a parasite that draws its food from its host. With all the nutrients passing into the parasite, its litter should be a banquet for microbes. But research by Yongge Yuan and Junmin Li at Taizhou University shows that Cuscuta australis litter decomposes more slowly than the host plant’s litter. Their results, published in November’s Soil Biology and Biochemistry, show that, even dead, a parasite can hinder the growth of others.
Yuan and Li wanted to examine how plant litter, the dead remains of plants, decomposes. If a parasite is helping itself to the nutrients of its host, then its litter should be rich in the kind of chemicals that fungi and microbes want when they break down the litter. The authors knew plenty of research shows how parasite litter decomposes faster than host litter but is it a general rule?
The scientists used Cuscuta australis, a parasite that climbs entwined around its host. As well as support, it gains water, carbon and nutrients by tapping into a host plant with highly modified roots, called haustoria. Earlier research by Junmin Li suggested that Cuscuta australis could be used as a control for the invasive plant Bidens pilosa. It sounds like a good solution, but if Cuscuta australis affected the decomposition of litter, then could there be an unexpected cost?
To find out, Yuan and Li set up a container with a bag of leaf litter in it. They then planted some Bidens pilosa infested with Cuscuta australis. The leaf litter bag was a small enough mesh that arbuscular mycorrhizal fungi associated with the plants could reach out to penetrate the bag, while plant roots could not.
The authors found that the Cuscuta australis litter decomposed more slowly than the Bidens pilosa litter. This was a puzzle.
Examining the chemical composition of the plants, the scientists found that Cuscuta australis litter had similar concentrations to Bidens pilosa. So why didn’t it decompose as quickly? A difference was in the concentrations of chemicals known as secondary metabolites. These are the chemicals that aren’t the basic structure of plants but are still necessary to survive. Complex molecules like plant hormones, defensive compounds or scents are all secondary metabolites. And you won’t find as many of them in Cuscuta australis.
Why the secondary metabolites should be necessary is unclear, but Yuan and Li have a few suggestions. One is that microbes could be waiting to detect these chemicals as litter decomposes. They point to research that shows flavonoids cause germination in arbuscular mycorrhizal fungi. If Cuscuta australis lacks these chemicals, a signal is missing from the soil. Without the fungi, other microbes lack a partner to help transport them to litter and break it down.
They also found that the gene abundances of anaerobic ammonium oxidation were significantly higher in bags with Bidens pilosa litter than those in bags with Cuscuta australis litter. These genes would be from bacteria that break down the litter. The variation in abundances showed that they could get more nitrogen from the Bidens pilosa than the Cuscuta australis.
Another issue they raise is earlier research that shows herbivory and nematode parasitism can decrease the proportion of carbon allocated to arbuscular mycorrhizal fungi. The parasites take carbon that the host plant would otherwise pass along to the fungi. It seems reasonable to ask if Cuscuta australis, a plant parasite, is also affecting the fungi and reducing their ability to work on decomposition elsewhere. Yuan and Li write that this is a new discovery.
“This study provided the first evidence of how parasitic plants induce a top-down cascading effect on the regulation of AMF on litter decomposition, as well as an example of how AMF differentially affected parasitic and host litter decomposition and how they responded to parasitism.”
Most work on parasitic plants has concentrated on how they extract life from their hosts, like vampires. Now it seems they still exert an influence in their afterlife.
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Yuan, Y. and Li, J. (2022) “Dodder parasitism limited the effect of arbuscular mycorrhizal fungi on litter decomposition,” Soil Biology & Biochemistry, 174(108837), p. 108837. https://doi.org/10.1016/j.soilbio.2022.108837