Close Encounters

Where does a hemiparasite’s food come from?

A parasitic plant's ability to steal carbon rests in their haustoria, specialised organs they use to tap into a host plant.
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Plants gain the carbon they need from the air and the oxygen and hydrogen they need from their roots. Sometimes they take shortcuts. Holoparasites are plants that take everything from their hosts. Hemiparasites, like mistletoe, are more of a puzzle. They have leaves, so they can photosynthesise. But do they take most of their carbon from the air, or do they steal it from their hosts?

Rhinanthus minor, Yellow Rattle. a root parasite that photosynthesises above ground. Image: Canva.

Previous research has examined hemiparasites with different methods of photosynthesis. There are three common methods of photosynthesis named after the way they build hydrocarbons, C3, C4 and CAM. Studying C3 parasites on C4 or CAM plants is relatively easy, as the different methods of photosynthesis discriminate against different isotopes, making it possible to ‘tag’ chemicals that enter the plants.

Giesemann and Gebauer have been working to find a way of tracking the chemicals C3 hemiparasites use on C3 hosts. In a paper published recently in Annals of Botany, they use a combination of holoparasites and hemiparasites to discover how the hemiparasites feed themselves.

They found that the hemiparasites varied in how much carbon they took from their host, from none to over 50%. The key to the difference was in their haustoria, the organs that hemiparasites can use instead of roots.

“Haustoria penetrate and invade the host tissue thereby facilitating a physio-physical bridge between host and parasite. Based on our results, the hemiparasites with less specialized haustoria, i.e. species belonging to the genera Euphrasia, Melampyrum and Bartisa, received much less xylem organic C than hemiparasites with more specialized haustoria, i.e. species belonging to the genera Rhinanthus and Pedicularis,” write Giesemann and Gebauer.

Understanding how parasites feed is important. Ecologists look at parasitic plants as ecosystem engineers. These plants can influence the reproduction of other plants and open up niches for other organisms. However, they can also cause a great deal of economic damage. Understanding how parasitic plants work opens the possibility of using their positive traits and mitigating their harms.

READ THE ARTICLE

Giesemann, P. and Gebauer, G. (2022) “Distinguishing carbon gains from photosynthesis and heterotrophy in C3-hemiparasite-C3-host pairs,” Annals of Botany, 129(6), https://doi.org/10.1093/aob/mcab153

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