Botanists find out why Yellow Rattle is so good at robbing so many different plants of their nutrients

Root hemiparasites have been known to play a crucial role in regulating plant communities and suppressing the growth of their host species. However, scientists have yet to uncover the roles of parasite genetic variation and plasticity in these interactions. Belén Moncalvillo and Diethart Matthies of Philipps-Universität Marburg, Germany, have recently studied the effects of genetic variation in European Yellow Rattle and found that plant predators might not be following the same rules as insect predators.

In a recent experiment, Moncalvillo & Matthies grew plants from eight families of the root hemiparasite Rhinanthus alectorolophus, European Yellow Rattle, with six potential host species (two grasses, two legumes and two forbs) and without a host. A root hemiparasite is a plant species that obtains part of its sustenance by tapping into the root systems of other plants. This parasitic behaviour allows it to absorb nutrients and water otherwise unavailable to it while still photosynthesising its own food. The biologists measured fitness-related and morphological traits of the parasite, host biomass and overall productivity.

The scientists found that although the parasite biomass and other traits showed strong plastic variation in response to different host species, their maternal parasite family also affected their performance. This difference indicates that there is genetic variation that could serve as the basis for adaptation to various host plants.

Interestingly, there were no negative correlations in the performance of families across different hosts, suggesting that the root hemiparasites have plastic generalist genotypes and are not constrained in their use of host species.

The only plant that bucked the trend was the forb Sinapis alba, White Mustard. Yellow Rattle grown with White Mustard actually grew worse than Yellow Rattle grown with no host at all. Moncalvillo & Matthies argue this indicates that White Mustard has some resistance to Yellow Rattle.

Scientists have discovered that the parasite families with the greatest biomass across all host species tend to be associated with the smallest hosts. This indicates that there may be a natural selection for increased negative effects on hosts, as opposed to the optimum degree of virulence and the prudent rather than maximum resource extraction proposed by botanists for hemiparasites.

However, if multiple parasite individuals are parasitising the same host individual, maximum resource extraction would be favoured, say Moncalvillo & Matthies. Maximum use of host resources by hemiparasites, which significantly reduces host growth, may be selected for as the hosts are not only a source of nutrients and water for the parasites but also potential competitors for light. For this reason, parasites having a stronger negative effect on small hosts than on larger ones may be due to selection for increased virulence.

Once again, the only plant that behaved differently was the White Mustard. Moncalvillo & Matthies write:

Surprisingly, there was a significant positive relationship between the size of parasites and Sinapis individuals grown in the same pot. As Sinapis was clearly an unsuitable host one would have expected that the negative effects of the Sinapis plants on R. alectorolophus due to competition for light and nutrients would increase with their size. A possible explanation for the positive correlation is that the parasites may nevertheless have obtained some resources by parasitism and this benefit may have been positively related to individual host size.

Moncalvillo & Matthies 2023.

The genetic variation that Moncalvillo & Matthies found in the effects on different hosts highlights the importance of the genetic diversity of hemiparasites to restore grassland diversity, as their specialisation may be limited by a lack of trade-offs in performance across hosts and by the substantial spatial and temporal variation in host species availability. If there are no trade-offs so that attacking this grass means you’re less well equipped to attack that grass, then there’s no drive to specialisation, so diversity allows your species to be equipped to tackle most things that come into your patch.

The experiment has shed light on the hidden secret of the root hemiparasites, providing new insight into their roles as keystone species in influencing plant community composition and productivity. 

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Moncalvillo, B. and Matthies, D. (2023) “Performance of a parasitic plant and its effects on hosts depends on the interactions between parasite seed family and host species,” AoB PLANTS, 15(2), p. lac063. Available at: https://doi.org/10.1093/aobpla/plac063.