festive parasites
regulating virulence
to preserve their hosts
Mistletoe is a parasitic plant often found growing on hardwoods, such as apple trees. Whilst able to photosynthesize itself, the majority of a mistletoe plant’s water and nutrients are taken from its host, putting strain on the host plant.
The burden of parasitism can be particularly hard on the host when environmental conditions are tough, for instance during a drought. Research by Nabity et al. (2021), however, has shown that the desert mistletoe (Phoradendron californicum) is able to adjust the balance between autotrophy (the amount it obtains resources for itself through photosynthesis) and heterotrophy (the amount it takes resources from its host).
During dry periods the researchers found that desert mistletoe plants increased the amount of photosynthesis they performed, limiting the burden they place on their environmentally stressed host, the velvet mesquite (Prosopsis velutina). In this way mistletoe plants increase the chances of their host plants surviving the harsh environmental conditions and, as a result, increase their own chances of survival.
The researchers also demonstrated evidence of competition for xylem resources between mistletoe plants on the same host, some of the first evidence of intraspecific competition in parasites. The mistletoe plants are able to detect other mistletoe plants on the same host and can adjust their virulence accordingly. Possible ways that mistletoe could detect one another include via scent (chemical compounds released through a plant’s pores) or through chemical compounds traveling along the host’s xylem.
The research also suggests that levels of relatedness between mistletoe plants sharing the same host may even affect virulence. More research is needed to clarify this, however, and to investigate whether the plants can actually detect relatedness or whether mistletoe’s method of seed dispersal simply means that plants sharing the same host are likely to have higher levels of relatedness than mistletoe plants on separate hosts.
Further reading: http://dx.doi.org/10.1016/j.cub.2021.01.034