Stealing genes to become better parasites

Parasitic plants can use genetic material stolen from host plants to more effectively siphon off the host’s nutrients, research shows.

In the case of the dodder (genus Cuscuta) studied by biologists from Penn State and Virginia Tech in the US, this includes stealing more than 100 functional genes, allowing the invasive vine to latch onto and steal nutrients from the host and even to send genetic weapons back into the host.

“Horizontal gene transfer, the movement of genetic material from one organism into the genome of another species, is very common in microbes and is a major way that bacteria can acquire antibiotic resistance,” says Penn State’s Claude dePamphilis. 

“We don’t see many examples of horizontal gene transfer in complex organisms like plants, and when we do see it, the transferred genetic material isn’t generally used. 

“In this study, we present the most dramatic case known of functional horizontal gene transfer ever found in complex organisms.”

The findings are published in the journal Nature Plants.

Dodder is well known for its parasitic tendencies, though research has also suggested it plays a positive role in plant-plant communication

Like other parasites it cannot live on its own; rather, it uses structures called haustoria to tap into a host plant’s supply of water and nutrients. It wraps itself around its host plant, growing into its vascular tissue, and often feeds on multiple plants at one time.

“Parasitic plants live very intimately in connection with their host, extracting nutrients,” says dePamphilis. “But they also get genetic material in the process, and sometimes they incorporate that material into their genome. 

“Previous studies focused on single transferred genes. Here, we used genome-scale datasets about gene expression to determine whether the large amount of genetic material coming over through horizontal gene transfer is actually being used.” 

The research team identified 108 genes that have been added to dodder’s genome by horizontal gene transfer and now seem to be functional in the parasite. Eighteen of these appear in all dodder species, suggesting, the researchers say, that these genes were originally stolen by the ancestral form of dodder and are maintained in modern species.

The genes contribute to haustoria structure, defence responses, and amino acid metabolism. One even produces small segments of RNA – known as micro RNA – that are sent back into the host plant, acting as weapons that may play a role in silencing host defence genes.

The team also identified 42 regions in the dodder genome that appear to result from horizontal gene transfer, but do not have any functional genes.

“Because such a huge quantity of genetic material has come over through horizontal gene transfer, we suspect that the parasitic plants cannot filter what is coming in,” says dePamphilis. 

“But natural selection is helping maintain the useful genes and filter out the less useful segments.”

The researchers are now investigating how genetic material is being transferred from host to parasite. They would also like to explore whether this transfer is a one-way street, or if the host can obtain genetic material from its parasite.