The arms race in the plant world is a brutal competition for dominance where failure means death. As scientific understanding of genetics has broadened over the decades, we’ve also come to realize that the organisms in nature have been utilizing even the most complicated of these mechanisms for their advantage in order to survive. Any tools we start using in agriculture or medicine are truly just copying existing systems that run rampant in the wild.
This is especially so in the fight between parasitic creatures and their hosts. Some of the most fascinating examples of this occur on the macro scale involving parasitic plants that themselves prey on other plants. Today we’ll be discussing the dodders, members of the taxonomic genus Cuscuta with over 100 different species. In truth, these parasites are members of the morning glory flower family, but they certainly do not take after their relatives.
Plant Parasites That Are Plants
Dodders can be found in tropical and subtropical climates, mostly shying away from colder environments. Their species names invariably come about from their hair-like structure that grows over other plants around them. They produce no true leaves and instead have their leaf-forming structures form into tiny scales. Their growth pattern finds them winding around the stems of plants before penetrating the fleshy tissue of their target with specialized structures known as haustoria.
These haustoria are used to siphon off the nutrients and water that the dodders need to survive, but things are a bit more complex than just that. Plant researchers at Penn State and Virginia Tech decided to collaborate to investigate more on how the dodders manage to so successfully penetrate past their host plant’s defenses. What they found was that dodders open up a genetic can of worms in order to win their life or death struggle.
Samples of the parasites were first obtained from a tomato farm in California, specifically of the Cuscuta campestris species. As the host subject to be infected, mutant varieties of the model organism Arabidopsis thaliana were used. Based on a whole genome sequencing done on the parasite, it was earlier found that there are a number of microRNAs (miRNAs) and small interfering RNAs (siRNAs) made in the Cuscuta genome that needed to be looked at more closely. Since these are commonly involved in gene silencing processes within the cell, the researchers suspected that this might be how the parasites achieve victory in their host invasion.
Therefore, they made sure to get several Arabidopsis mutants that had gene knockouts in genes connected to gene silencing and RNA transcription. Then the two plants were grown together to start the infection process. C. campestris that were able to make at least two coils worth of connections to the Arabidopsis with haustoria injection being evident were used in the study, with uninfected Arabidopsis serving as controls. Tissue was harvested from the parasites at their point of connection with their host and the samples were pooled together to collect the small RNAs that were expected to exist in abundance in these tissues.
Hijacking Silencing Systems
What their sequencing and analysis discovered is that C. campestris do indeed use miRNAs at the point of interface with their host. Unique ones as well, clocking in at only 22 nucleotides long, a surprising size that is normally only seen when attempting targeted gene silencing, as expected. It appears that these C. campestris miRNAs then target working mRNAs for certain genes and cleave them into non-functionality by taking control of the plant’s natural Dicer system for miRNAs.
These cleaved Arabidopsis mRNAs are then turned into siRNAs that target their original gene sequences for silencing. Therefore, in one fell swoop, the parasites have destroyed any working mRNAs for the genes and resulting proteins they are targeting and also have created siRNAs to silence those same genes for good in their host genome. One of those target genes, SEOR1, was found in mutant Arabidopsis with the gene already knocked out to cause the hosts to be more immediately susceptible to C. campestris attack and caused higher growth rates of the parasite.
Homolog equivalent genes to those targeted in Arabidopsis were also found in a diverse selection of plants, indicating that they could be attacked by the parasite as well. In short, C. campestris is a parasitic plant capable of trans-species RNA and gene silencing, thereby regulating gene expression in organisms outside of its own cells by using an injected invasion of miRNAs.
The Plant War
This sort of host-induced gene silencing (HIGS) is generally used by plants as protection against bacterial, fungal, insect, and nematode invaders, allowing the plant to shut off the virulence genes of its attackers. Now, we have a parasitic plant that flips around this plant-based capability and uses it as a weapon against the hosts it wants to siphon nutrients from. It can be said to be a natural evolution of the fight, where the next step may be for plants to develop methods of regulating and reversing their own silencing systems to prevent this sort of assault, perhaps.
Agricultural scientists hope to be able to borrow these defensive capabilities to act as crop protection against pests and medical researchers hope to use similar RNA and gene silencing methods to prevent the onset of genetic diseases. Every new piece of information we learn about the natural world showcases its complexity, but also gives us the opportunity to use its abilities for our own benefit and for the future we strive for.
Photo CCs: Cuscuta campestris from Wikimedia Commons