The parasitic plant haustorium: a trojan horse releasing microRNAs that take control of the defense responses of the host

Organisms in a symbiotic relationship have evolved in an arms race to modulate each other’s physiology. In most cases, symbiotic interactions elicit a local and a systemic response that involves an extensive transcriptional reprogramming of both organisms. Although the genetic component of these interactions has been studied for years, the potential exchange of genetic information between symbionts has remained elusive till now. The theory that an exchange of RNA could exist was an exciting, yet difficult-to-prove, theory. However, in recent years, due to both the development of high-throughput sequencing and increase in genome sequence information, it has been confirmed that both messenger RNAs (mRNAs) and small RNAs (sRNAs) are exchanged between symbionts. Most strikingly, these foreign sRNAs are functional in the host cellular environment and, in most of the cases, they are involved in the regulation of pathogenicity and virulence. Here, we review the recent discovery of the exchange of sRNAs between a parasitic plant, Cuscuta campestris, and its host Arabidopsis thaliana (1), which opens new perspectives and questions both at the agricultural and biological levels.