PLants use a variety of mechanisms to communicate with other organisms, including each other. Volatile compounds can signal flowering and attract pollinators, for example, and networks of mycorrhizal fungi can send warnings or transfer resources. Small RNAs are on this list of communication molecules, and new findings confirm their potential: according to an article published Oct. 14 in Natural plants, the plant Arabidopsis thaliana secretes microRNAs (miRNAs) – a type of small single-stranded RNA – into its liquid growth medium. Nearby individuals then appropriate these RNAs, which alter their gene expression patterns by binding to messenger RNAs and preventing certain genes from being translated into proteins (a process known as RNA interference).

Hailing Jin, a plant molecular geneticist at the University of California at Riverside, who was not involved in the study, says it’s exciting how plants can take up microRNAs from the environment, including those “secreted by other plants through the roots”.

That small RNAs can be exchanged between different organisms is not new. In addition to their role as regulators of gene expression within an individual, in development or in response to stress, they have been implicated in the defense against pathogens in recent years. For example, Arabidopsis cells infected with the pathogenic fungus Botrytis cinerea secrete small RNAs packaged in extracellular vesicles which, when delivered to their attacker, inhibit its virulence. Plants are also able to take up pulverized RNA molecules targeting pathogen genes. The recent findings are the first evidence that plants take up RNA secreted by other plants into the environment.

See “RNA interference between realms”

“The results were totally unexpected,” writes Pierdomenico Perata, plant physiologist at the Sant’Anna School of Advanced Studies in Pisa, Italy, and study co-author, in an email to The scientist. Given the reputation of RNAs as “very unstable” molecules outside a cell, he writes that his team “expected miRNAs to be incompatible with a non-sterile environment such as medium. growth “.

Perata says his team was working “on a completely unrelated subject” – exploring the role of RNA interference under limited oxygen availability – and it was for this purpose that they grew up in hydroponics. Arabidopsis plants designed to produce large amounts of specific miRNAs. Since they just wanted them to produce seeds, he adds, the researchers “didn’t bother to place different lines of plants in separate trays.” But then they noticed that the wild-type plants sharing the hydroponic solution of the mutants had different phenotypes than expected – for example, those that grew alongside mutants that overexpressed miRNAs targeting developmental genes had their own period of growth. modified flowering. According to Perata, this was when he and his colleagues wondered “if miRNAs could be released into the liquid growth medium, thereby affecting the phenotype of wild-type plants.”

The researchers tested the hydroponic solution and lo and behold, they detected miRNAs. These miRNAs were present whether the plants growing in the solution were wild type or mutated to overexpress them, although more RNA was detected in the mutant solution. In addition, culturing both lines in the same solution resulted in wild type plants with significantly lower expression levels of the genes targeted by the mutant-boosted miRNA molecules. Application of miRNAs extracted from mutants or chemically synthesized equivalents also reduced gene expression.

Why would one plant need to affect the gene expression of another plant? One possibility, Perata argues, is that “sharing information by exchanging RNA would allow plants under stress to warn neighboring plants, not yet affected by the stress.” Competition could be another explanation, he writes; for example, if a miRNA-releasing plant “could inhibit the physiological functions of a neighboring plant”, it could gain “a competitive advantage for the use of resources”.

One unanswered question is how plants absorb these tiny molecules from the environment. Previous work studying RNA exchange between plants and pathogens suggests that exosomes, a type of vesicle that can serve as delivery vehicles, may be involved in the process. However, the researchers found that the application of extracted, presumably naked miRNAs or synthetic RNAs had an effect on gene expression, suggesting that exosomes are not needed for uptake.

Hui-Shan Guo, a plant microbiologist at the Institute of Microbiology of the Chinese Academy of Sciences, says evidence from studying naked RNA uptake confirms previous reports of gene silencing via sprayed RNA . She suggests in an email to The scientist that, as with nutrients, plants could actively assimilate small RNAs from the environment. But unlike substances that plants are known to import, naked RNA molecules “were considered unstable,” she says, so “RNA uptake was ignored or underestimated.”

See “Exosomes make their debut in plant research”

Jin agrees that the evidence in the article supports the hypothesis that plants can take up naked miRNAs, but she says she questions whether their secretion always occurs via root exosomes, a question the authors did not say. have not explored. She adds that she also suspects that these vesicles might protect miRNAs, helping plants achieve more efficient uptake. Otherwise, the molecules could be more easily broken down in the soil and in the environment, she speculates.

Guo points out that, as this mechanism has only been explored in plants grown in hydroponics, it is not yet clear “whether the seedlings grow in the soil.” . . would have effects on the regulation of gene expression in [nearby] plants ”—something that future studies might examine.

Jin adds that these new findings open up a lot of new questions, and that there is probably a lot more to learn about the role of RNA in plant communication. What we currently know is only the “tip of the iceberg,” she concludes.