The DNA Series:
Hello everyone, this is going to be a series of blog posts. Stay tuned so you don’t lose a single one! For this series, the blog posts will be:
Interference RNA
Interference RNA, known as RNAi, is a class of molecules that naturally occurs to interfere with protein expression, resulting in gene silencing. This natural mechanism is usually triggered by the presence of a foreign DNA, a micro RNA (miRNA) or a double stranded RNA (dsRNA) of viral origin, suggesting an important role in pathogen resistance. The RNAi can cause a process called Post-Transcriptional Gene Silencing (PTGS), where the gene already started to be expressed, but is stopped in the way to encode a protein.
In the genome, most genes that encode proteins are transcribed by RNA polymerase II first into messenger RNA (mRNA) that is exported to the nucleus of the cell to the cytoplasm, where rybosomes catalyse translation of the mRNA into polypeptide chains that fold into proteins. This is where some forms of RNA can have silencing effects. There are several types of regulatory small RNA, such as small inteferring RNA, also known as siRNA, which are derived from long double stranded RNA, or a miRNA. Those siRNA binds to dicer, an endonuclease protein that cuts the RNA into short segments (most siRNA’s are aproximately 21 nucleotides long). After that, it binds to an argonaut (AGO) protein where only one strand of the RNA will stay to work as a guide, called the RNA Induced Silencing Complex (RISC). siRNA’s direct RISC to bind to an specific mRNA by matching the nucleotide sequence between them. siRNA are usually perfectly complementary to their target mRNA. AGO then catalyses the cleavage of mRNA that will then be degraded, and therefore translation being inhibited.
This mechanism is found in plants, animals, fungi and some bacteria and this is how it naturally occurs, but scientists can design a double stranded RNA to target a specific mRNA and insert it into the cell to silence the expression of a specific gene. The first report of the RNAi system was made in 1990 by Napoli and Jorgensen in their research to generate violet petunia flowers. In animals, RNA silencing was first reported by Guo and Kemphues in roundworms in 1995.
According to ISAAA database, there are 42 GM events developed with RNAi in 9 different crops, as shown in the table below.
Crop | GM trait | Event Name | |
|---|---|---|---|
Papaya | Resistance to disease | Replicase domain of the papaya ringspot virus (PRSV) | |
Potato | Resistance to disease | Putative helicase/replicase domain of the potato leaf roll virus (PLRV) | |
Modified product quality | Generate dsRNA to downregulate transcripts to lower reducing sugars, lower black spot bruise development, and lower asparagine formation | ||
Production of granule-bound starch synthase enzyme is suppressed by a gene silencing mechanism | |||
No functional granule-bound starch synthase (GBSS) enzyme is produced; production of GBSS enzyme is suppressed by a gene silencing mechanism | |||
Corn | Insect resistance | Down-regulation of the function of the targeted gene leading to Western Corn Rootworm mortality | |
Soybean | Modified product quality | Production of FATB enzymes or acyl-acyl carrier protein thioesterases is suppressed by RNA interference | |
Production of delta-12 desaturase enzyme is suppressed by RNA interference | |||
Blocks the formation of linoleic acid from oleic acid (by silencing the fad2-1 gene) and allows accumulation of oleic acid in the seed | |||
Apple | Modified product quality | Suppresses polyphenol oxidase (PPO) resulting in apples with a non-browning phenotype | |
Cotton | Modified product quality | dsRNA that suppresses the expression of d-cadinene synthase gene that encode d-cadinene synthase, a key enzyme involved in gossypol biosynthesis | |
Pineapple | Modified product quality | Increases lycopene accumulation using RNAi technology | |
Safflower | Modified product quality | Production of FATB enzymes or acyl-acyl carrier protein thioesterases is suppressed by RNA interference | |
Production of delta-12 desaturase enzyme is suppressed by RNA interference | |||
Tomato | Modified product quality | Inhibits the production of polygalacturonase enzyme responsible for the breakdown of pectin molecules in the cell wall, and thus causes delayed softening of the fruit by gene silencing mechanism | |
* Here, gene silencing was considered as RNAi except for those traits that stated single stranded RNA was used.
The most common modification made using RNAi was modified product quality, that represented aproximately 76% of all the events using RNAi, followed by virus resistance and insect resistance, respectively.
The majority of RNAi technology application in agriculture are based on the generation of transgenic plants that express dsRNAs. But that is not the only way RNAi can be applied in agriculture. Exogenous dsRNAs can be introduced through microinjections, soaking or direct spraying on leaves. As an example, papaya was successfully sprayed with dsRNA derived from the coat protein of papaya ringspot virus-Tirupai to protect papaya against PRSV infection.
In the case where a modification is made so the plant starts expressing a designed dsRNA, the resulting plant will be considered transgenic and, therefore, will fall into the same regulations as GMOs do worldwide.
Interested in learning more?
We invite you to watch these TED-Ed video and Nature video about interference RNA.
