RNA silencing in plant biotechnology

RNA silencing or post transcriptional gene silencing (PTGS) plays an important biological role in plants which includes the following:

• defense against viruses
• epigenetic control of chromatin modifications
• control of the expression of developmentally regulated genes
• regulation of biotic & abiotic stress, and
• defense against nonviral pathogens and insects.

RNA silencing pathways or RNA interference (RNAi) are mediated by small homologous RNA molecules that are 21 to 24 nt long. These RNA molecules are processed by dicers or dicer-like endonucleases (DCL) and RNA dependent RNA polymerases (RDRs). They bind with AGO (Argonaute) proteins to form RISC (RNA induced silencing complex) and repress/modify the gene expression at transcriptional, post transcriptional and translational level. RNA silencing pathways involve various types of small RNAs which includes

• micro RNAs (mi RNAs) that have important biological role in plants including auxin regulator and accumulator of transcription factors that involves in plant development [1]
• endogenous trans acting si RNAs/TAS (ta-siRNAs)
• natural cis antisense transcripts associated si RNAs (nat si RNAs), and
• heterochromatic si RNAs (hc si RNA).

The micro RNAs are produced by DCL1 and binds with AGO1 to cleave target mRNA and repress the translation. The ta-si RNA is produced by DCL4 and RDR6 to cleave TAS gene and control the gene expression in plants [2]. The hc si RNAs are produced by plant specific transcript POL4, RDR2 and DCL3 and then binds with AGO4, AGO6 and AGO9 to direct DNA methylation. Through these RNA silencing pathways plants are defended against invaded viral nucleic acid/transgenes. During this defense mechanism viral RNA is diced by DCL4&DCL2  to produce the si RNAs to degrade the viral RNA [3].

RNAi is now widely used in industrial plant biotechnology for production of  disease and pathogen resistant plants, for generation of male sterility to produce hybrid seeds, for crop production with improved nutritional contents and in various other aspects. Industrially RNAi has also been used to modify the metabolic pathways of plants to enhance nutrient accumulation and decrease toxin generation. For example, lyc gene and DET1 gene in tomato are engineered by RNAi to increase lycopene (antioxidant), flavonoid and beta carotene concentration. FAD2 and SAD1 gene in canola, cotton, and peanut plant is manipulated by RNAi to  increase the oleic acid and stearic acid concentration [4]. In 2015 a group of scientists  at Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO) explained the use of RNAi to increase the healthier monounsaturated fatty acid (MUFA) level in flaxseeds [5]. CSIRO used RNAi to reduce the function of 3 genes in cottonseed that usually convert oleic acid to bad fatty acids. This resulted in high oleic acid containing cottonseed oil that is free from cholesterol raising trans fatty acids [6].

Other than plants RNAi technology is also applied for genome analysis and drug target validation for  therapeutic development and hence it is termed as "breakthrough in biotech industry".

References:

1. https://www.researchgate.net/profile/Olivier_Voinnet/publication/7212300_The_diversity_of_RNA_silencing_pathways_in_plants/links/0deec51e64ff16726e000000.pdf
2. http://www.plantphysiol.org/content/147/2/456.full
3. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3181474/
4. https://isaaa.org/resources/publications/pocketk/34/default.asp
5. https://www.geneticliteracyproject.org/2015/01/29/gene-silencing-technology-boosts-levels-of-monounsaturated-fats-in-flax/
6. http://www.csiro.au/en/Research/Farming-food/Innovation-and-technology-for-the-future/Gene-technology/RNAi