Viruses infected or defective in sRNA biosynthesis exhibit similar phenotypes

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(Contributed by Yan Zhuang)

Plantviruses are mainly RNA viruses, of which the ssRNA can be catalyzed into dsRNAby RNA-dependent RNA polymerases of plants after infection1. This dsRNA isrecognized by host’s Dicers, processed into 21–24nt siRNAs and furtherlyincorporated into RISC silencing complex2. As aconsequence, the siRNAs target and silence RNAs originated from viruses3. This is themechanism underlying antiviral immunity in plants.

Inorder to compete with this immune mechanism, viruses have evolved suppressorsof RNA silencing4. The firstfound viral suppressor of gene silencing is P1/HC-Pro, which originates from Turnip mosaic virus 5-8. Scientistsfound that plants infected by Turnipmosaic virus exhibited phenotypes similar to those of dcl mutants, including reduced miRNA level and increased miRNA targetgene level7. The P1/HC-Prowas supposed to interference with components that functions in the antiviralRNA-silencing pathway. Intriguingly, overexpression of DCL1 was unable to rescue the reduction of endogenous miRNAs9. Therefore,plants infected by viruses are defective in small RNA biogenesis and phenocopieswith mutants in small RNA biosynthesis.

Questionscome to how these suppressors of RNA silencing function in plants. After 30 yearsof study, scientists figured out that different kinds of suppressors inhibitdistinct points of the RNA silencing pathway2 (Fig.). Theseincludes suppressors binding to dsRNA to prevent the recognition by Dicers,disturbing the formation of RISC complex and interfering AGO binding ability etc.2

Fig. Suppression of antiviral immunity by viruses

Reference:

1.  Baulcombe, D. RNA silencing in plants. Nature 431, 356-363 (2004).

2.  Burgyan, J. & Havelda,Z. Viral suppressors of RNA silencing. TrendsPlant Sci. 16, 265-272 (2011).

3.  Ding, S. W. & Voinnet,O. Antiviral immunity directed by small RNAs. Cell 130, 413-426(2007).

4.  Vargason, J. M., Burch, C.J. & Wilson, J. W. Identification and RNA binding characterization of plantvirus RNA silencing suppressor proteins. Methods64, 88-93 (2013).

5.  Anandalakshmi, R. et al. A viral suppressor of genesilencing in plants. Proc. Natl. Acad.Sci. USA 95, 13079-13084 (1998).

6.  Mallory, A. C., Reinhart,B. J., Bartel, D., Vance, V. B. & Bowman, L. H. A viral suppressor of RNAsilencing differentially regulates the accumulation of short interfering RNAsand micro-RNAs in tobacco. Proc. Natl.Acad. Sci. USA 99, 15228-15233(2002).

7.  Kasschau, K. D. et al. P1/HC-Pro, a viral suppressor ofRNA silencing, interferes with Arabidopsis development and miRNA unction. Dev Cell 4, 205-217 (2003).

8.  Kasschau, K. D. &Carrington, J. C. A counterdefensive strategy of plant viruses: suppression ofposttranscriptional gene silencing. Cell95, 461-470 (1998).

9.  Mlotshwa, S. et al. Ectopic DICER-LIKE1 expressionin P1/HC-Pro Arabidopsis rescues phenotypic anomalies but not defects inmicroRNA and silencing pathways. PlantCell 17, 2873-2885 (2005).