Combined Activity of DCL2 and DCL3 Is Crucial in the Defense against Potato Spindle Tuber Viroid

PLoS Pathog. 2016 Oct 12;12(10):e1005936. doi: 10.1371/journal.ppat.1005936. eCollection 2016 Oct.

Abstract

Viroids are self replicating non-coding RNAs capable of infecting a wide range of plant hosts. They do not encode any proteins, thus the mechanism by which they escape plant defenses remains unclear. RNAi silencing is a major defense mechanism against virus infections, with the four DCL proteins being principal components of the pathway. We have used Nicotiana benthamiana as a model to study Potato spindle tuber viroid infection. This viroid is a member of the Pospiviroidae family and replicates in the nucleus via an asymmetric rolling circle mechanism. We have created knock-down plants for all four DCL genes and their combinations. Previously, we showed that DCL4 has a positive effect on PSTVd infectivity since viroid levels drop when DCL4 is suppressed. Here, we show that PSTVd levels remain decreased throughout infection in DCL4 knockdown plants, and that simultaneous knockdown of DCL1, DCL2 or DCL3 together with DCL4 cannot reverse this effect. Through infection of plants suppressed for multiple DCLs we further show that a combined suppression of DCL2 and DCL3 has a major effect in succumbing plant antiviral defense. Based on our results, we further suggest that Pospoviroids may have evolved to be primarily processed by DCL4 as it seems to be a DCL protein with less detrimental effects on viroid infectivity. These findings pave the way to delineate the complexity of the relationship between viroids and plant RNA silencing response.

MeSH terms

  • Blotting, Northern
  • Nicotiana / virology*
  • Oligonucleotide Array Sequence Analysis
  • Plant Diseases / immunology*
  • Plant Diseases / virology
  • Plant Proteins / metabolism*
  • Polymerase Chain Reaction
  • Viroids / immunology*
  • Viroids / metabolism
  • Virus Diseases / immunology*

Substances

  • Plant Proteins

Grants and funding

This project was partly funded by the Greek 'Ministry of education and life-long learning' through a NSRF/ESPA grant (Viroid-miR, Aristeia II-4499). WD is a postdoctoral fellow of the Fund for Scientific Research Flanders (FWO). This project was supported by the Fund for Scientific Research Flanders (FWO) (grant G009312N to TVL) and grant G053815N to (TVL and WD). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.