Did Silencing Suppression Counter-Defensive Strategy Contribute to Origin and Evolution of the Triple Gene Block Coding for Plant Virus Movement Proteins?

Abstract

Comparison of gene silencing in tissues and whole organisms shows intriguing similarities between plants and animals (Cohen and Xiong, 2011; Hyun et al., 2011; Jose et al., 2011; Melnyk et al., 2011; Molnar et al., 2011) despite that they are very different from each other in many aspects related to the cell-to-cell communications (Ritzenthaler, 2011). Interestingly, one of the shared mechanisms is the reprogramming of intracellular silencing pathways and intercellular communications during development of virus infections. As a part of their counter-defensive strategy, viruses encode silencing suppressors to inhibit various stages of the silencing process. These suppressors are diverse in sequence and structure and act via different molecular mechanisms including, particularly, blockage of intercellular and systemic spread of mobile small interfering RNAs (siRNAs; Li and Ding, 2006; Bivalkar-Mehla et al., 2011; Burgyan and Havelda, 2011; Shimura and Pantaleo, 2011; Song et al., 2011). Importantly, plant, insect, and animal virus suppressors can substitute for each other in different eukaryotic model systems (Schnettler et al., 2008; Jing et al., 2011; Maliogka et al., 2012; Zhu et al., 2012). Many viral proteins that in the past were characterized as proteins involved in systemic plant invasion are now known to be suppressors of gene silencing. For example, tombusvirus P19 blocks the intercellular movement of the silencing signal by binding DCL4-dependent 21-nt siRNA. Cucumovirus 2b protein inhibits the systemic movement of RNA silencing by either binding dsRNA/siRNA or inhibiting the slicer activity of AGO1. Potato virus X P25 protein also inhibits the systemic movement of RNA silencing (Li and Ding, 2006; Burgyan and Havelda, 2011; Shimura and Pantaleo, 2011). Direct link between the viral suppressor activity and the ability of virus to move cell-to-cell and long-distance is further strengthened by the discovery of plant movement proteins (MPs) acting also as silencing suppressors (Bayne et al., 1995; Voinnet et al., 1999; Yaegashi et al., 2007; Powers et al., 2008; Lim et al., 2010; Wu et al., 2010; Senshu et al., 2011; Renovell et al., 2012). On the other hand, it has been shown that the MPs of certain viruses act as viral enhancers of RNA silencing by promoting the propagation of RNA silencing from cell to cell (Vogler et al., 2008; Zhou et al., 2008; Lacombe et al., 2010; Amari et al., 2012).