Small Talk: On the Possible Role of Trans-Kingdom Small RNAs during Plant-Virus-Vector Tritrophic Communication

Abstract

Small RNAs (sRNAs) are the hallmark and main effectors of RNA silencing and therefore are involved in major biological processes in plants, such as regulation of gene expression, antiviral defense, and plant genome integrity. The mechanisms of sRNA amplification as well as their mobile nature and rapid generation suggest sRNAs as potential key modulators of intercellular and interspecies communication in plant-pathogen-pest interactions. Plant endogenous sRNAs can act in cis to regulate plant innate immunity against pathogens, or in trans to silence pathogens' messenger RNAs (mRNAs) and impair virulence. Likewise, pathogen-derived sRNAs can act in cis to regulate expression of their own genes and increase virulence towards a plant host, or in trans to silence plant mRNAs and interfere with host defense. In plant viral diseases, virus infection alters the composition and abundance of sRNAs in plant cells, not only by triggering and interfering with the plant RNA silencing antiviral response, which accumulates virus-derived small interfering RNAs (vsiRNAs), but also by modulating plant endogenous sRNAs. Here, we review the current knowledge on the nature and activity of virus-responsive sRNAs during virus-plant interactions and discuss their role in trans-kingdom modulation of virus vectors for the benefit of virus dissemination.

Keywords: Small RNA-based communication; plant viral diseases; plant viruses; trans-kingdom RNA silencing; virus–plant interactions.

Figure 1

Implicated roles of virus-derived siRNAs (vsiRNAs) in plant–RNA virus interaction. During antiviral RNA silencing, generated vsiRNAs not only target the virus genome but can also modulate the expression of host genes. In the case of RNA virus infection (represented in this figure), viral replicative intermediate double-stranded RNAs (dsRNAs) are processed by Dicer-like proteins (DCL) into ~ 21 nucleotide (nt) primary vsiRNAs. These vsiRNAs can be loaded into AGO1 to form an active RNA-induced silencing complex (RISC) that mediates the silencing of the respective virus genome (left). Plant RDR proteins are involved in the generation of secondary vsiRNAs. Both primary and secondary vsiRNAs can be loaded into different AGOs for the amplification of RNA silencing. Plant virus-derived siRNAs may also act in trans to modulate the viral–host interplay when functional vsiRNAs, exhibiting complementarity to host genes, are loaded into the RNA silencing complex, change host gene expression and, consequently, affect different plant biological processes, depending on the nature of the host target mRNAs (panels at the right hand side). For example: (I) silencing of chloroplast-related genes by vsiRNAs may induce symptoms such as yellowing of the leaves, turning plants more attractive to insect vectors; (II) silencing of stress-related genes may lead to increased susceptibility of the host to virus infection; (III) silencing of important regulators of autophagy may activate the antiviral autophagy mechanism that inhibits viral infection; and (IV) silencing of ROS regulators may amplify ROS signaling that promotes cell death.

Figure 2

Implicated roles of virus-activated plant endogenous sRNAs in plant–virus interaction. In response to virus infection or activated by it, plants generate altered endogenous sRNAs profiles which modulate/fine-tune the expression of endogenous genes through RNA-mediated silencing. Besides the production of viral siRNAs, virus infection is accompanied by the accumulation of viral-activated host siRNA (vasiRNAs), microRNAs (vamiRNAs), and trans-acting siRNA (tasiRNAs). (A) This diverse group of plant sRNAs causes modulating effects on host genes that either support the establishment of virus infection or activate antiviral host defense mechanisms. Viral suppressors of RNA silencing (VSRs) play a role in modulating the accumulation and profile of host sRNAs. For example, the tombusvirus p19 VSR induces the overexpression of miR168, which arrests the translation AGO1-encoding mRNA, thus alleviating the antiviral RNAi pressure on virus replication (left panel). Plant RDRs also play roles in the production of vsiRNA. Production of virus-activated and RDR1-dependent siRNAs was observed to be involved with plant broad-spectrum resistance by widespread silencing of host genes (right panel). (B) Plant viruses rely on insect vectors for their dissemination, and during acquisition access feeding on virus-infected plants, the insect vectors may also acquire sRNAs. Since viruses utilize a plethora of strategies to increase their survival and spread, this tempts to hypothesize that during the modulation of plant sRNA profiles, some of these sRNAs are also transferred to insect vectors, where they modulate insect vector behavior for the benefit of virus dissemination (trans-kingdom RNAi). This hypothesis requires the examination of sRNA crosstalk up to the level of tritrophic interaction between virus, vector, and host. In case of viruses that are transmitted in a propagative manner (i.e., replicating in the insect vector as well), and not presented/explained in this figure, this could involve bi-directional trans-kingdom RNAi (virus-induced and altered sRNA profiles in both host plant and insect vector species).