Interaction between viroids and RNAi

Likewise virus infections, viroid infections initiate a defence mechanism in the host plant that was termed RNA interference (RNAi) [RNA interference (RNAi)]. To block this defence mechanism almost all plant viruses encode silencing suppressor proteins (Viral suppressors of RNAi). These proteins prevent the premature degradation of the viral RNA, thereby enabling successful replication of the corresponding virus. Viroids do not code for proteins and are, thus, unable to use a protein-based silencing suppressor strategy. Yet, they replicate and systemically infect the host plant. It is reasonable to assume that the viroid RNA itself possesses a silencing suppressor activity. This assumption is supported by the finding that viroids induce symptoms (Fig. 1) that frequently resemble those induced by viruses. Today we know that RNAi does not only function as a plant defence mechanism, but also has a major impact on normal plant gene regulation. In plants, impairment of defined RNAi processes caused severe developmental dysfunctions. The appearance of such plants resembled the phenotype of virus-infected plants. We recently started a project that could provide information about possible silencing suppressor activities of viroids. From viroid-infected plants, we would like to isolate complexes of RNAi components (e.g. RNA-directed RNA polymerases (RDRs), Dicer-like (DCL) and Argonaute (AGO) proteins) by immunoprecipitation to analyze whether these components are directly associated with viroid RNA. In addition, we would like to examine whether systemic silencing of a transgene is affected in viroid-infected plants. After local initiation of transgene silencing a silencing signal is produced that spreads throughout the plant. Spreading of the signal finally results in the complete silencing of the upper part of the plant. In transgenic plants, systemic silencing can be induced by introduction of silencer constructs. Here, we will initiate systemic silencing of a green fluorescent protein- (GFP-) expressing and PSTVd-infected Nicotiana benthamiana plant by agroinfiltration using a GFP silencer construct.

Figure 1: Phenotype of a PSTVd-infected tomato plant (Money Maker) (infected, left and PSTVd-free, right)

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