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Members of the Luteoviridae family occupy a very special position among plant viruses. Unlike most plant viruses that can infect almost all cell types, these viruses exhibit a specific tropism restricted to vascular tissues. The infection of these tissues is maintained by a piercing-sucking insect vector, an aphid that promotes viral plant-to-plant transmission by feeding on phloem sap. This review focuses on the movement in the phloem of viruses belonging to the Luteoviridae family underlining the roles of viral proteins in this process. A second part is dedicated to the unique mode of action of the silencing suppressor of the Polerovirus genus (one of the three Luteoviridae genera). Finally, several hypotheses are discussed in order to explain the phloem restriction of these peculiar viruses.

Posttranscriptional gene silencing (PTGS) mediated by siRNAs is an evolutionarily conserved antiviral defense mechanism in higher plants and invertebrates. In this mechanism, viral-derived siRNAs are incorporated into the RNA-induced silencing complex (RISC) to guide degradation of the corresponding viral RNAs. In Arabidopsis , a key component of RISC is ARGONAUTE1 (AGO1), which not only binds to siRNAs but also carries the RNA slicer activity. At present little is known about posttranslational mechanisms regulating AGO1 turnover. Here we report that the viral suppressor of RNA silencing protein P0 triggers AGO1 degradation by the autophagy pathway. Using a P0-inducible transgenic line, we observed that AGO1 degradation is blocked by inhibition of autophagy. The engineering of a functional AGO1 fluorescent reporter protein further indicated that AGO1 colocalizes with autophagy-related (ATG) protein 8a (ATG8a) positive bodies when degradation is impaired. Moreover, this pathway also degrades AGO1 in a nonviral context, especially when the production of miRNAs is impaired. Our results demonstrate that a selective process such as ubiquitylation can lead to the degradation of a key regulatory protein such as AGO1 by a degradation process generally believed to be unspecific. We anticipate that this mechanism will not only lead to degradation of AGO1 but also of its associated proteins and eventually small RNAs.