Abstract
Although the majority of plant viruses are transmitted by arthropod vectors and invade the host plants through the aerial parts, there is a considerable number of plant viruses that infect roots via soil-inhabiting vectors such as plasmodiophorids, chytrids, and nematodes. These soil-borne viruses belong to diverse families, and many of them cause serious diseases in major crop plants. Thus, roots are important organs for the life cycle of many viruses. Compared to shoots, roots have a distinct metabolism and particular physiological characteristics due to the differences in development, cell composition, gene expression patterns, and surrounding environmental conditions. RNA silencing is an important innate defense mechanism to combat virus infection in plants, but the specific information on the activities and molecular mechanism of RNA silencing-mediated viral defense in root tissue is still limited. In this review, we summarize and discuss the current knowledge regarding RNA silencing aspects of the interactions between soil-borne viruses and host plants. Overall, research evidence suggests that soil-borne viruses have evolved to adapt to the distinct mechanism of antiviral RNA silencing in roots.
Highlights
Most plant virus transmissions in nature are facilitated by biological vectors, and the site of virus entry into the host plant differs according to these transmission vectors (Hull, 2013)
We summarize the current information on the molecular aspects of antiviral RNA silencing in roots, with emphasis on the interactions between host antiviral defense and soil-borne viruses
The observations from the studies described in this review provide evidence for divergent operations of RNA silencing in roots, the primary factors responsible for the distinct regulation of RNA silencing activities in roots remain an open question
Summary
Most plant virus transmissions in nature are facilitated by biological vectors, and the site of virus entry into the host plant differs according to these transmission vectors (Hull, 2013). All known vector-transmitted soil-borne viruses have positive-sense ssRNA genomes except for the members of two genera, Ophiovirus and Varicosavirus, that have negative sense ssRNA genomes (Verchot-Lubicz, 2003; Kormelink et al, 2011) (Figure 1) It appears that the members with multipartite ssRNA genomes dominate the soil-borne viruses as they are more evident in the viruses that belong to the families Rhabdoviridae and Potyviridae, wherein the members having monopartite genomes and arthropod vectors (such as aphids, whiteflies, leaf- and planthoppers) are the majority in these virus families (Bragard et al, 2013). Beet necrotic yellow vein virus (BNYVV; genus Benyvirus) infection in sugar beet causes the economically significant rhizomania disease which spreads worldwide (Tamada, 2016) It is typically characterized as a massive proliferation of lateral roots and rootlets (“bearded”-like appearance) and severely stunted taproots (Tamada, 1999). Tobacco rattle virus (TRV, genus Tobravirus) can infect variety of crops and causes the major diseases of potato (spraing) and ornamental bulbs (Macfarlane, 2008)
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