Long-Distance Movement, Virulence, and RNA Silencing Suppression Controlled by a Single Protein in Hordei- and Potyviruses: Complementary Functions between Virus Families

Abstract

RNA silencing is a natural defense mechanism against genetic stress factors, including viruses. A mutant hordeivirus (Barley stripe mosaic virus [BSMV]) lacking the ␥b gene was confined to inoculated leaves in Nicotiana benthamiana, but systemic infection was observed in transgenic N. benthamiana expressing the potyviral silencing suppressor protein HCpro, suggesting that the ␥b protein may be a long-distance movement factor and have antisilencing activity. This was shown for ␥b proteins of both BSMV and Poa semilatent virus (PSLV), a related hordeivirus. Besides the functions in RNA silencing suppression, ␥b and HCpro had analogous effects on symptoms induced by the hordeiviruses. Severe BSMV-induced symptoms were correlated with high HCpro concentrations in the HCpro-transgenic plants, and substitution of the ␥b cistron of BSMV with that of PSLV led to greatly increased symptom severity and an altered pattern of viral gene expression. The efficient systemic infection with the chimera was followed by the development of dark green islands (localized recovery from infection) in leaves and exemption of new developing leaves from infection. Recovery and the accumulation of short RNAs diagnostic of RNA silencing in the recovered tissues in wild-type N. benthamiana were suppressed in HCpro-transgenic plants. These results provide evidence that potyviral HCpro and hordeivirus ␥b proteins contribute to systemic viral infection, symptom severity, and RNA silencing suppression. HCpro's ability to suppress the recovery of plants from viral infection emphasizes recovery as a manifestation of RNA silencing.The first observations of posttranscriptional gene silencing, or RNA silencing, were made more than 10 years ago. At that time, the phenomenon was described as "cosuppression" of endogenous genes caused by expression of homologous sequences in transgenic plants (43,66). RNA silencing is no longer associated only with transgenic plants but is known as a natural defense system against genetic stress factors, such as viruses and transposable elements, in multicellular eukaryotic organisms (5,6,11,20,35,39,68,70,73). RNA silencing is characterized by rapid and specific degradation of cytoplasmic RNAs. The accumulation of small 21-to 25-nucleotide RNA fragments (small interfering RNAs [siRNAs]) originating from the target sequence is diagnostic of RNA silencing (24, 37). The most potent inducer of RNA silencing is double-stranded RNA (dsRNA) (5,6,11,20,35,39,63,68,70,73). The onset of RNA silencing is followed by a propagation phase during which a systemic signal is delivered from the tissues undergoing silencing and is transported to other parts of the plant where homologous RNA molecules will be silenced. The nature of the signal is yet unknown, but the signaling pathway follows the transport of macromolecules and viruses through plasmodesmata between cells and via phloem over long distances (20,53,64).

The Road to RNA Silencing is Paved with Plant-Virus Interactions

“…These experiments demonstrated that DGIs are a manifestation of RNA silencing, although why they occur in such islands is not known. Interestingly, the formation of DGIs against a chimeric hordeivirus was either reduced or inhibited in transgenic N. benthamiana plants expressing a VSR, the HC-Pro of potato virus A (Yelina et al, 2002); however the DGIs induced by infection with either TMV or CMV were not affected by the transgenic expression of PVY HC-Pro in tobacco (Shams-Bakhsh et al, 2007). Thus, DGI formation can be affected by VSR in some cases, as was the cycling of recovered and symptomatic tissues after infection by CMV (Fukuzawa et al, 2010), further linking these various phenomena involving plant-virus interactions.…”

Mechanisms of resistance to viruses

“…Localized recovery, or dark green islands (DGIs), may develop on chlorotic, virusinfected leaves. No viral nucleic acids and proteins are detected in DIGs, they are resistant to infection with the same virus (MOORE et al 2001), and siRNA specific to the virus accumulate in them (YELINA et al 2002). In tobacco plants, silencing of the gene NtRDRP1 that encodes an RNA-dependent RNA polymerase (a host factor required for RNA silencing), prevents the formation of DGIs after infection with TMV (XIE et al 2001).…”

“…It is hypothesized that co-evolution of viruses and their plant hosts has resulted in mechanisms that allow the host to inhibit, restrict or tolerate viral infections and the viruses to evade host defenses (YELINA et al 2002). The R gene-mediated resistance has evolved to inhibit infection with specific viruses, but viruses can overcome it due to mutations (amino acid substitutions) introduced to the viral AVR proteins during virus replication.…”

“…In tobacco plants, silencing of the gene NtRDRP1 that encodes an RNA-dependent RNA polymerase (a host factor required for RNA silencing), prevents the formation of DGIs after infection with TMV (XIE et al 2001). A viral RNA suppressor alone reduces DGI formation and is sufficient to prevent systemic recovery of plants from viral infection (YELINA et al 2002).…”

Mechanical vs Agroinfiltration: Efficient mechanism of plant RNA virus infection

“…Once the virus is encapsidated, it either moves from one cell to another using plasmodesmata, or infects other plants through vectors [2,18]. Though movement of viruses or nucleoprotein complex can also takes place using complexes of virus encoded movement protein (MP) [19], capsid protein (CP), HC-Pro [20] etc. In case of vector transmission, most of the plant viruses use insect vectors but do not replicate in insect vectors.…”