RNA-dependent RNA polymerases (RdRPs) have been implicated in posttranscriptional gene silencing (PTGS) and antiviral defense. An Arabidopsis RdRP (SDE1/SGS2) has been previously shown to be required for transgene-induced PTGS but has no general role in antiviral defense. On the other hand, we have recently shown that transgenic tobacco deficient in an inducible RdRP (NtRdRP1) activity became more susceptible to both Tobacco mosaic virus and Potato virus X. Thus, different RdRPs may have distinct roles in closely related PTGS and antiviral defense. In the present study, we analyzed roles of a newly identified Arabidopsis RdRP gene (AtRdRP1) in plant antiviral defense. AtRdRP1 encodes an RdRP closely related structurally to NtRdRP1 and is also induced by salicylic acid treatment and virus infection. A T-DNA insertion mutant for AtRdRP1 has been isolated and analyzed for possible alterations in response to viral infection. When infected by a tobamovirus and a tobravirus, the knockout mutant accumulated higher and more persistent levels of viral RNAs in both the lower, inoculated and in upper, systemically infected leaves than did wild-type plants. These results suggest that the inducible AtRdRP1 is the Arabidopsis ortholog of NtRdRP1 and plays a role in antiviral defense. Examination of short viral RNAs and silencing studies using a viral vector harboring an endogenous plant gene suggest that, while not required for virus-induced PTGS, AtRdRP1 can apparently promote turnover of viral RNAs in infected plants.

Previously, it was shown that the upper leaves of plants infected with nepoviruses and caulimoviruses are symptom free and contain reduced levels of virus. These leaves are said to be recovered. Recovery is associated with RNA-mediated cross-protection against secondary virus infection. Here, by analyzing plants infected with viruses that are quite distinct from the nepovirus or caulimovirus groups, we demonstrate that this RNA-mediated defense is a general response to virus infection. Upon infection with a tobravirus, plants exhibited RNA-mediated cross-protection and recovery, as occurs in nepovirus-infected plants. However, upon infection with a potexvirus, plants exhibited RNA-mediated cross-protection without recovery. In both instances, a transient gene expression assay showed that RNA-mediated cross-protection was functionally equivalent to post-transcriptional gene silencing. Combined, these data provide direct evidence that post-transcriptional gene silencing of nuclear genes is a manifestation of a natural defense mechanism that is induced by a wide range of viruses. INTRODUCTIONSeveral lines of evidence suggest a link between post-transcriptional gene silencing (PTGS) in transgenic plants and viruses (Baulcombe, 1996a;Pruss et al., 1997). For example, transgene-induced PTGS causes resistance against viruses that have nucleotide sequences similar to that of the transgene (Smith et al., 1994;Mueller et al., 1995;English et al., 1996; Goodwin et al., 1996). This type of transgenic virus resistance is referred to as RNA homology-dependent resistance. Viruses can also induce PTGS of homologous transgenes (Angell and Baulcombe, 1997;Al-Kaff et al., 1998;Atkinson et al., 1998;Kjemtrup et al., 1998;Ruiz et al., 1998). In some transgenic plants, the virus can be both an inducer and a target of gene silencing. The lower leaves of these plants display the normal viral symptoms. However, upper leaves emerging after systemic infection are symptom and virus free. These upper leaves are resistant to secondary infection by the inducing virus and are said to be "recovered" (Lindbo et al., 1993; Guo and Garcia, 1997).There is also a link between PTGS and viruses in nontransgenic plants. For example, PTGS is induced by recombinant virus vectors carrying inserts that are homologous to endogenous genes. This virus-induced gene silencing may be mediated by tobacco mosaic virus (TMV; a tobamovirus) (Kumagai et al., 1995) and potato virus X (PVX; a potexvirus) (Ruiz et al., 1998) vectors with RNA genomes and by tomato golden mosaic virus (a geminivirus) (Kjemtrup et al., 1998) with a DNA genome. A PTGS-like mechanism is also induced by nepoviruses and caulimoviruses (Covey et al., 1997;Ratcliff et al., 1997) that do not have homology to endogenous genes. In these examples, the infected plants exhibit a response very similar to the virus-induced recovery on transgenic plants in that the upper leaves are symptom free and contain reduced levels of virus. In nepovirus-infected plants, the recovered leaves exhibit homology-depend...

SummaryVirus-induced gene silencing (VIGS) is an attractive reverse-genetics tool for studies of gene function. However, efficient VIGS has only been accomplished in a few plant species. In order to extend the application of VIGS, we examined whether a VIGS vector based on Pea early browning virus (PEBV) would produce recognizable phenotypes in Pisum sativum. A plasmid vector of PEBV was modified to allow agro-inoculation and insertion of heterologous sequences. cDNA fragments of the P. sativum phytoene desaturase (PDS), LEAFY (LFY) and KORRIGAN1 (KOR1) homologues were inserted into the PEBV RNA2 vector, replacing the genes required for nematode transmission. Pisum sativum inoculated with PEBV carrying a fragment of PsPDS developed characteristic photo-bleached leaves and this phenotype was associated with a significant reduction in PsPDS mRNA. The P. sativum homologue of LFY is known as UNIFOLIATA (UNI). Plants inoculated with PEBV carrying a fragment of UNI developed distorted flowers and leaves with modified architecture, which are also observed in UNI-mutants. In Arabidopsis thaliana, the KOR1-mutant is characterized by an extreme dwarf phenotype. Pisum sativum plants inoculated with PEBV carrying a fragment of PsKOR1 displayed a significant reduction in height and inhibition of root growth. The PEBV VIGS vector did not affect the ability of P. sativum to flower, set seeds, and form nodules characteristic of symbiosis with rhizobium. These results suggest that the PEBV vector can be applied to functional genomics in a legume species to study genes involved in a wide range of biological processes.

Most plant viruses rely on vector organisms for their plant-to-plant spread. Although there are many different natural vectors, few plant virus-vector systems have been well studied. This review describes our current understanding of virus transmission by aphids, thrips, whiteflies, leafhoppers, planthoppers, treehoppers, mites, nematodes, and zoosporic endoparasites. Strategies for control of vectors by host resistance, chemicals, and integrated pest management are reviewed. Many gaps in the knowledge of the transmission mechanisms and a lack of available host resistance to vectors are evident. Advances in genome sequencing and molecular technologies will help to address these problems and will allow innovative control methods through interference with vector transmission. Improved knowledge of factors affecting pest and disease spread in different ecosystems for predictive modeling is also needed. Innovative control measures are urgently required because of the increased risks from vector-borne infections that arise from environmental change.

Plant hormones play a vital role in plant immune responses. However, in contrast to the relative wealth of information on hormone-mediated immunity in dicot plants, little information is available on monocot-virus defense systems. We used a high-throughput-sequencing approach to compare the global gene expression of Rice black-streaked dwarf virus (RBSDV)-infected rice plants with that of healthy plants. Exogenous hormone applications and transgenic rice were used to test RBSDV infectivity and pathogenicity. Our results revealed that the jasmonic acid (JA) pathway was induced while the brassinosteroid (BR) pathway was suppressed in infected plants. Foliar application of methyl jasmonate (MeJA) or brassinazole (BRZ) resulted in a significant reduction in RBSDV incidence, while epibrassinolide (BL) treatment increased RBSDV infection. Infection studies using coi1-13 and Go mutants demonstrated JA-mediated resistance and BR-mediated susceptibility to RBSDV infection. A mixture of MeJA and BL treatment resulted in a significant reduction in RBSDV infection compared with a single BL treatment. MeJA application efficiently suppressed the expression of BR pathway genes, and this inhibition depended on the JA coreceptor OsCOI1. Collectively, our results reveal that JA-mediated defense can suppress the BR-mediated susceptibility to RBSDV infection.