Solanum elaeagnifolium and S. rostratum as potential hosts of the tomato brown rugose fruit virus.

Invasive weeds cause significant crop yield and economic losses in agriculture. The highest indirect impact may be attributed to the role of invasive weeds as virus reservoirs within commercial growing areas. The new tobamovirus tomato brown rugose fruit virus (ToBRFV), first identified in the Middle East, overcame the Tm-22 resistance allele of cultivated tomato varieties and caused severe damage to crops. In this study, we determined the role of invasive weed species as potential hosts of ToBRFV and a mild strain of pepino mosaic virus (PepMV-IL). Of newly tested weed species, only the invasive species Solanum elaeagnifolium and S. rostratum, sap inoculated with ToBRFV, were susceptible to ToBRFV infection. S. rostratum was also susceptible to PepMV-IL infection. No phenotype was observed on ToBRFV-infected S. elaeagnifolium grown in the wild or following ToBRFV sap inoculation. S. rostratum plants inoculated with ToBRFV contained a high ToBRFV titer compared to ToBRFV-infected S. elaeagnifolium plants. Mixed infection with ToBRFV and PepMV-IL of S. rostratum plants, as well as S. nigrum plants (a known host of ToBRFV and PepMV), displayed synergism between the two viruses, manifested by increasing PepMV-IL levels. Additionally, when inoculated with either ToBRFV or PepMV-IL, disease symptoms were apparent in S. rostratum plants and the symptoms were exacerbated upon mixed infections with both viruses. In a bioassay, ToBRFV-inoculated S. elaeagnifolium, S. rostratum and S. nigrum plants infected tomato plants harboring the Tm-22 resistant allele with ToBRFV. The distribution and abundance of these Solanaceae species increase the risks of virus transmission between species.

Analysis of the RNA-Dependent RNA Polymerase 1 (RDR1) Gene Family in Melon.

RNA-dependent RNA polymerase 1 (RDR1) plays a crucial defense role against plant viruses by secondary amplification of viral double-stranded RNA in the gene-silencing pathway. In this study, it was found that melon (Cucumis melo) encodes four RDR1 genes (CmRDR1a, b, c1 and c2) similar to the CsRDR1 gene family of cucumber (C. sativus). However, in contrast to cucumber, melon harbors a truncated CmRDR1b gene. In healthy plants, CmRDR1a was expressed, whereas the expression of CmRDR1c1/c2 was not detected. CmRDR1a expression level increased 20-fold upon cucumber mosaic virus (CMV) infection and was not increased in melon plants infected with zucchini yellow mosaic virus (ZYMV), cucumber vein yellowing virus (CVYV) and cucumber green mottle mosaic virus (CGMMV). The expression of CmRDR1c1/c2 genes was induced differentially by infection with viruses from different families: high levels of ~340-, 172- and 115-fold increases were induced by CMV, CVYV and CGMMV, respectively, and relatively low-level increases by potyvirus infection (4- to 6-fold). CMV mutants lacking the viral silencing suppressor 2b protein did not cause increased CmRDR1c/c2 expression; knockout of CmRDR1c1/c2 by CRISPR/Cas9 increased susceptibility to CMV but not to ZYMV. Therefore, it is suggested that the sensitivity of melon to viruses from different families is a result of the loss of function of CmRDR1b.

Knockout of SlTOM1 and SlTOM3 results in differential resistance to tobamovirus in tomato.

During tobamovirus-host coevolution, tobamoviruses developed numerous interactions with host susceptibility factors and exploited these interactions for replication and movement. The plant-encoded TOBAMOVIRUS MULTIPLICATION (TOM) susceptibility proteins interact with the tobamovirus replicase proteins and allow the formation of the viral replication complex. Here CRISPR/Cas9-mediated mutagenesis allowed the exploration of the roles of SlTOM1a, SlTOM1b, and SlTOM3 in systemic tobamovirus infection of tomato. Knockouts of both SlTOM1a and SlTOM3 in sltom1a/sltom3 plants resulted in an asymptomatic response to the infection with recently emerged tomato brown rugose fruit virus (ToBRFV). In addition, an accumulation of ToBRFV RNA and coat protein (CP) in sltom1a/sltom3 mutant plants was 516- and 25-fold lower, respectively, than in wild-type (WT) plants at 12 days postinoculation. In marked contrast, sltom1a/sltom3 plants were susceptible to previously known tomato viruses, tobacco mosaic virus (TMV) and tomato mosaic virus (ToMV), indicating that SlTOM1a and SlTOM3 are not essential for systemic infection of TMV and ToMV in tomato plants. Knockout of SlTOM1b alone did not contribute to ToBRFV and ToMV resistance. However, in triple mutants sltom1a/sltom3/sltom1b, ToMV accumulation was three-fold lower than in WT plants, with no reduction in symptoms. These results indicate that SlTOM1a and SlTOM3 are essential for the replication of ToBRFV, but not for ToMV and TMV, which are associated with additional susceptibility proteins. Additionally, we showed that SlTOM1a and SlTOM3 positively regulate the tobamovirus susceptibility gene SlARL8a3. Moreover, we found that the SlTOM family is involved in the regulation of plant development.