A Barley mild mosaic virus (BaMMV) isolate from France (BaMMV-Sil) capable of overcoming rym5-controlled resistance was inoculated to barley genotypes carrying various genes for resistance to the barley mosaic viruses. BaMMV-Sil was unable to infect genotypes carrying rym1, rym4, rym8, rym9, or rym11 but genotypes carrying rym3, rym5, rym6 or no known bymovirus resistance gene were susceptible. Plants carrying rym7 or rym10 showed partial resistance with delayed virus accumulation. The two genomic RNAs of BaMMV-Sil were sequenced and compared to published sequences and those of a further common strain isolate from the UK. Four amino acid differences were observed between BaMMV-Sil and European common strain isolates in the polypeptide encoded by RNA1, the RNA species which determines pathogenicity on the rym5 genotypes. Only two of these differences are likely to be functionally important (His rather than Gln at position1217 in the VPg cistron; His rather than Asp at position 1776 in the NIb cistron). Comparisons with related viruses in the genera Bymovirus and Potyvirus suggest that the change in the VPg, which occurs within a motif conserved amongst all viruses within the family Potyviridae, is the more likely cause of rym5 resistance-breaking.
Cucurbit yellow stunting disorder virus (CYSDV) and Watermelon chlorotic stunt virus (WmCSV) are the most widespread and damaging viruses to cucurbits in the Middle East. CYSDV and WmCSV are cucurbit-infecting bipartite whitefly-transmitted begomoviruses. Post-transcriptional gene silencing (PTGS) is a universal mechanism by which plants are able to systemically switch off the expression of targeted genes via the reduction of steady-state levels of specific RNAs. PTGS was used in this study to control the two viruses. In this study, the efficiency of the dsRNA for the ability to trigger resistance against the CYSDV and WmCSV was investigated. Three regions of three genes of CYSDV genome were selected; the coat protein gene (CP), heat shock gene (Hsp70) and ORF3, while the two regions of two genes of WmCSV genome were selected; CP gene and rep gene. Bioassay, dot-blot hybridization and polymerase chain reaction (PCR) methods were capable to evaluate the resistance against viruses. Clear symptoms on tobacco plants took two to three weeks to appear and all non-infiltrating tobacco plants (positive control) showed viral symptoms after inoculation. Most of the agro-infiltrating sense/antisense constructs did not yield symptoms of the viruses. Dot-blot hybridization, showed that negative hybridization was obtained with infiltrating tobacco plants with prepared constructs compared to those non-infiltrating tobacco plants used as the control. Only one out of five gave positive signals with the construct pasCYSDV-Hsp70. Using PCR, positive reactions of the expected size of 500 bp fragment with WmCSV and 800 bp with CYSDV were obtained with the infiltrating tobacco plants with sense constructs, which pointed out the existence of viral genome in challenging tobacco plants. Infiltrating tobacco plants with sense/antisense constructs gave negative PCR pointed out the lack of the viral genome.
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