No abstract

Transformed lines of Nicotiana benthamiana expressing a translatable transgene (CPtrans) encoding the coat protein (CP) gene of potato mop-top virus (PMTV) differed considerably in steady-state levels of transgene RNA transcript and CP that were positively correlated. A mean of 88% of transgenic T1 plants from seven transgenic lines were resistant to manual PMTV inoculation. Resistance was identified by failure to develop symptoms in plants and inability to recover infectious virus with a sensitive infectivity assay: many of the 12% of transgenic plants that gave a positive infectivity assay did not produce symptoms and virus was not detected in repeat assays on the same plants. Resistant plants did not express a “recovery” phenotype and resistance was not overcome by use of RNA inoculum. Strong resistance was expressed in all CPtrans lines irrespective of the amount of transcript RNA or CP detected, and there was no evidence that resistance was mediated by a “transgene silencing” mechanism. Lines transformed with PMTV CP gene sequence in a nontranslatable form (CPnontrans) also contained differing steady-state levels of RNA transcript, but were not resistant to PMTV, although in some lines symptoms were slightly milder and were slower to develop than those in nontransgenic control plants. These results suggest that CPtrans is unique because, although it depends on CP translation to be effective, it mediates very strong resistance in 10 independent transformed lines (tested here and in other work).

Background: RNA interference (RNAi) in animals and post-transcriptional gene silencing (PTGS) in plants are related phenomena whose functions include the developmental regulation of gene expression and protection from transposable elements and viruses. Plant viruses respond by expressing suppressor proteins that interfere with the PTGS system.

S U M M A R YNarcissus late season yellows virus (NLSYV) was obtained, apparently unmixed with other viruses, in selected naturally infected narcissus plants. The virus was not transmitted by manual inoculation to any of the 12 herbaceous species tested but was transmitted by Myzuspersicae to virus-tested narcissus plants, which then developed symptoms characteristic of late season yellows. Typically, symptoms develop late in the growing season but their expression is unreliable. NLSYV particles, which measured c. 750 x 12 nm, were purified from fresh leaf extracts of narcissus in 0-3 M sodium citrate containing 10 mM sodium EDTA and 10 mM sodium DIECA by differential centrifugation followed by clarification with diethyl ether, gel filtration and isopycnic banding in caesium sulphate. Using antibodies from antisera prepared to virus particles, NLSYV was detected by ELISA in 14 of the 18 cultivars examined and nine seemed totally infected. Detection of the virus by immunosorbent electron microscopy was as sensitive as by ELISA but neither test detected all infections. The virus was detected by ELISA only in samples collected after flowering and capsid antigen concentration was greatest in the distal region of leaves. The reliability of detection depended on the cultivar. Immuno-electron microscopy tests confirmed the Occurrence of a distinct potyvirus, presumably narcissus degeneration, in narcissus cv. Grand Soleil d'Or, failed to detect any virus other than NLSYV in narcissus plants affected by white streak disease, and showed that NLSYV is related to three of five other potyviruses tested.

SummaryAs previously reported, narcissus latent virus (NLV) has flexuous filamentous particles measuring c. 650 nm × 13 nm, is manually transmissible to Nicotiana clevelandii and Tetragonia expansa, and is transmitted by the aphid Myzus persicae following brief acquisition access periods. In contrast to previous reports the virus particle protein has an apparent mol. wt of c. 45 kD. Moreover, infected cells in N. clevelandii leaves contain cytoplasmic inclusion bodies resembling those of potyviruses. In vitro translation of NLV RNA produced only one major product (mol. wt c. 25 kD) which was not precipitated by antisera to virus particle protein or to cytoplasmic inclusion protein. Antisera to 12 potyviruses and nine carlaviruses failed to react with sap containing NLV particles. Similarly antiserum to NLV particles did not react with particles of seven potyviruses or four carlaviruses.A weak reaction was detected between NLV particles and antiserum to particles of maclura mosaic virus (MMV), a virus which resembles NLV in particle morphology and particle‐protein size, and in inducing pinwheel inclusions. The cytoplasmic inclusion proteins (CIPs) of NLV, MMV and from narcissus plants with yellow stripe symptoms were serologically inter‐related. These proteins were also serologically related to, and had mol. wt similar to, the CIP of members of the potyvirus group. Particles with the size and antigenic specificity of those of NLV were found consistently in narcissus plants with yellow stripe disease.Narcissus latent and narcissus yellow stripe viruses therefore seem to be synonymous and, together with MMV, have properties distinct from those of any previously described virus group.