Mechanisms of Cross-Protection between Plant Virus Strains. In: Evered D, Harnett S, editors. Ciba Foundation Symposium 133 - Plant Resistance to Virus. Chichester: John Wiley & Sons, Ltd.; 2007. pp. 136-50. [DOI: 10.1002/9780470513569.ch10]

Development and Evaluation of Stable Sugarcane Mosaic Virus Mild Mutants for Cross-Protection Against Infection by Severe Strain

Sugarcane mosaic virus (SCMV; genus Potyvirus) induces maize dwarf mosaic disease that has caused serious yield losses of maize in China. Cross-protection is one of the efficient strategies to fight against severe virus strains. Although many mild strains have been identified, the spontaneous mutation is one of the challenging problems affecting their application in cross-protection. In this study, we found that the substitution of cysteine (C) at positions 57 or 60 in the zinc finger-like motif of HC-Pro with alanine (A; C57A or C60A) significantly reduced its RNA silencing suppression activity and SCMV virulence. To reduce the risk of mild strains mutating to virulent ones by reverse or complementary mutations, we obtained attenuated SCMV mutants with double-mutations in the zinc finger-like and FRNK motifs of HC-Pro and evaluated their potential application in cross-protection. The results showed that the maize plants infected with FKNK/C60A double-mutant showed symptomless until 95 days post-inoculation and FKNK/C60A cross-protected plants displayed high resistance to severe SCMV strain. This study provides theoretical and material bases for the control of SCMV through cross-protection.

Appraisal of Cocoa swollen shoot virus (CSSV) Mild Isolates for Cross Protection of Cocoa Against Severe Strains in Ghana

Two mild strains of Cocoa swollen shoot virus (CSSV), N1 and SS365B, were assessed for their capability to mitigate the devastating effects of CSSV in field trials at the Cocoa Research Institute of Ghana in comparison with noninoculated cocoa plants. Analysis of growth, mortality, and yield of cocoa trees inoculated with the mild strains over a period of two decades between 1996 and 2014 indicated that the cocoa plants inoculated with mild strains performed significantly (P < 0.05) better in the presence of the severe strains (CSSV 1A) than noninoculated cocoa, albeit degenerated over time. The development of cocoa hybrids studied in the trial showed differences in their performance in the presence of the mild-strain viruses. Spread of the severe virus increased and deteriorated the effects of the mild isolates over time, thereby decreasing the economic life span of the cocoa trees. This observation was consistent in all the trials regardless of the type of mild strain involved. Apparently, the immunity and protection conferred by mild strains of CSSV (SS365B and N1) on cocoa declined over time, and we further confirmed 'Amelonado' cocoa to be very susceptible to CSSV infection in terms of effects on growth and yield.

Co-infection and Disease Severity of Ohio Maize dwarf mosaic virus and Maize chlorotic dwarf virus Strains

Two major maize viruses have been reported in the United States: Maize dwarf mosaic virus (MDMV) and Maize chlorotic dwarf virus (MCDV). These viruses co-occur in regions where maize is grown, such that co-infections are likely. Co-infection of different strains of MCDV is also observed, and a synergistic enhancement of symptoms in co-infected plants was previously reported. Here, we examined the impact of co-infections of two strains of MCDV (MCDV-S and MCDV-M1, severe and mild, respectively), and co-infections of MCDV and MDMV in the sweet corn hybrid 'Spirit' in greenhouse experiments. Quantitative plant growth and development parameters were measured and virus accumulation was measured by reverse-transcriptase quantitative polymerase chain reaction. Virus symptoms were enhanced and plants showed no recovery over time in co-infections of MDMV-OH and MCDV-S but virus titers and quantitative growth parameters did not indicate synergy in co-infected plants. MCDV-M1 co-infections with either MDMV-OH or MCDV-S did not show symptom enhancement or evidence of synergism.

Pepper mild mottle virus pathogenicity determinants and cross protection effect of attenuated mutants in pepper

To determine the pathogenicity domain and to apply cross protection, Pepper mild mottle virus (PMMoV) point-mutations in the replicase (REP) gene between the methyl-transferase and helicase domains, and deletions truncating pseudoknots in the 3' non-coding region (NCR), were constructed. Some mutants substituting a single amino acid in REP residue 348 exhibited mild symptoms in Nicotiana benthamiana or pepper plants. Accumulation of these mutants was higher than that of other REP mutants or wild-type PMMoV. Deletion mutants in the 3' NCR pseudoknot showed the lowest virus replication and accumulation among the mutants tested. Six attenuated mutants, which combined 3' NCR deletions and single or double REP substitution mutations were constructed to investigate cross protection effects on pepper plants. All six of the attenuated mutants showed milder symptom development than wild-type virus. These results suggest that REP and the pseudoknot in the 3' NCR are major pathogenicity determinants of the virus, and engineered PMMoV attenuated mutants can be useful for protection against the virus in pepper plants.

Nonsense mutations of replicase and movement protein genes contribute to the attenuation of an avirulent tomato mosaic virus

Three recovery mutants of an avirulent Tomato mosaic virus genus: (Tobamovirus) (ToMV-K) with back mutations of the replicase and/or movement protein (MP) genes, have been constructed by site-directed mutagenesis, and infectious plasmids (pToMV-K) were obtained. The rescued phenotypes of the progeny viruses showed that the replicase and MP recovery mutant (ToMV-K(rase-mp)) induced severe symptoms on both systemic and necrotic plants similar to those induced by the virulent strain. The replicase back mutant (ToMV-K(rase)) produced chlorosis and mosaic symptoms on N. tabacum cv. Huangmiaoyu (systemic host), while the MP recovery mutant (ToMV-K(mp)) produced no systemic symptoms on Huangmiaoyu tobacco. Sequencing of the cDNA of progeny viruses revealed that the "back mutants" maintained these mutation sites during infection. Protein immunoblots indicated that the 98 and 126 kDa proteins were expressed in the plants systemically infected by ToMV-K and pToMV-K, whereas no 98 kDa protein was detected in the plants infected by ToMV. The MPs (27 kDa) of ToMV-K and pToMV-K in the plants were smaller in size than those (30 kDa) of ToMV and pToMVK(rase-mp). These data suggest that ToMV-K replicates and spreads by expressing the truncated 98 and 126 kDa replicases and 27 kDa MP in plants. The opal mutation at nucleotides (nt) 2670-2672 of the replicase gene mainly contributes to the attenuation of ToMV-K, whereas the mutations at nt 5632-5664 of the MP gene attenuate the induced symptoms.

Tobacco mosaic virus assembly and disassembly: determinants in pathogenicity and resistance

The structural proteins of plant viruses have evolved to self-associate into complex macromolecules that are centrally involved in virus biology. In this review, the structural and biophysical properties of the Tobacco mosaic virus (TMV) coat protein (CP) are addressed in relation to its role in host resistance and disease development. TMV CP affects the display of several specific virus and host responses, including cross-protection, systemic virus movement, hypersensitive disease resistance, and symptom development. Studies indicate that the three-dimensional structure of CP is critical to the control of these responses, either directly through specific structural motifs or indirectly via alterations in CP assembly. Thus, both the structure and assembly of the TMV CP function as determinants in the induction of disease and resistance responses.

A Point Mutation in the FRNK Motif of the Potyvirus Helper Component-Protease Gene Alters Symptom Expression in Cucurbits and Elicits Protection Against the Severe Homologous Virus

Sequence comparison had previously shown three amino acid changes in conserved motifs in the 455-amino acid sequence of the helper component-protease (HC-Pro) between a severe field strain of Zucchini yellow mosaic virus (ZYMV-NAT) and a mild field strain of ZYMV (ZYMV-WK). In this study, exchange of fragments and site-directed mutagenesis within the HC-Pro gene in an infectious clone of ZYMV enabled the effects of the mutations on symptom expression to be mapped. The substitution of Ile for Arg at position 180 in the conserved motif Phe-Arg-Asn-Lys (FRNK) of potyviruses was found to affect symptom expression. Infection of cucurbits with the engineered ZYMV (ZYMV-AG) that contained this mutation caused a dramatic symptom change from severe to mild in squash and to a symptom-free appearance in cucumber, melon, and watermelon. The Ile to Arg mutation was found to be stable, and no revertant virus was found after several passages through plants after long incubation periods. The AG strain was detected 4 days postinoculation and accumulated in cucurbits to a level and with kinetics similar to that of the wild-type ZYMV-AT strain. Cucurbit plants infected with the AG strain were protected against infection by the severe strain.

Evidence of Cross-Protection Between Beet Soilborne Mosaic Virus and Beet Necrotic Yellow Vein Virus in Sugar Beet

A high degree of reciprocal cross-protection occurred in sugar beets between beet soilborne mosaic virus (BSBMV) and beet necrotic yellow vein virus (BNYVV) in greenhouse experiments. The incidence of cross-protection depended on the interval between inoculations with protecting and challenging virus; longer inoculation intervals enhanced the incidence of cross-protection. Cross-protection was most effective when the inoculation interval was between 5 and 10 days, a period during which virus accumulated to a maximum level in plants singly infected with BSBMV or BNYVV. Results obtained by enzyme-linked immunosorbent assay (ELISA) and Western blotting were consistent and indicated that cross-protection affected viral capsid protein accumulation. RNA of both protecting and challenging viruses was detected in doubly infected plants by using reverse transcription-polymerase chain reaction (RT-PCR), indicating that RNA of the challenge virus was present in protected plants, but capsid protein was undetected by serological tests. BSBMV is a closely related but serologically distinct virus that provides an excellent system for studies of cross-protection with BNYVV.