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Light is an important environment factor controlling plant growth, development, and nutritional quality and is also one of the most important factors inducing plant defence. In this study, we assayed the potential effects of light quality on the interaction between Nicotiana tabacum and cucumber mosaic virus (CMV). Our results indicated that white light-treated N. tabacum plants displayed obvious symptoms at early stage postinoculation, while the symptoms were significantly inhibited under red light and blue light. Western blotting and quantitative real-time PCR (qRT-PCR) analyses showed that blue light and red light can effectively delay the replication of CMV compared with white light. The activities of various reactive oxygen species (ROS)-scavenging enzymes and reducing substances [reduced glutathione (GSH) and ascorbic acid (ASA)] were increased under blue light and red light. In addition, hormone measurements and qRT-PCR analyses revealed that salicylic acid (SA)-mediated signalling pathway plays positive role in the related regulation, and cytokinin (CTK) may also participate in them. Furthermore, we found that the formation of dark green islands (DGIs) was significantly suppressed in plants under red light and blue light at 30 days postinoculation (Dpi). However, the accumulation of virus in plants under different light conditions had no notable differences at later stage of postinoculation. Taken together, these results indicated that red light and blue light could effectively delay symptom expression and replication of CMV on N. tabacum at the relatively earlier stage postinoculation.

Cross-protection has been used successfully and commercially to control a range of virus diseases for which the selection of suitable mild strains of plant viruses is necessary. Turnip crinkle virus (TCV) is highly pathogenic on Arabidopsis plants and its silencing suppressor-defective mutant, TCVCP, can induce highly localized RNA silencing which is differs from that of other protective strains. We found that TCVCP provides some protection against wild-type TCV but lacks complete protection, and the relative locations of the protective virus and challenge virus affect the degree of cross-protection. However, similar cross-protection afforded by TCVCP is not observed in Nicotiana benthamiana plants. As expected, TCVCP pre-infected Arabidopsis plants fail to protect against infection with the unrelated Cucumber mosaic virus, strain Fhy. It appears that cross-protection afforded by TCVCP requires that the challenge virus be very similar in sequence, which is a characteristic of RNA silencing. In order to investigate whether the protection is associated with the highly localized RNA silencing, mutant plants involved in key silencing pathway genes of RNA silencing machinery, including dcl2, dcl4 and triple dcl2/dcl3/dcl4 mutants were used. The results demonstrate that cross-protection afforded by TCVCP is dependent on host RNA silencing, and both DCL2 and DCL4 play important roles in this process.

The interaction between Cucumber mosaic virus (CMV) and Turnip crinkle virus (TCV) in Arabidopsis thaliana is reported. Although TCV coat protein accumulates to a similar level in singly or doubly infected plants, CMV coat protein is significantly decreased in doubly infected plants, which develop symptoms similar to those of TCV infection alone. TCV thus strongly interfers with infection by CMV. A significant reduction in CMV 2b gene expression also occurs in co-infected plants. A defence signalling initiator, salicylic acid (SA), further inhibits CMV accumulation during CMV-TCV co-infection. This interference is correlated with both enhanced virus gene silencing and defence signalling.

Mixed infections of Nicotiana benthamiana plants by Tobacco necrosis virus (TNV) and Turnip crinkle virus (TCV) exhibited an interference interaction. Accumulation of TNV (+)RNA as well as capsid protein in mixed infection were considerably lower than that of singly infected plants. There were also a slight reduction in the levels of TCV (+)RNA and capsid protein in doubly infected plants, which displayed the concentration of both viruses decreased in dually infected plants. Tissue immunoblot analysis of systemic N. benthamiana leaves infected by TNV and TCV singly or doubly showed the interference between the two viruses in situ, which exhibited the decrease of both viruses in doubly infected leaves although the distribution of them did not change remarkably. These results were consistent with the hybridization analysis of viral genomic RNA and coat protein. Both cross-protection test and mixed infection of the two viruses confirmed TCV had relatively stronger interference to the infection of TNV. Interference infection by TNV and TCV induced higher increase in the levels of cytochrome pathway respiration and alternative pathway respiration in host plants, especially the latter. Interference often occurred in different strains of one kind of virus or two different closely related viruses in one genus. Our results showed that interference could also occur in different viruses belonging to different genera.

Mixed infections of Nicotiana benthamiana plants by Cucumber mosaic virus (CMV) and Tobacco necrosis virus (TNV) exhibit a synergistic interaction and result in symptom enhancement. Accumulation of CMV(+) RNA as well as capsid protein (CP) in mixed infection was considerably higher than that of singly-infected plants. There was also a slight increase in TNV(+) RNA and CP levels in doubly infected plants. Synergistic infection by CMV- and TNV-induced higher increase in the levels of malonyldialdehyde, hydrogen peroxide (H 2 O 2 ) and more decline in the activities of catalase than singly infected ones. Both peroxidase and superoxide dismutase activities increased rapidly for the first 10 days post inoculation (dpi) in doubly-infected plants and then declined, whereas the enzyme activities continued to increase after 10 dpi in singly infected plants and had higher enzyme activities in the late stages than that of co-infected plants. These results suggest that synergistic infection by CMV and TNV produced severes oxidative stress in N. benthamiana plants and the synergy between the two viruses was mutual.