Superinfection exclusion by Citrus tristeza virus does not correlate with the production of viral small RNAs

Superinfection exclusion (SIE), a phenomenon in which a preexisting viral infection prevents a secondary infection with the same or closely related virus, has been described for different viruses, including important pathogens of humans, animals, and plants. Several mechanisms acting at various stages of the viral life cycle have been proposed to explain SIE. Most cases of SIE in plant virus systems were attributed to induction of RNA silencing, a host defense mechanism that is mediated by small RNAs. Here we show that SIE by Citrus tristeza virus (CTV) does not correlate with the production of viral small interfering RNAs (siRNAs). CTV variants, which differed in the SIE ability, had similar siRNAs profiles. Along with our previous observations that the exclusion phenomenon requires a specific viral protein, p33, the new data suggest that SIE by CTV is highly complex and appears to use different mechanisms than those proposed for other viruses.