New insights on how a viral protein neutralizes a plant antiviral defense response

A study published in the journal New Phytologist by the group of  Dr. Tomás Canto at the Margarita Salas Biological Research Center (CSIC) reveals the mechanism by which the viral protein HCPro of potyvirus facilitates the spread of infections by this virus in plants by suppressing their defensive response against the virus.

Plant viruses need to overcome plant defenses in order to infect them systemically.  One of those defenses is the gene silencing response that is mediated by the small RNAs (sRNAs) that the plant synthesizes in response to viral infections.  They have sequences complementary to those of the virus genome, targeting the latter for degradation. 

Through co-evolution with plants, viruses have developed strategies to evade this antiviral defense: specific viral proteins have functions that disable the complex gene silencing mechanisms.  They are called silencing suppressors

One of those suppressor proteins is HCPro from potyviruses.  This is a multifunctional protein that is also required for the dissemination of infection between plants using insects as vectors.  It allows infections by potyviruses to spread within and between plants, and thus to remain in the environment.

The mechanism by which HCPro performs its suppression function remains unclear.  Previous works show that it could bind to sRNAs.  However, the molar excess of sRNAs of viral sequence to HCPro would make it implausible that binding could prevent their role in gene silencing.

Purifying the sRNAs that HCPro associates when expressed from either a potyvirus, a potexvirus (another type of plant virus), or T-DNAs, del Toro et al. have shown that HCPro binding targets sRNAs that are of viral sequence, of 21 or 22 nt in length, and with 5´-end adenines, rather than those that are of plant sequence, have other 5´-ends, or nt lengths.  The sRNAs preferred by HCPro are precisely those that would associate with ARGONAUTE2 (AGO2), which is a key factor in antiviral silencing.

In addition, the authors demonstrate that HCPro reduces the methylation of sRNAs of viral sequence (or perhaps of those being generated de novo), which also prevents them from associating with AGO2. 

This dual mode of action could explain how HCPro performs its silencing suppression function.

Reference: In planta vs. viral expression of HCPro affects its binding of non-3 plant 21-22 nt small RNAs, but not its preference for 5´-terminal 4 adenines, or its effects on small RNA methylation. del Toro, F. J., Sun, H., Robinson, C., Covielles, E., Higuera, T., Aguilar, E., Tenllado, F., Canto, T (2022) New Phytologist. 233:2266–2281.  doi: 10.1111/nph.17935