Family - Secoviridae

Abstract

This chapter focuses on Secoviridae family whose member genuses are Comovirus, Fabavirus, and Nepovirus. The virions of this family are nonenveloped, are of 25–30 nm in diameter, and exhibit icosahedral symmetry. Various virus preparations contain empty virus particles and in the case of viruses with a bipartite genome, the two RNAs are encapsidated in separate virions. The genome consists of one or two molecules of linear positive sense ssRNA. Nepoviruses have a single coat protein (CP) of 52–60 kDa, and comoviruses, fabaviruses, sadwaviruses, and strawberry latent ringspot virus have two CPs of 40–45 kDa and 21–29 kDa. The cheraviruses, torradoviruses, sequiviruses, and waikaviruses have three CPs of similar sizes. Unfractionated viral RNA is highly infective and in the case of viruses with a bipartite genome, neither RNA species alone can infect plants systemically. Viral proteins are usually expressed as large polyproteins, which are cleaved by 3C-like proteinases. Each RNA usually encodes a single polyprotein, a notable exception is the RNA2 of torradoviruses, which contains two open reading frames. Within the polyproteins, protein domains are organized in a manner that is common to that of other members of the order Picornavirales. Virus preparations are usually good immunogens and polyclonal antibodies prepared against purified virus particles recognize all CPs. All members of the family infect plants and the host range and symptoms vary with the genera and viruses considered.