Viruses can be potent infectious agents, but plants have evolved responses to viral infection—including a hypersensitive response—that are much like those mounted against other pathogens. In addition, plants have a form of defense that targets the virus itself (Fig. 32.8). Most plant viruses have genomes made of single-stranded RNA (ssRNA). During the replication of the viral genome inside the plant cell, double-stranded RNA molecules (dsRNA) are formed. Because plant cells do not normally make double-stranded RNA, the replicating viral genomes are identified as foreign. Enzymes produced by the plant cell cleave the double-stranded RNA molecules into small pieces of 20 to 25 nucleotides, forming fragments called small interfering RNA, or siRNA (Chapter 3). These fragments bind with specific protein complexes in the cell. The RNA–protein complexes then play a role in targeting and destroying single-stranded RNA molecules that have a complementary sequence—that is, the viral genome (Chapter 19). The virus is unable to replicate and thus cannot spread. In this way, the plant cell uses RNA chemistry to eliminate viral infections.

When a cell is attacked by a virus, the siRNA molecules that are produced in response to the initial infection can also spread, allowing the plant to acquire immunity against specific viruses. The siRNA molecules move through plasmodesmata, enter the phloem, and from there spread throughout the plant. Thus, the systemic response to viral infection consists of both general defense responses triggered by salicylic acid and the transport of highly specific molecules that can target and destroy viral genomes throughout the plant.