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RNA Viruses - These viruses are not high on the list of potential vectors for plant gene transfer. They do have a few properties which may be exploited. There are two basic types of single-stranded RNA viruses.

The monopartite viruses have undivided genomes containing all the genetic information present in these viruses and are usually fairly large (e.g., tobacco mosaic virus is of the order of 2 x 10 MW).

Their large size is a major disadvantage rendering in vitro manipulation difficult. The multipartite viruses, as the name suggests, have their genome divided among small RNAs, either in the same particle or in separate particles.

For example, brome mosaic virus (BMV) contains four RNAs divided between three separate particles. The RNA components of multipartite genomes are small and appear to be able to self-replicate in plants.

With some members of the group, the genes encoding the coat protein may be dispensable; the loss does not affect viral DNA multiplication. From the vector construction point of view this would enable the RNA to move systematically through the plant in an unencapsidated form.

French et al. (1986) developed an in vitro transcription system to express brome mosaic virus from cloned cDNA. Like the alfalfa mosaic virus, the brome mosaic virus genome consists of three RNA molecules but only two are necessary for replication in protoplasts of monocotyledons.

The third RNA, which codes for the coat protein of the virus, can be replaced with desirable coding sequences and provides a high level of expression of, the inserted gene. Coding sequences for the bacterial chloramphenicol acetyl transferase gene have been inserted in place of the BMV coat genes, downstream of its AUG initiation codon.

Expression of cDNA in barley protoplasts suggests that the RNA virus may be a useful vector for gene manipulation. The second group, subgenomic RNAs (for example, RNA IV of BMV), is unlikely to find application as cloning vectors as they are unable to self replicate in infected plants.

The third group, satellite RNAs, has perhaps the greatest potential because totally dispensable to the virus. They vary in size from 270 bases (tobacco ringspot virus satellite) to 1.5 kb (tomato black ring virus satellite). These satellites appear to share little homology with the viral genomic RNAs, have the templates for their own replication, and utilize the machinery for replication.

They are not required for virus replication but are capable of altering the pathogenicity of the viral infection. These satellite RNAs have a number of other unusual properties, including the ability to code for proteins and stability in the plant in the absence of other viral components.

 

 
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