ABSTRACT
In addition to their utility as "toolboxes" in providing sequences of value in plant genetic engineering plant viruses also offer the possibility of providing convenient and readily manipulated vectors for the rapid and high-level expression of foreign genes in plants. Geminiviruses are plant viruses with a genome that comprises either one or two single-stranded (ss), circular DNAs that encapsidate in virions which appear as twinned (geminate) icosahedra. The next advance in the evolution of plant virus-based vectors was the move from gene replacement vectors to gene insertion vectors to avoid the adverse consequences associated with gene deletion. Conversely, in applications of viral vectors that are aimed at exploiting gene silencing to knock out expression of endogenous genes it is advantageous if the vector is a strong inducer of silencing and does not express a functional anti-silencing protein.
