Sum ma ry
Traditionally, plant viruses are viewed as harmful, undesirable pathogens. However, their genomes can provide several useful 'designer functions' or 'sequence modules' with which to tailor future gene vectors for plant or general biotechnology. The majority (77%) of known plant viruses have single-stranded RNA of the messenger (protein coding) sense as their genetic material. Over the past 4 years, improved in vitro transcription systems and the construction of partial or fulllength DNA copies of several plant RNA viruses have enhanced our ability to manipulate and study their genomes, particularly in the context of their pathogenic interactions with host plants. Recently, two forms of genetically engineered protection against plant virus infections have been reported. In both, a virus-related ' interfering' molecule was stably introduced into plants via the DNA-transfer mechanism of Agro- bacterium tumefaciens. To date, the choice o f ' interfering' molecule has been guided by empirical jield-observations and each is eflective against only a narrow range of closely-related viruses. As yet, we do not fully understand the molecular mechanism(s) responsible for the observed protection.
The ability to manipulate the plantpathogen relationship is a powerful tool to increase our knowledge and improve future strategies for unconventional cropprotection by genetic engineering techn iques .