AN OVERVIEW OF PATHOGEN-DERIVED RESISTANCE
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Management of virus disease in plants involves deployment of resistance genes, phytosanitary practices and cross-protection. A natural extension of cross-protection was the development in the mid-1980s of genetically engineered resistance in susceptible plants using viral genes (transgenes) or selected nucleotide sequences to express proteins which acted to protect plants against infection by related strains. This process, designated 'pathogen-derived resistance' (PDR) is being used to minimize the effects of infection by a range of viruses, e.g. single-stranded (ss) RNA viruses (tobamo-, cucumo-, potex-, carla- and tospo-virus genera), and several ss DNA viruses (geminiviruses). Three general classes of PDR have been designated.
Use of PDR may involve elements of risk. In laboratory experiments, viral transgenes have been shown to recombine with infecting viruses, leading in some instances to recombinants with altered host range or increased virulence. Transgenic potato expressing the 5' proximal regions of two potyviruses, when inoculated with PVX, expressed symptoms characteristic of the synergistic interaction between PVX and potato virus Y. The latter results suggest that severe disease may result when transgenic crops are infected by certain non-target viruses. Additional concerns about deployment of PDR strategies include possible spread of transgenes to related wild plants, and the intense, ongoing public debate about labelling and marketing of genetically engineered food products destined for domestic and international trade. Coincident with the development of PDR strategies, there is increasing interest in transforming plants with other agents for disease management, e.g. cloned ribozymes to provide resistance to potato spindle tuber viroid in potato, expression of the cloned rice gene Xa21 in transgenic rice for control of a broad range of isolates of the bacterial blight pathogen, and expression of a chimaeric fusion protein containing the secretory sequence from barley alpha-amylase and a modified cecropin-coding sequence to provide resistance to the tobacco wildfire bacterium. Continued research and development will undoubtedly improve current PDR strategies, and provide new and novel ones for more effective disease management.