Strategies to protect crop plants against viruses: pathogen-derived resistance blossoms.

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Abstract
Since 1986, the ability to confer resistance against an otherwise devastating virus by introducing a single pathogen-derived or virus-targeted sequence into the DNA of a potential host plant has had a marked influence on much of the research effort, focus, and short-term objectives of plant virologists throughout the world. The vast literature on coat protein-mediated protection, for example, attests to our fascination for unraveling fundamental molecular mechanism(s), our (vain) search for a unifying hypothesis, our pragmatic interest in commercially exploitable opportunities for crop protection, and our ingenuity in manipulating transgene constructions to broaden their utility and reduce real or perceived environmental risk issues. Other single dominant, pathogen-derived plant resistance genes have recently been discovered from a wide variety of viruses and are operative in an ever-increasing range of plant species. Additional candidates seem limited only by the effort invested in experimentation and by our ingenuity and imagination. This review attempts to consider, in a critical way, the current state of the art, some exceptions, and some proposed rules. The final impression, from all the case evidence considered, is that normal virus replication requires a subtle blend of host- and virus-coded proteins, present in critical relative concentrations and at specific times and places. Any unregulated superimposition of interfering protein or nucleic acid species can, therefore, result in an apparently virus-resistant plant phenotype.