TYPES OF RESISTANCE TO DISEASE IN PLANTS

Resistance to disease of plants has historically been divided into two major categories: non-host resistance and host resistance. The former, which encompasses the case where all genotypes within a plant species show resistance or fail to be infected by a particular virus, specifically signifies the state where genetic polymorphism for susceptibility to a particular virus has not been identified in a host taxon. Clearly, most plant species are resistant to most plant viruses. Susceptibility is the exception to the more general condition of resistance or failure to infect.

Although underlying mechanisms of non-host resistance to viruses are largely unknown and are likely as diverse for viruses as they are for other classes of plant pathogens, improved understanding of the ways in which infection fails in these interactions may be particularly important for breakthroughs in the development of plants with durable broad-spectrum disease resistance. Host resistance to plant viruses has been more thoroughly investigated, at least in part because, unlike non-host resistance, it is genetically accessible. This general case, termed host resistance, specific resistance, genotypic resistance, or cultivar resistance, occurs when genetic polymorphism for susceptibility is observed in the plant taxon, i.e., some genotypes show heritable resistance to a particular virus whereas other genotypes in the same gene pool are susceptible.

In resistant individuals, the virus may or may not multiply to some extent, but spread of the pathogen through the plant is demonstrably restricted relative to susceptible hosts, and disease symptoms generally are highly localized or are not evident. The distinction between resistance to the pathogen and resistance to the disease is important to articulate. Resistance to the pathogen typically leads to resistance to the disease; however, resistant responses involving necrosis can sometimes be very dramatic, even lethal, e.g., the N gene in tobacco for resistance to Tobacco mosaic virus (TMV) or the I gene in Phaseolus vulgaris for resistance to Bean common mosaic virus. In the case of resistance to disease symptoms or tolerance to the disease, the virus may move through the host in a manner that is indistinguishable from that in susceptible hosts, but disease symptoms are not observed. If the response is heritable, these plants are said to be tolerant to the disease, although they may be fully susceptible to the pathogen. This host response is very prevalent in nature, and has been used to considerable benefit in some crops, e.g., the control of Cucumber mosaic virus (CMV) in cucumber, even though the genetic control of this response is typically difficult to study. The genetics of tolerant responses are not be considered further due to the complexity of the biology and relative lack of information. More recently, a third important category of host resistance has been identified, initially in studies involving TMV: systemic acquired resistance (SAR).

This response can be activated in many plant species by diverse pathogens that cause necrotic cell death, resulting in diminished susceptibility to later pathogen attack. As SAR has recently been reviewed, this topic is not discussed further here. Virus-induced gene silencing, another induced defense mechanism to virus disease, has also been reviewed recently. Transgenic approaches to plant virus resistance have been widely explored since the earliest experiments where by transgenic tobacco plants expressing TMV coat protein (CP) were challenged with TMV and shown to be resistant. It is now possible to engineer resistance and tolerance to plant viruses using transgenes derived from a wide range of organisms including plant-derived natural R genes, pathogen-derived transgenes, and even non-plant and non-pathogen-derived transgenes.