Resistance of Plants to Infectious Agents

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Abstract
The cuticle and other physical and chemical barriers to penetration, the presence of preformed inhibitors, the production of inhibitors, as well as the ability to establish a compatible metabolism between host and pathogen, may all be important for plant resistance under some conditions. Under particular conditions, which would be of rare occurrence in nature, none of the above may be effective. All mechanisms for resistance or susceptibility are genetically controlled. The presence of the genetic code for resistance however, does not assure its expression or effectiveness in a particular host-pathogen interaction. The production of a toxin by a microorganism may overcome an otherwise effective resistance mechanism. Unsuitable environment, and the dependence upon the release of a metabolic brake or trapping of repressor may metabolically prevent the expression of the genetic code, and thereby prevent the expression of resistance. Three distinct mechanisms for the disease resistance of plants were discussed in this review. Physical or chemical barriers, including preformed inhibitors of microbial growth, detoxication of enzymes or toxins, and the lack of necessary growth factors, all represent a nondynamic mechanism. This mechanism has a minimum of specificity and provides general protection for plants. A 2nd mechanism for resistance, based on physiological stress, includes the production, liberation, or mobilization of compounds following infection or injury. It has a degree of specificity since different microorganisms as well as different modes of injury can induce different degrees of response. The 3rd mechanism includes the unmasking of host DNA by a specific microorganism. This gives rise to specific RNA and hence metabolic function. This would be a highly specific mechanism for susceptibility or resistance.