Fungal Regulation of Disease Resistance Mechanisms in Plants

Abstract

We share this world with a myriad of living organisms each equally well adjusted for survival in its ecological niche. We constantly interact with each other either directly or indirectly and in the overall scheme of life this interaction results in survival. Life, nevertheless, is not free of disease. Disease implies a harmful effect on the life processes of an organism, but a harmful effect on one organism may be beneficial to another. Disease may limit realization ofthe genetic potential for yield and quality in agricultural crops, but in nature it is seldom the determinant of survival for both plant and pathogen. With a different perspective, the title of this article could be plant regulation of mechanisms for parasitism in fungi. Specificity of a pathogen for a plant is as complex and important a phenomenon as is the specificity of host reaction to a pathogen. It is difficult to imagine two organisms in close association, as would be the case of a fungus which penetrated into the tissues of a green plant, without an interaction and influence on the growth and development of each. It is an apparent paradox, therefore, that in successful parasitism, the compatibility ofthe host and pathogen is frequently prolonged, at least during initial stages of the parasite's development. As would be expected, the highly attuned metabolism of host and parasite is rarely encountered. Resistance is the common phenomenon in nature and susceptibility is the rare exception. A critical consideration in the health of a plant or development of a parasite would be the rapid recognition of its partner as non-self. From our perspective we often are most concerned with the plant recognizing the pathogen as non-self and reacting to restrict its development. The recognition of the plant as non-self by the pathogen may, however, be of importance in determining the success of the pathogen. The longer the pathogen avoids or suppresses recognition by the host, the more likely the pathogen will develop and reproduce within the host. Having recognized the pathogen as non-self, the plant activates metabolic pro? cesses which may restrict the pathogen's development. In turn, the pathogen can suppress such processes or detoxicate its products. The speed and magnitude with which the plant activates its mechanisms for disease resistance are critical to the development of the pathogen. To survive the selection pressure of evolution, it is also likely that disease resistance in plants depends upon the activation of multicomponent mechanisms for resistance. A single mechanism or compound with a single metabolic site of