Host-pathogen interaction as a special case of interrelations between organisms

Abstract

Plants are open systems and therefore maintain a vast number of complex relationships with the environment. Among these are: 1. Dependencies upon the external environment. 2. Strong influences on the environment. 3. Diverse interrelations with other organisms at different levels, of plant species among each other, and of plants with animals including human beings. According to the nature of the interaction various types of interrelations can be distinguished: (a) neutral relations (e.g. epiphytes), (b) mutualistic relations (fertilization, symbiosis), and (c) parasitic relations (incompatibility, the phytopathological host-parasite interactions). Pathogens can be defined as organisms out of place. As a result of special conditions defense reactions are observed in the stricken individuals such as irregular reactions on the metabolic level, formation of phytoalexins, competition for food, space, etc., involving also genetic interrelationships between hosts and their parasites. Such interrelations occurring on the cellular level include the problems of cell recognition, adherence of the cell surfaces to each other, and intercellular bridging. Interactions on the individual population and ecosystem levels can be reduced to aspects of the interactions, which involve mutual gene regulation.The position ofphytopathology within the biological sciences can be in a way described as an association between botany and biochemistry, an association which includes close relations with ecology, zoology, agricultural technology, and biophysics. Moreover, phytopathology deals with a general problem of biology, namely, the interaction of living cells. Organisms as open systems