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Plants are open systems and therefore maintain a vast number of complex relationships with the environment. Among these are: 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. Planten zijn open systemen en staan daardoor op velerlei ingewikkelde wijzen met hun omgeving in betrekking. Hieronder vallen: Naar de aard van de interactie kunnen verschillende typen van interrelatie worden onderscheiden: (a) neutrale relaties (bijv. epifyten), (b) mutualistische relaties (bevruchting, symbiose), en (c) parasitaire relaties (incompatibiliteit, fytopathologische gastheer-parasiet verhoudingen). Pathogenen kunnen gedefinieerd worden als organismen “out of place”. Bij de betrokken gastheer worden afweerreacties waargenomen, zoals ongewone stofwisselingsreacties, vorming van fytoalexinen, concurrentie op het gebied van voedsel, ruimte enz., waarbij ook genetische interrelaties tussen gastheer en parasiet een rol spelen. Dergelijke interrelaties op celniveau betreffen het herkennen en samengaan van cellen, via het celoppervlak of door intercellulaire “bruggen”. Interacties op het niveau van individu, populatie en oecosysteem kunnen worden teruggebracht tot aspecten van intercellulaire interacties, waarbij wederzijdse gen-regulatie een rol speelt.

Iron, as participant of many biological processes is a prerequisite for life. Uptake, internal transport and storage by organisms is handled by highly specialized chemical systems endowed with strong metal binding affinities. Apart from the homeostatic function of iron-binding compounds they appear of significance for inter-species interactions. Thus, by tight binding transferrin withholds the iron from invading microorganisms required for their optimal growth. This bacteriostatic property of the iron transport protein is however partially overcome by small molecular substances synthesized by bacteria and successfully competing for the metal. The balance of such interaction is a complex one. Yet, strong evidence points to the crucial importance of the amount of iron offered by a host to infecting agents for determining the fate of bacterial disease.