7 – Parasites and Mutualistic Symbionts
This chapter provides an overview of symbiotic relationships, including parasitic, in which only one species benefits, and mutualistic, where both partners benefit. A parasite that causes perceptible damage to its host is termed a pathogen. The fungi that live as airborne symbionts of aerial plant surfaces depend on the effectiveness of their mechanisms for spore production, liberation, dispersal in air, and reinfection. Environmental factors affect spore germination, and hence the behavior of a pathogenic fungus in nature. Necrotrophic pathogens first kill, and then feed upon the host's cells. Toxins of pathogenic fungi are secondary metabolites. Enzymes produced by pathogens enable them to counteract host resistance and degrade host tissues. Mycorrhizal fungi become endemic in soils where their hosts grow, and form extensive networks that connect to roots of plants of different species. Some fungal inhabitants of plants do not cause disease, but inhabit the apoplastic spaces of their plant hosts. Plants have features that provide protection against fungal invasion, such as the physical barrier of the cuticle and its water-repellent surface, and the chemical substances found in plant cells. The most important effect of fungi on human activities is the destruction of crops. Disease can be controlled in crop plants by breeding for the characteristics that confer resistance. Fungal diseases can be classified into three groups of increasing severity: superficial, subcutaneous and systemic infections. Fungi have biosynthetic and degradative abilities that are lacking in most insects. They synthesize vitamins including those of the B group, sterols, and act as food for animals.