The Differentiation of Infection Structures as a Result of Recognition Events between some Biotrophic Parasites and their Hosts

Most uredospores of rust fungi develop infection structures in a typical pattern so that they can infect the host plant. The function of these infection structures is divided into the following three phases:

• 1 In the recognition phase, the germ tube recognizes the cuticle and the stoma. This process may occur independently from the host plant since copies of the cuticle induce similar reactions of the fungus. During fungal growth on the epidermis, unspecific stress responses of the plant are triggered.
• 2 In the signal phase, the fungal substomatal vesicle and infection hypha(e) contact the host cells within the leaf parenchyma. A signal from the host induces further development of the fungus. Haustorium mother cell differentiation is effected and haustorium formation is initiated. At the same time, the fungus suppresses the synthesis of stress metabolites by the plant.
• 3 In the parasitic phase, the fungus penetrates the host cell and complex interactions between host and parasite begin. A highly specialized interface around the haustorium develops presumably in order to allow a more efficient nutrient transfer from host to parasite. Eventual defence reactions of the plant, generally on the race-cultivar level, fail to be evoked or are suppressed in compatible combinations.