Light and electron microscopy of the interaction between the sunflower rust fungus (Puccinia helianthi) and leaves of the nonhost plant, French bean (Phaseolus vulgaris)

Growth of the sunflower rust fungus (Puccinia helianthi Schw.) was compared by light microscopy in sunflower leaves, in untreated French bean leaves, in bean leaves given a preinoculation heat treatment, and on collodion membranes. Results suggested that fungal growth was slightly reduced and the formation of haustorial mother cells was inhibited in untreated bean leaves. Haustorial mother cells, when present, did not form haustoria and adjacent mesophyll cell walls usually were highly refractive. Preinoculation heat treatment reduced the incidence of refractive cell walls and increased that of haustorial mother cells and haustoria. Ultrastructurally, infection hyphae in unheated bean leaves appeared unusually vacuolate and often contained wall appositions where they touched the plant cells. Silicalike deposits were present in and on mesophyll cell walls at most infection sites. In heated plants, necrotic haustoria with small bodies were seen at the few sites that lacked silicalike deposits. At other sites, the fungus appeared to have stopped growing during the formation of the penetration peg or the haustorial neck, and such necks were encrusted with silicalike material. At most sites, penetration pegs were occluded resulting in the unusual situation in which the haustorial mother cell remained seemingly alive in spite of the necrosis of the haustorium.