Relationship Between Heat-Induced Fungal Death and Plant Necrosis in Compatible and Incompatible Interactions Involving the Bean and Cowpea Rust Fungi

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Abstract
Postinoculation heat treatment of bean [Phaseolus vulgaris] and cowpea [Vigna sinensis] leaves infected with their respective compatible rust fungi [Uromyces phaseoli] resulted in the seemingly rapid death of the fungus and the encasement of haustoria. Browning of invaded cells was rare and, although discolored flecks developed on bean leaves heated during uredium formation, this was caused by the browning of the fungus and cell walls of the plant, rather than the plant cytoplasm. These results suggest that bean and cowpea rust fungi do not release products during death that cause significant necrosis in susceptible tissue. In an incompatible combination of bean and the bean rust fungus, and in infections of the same fungus in the nonhost species, cowpea, fluorescence microscopy revealed no signs of haustorium death before that of the invaded plant cell. Postinoculation heat treatment applied to these plant-fungus combinations inhibited the normal plant cell necrotic reaction if applied early enough. Heat treatment applied later had no effect on the frequency or extent of plant browning, indicating either that the browning had been irreversibly triggered prior to heating or that the fungus had reached a stage of development at which recently-formed, constitutive, necrosis-causing factors were released during fungal death. It is suggested that the former hypothesis is the more likely and that the initiation of plant necrosis in the incompatible interactions examined requires some activity of the living fungus.