Effect of Fungal Death or Inhibition Induced by Oxycarboxin or Polyoxin D on the Interaction between Resistant or Susceptible Bean Cultivars and the Bean Rust Fungus

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Abstract
Light and electron microscopy of resistnt or susceptible bean [Phaseolus 4vulgaris L.] leaves inoculated with the bean rust fungus [Uromyces appendiculatus (Pers.) Unger var. appendiculatus] and then treated with the fungicide oxycarboxin or the chitin synthesis inhibitor polyoxin D confirmed that the typical plant response to the dead fungus is the encasement of haustoria in the absence of plant cell necrosis. In the untreated resistant cultivar, necrosis commonly was detected first in cells adjacent to those containing haustoria at about 2 days after inoculation; the chemicals inhibited such necrosis if applied 1 day after inoculation. Polyoxin D treatment at this time allowed more infection sites to develop necrotic cells than did oxycarboxin application, apparently because after treatment with the former, some haustoria remained alive and continued to grow. These results indicate that haustorium walls do not contain chitin, that only the living haustorium elicits necrosis, and that it does so a few hours after it becomes mature. Ultrastructurally, the incompatible interaction was characterized by silica deposition in plant cells next to dead ones, wide electron-opaque extrahaustorial matrices, and the widespread deposition of callose-like material along the wall of haustorium-containing cells. Polyoxin-induced fungal death in the compatible cultivar did not elicit ultrastructural features typical of the incompatible interaction except for a slight increase in electron-opaque material in the extrahaustorial matrix.