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Nuclear Behavior of the Cowpea Rust Fungus During the Early Stages of Basidiospore- or Urediospore-Derived Growth in Resistant or Susceptible Cowpea Cultivars. Michèle C. Heath, Department of Botany, University of Toronto, 25 Willcocks Street, Toronto, ON M5S 3B2, Canada; Haixin Xu(2), and Tamar Eilam(3). (2)Department of Botany, University of Toronto, 25 Willcocks Street, Toronto, ON M5S 3B2, Canada; (3)Institute for Cereal Crop Improvement, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel. Phytopathology 86:1057-1065. Accepted for publication 29 July 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-1057.

The relationship between fungal nuclear condition and growth of the two parasitic stages of the cowpea rust fungus was studied by light microscopy in living and fixed material. Uninucleate teliospores of the cowpea rust fungus germinated to form a promycelium that produced four basidiospores, each containing two nuclei as the result of a nuclear division during nuclear migration into the developing spore. Basidiospore-derived hyphae within an epidermal cell of a susceptible host cultivar rapidly became monokaryotic by (i) synchronous mitosis of the two basidiospore nuclei after they had migrated into the invasion hypha and the latter had grown to about 40 µm; (ii) the subsequent immediate division by septa of the invasion hypha into one binucleate and two uninucleate cells; and (iii) the disappearance of one of the nuclei in the binucleate cell. However, in the same host cultivar, the intercellular infection hyphae, secondary hyphae, and first two haustorial mother cells (HMCs) derived from dikaryotic urediospores of the same fungus had variable nuclear numbers due to nuclear degradation and asynchronous nuclear divisions. Nevertheless, by 3 days after inoculation, all subsequently formed intercellular hyphae, HMCs, and haustoria were binucleate. In the three resistant cultivars tested, the presence or absence of nuclear division in the basidiospore-derived, intracellular invasion hyphae depended on the degree of maximum fungal growth; growth inhibition was related to plant cell death in two of the cultivars and to callose encasement in cultivar Queen Anne. In urediospore-derived infections of these same cultivars, the intercellular infection hyphae and HMCs developed normally, but nuclear migration into the haustorium was reduced, often before callose encasement of the haustorium in Queen Anne. The data suggest that in Queen Anne, fungal encasement plays a greater role in resistance to the basidiospore-derived than to the urediospore-derived stage of infection.

Additional keywords: dikaryon, hypersensitive response, monokaryon, Uromyces vignae.