NUCLEAR BEHAVIOR OF THE COWPEA RUST FUNGUS DURING THE EARLY STAGES OFBASIDIOSPORE-DERIVED OR UREDIOSPORE-DERIVED GROWTH IN RESISTANT OR SUSCEPTIBLE COWPEA CULTIVARS

The relationship between fungal nuclear condition and growth of the tw o parasitic stages of the cowpea rust fungus was studied by light micr oscopy in living and fixed material. Uninucleate teliospores of the co wpea rust fungus germinated to form a promycelium that produced four b asidiospores, each containing two nuclei as the result of a nuclear di vision during nuclear migration into the developing spore. Basidiospor e derived hyphae within an epidermal cell of a susceptible host cultiv ar rapidly became monokaryotic by (i) synchronous mitosis of the two b asidiospore nuclei after they had migrated into the invasion hypha and the latter had grown to about 40 mu m; (ii) the subsequent immediate division by septa of the invasion hypha into one binucleate and two un inucleate cells; and (iii) the disappearance of one of the nuclei in t he binucleate cell. However, in the same host cultivar, the intercellu lar infection hyphae, secondary hyphae, and first two haustorial mothe r cells (HMCs) derived from dikaryotic urediospores of the same fungus had variable nuclear numbers due to nuclear degradation and asynchron ous nuclear divisions. Nevertheless, by 3 days after inoculation, all subsequently formed intercellular hyphae, HMCs, and haustoria were bin ucleate. In the three resistant cultivars tested, the presence or abse nce of nuclear division in the basidiospore-derived, intracellular inv asion hyphae depended on the degree of maximum fungal growth; growth i nhibition was related to plant cell death in two of the cultivars and to callose encasement in cultivar Queen Anne. In urediospore-derived i nfections 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 Que en Anne. The data suggest that in Queen Anne, fungal encasement plays a greater role in resistance to the basidiospore-derived than to the u rediospore-derived stage of infection.