Completion of the sexual cycle and demonstration of genetic recombination in Ustilago maydis in vitro

The heterobasidiomycetes responsible for plant smuts obligatorily require t heir hosts for the completion of the sexual cycle. Accordingly, the sexual cycle of these fungi could so far be studied only by infecting host plants. We have now induced Ustilago maydis, the causative agent of corn smut, to traverse the whole life cycle by growing mixtures of mating-compatible stra ins of the fungus on a porous membrane placed on top of embryogenic cell cu ltures of its host Zea mays. Under these conditions, mating, karyogamy and meiosis take place, and the fungus induces differentiation of the plant cel ls. These results suggest that embryogenic maize cells produce diffusible c ompounds needed for completion of the sexual cycle of U. maydis, as the pla nt does for the pathogen during infection.