The biological cycle of Sporisorium reilianum f.sp. zeae: an overview using microscopy.

Sporisorium reilianum f.sp. zeae is the causal agent of maize head smut. Using microscopy, we describe the development of the fungus during its saprophytic and parasitic phase. When compatible, the yeast forms fused to produce dicaryotic hyphae. These hyphae were infectious and penetrated the maize in the root. Surprisingly, the formation of conjugation tubes was rarely observed in vitro. In contrast, extensive development of long hyphae was observed from the haploid form of the yeast, these hyphae being able to fuse when arising from compatible strains. In planta, the fungus acted as a biotrophic endophyte until sporogenesis, which occurred in the floral meristem of the maize. The symptoms of the infection were reduced. Penetration in the root was never accompanied by drastic damage of the host cell and we did not observe thickening or apposition of plant material to reinforce the wall structure. Moreover, the fungus was embedded in an amorphous matrix and thus appeared isolated from the host cell. In the floral meristem, radical changes were observed, the host cell was totally invaded by the fungus in the course of sporogenesis. The deposits observed on the fungal wall are likely related to the echinulation of the teliospores.