In vivo observation of conidial germination at the oxic-anoxic interface and infection of submerged reed roots by Microdochium bolleyi.

The underground plant parts of reed (Phragmites australis) growing in anoxic soil of the littoral zone of lakes are provided with oxygen via an aerenchyma. Some of this oxygen is released into the rhizosphere, which creates a potential microhabitat for aerobic fungi. Although fungal endophytes of reed have been shown to occur also in roots of flooded habitats, it is not known whether or how fungi can infect roots growing in anoxic or hypoxic soil. To study fungal infection of reed roots in the laboratory, we developed an incubation chamber to expose reed roots to conidia of Microdochium bolleyi in an anoxic agar medium and to observe fungal infection in vivo. Germination rates of conidia were high close to living roots, but decreased to zero in anoxic areas of the chamber. Conidial germ tubes located up to 200 microm from the roots grew preferentially towards the living roots. Conidia also germinated close to air-filled Teflon tubes and exhibited germ-tube tropism, but not as distinctly as on living reed roots. Conidia did not germinate in the neighbourhood of dead roots in anoxic agar. However, in the aerated margin of the incubation chamber most conidia germinated and exhibited tropic growth towards dead roots. Penetration of M. bolleyi through several cell layers of living roots was observed in cryo-microtome sections. Penetration was significantly deeper with illuminated plants than with plants kept in the dark; in some cases even the stele was reached. This is the first observation of oxygen released from roots to support growth of an aerobic fungus and of fungal penetration into root tissue in an anoxic environment.