Ultrastructural analysis of the behavior of the dimorphic fungus Microbotryum violaceum in fungus-induced anthers of female Silene latifolia flowers

Abstract

The development of male organs is induced in female flowers of the dioecious plant Silene latifolia by infection with the fungus Microbotryum violaceum. Stamens in a healthy female flower grow only to stage 6, whereas those in an infected female flower develop to the mature stage (stage 12), at which the stamens are filled with fungal teliospores instead of pollen grains. To investigate these host-parasite interactions, young floral buds and fungus-induced anthers of infected female flowers were examined by electron microscopy following fixation by a high-pressure freezing method. Using this approach, we found that parasitic hyphae of this fungus contain several extracellular vesicles and have a consistent appearance up to stage 8. At that stage, parasitic hyphae are observed adjacent to dying sporogenous cells in the infected female anther. At stage 9, an increased number of dead and dying sporogenous cells is observed, among which the sporogenous hyphae of the fungus develop and form initial teliospores. Several types of electron-dense material are present in proximity to some fungi at this stage. The initial teliospores contain two types of vacuoles, and the fungus cell wall contains abundant carbohydrate, as revealed by silver protein staining. The sporogenous cell is probably sensitive to infection by the fungus, resulting in disruption. In addition, the fungus accelerates cell death in the anther and utilizes constituents of the dead host cell to form the mature teliospore.

Genomic and transcriptomic analyses of Microbotryum lychnidis-dioicae provide insights into the biology of a fascinating fungal phytopathogen.

“…Therefore the sepals were collected as the <1-mm group after all buds larger than 1 mm were removed. Based on our measurements, these buds were clearly Stage 8 (Farbos et al, 1997;Uchida et al, 2005;Uchida et al, 2003) or earlier in floral development (Fig. 4-2, 3 and 4A).…”

“…The healthy host plants display strong sexual dimorphism in vegetative (Zluvova et al, 2010) and reproductive traits (Delph et al, 2010). When infected, the fungus replaces the pollen in a male plant; in contrast, the gynoecium is suppressed in infected female flowers, and pseudo-anthers develop to house the teliospores, much like in the male plant (Ruddat et al, 1991;Uchida et al, 2005;Uchida et al, 2003). Infection of females may thus produce at least a partial sex reversal (Ruddat et al, 1991;Uchida et al, 2005;Uchida et al, 2003).…”

“…When infected, the fungus replaces the pollen in a male plant; in contrast, the gynoecium is suppressed in infected female flowers, and pseudo-anthers develop to house the teliospores, much like in the male plant (Ruddat et al, 1991;Uchida et al, 2005;Uchida et al, 2003). Infection of females may thus produce at least a partial sex reversal (Ruddat et al, 1991;Uchida et al, 2005;Uchida et al, 2003). The infection renders both the male and female plants sterile, in the former due to the lack of pollen, while in the latter the rudimentary ovary that can no longer set seeds.…”

“…The definition of the stages was informed by graphical and text illustration of previouslypublished studies (Farbos et al, 1997;Uchida et al, 2005;Uchida et al, 2003).…”

“…Since the teliospores occupy the space of the pollen in the anthers, infection in the female then seems futile, as there is no stamen. To overcome that, the fungus alters the female reproductive organs by suppressing the gynoecium and makes the supposedly rudimentary stamen continue its development into anthers that can house the teliospores, effecting a sex change in the female host (Ruddat et al, 1991;Uchida et al, 2005;Uchida et al, 2003).…”