Introduction to Fungi, Third Edition

654 USTILAGINOMYCETES: SMUT FUNGI AND THEIR ALLIES
Fig 23.15 Fimbriae of M. violaceum. Shadowing-TEM
view. Reprinted from Gardiner and Day (1988) by
copyright permission of the National Research
Council of Canada; original print kindly provided by
A.W. Day.
fimbriae, and that these have a similar biochemical
composition. Fimbriae can be of variable
length (up to 20 mm) but have a uniform diameter
of 7 nm. Most of the biochemical work on fungal
fimbriae was done with M. violaceum, and this
subject has been well reviewed by Celerin and Day
(1998).
Most fungal fimbriae consist of three structural
components. The main component is
proteinaceous, with strong homology to collagen
which had been thought previously to exist only
in animals (Celerin et al., 1996). The second
component consists of polysaccharide glycosylation
chains on the collagen protein. Whereas
deglycosylated fungal collagen can still polymerize
to form fimbriae, these become unstable in
extreme conditions. Further, the glycosylation
chains are involved in the specificity of mating
recognition in M. violaceum, with the fimbrial
protein component of a1 sporidia binding to
the mannose residues on the proteins of the
a2 fimbriae, i.e. acting as a lectin (Castle et al.,
1996). The third component of fungal fimbriae is
a short (30 nucleotides) single-stranded fimbrial
RNA (fRNA), the function of which is as yet
unknown (Celerin et al., 1994). The fimbriae of
M. violaceum do not traverse the cell wall, but
are anchored to it by means of a protein which
may be functionally analogous to mammalian
fibrinogen.
Whereas M. violaceum produces only one
type of fimbria, other fungi have several
morphologically distinguishable types (Gardiner
& Day, 1988). In Candida albicans (p. 277), fimbriae
play an important part in adhesion to human
tissue and initiation of infection, and the
fimbriae involved in this process are not made
up of collagen (Yu et al., 1994).
Host pathogen interactions
Initial infection of the host plants by M. violaceum
occurs if a pollinating insect carries teliospores
from a diseased to a healthy flower (Jennersten,
1988). Anther smut is thus a sexually transmitted
disease. The entire flower is colonized by intercellular
dikaryotic mycelium. Infection results
in reproductive sterility of the host; in male
flowers, the anthers become converted to teliospore
sori whereas in female flowers, the female
traits are suppressed and anthers are formed
which carry teliospores. This development has
been described in detail for infections of Silene
latifolia (Uchida et al., 2003). Diploid female
plants carry two X chromosomes in addition to
22 autosomes, whereas male plants contain
22 autosomes plus one X and one Y chromosome.
The Y chromosome encodes genes that suppress
the development of female traits and promote
stamen development, thereby making the flower
male. Microbotryum violaceum must therefore produce
a signal called male-sterility restoration
factor that substitutes for the Y chromosome in
female flowers (Uchida et al., 2003). Its identity is
as yet unknown, but it must be a signal molecule