Introduction to Fungi, Third Edition

UREDINALES: THE RUST FUNGI
617
Fig 22.6 Cryo SEM of freeze-fractured haustoria
of Uromyces viciae-fabae.(a)Intacthaustorium
(Hau) connected to the intercellular haustorial
mother cell (HMC) by a thin penetration tube.The
neck-band (NB) is visible in the penetration tube.
The vacuole (Vac) of the infected plant cell is also
obvious. (b) Fracture through a haustorium,
revealing one of its two nuclei (HN).The nucleus
of the infected plant cell (PN) is closely associated
with the haustorium, a feature frequently
observed in rust infections.The host cell wall
(CW), vacuole (Vac) and a Golgi stack (G) are also
visible. Both images to same scale. Previously
unpublished images very kindly provided by
E. Kemen and K. Mendgen.
which swells inside the host cell to form the
haustorial body. This contains a full complement
of organelles, including two nuclei. In some
rusts, especially the cereal-infecting species,
these two nuclei sometimes fuse into one diploid
nucleus (Harder & Chong, 1984). From the inside
outwards, the haustorial cytoplasm is surrounded
by the haustorial membrane, the
haustorial wall, the extrahaustorial matrix and
the extrahaustorial membrane (i.e. the modified
plant plasmalemma). The haustorial matrix is
sealed against the apoplast outside the infected
plant cell by means of a neckband (Fig. 22.6a).
The host nucleus is often closely associated with
the extrahaustorial membrane (Fig. 22.6b).
As in the Erysiphales (see p. 398), the extrahaustorial
membrane surrounding D-haustoria
seems to lack ATPase activity (Baka et al., 1995),
thus indicating that the infected plant cell has no
effective means to restrict the efflux of metabolites
into the haustorial matrix. In contrast,
ATPase activity in the fungal haustorial
membrane may actually be increased relative to
normal hyphae (Struck et al., 1996). Not surprisingly,
proton-driven hexose and amino acid
uptake appears to occur from the matrix across
the haustorial membrane into the haustorium
(Fig. 22.7; Voegele & Mendgen, 2003). The uptake
mechanism is thus equivalent to the uptake of
solutes into growing hyphae (see Fig. 1.11).
Whereas powdery mildews appear to rely on the
host plant for the hydrolysis of the transport
disaccharide sucrose into the hexoses fructose
and glucose prior to uptake into the haustorium
(p. 398), Voegele and Mendgen (2003) have
suggested that the rust haustorium secretes