Introduction to Fungi, Third Edition

618 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
Fig 22.7 Diagram showing nutrient uptake mechanisms in a D-haustorium. Enzymatic reactions and proton-driven pumping
are indicated by solid arrows; dotted arrows indicate diffusion or translocation processes. Sucrose (Sucr.) is hydrolysed to fructose
(Fruct.) and glucose (Gluc.) by invertase (Inv.) secreted into the extrahaustorial matrix.These monosaccharides as well as amino
acids (AA) are taken up across the haustorial membrane by specialized porter proteins fuelled by a transmembrane H þ gradient.
Fructose is converted to the fungal transport compound mannitol (Mann.) by alcohol dehydrogenase within the haustorium.
Modified and redrawn fromVoegele and Mendgen (2003).
invertase into the matrix to perform the necessary
hydrolysis (Fig. 22.7).
All rusts are obligately biotrophic in nature,
i.e. they need to parasitize living host plants in
order to complete their life cycle. The fact that
many of them can now be cultivated on agar
media in the laboratory does not alter their
status as obligate biotrophs in nature (P. G.
Williams, 1984). The initial report of the cultivation
of P. graminis on a simple medium (Williams
et al., 1966) raised high hopes for a breakthrough
in research on rusts, but interest has waned in
recent years. Excellent accounts recalling the
excitement of discovery have been written by
Maclean (1982) and P. G. Williams (1984). It is
now possible to cultivate rust fungi on relatively
simple agar media based on minerals (e.g.
Czapek-Dox agar) with sucrose or glucose as
carbon source and yeast extract, peptone or
certain amino acids as a nitrogen source.
22.2.5 Host resistance
Resistance against rust infections can manifest
itself at different stages of the infection process.
Most commonly it becomes evident as a hypersensitive
response when the infectious hypha
attempts to breach the first host cell wall (Heath
& Skalamera, 1997). With monokaryotic stages of
rust fungi, this is the direct penetration through
the wall of the epidermal cell, whereas the
resistance response to an incompatible dikaryotic
strain occurs later, during or after the