INFECTION STRATEGIES OF PLANT PARASITIC FUNGI - The Epidemiology of Plant Diseases - page 129

from living tissue. This group represents economically the most important plant
parasitic fungi including the downy mildews, powdery mildews and rusts. Besides
the loss of photosynthetically active leaf area, the enormous spore production of
these fungi results in a significant loss of biomass by the diseased crop.
Hemibiotrophic pathogens are typified by a transient phase of the biotrophic life
style. These fungi have initial biotrophic growth phases before switching to killing
the host. Examples of hemibiotrophs are Phytophthora infestans , Cladosporium
fulvum , Colletotrichum spp. and Magnaporthe grisea . A predominantly biotrophic
phase until reproduction occurs has been observed in parasites such as Septoria spp.,
Claviceps spp. and Venturia inaequalis (Parbery, 1996).
In contrast to hemibiotrophs, obligate biotrophic fungi require living host plants
to complete their life cycle. A particular requirement for infection success is the
ability to keep host cells alive. Obligate pathogens form stable intimate associations
with their hosts with which they may co-exist for a period of time. Thus, they cause
very little systemic damage to host plants. A comparison of plant infection of
biotrophs and hemibiotrophs is given by Mendgen and Hahn (2002).
(a) Nutrient uptake at biotrophic plant-fungus interfaces
The biotrophic way of life requires a high degree of adaptation to the metabolism of
the living host to ensure the supply of nutrients from the living plant cell. A pre-
requisite for nutrient movement between host and biotrophic pathogen is the
development of a functional interface which promotes the nutrient transfer. During
infection some of these pathogens, such as downy mildews, powdery mildews and
rust fungi, form specialized physiological and morphological adaptations - the
haustoria - that represent the host-parasite interface specialized in nutrient uptake
(Harder and Chong, 1984; Staples, 2001).
A typical example is the powdery mildew fungus, Blumeria graminis ,
because the haustoria of this species provide the only interface with the host
cell, while the mycelium develops on the leaf surface (for review, see Aist and
Bushnell, 1991).
The rust fungus Uromyces fabae is one of the best studied examples of
functional aspects of haustoria (Mendgen et al. , 2000). Differential screening of a
haustorium-specific cDNA library of the rust fungus Uromyces fabae resulted in
the isolation of a large number of rust genes, showing preferential expression in
parasitically growing hyphae and haustoria (Hahn and Mendgen, 1997). Several of
these genes were further analysed to obtain more insight into the functional
aspects of haustoria. The role of nutrient transfer across the haustorial interface
was confirmed by several proton-symport-driven transporters. Both the genes
Hxt1 , encoding a hexose transporter (Voegele et al. , 2001) and AAT2 , encoding an
amino acid transporter (Hahn et al. , 1997) are preferentially expressed in the
haustorial membrane. Furthermore, using immuno-localisation it was shown that
both of the transporters were restricted to the haustoria; the signals were absent in
other fungal structures. Functional characterisation revealed that HXT1p has a
transport preference for D-glucose and D-fructose, suggesting that these might be