INFECTION STRATEGIES OF PLANT PARASITIC FUNGI - The Epidemiology of Plant Diseases - page 130

the main carbohydrates assimilated by haustoria of U. fabae . Two further amino
acid transporters (AATp1 and AATp3) showed strongest expression in haustoria
and weak expression in intercellular hyphae. Interestingly, both amino acid
transporters showed broad substrate affinities with preferences for in planta
apoplast-scarce amino acids such as histidine and lysine in the case of AAT1p and
leucine and the sulphur-containing amino acids cysteine and methionine in the
case of AAT3p (Struck et al. , 2002; Struck et al. , 2004a). Furthermore, AAT1p
showed no transport of cysteine and only intermediate transport of methionine and
leucine. On the other hand, AAT3p showed no transport of lysine and had only
weak affinity for histidine. Thus, it appears that these amino acid transporters
complement each other. This might reflect specific adaptations of the fungus to its
environment, the host plant.
However, there is little information concerning both the nutrient supply of
biotrophic pathogens and the composition of nutrients (Solomon et al. , 2003). Only
recently, a comparison of expression profiles of genes involved in the primary
metabolic pathways of Blumeria graminis f.sp. hordei performed by microarray
analysis was published (Both et al. , 2005). The results showed a dynamic regulation
of genes encoding key metabolic steps during the different developmental stages of
germination, penetration, infection establishment and conidial production. Studies of
genes encoding glycolytic enzymes confirmed that glucose is the predominant
source of carbon (Sutton et al. , 1999) taken up by haustoria; they also showed that
B. graminis had not lost metabolic capacity nor had it lost the ability to modulate its
metabolism. Thus, the question remains: Why is B. graminis an obligate pathogen?
(b) Suppression of plant defense reactions
To establish a successful infection, biotrophic pathogenic fungi are dependent on
living host cells. Thus, the fatal defense reaction of plants is the so-called
hypersensitive reaction - a localized programmed cell death at infection sites. For a
long-term biotrophic interaction the suppression or avoidance of the plant defense is
an important pre-requisite (Panstruga, 2003). Nevertheless, several fungal avirulent
genes have been described that may prevent a compatible interaction in host plants
containing the corresponding resistance gene. The best studied examples of this
group are the avirulence (Avr) genes of Cladosporium fulvum (Laugé and de Wit,
1998). Only recently, a direct interaction between C. fulvum avirulence proteins and
tomato disease resistance proteins leading to the activation of the hypersensitive
reaction was demonstrated (Rooney et al. , 2005).
Do the obligate biotrophic fungi have the competence to influence processes in
their hosts? Interestingly, the first example of a possible fungal effector protein of
the rust fungus Uromyces fabae was recently reported: the protein (RTP1p: rust
transferred proteins) with a potential signal sequence was shown to be translocated
from the extrahaustorial matrix to the cytoplasm of the infected host cell (Struck
et al. , 2004b). This protein might be involved in signalling phenomena between host
and parasite.