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New Insights Into Biotrophy : The Story Rust Fungi Have to Tell Us !
R.T. Voegele(1), S. Wirsel(2), C. Struck(1), M. Hahn(3), and K.
Obligate biotrophic parasites are among the most serious plant pathogens
world wide. Yet, very little is known about the molecular mechanisms underlying
this specific life style. This is mainly due to the fact that these organisms
cannot be grown in culture, at least not to a point equivalent to the
true biotrophic phase. In order to develop new strategies in disease suppression
it would be highly desirable to understand how these pathogens manage
to maintain a symbiosis-like, prolonged association with their hosts.
Obligate biotrophs are best exemplified by the powdery mildew - and the
rust fungi. Both classes of fungi are characterized by the formation of
specially differentiated hyphae, so called haustoria, which differentiate
within the host cell. We have chosen the model systems Uromyces fabae
/ Vicia faba and U. striatus / Medicago truncatula to study the role of
these structures in maintaining the biotrophic life style. Based on the
isolation of haustoria and the construction of a haustorium-specific cDNA-library,
we were able to compare this blue print of the parasitic phase with other
stages of pathogen development at a molecular level. This comparison enabled
us to identify genes which are preferentially or exclusively linked to
the parasitic growth phase. We were able to show that rust haustoria are
small molecular power stations involved in biosynthetic processes and
that they are the exclusive sites for the uptake of carbohydrates. However,
the capacity of haustoria stretches beyond simple uptake and metabolic
duties. A fungal invertase is expressed in early stages of fungal growth
long before the hexose transporter is present. This might be a way for
the fungus to pre-condition colonized tissue for a source-sink conversion.
Support for this hypothesis stems from the influence of the parasite on
the expression of various host genes, including invertases, even in plant
organs far remote from the site of infection. Another interesting aspect
of haustorial function is the production of large quantities of sugar
alcohols. These alcohols act as mobile carbohydrate storage compounds.
However, they are also excreted into the apoplast where they presumably
act as scavengers of reactive oxygen species. A further focal point of
our research regarding the suppression of host plant defense responses
is the elucidation of the role of specific rust transferred proteins (RTPs)
which are secreted from the fungus into the infected host cell. Although
known from bacterial pathogens, this represents a novelty for fungal pathogens.
However, a link to biotrophy remains to be established.