The first barrier to penetration and proliferation
of pathogenic microorganisms is the plant cuticle.
To breach the plant cuticle, fungi can either utilize
coordinately regulated cell wall-degrading
enzymes (Tonukari et al. 2000) or generate invasive
forces (by accumulating glycerol up to 3 M in
the appressorium generating a very high hydrostatic
pressure) to break the cuticle mechanically
(Bechinger et al. 1999). Melanin-deficient
mutants do not generate appressorial turgor and
are non-pathogenic (Thines et al. 2000).