INFECTION STRATEGIES OF PLANT PARASITIC FUNGI - The Epidemiology of Plant Diseases - page 125

mother cells (Heiler et al. , 1993). Also, the production of the pectic enzymes pectin
methylesterase and polyglacturonate lyase (Deising et al. , 1995) and extracellular
proteases (Rauscher et al. , 1995) of this rust fungus depends on the differentiation of
infection structures. Apparently, the concerted action of CWDEs enables the hyphal
growth through the leaf tissue but prevents extensive cell wall maceration and cell
death which would interfere with the biotrophic lifestyle of the fungus.
For the biotrophic fungus Blumeria graminis f.sp. hordei there are clear
indications of the function of CWDEs. The cellobiohydrolase I is present at the tip
of the appressorial germ tube whereas isoform II is present at the tip of the primary
germ tube (Pryce-Jones et al. , 1999).
During its biotrophic phase, the mycelium of Venturia inaequalis does not
macerate the host tissue and is restricted to the area between the cuticle and the outer
epidermal cell wall. Low amounts of CWDEs are necessary to remove physical
barriers and to release nutrients. Kollar (1994) detected a pattern of twelve cellulase
isoenzymes that are produced constitutively in very low amounts in situ as well as in
in vitro cultures of different isolates of V. inaequalis . This complex cellulytic
system, with low variability, appears to be correlated with virulence of the fungus or
may give flexibility with properties that contribute to the performance of the
enzyme.
Xylan is the predominant hemicelluose in the cell walls of plants containing 1,
4-ß linked xylose residues, which can be substituted by different side groups. The
biodegradation of the xylan backbone depends mainly on two classes of enzymes,
endo - 1,4-ß - xylanases (EC 3.2.1.8), which hydrolyse the 1,4-ß - linked xylose
backbone, and ß-xylosidases (EC 3.2.1.37), which hydrolyse xylobiose and other
short xylooligosaccharides resulting from the action of endoxylanases.
Because of their potential role in fungal pathogenicity, xylanases have been
purified and characterised from an increasing number of phytopathogenic fungi and
the genes encoding xylanases have been cloned and characterised. Wu et al. (1997)
reported as many as five xylanases from the rice blast fungus Magnaportha grisea,
and at least four different xylanases have been identified from the maize leaf spot
fungus, Cochliobolus carbonum (Apel et al., 1993; Apel-Birkhold and Walton,
1996), each differing in molecular weight and pI values. Mutants of the maize
pathogen C. carbonum that specifically lacked a functional gene for a xylan-
degrading enzyme showed 85-94% reduced activity but growth of this strain was
indistinguishable from the wild-type in media containing corn cell walls or xylan as
the sole carbon source (Apel et al. , 1993). Some of the xylanases are induced only
during infection (Apel-Birkhold and Walton, 1996) suggesting different sets of
endoxylanases function in saprophytic and pathogenic growth of fungi.
Vast amounts of CWDEs are secreted by saprophytes and necrotrophic
pathogens. In contrast to biotrophic fungi, the involvement of these enzymes in the
penetration process of necrotrophic fungi is unclear. These fungi produce a high
level of CWDEs during infection. Thus, it is difficult to make a distinction between
the penetration of the plant surface and the maceration of plant tissue for nutrient
supply. Due to the importance of CWDEs, especially the pectic enzymes, for
colonising plant tissue, this subject will be discussed later.