Agriculture Reference
In-Depth Information
cinerea
(Van Kan
et al.
, 1997) have questioned the importance of cutinase for
fungal pathogenicity. These results suggest a saprophytic role for the cutinases and
imply that these enzymes are not important in plant infection. Thus, there may be
additional cutinases that function during pathogen ingress in a localized manner.
Only recently Li
et al
. (2003) provided molecular evidence for a requirement of
cutinase for the pathogenicity of
Pyrenopeziza brassicae,
cause of light leaf spot on
oilseed rape. A cutinase deficient mutant failed to penetrate the cuticle and was
unable to develop disease symptoms. The complementation of this mutant with the
single-copy
P. brassicae
cutinase gene
Pbc1
restored both cutinase activity and
pathogenicity.
However, the general importance of fungal cutinases for the direct penetration of
plant surfaces remains unclear. The lifestyle of the pathogen - saprophytic or
parasitic - may play a role and/or the site of pathogen ingress.
Pyrenopeziza
brassicae
does not enter the leaf through stomata, but directly penetrates the cuticle
(Li
et al.
, 2003). Thus, enzymatic degradation of the cuticle might be important.
Besides the role of fungal cutinolytic enzymes for the penetration process, the
regulation, especially the induction of these enzymes has been questioned. Host
factors such as cutin monomers could serve as signals that activate the pathogen
enzyme synthesis (Kolattukudy
et al.
, 1995). Furthermore, for some pathogens cutin
monomers have been described as being important for the induction of appressorium
formation.
Another possible aspect of cutinase function was reported for
Monilia fructicola
.
The cutinase production was inhibited by antioxidants such as caffeic acid at the
transcription level (Wang
et al.
, 2002). Thus, it could be assumed that the host
microenvironment, for example the ripening state, is an important factor for cutinase
activity.
Cell wall degrading enzymes (CWDEs)
. In general, the degradation of the plant cell
wall involves a concerted and/or synergistic action of several enzyme families
including cellulases, xylanases, pectic enzymes and proteases adapted to the
different cell wall polymers (Walton, 1994; Annis and Goodwin, 1997). Matching to
the complexity of the components that make up the plant cell wall, fungal plant
pathogens are able to produce a broad range of extracellular enzymes capable of
degrading the plant cell wall. These enzymes may be essential for pathogenicity.
However, similar to the cutinolytic enzymes, the role of CWDEs is not clear. The
redundancy - most enzymes are encoded by multigene families - makes it difficult
to prove their importance for the infection process.
Nevertheless, penetration by obligately biotrophic parasites, such as rust fungi
and powdery mildews or some hemibiotrophs, requires only minor damage of the
cell wall. Degradation of the cell wall is limited to the site of penetration as shown
by Xu and Mendgen (1997). Secretion of cellulytic enzymes of these pathogens is
either developmentally regulated or triggered by environmental signals (Mendgen
et al.
, 1996).
For example, cellulase activity of
Uromyces fabae
germlings has been shown to
be strictly regulated by differentiation. It increases during appressorium formation
and reaches a maximum during development of infection hyphae and haustorial
