Agriculture Reference
In-Depth Information
In conclusion, CWDEs are important determinants of the life style of numerous
plant pathogenic fungi and may be involved in the virulence of certain plant
pathogens. However, evidence of a role for these enzymes in any aspect of
pathogenesis is difficult to obtain. Nevertheless, an alternative approach to the
disruption of individual genes encoding CWDEs was shown by Tonukari
et al.
(2000), who analysed a gene encoding a protein kinase (
SNF1
) in
C. carbonum
known as a regulatory element of catabolite repressed genes including the CWDEs;
their results indicated that this gene was important for pathogenesis especially for
the penetration process.
4.4 STRATEGIES FOR COLONIZING THE HOST TISSUE
Plant pathogens can be broadly divided into those that kill the host and feed on the
contents, the so-called necrotrophs, and those that require a living host to complete
their life cycle, the biotrophs.
4.4.1 Necrotrophs
Necrotrophic parasites obtain their nutrients from dead plant tissues. These fungi
establish themselves inside the host tissue by releasing macerating enzymes and/or
detoxifying enzymes and phytotoxins, which disrupt cell integrity and cause cell
death immediately.
(a) Colonization supported by enzymes
Different groups of fungal enzymes can play important roles during any or all stages
of infection: initial penetration as described above, supporting the spread through the
host tissue and in addition, especially in the case of the necrotrophic lifestyle,
providing a food source. Cuticle degrading enzymes and CWDEs are purified from a
wide range of fungi. One of the most important groups of enzymes of necrotrophs
are the pectic enzymes causing maceration of plant tissue.
Other types of enzymes that might contribute to colonising plant tissue are those
that degrade antifungal substances of host plants - the so-called detoxifying
enzymes.
Pectic enzymyes
. For a long time research on cell wall degrading enzymes has
focused on pectinolytic enzymes (endo- and exo-pectin lyase, endo- and exo-
polygalacturonases and pectin methyl esterases) from pathogens of dicotyledonous
plants because pectin is the main compound of the middle lamella and primary cell
wall of dicotyledonous plants; thus these enzymes are able to cause maceration.
For example, during all stages of infection
B. cinerea
produces a broad set of
pectinases, including pectin methylesterase (Reignault
et al.
, 1994), pectin lyase
(Movahedi and Heale, 1990), and exo- and endopolygalacturonases (Johnston and
Williamson, 1992). All are believed to degrade the cell wall in concerted action but
their involvement in pathogenicity is still unknown. As shown by Ten Have
et al.
