Biomedical Engineering Reference
In-Depth Information
the presence or absence of microconidia and chlamydospores, colony
colour, and conidiophore structure (Windels 1992). The diffi culty in
delineating species based on these features is evidenced by the different
systems that have been proposed, recognizing anywhere from 30 to 101
species (Nelson et al
.
1983).
F. oxysporum
is more commonly isolated fungi
from asymptomatic roots of crop plants (Gordon and Martyn 1997).
F.
oxysporum
has a well-documented ability to persist without recourse to
pathogenesis. Both pathogenic and non-pathogenic strains of
F. oxysporum
are
found in agricultural soils throughout the world and have received
a lot of attention from researchers. However, substantial populations of
F. oxysporum
are also found in many native plant communities, in soils
that have never been cultivated (Gordon and Martyn 1997). This group
of cosmopolitan, soil borne fi lamentous fungi is economically important
because many members are the causal agents of vascular wilt or root rot
diseases in agricultural and ornamental crops throughout the world (Woo
et al. 1998). The typical symptoms caused by
Fusarium oxysporum
include
wilting, yellowing and xylem discoloration.
The capacity of a fungal pathogen such as
F. oxysporum
to establish
infection on its host depends in part on its capacity to evade the toxicity of
host defense proteins and their respective coding genes.
Fusarium
genes that
control resistance to these molecules would therefore refl ect co-evolutionary
pressures and include genes that control mutual recognition, avoidance,
and detoxifi cation. The role of enzymes in the infection process has already
proved. Thorough research has been focused on trying to determine the
role and importance of extra-cellular cell wall-degrading enzymes (CWDE)
related to the virulence of plant pathogenic fungi with respect to their
respective coding genes for causing wilt diseases of plants.
PATHOGENICITY OF
FUSARIUM OXYSPORUM
WITH
RESPECT TO CELL WALL DEGRADING ENZYMES
Fusarium
spp. secretes a number of hydrolytic enzymes capable of
degrading cell wall polymers in order to invade the plant tissue and
feed from the released nutrients. Numerous virulence factors and their
corresponding genes have been identifi ed and characterized to varying
degrees for
F.
oxysporum
(Lu et al. 2003, Roncero et al. 2003). Among these,
pectic enzymes are extensively studied and considered to be virulence
factors for phytopathogenic
Fusarium oxysporum
(García-Maceira et al 2001,
Ruiz-Roldán et al. 2001). Gómez-Gómez et al. (2001, 2002) suggested a
similar role for xylan degrading enzymes in infection of vascular wilt
of plants. It is assumed that xylanases might be a virulence factor for
pathogens as xylan is the most abundant component of the cell wall in
