Biology Reference
In-Depth Information
type B trichothecene, deoxynivalenol (DON), is considered as virulence factor in
infection of plants (Bai et al. 2002 ). The toxin also acts as an elicitor and triggers
generation of hydrogen peroxide, deposition of callose, accumulation of salicylic
acid, activation of mitogen-activated protein kinases, and expression of PR-1 and
PR-2 genes (Nishiuchi et al. 2006 ). Another plant pathogen Alternaria alternata
f. sp. lycopersici produces a phytotoxin, AAL toxin. The AAL toxin triggers cytolysis
and also triggers expression of defense genes (Gechev et al. 2004). The host-
selective toxin victorin produced by Cochliobolus victoriae , the maize victoria
blight pathogen, induces defense-related responses such as extracellular alkaliniza-
tion, generation of ROS and nitric oxide (NO), and production of phytoalexin (Tada
et al. 2005 ). Collectively these studies suggest that the cytolytic toxins play dual
roles in plant-pathogen interactions as virulence determinants (effectors) and as
nonself recognition determinants (PAMPs) for the activation of plant innate immune
responses (Qutob et al. 2006 ).
2.5
PAMP-Induced HAMPs (DAMPs/MIMPs/PAMP
Amplifi ers/Endogenous Elicitors)
Polygalacturonases and cellulases are produced by a wide range of pathogens and
they act as effectors and also function as general elicitors (Rotblat et al. 2002 ;
Boudart et al. 2003 ; Poinssot et al. 2003 ). During host-pathogen interaction, many
pathogens secrete these cell-wall-degrading enzymes (Vidal et al. 1998 ; Furman-
Matarasso et al. 1999 ; Boudart et al. 2003 ; Poinssot et al. 2003 ). These enzymes
can themselves function as elicitors (Rotblat et al. 2002 ; Poinssot et al. 2003 ), but
their enzymatic products are also known to be general elicitors of plant defense
responses (Shibuya and Minami 2001 ). These enzymes degrade the plant cell wall
structure and some of the degradation products such as pectin-derived oligogalact-
uronides (OGs) and cellodextrins act as potent elicitors of innate immunity. These
host-derived elicitors function almost in the same fashion as the PAMPs function
in plant innate immunity.
The host-derived elicitors are called by different names by different authors.
They are called host-associated molecular patterns (HAMPs) as they are of host
origin (Galletti et al. 2009 ), damage-associated molecular patterns (DAMPs) as
they are also induced by cellular damage (Zipfel 2009 ), or endogenous/internal
elicitors (Ryan et al. 2007 ; Huffaker et al. 2011 ). It was also suggested that these
plant cell-wall degradation products can be called microbe-induced molecular
patterns (MIMPs) recognized through receptors as 'pathogen-induced modifi ed
self' (Mackey and McFall 2006 ; Aziz et al. 2007 ). It may be better to call the
MIMPs as pathogen-induced molecular patterns (PIMPs) as these oligosaccha-
rides are formed during pathogenesis by the production of cell-wall-degrading
enzymes (Vidal et al. 1998 ; Furman-Matarasso et al. 1999 ; Boudart et al. 2003 ;
Poinssot et al. 2003 ). Most of the endogenous elicitors have the property of
inducing expression of their own genes to initiate a feedback mechanism to the
Search WWH ::




Custom Search