PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 40

2.8.3 Oomycete Cell Wall Glucans as PAMPs

A glucan fragment -1,6-1,3 heptaglucan from Phytophthora sojae induced defense

responses in soybean and the minimal structural requirements for the elicitation of
defense responses by this glucan were established as a succession of fi ve

β

β -1,6-

linked glucosyl residues with two side branches of β -1,3 glucan (Cheong et al.

1991). A glucan fragment, hexa- -glucopyranosyl-D-glucitol isolated from the cell

walls of Phytophthora megasperma f. sp. glycinea , acted as PAMP triggering
defense responses (Sharp et al. 1984 ). A doubly-branched hepta-

β

-glucoside gener-
ated from P . megasperma f. sp. glycinea glucan is a highly active PAMP (Cheong
et al. 1991); however its elicitor action is restricted to the plants belonging to the
family Fabaceae (Cote et al. 2000).

β

-1,3 glucans induced a variety of defense reac-
tions in a wide range of host plants (Klarzynski et al. 2000 ; Ménard et al. 2004 ; Aziz
et al. 2003 ). The biological activity of

β

-1,3 glucans is dependent on the degree of
polymerization (DP) and decorations carried by the sugar backbone (Inui et al.
1997 ; Ménard et al. 2004 ).

β

2.8.4 Cell Wall Glycoprotein CBEL with CBD

Motifs as PAMPs

A cell wall glycoprotein named CBEL (for Cellulose-Binding Elicitor Lectin) iso-
lated from the root rot pathogen Phytophthora parasitica var. nicotianae has been
identifi ed as an elicitor of defense responses in tobacco (Séjalon-Delmas et al.
1997 ; Villalba Mateos et al. 1997 ). It shows cellulose-binding and lectin-like activ-
ities (Villalba Mateos et al. 1997 ). CBEL binds to crystalline cellulose and isolated
plant cell walls, but it is devoid of enzyme activity on cellulose and various glu-
cans. CBEL was found to be localized in the inner and outer layers of the cell walls
of the oomycete, and it was present in close contact to the host cell wall during
infection (Séjalon et al. 1995 ).
The protein portion of CBEL is composed of two cysteine-rich domains, each
containing subdomain homologous to cellulose-binding domains (CBDs) of fungal
glucan hydrolases. CBDs belong to the Carbohydrate Binding Module 1 family,
which is found exclusively in fungi and oomycetes (Gaulin et al. 2006 ). CBDs of
CBEL have been shown to be essential and suffi cient to stimulate defense responses
and CBDs are considered as a novel class of PAMPs (Gaulin et al. 2006 ).
CBEL triggers the expression of various defense genes encoding lipoxygenase,
peroxidase, sesquiterpene cyclase, and anthranilate synthase and accumulation of
defense proteins including PR-1, PDF1.2, and hydroxyproline-rich glycoprotein
(HRGP) (Villalba Mateos et al. 1997 ; Khatib et al. 2004 ). CBEL has been shown to
be necessary for the structure of the hyphal cell walls (Gaulin et al. 2002 ). CBEL
homologues have been detected in various Phytophthora species and these homo-
logues contain highly conserved CBD motifs (Khatib et al. 2004 ).