Biology Reference
In-Depth Information
Alternaria brassicicola
, and
Fusarium oxysporum
in
A
.
thaliana
(Llorente et al.
2005
; Trusov et al.
2006
). In contrast, mutations in the
Arabidopsis
β
subunit AGB1
and the Arabidopsis
subunit AGG1 enhanced plant susceptibility to necrotrophic
fungal pathogens (Llorente et al.
2005
; Trusov et al.
2006
,
2007
). These results
suggest that the G
γ
α
subunit is a negative regulator, while G
β
and G
γ
subunits are
positive regulators of disease resistance.
Besides the three subunits of the heterotrimeric G-proteins,
Arabidopsis
has three
unique G
-like proteins, known as Extra Large G-protein 1 (XLG1), XLG2, and
XLG3 (Zhu et al.
2009
).
XLG2
and
XLG 3
were rapidly induced by infection with the
bacterial pathogen
Pseudomonas syringae
, whereas the
XLG1
transcript level was not
affected by the pathogen infection. The
xlg2
loss-of-function mutation caused
enhanced susceptibility to
P
.
syringae
(Zhu et al.
2009
). The
xlg2
mutation affected
pathogen-triggered induction of a small set of defense-related genes (Zhu et al.
2009
).
Constitutive overexpression of
XLG2
leads to the accumulation of transcripts from
multiple defense-related genes. These results suggest that XLG2 is involved in trig-
gering immune responses. In contrast,
xlg1
and
xlg3
mutants showed no difference
from wild-type in resistance to
P
.
syringae
, suggesting that XLG1 and XLG3 are not
involved in triggering defense responses against the pathogen (Zhu et al.
2009
).
AGB1, the sole G
α
subunit in
Arabidopsis
has been found to be a positive regula-
tor in resistance against necrotrophic fungal pathogens. The
agb1
mutant impaired
in the G
β
subunit shows enhanced susceptibility to these pathogens (Llorente et al.
2005
; Trusov et al.
2006
,
2009
). The G
β
β
subunit forms an obligate dimer with either
one of the
Arabidopsis
G
2/AGG2). The
agg1 agg2
dou-
ble mutant is as susceptible as
agb1
plants to
Plectosphaerella cucumerina
(Delgado-
Cerezo et al.
2012
). This heteromeric G-protein-mediated resistance was found to
be independent of SA-, JA-, ethylene-, and abscisic acid-mediated signaling path-
ways. However, this G-protein-mediated resistance was found to be modulated by
cell wall defense responses. The xylose content was lower in
agb1
and
agg1 agg2
mutants than in wild-type plants, suggesting that cell wall modifi cations may be the
immune response triggered by G
γ
subunits (
γ
1/AGG1 and
γ
β
and G
γ
subunits of the heteromeric G-proteins
(Delgado-Cerezo et al.
2012
).
The rice
dl
mutant, which is defi cient in the G
subunit, exhibited a reduced
hypersensitive response after infection by the rice blast pathogen
Magnaporthe ory-
zae
(formerly known as
M. grisea
) (Suharsono et al.
2002
, Iwata et al.
2002
) and
bacterial blight pathogen
Xanthomonas oryzae
pv.
oryzae
(Komatsu et al.
2004
).
These results suggest that the heteromeric G
α
subunit may play an important role in
both fungal and bacterial disease resistance (Izawa et al.
2010
).
α
3.6
Small G-Proteins Activate Plant Innate Immunity
The small G-proteins have been reported to play important role in activating immune
responses against fungal, oomycete, bacterial, and viral diseases in different plants
(Sano and Ohashi
1995
; Ono et al.
2001
; Suharsono et al.
2002
; Shirasu and
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