Biology Reference
In-Depth Information
7.3.4
Arabidopsis MKK7 Positively Regulates
SA-Mediated SAR
Another MAPKK gene detected in
Arabidopsis
,
MKK7
, positively regulates plant
basal and systemic acquired resistance. MKK7 has been shown to trigger accumula-
tion of SA and the increases in SA levels resulted in enhanced expression of PR
genes. Overexpression of MKK7 induced
PR
gene expression and triggered resis-
tance to the bacterial pathogen
Pseudomonas syringae
pv.
maculicola
and the
oomycete pathogen
Hyaloperonospora parasitica
(Zhang et al.
2007c
). The
enhanced expression of disease resistance and PR gene expression induced by
MKK7 was shown to be dependent on SA and NPR1 (Zhang et al.
2007c
). The
results suggest that the MAPKK is involved in triggering SA signaling system to
confer disease resistance.
The expression of the
MKK7
gene in
Arabidopsis
was induced by pathogen
infection. Reducing mRNA levels of
MKK7
by antisense RNA expression blocked
the induction of SA-dependent systemic acquired resistance (SAR) in plants.
Ectopic expression of
MKK7
in local tissues induced
PR
gene expression and
disease resistance to the bacterial pathogen in systemic tissues (Zhang et al.
2007c
).
The results suggest that
MKK7
is involved in generating the mobile signal of SAR.
The homologue of AtMKK7 in tomato, LeMKK4, is involved in
Pto
-mediated
immune responses (Pedley and Martin
2004
).
7.3.5
Cotton GhMKK5 Triggers ROS-Mediated Signaling
Systems
A MAPKK encoding gene
GhMKK5
from cotton has been isolated and characterized.
The expression of
GhMKK5
is induced by pathogen infection and defense-related
signal molecules. The overexpression of
GhMKK5
in
Nicotiana benthamiana
showed enhanced resistance against the bacterial pathogen
Ralstonia sola-
nacearum
but the transgenic plants were susceptible to the oomycete pathogen
Phytophthora parasitica
var.
nicotianae
(Zhang et al.
2012b
).
GhMKK5
-
overexpressing plants showed enhanced expression of
NtRbohA
gene encoding
NADPH oxidase involved in accumulation of ROS. The transgenic plants showed
accumulation of H
2
O
2
, suggesting that MKK5 may be involved in ROS-mediated
immune signaling system (Zhang et al.
2012b
). GhMKK5 is signifi cantly induced
by SA, methyl jasmonate and ethephon (Zhang et al.
2012b
). The results suggest
that GhMKK5 might be involved in the SA or JA/ET signaling pathways. The
expression of SA signaling system-inducible
PR1a
and
PR5
and JA signaling
system-inducible
PR-4
genes were greatly elevated in
GhMKK5
-overexpressing
plants (Zhang et al.
2012b
). Another SA signaling pathway gene
NPR1
, which is
involved in SAR response, was also signifi cantly increased in
GhMKK5
-
overexpressing plants (Zhang et al.
2012b
).
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