Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 334

ABA signaling. MPK9 and MPK12 function downstream of ROS to regulate ABA
signaling (Jammes et al. 2009 ). ABA-induced stomatal closure plays an important
role in bacterial disease resistance. The mpk9/mpk12 double mutants have been
shown to be highly susceptible to P . syringae pv. tomato DC3000 compared to
wild-type Arabidopsis thaliana plants (Jammes et al. 2011 ). The results suggest that
the regulation of stomatal apertures by MPK9 and MPK12 contributes to the fi rst
line of defense against pathogens.

7.2.5 Soybean GmMPK4 Negatively Regulates SA

and ROS Signaling Systems

Soybean homolog of MPK4, GmMPK4, negatively regulates defense responses.
Silencing GmMPK4 resulted in activation of immune responses and it also led
to elevated levels of SA and H 2 O 2 . In soybean, GmMPK4 is a negative regulator
of SA, ROS, and defense responses. Thus the functions of MPK4s from
Arabidopsis and soybean are almost similar in negatively activating SA signaling
system (Liu et al. 2011 ).

7.2.6 Rice OsMPK6 Positively Regulates Local Resistance

and Negatively Regulates Systemic Acquired Resistance

Rice, OsMPK6, an ortholog of AtMPK4, functions both as an activator and a repres-
sor in conferring resistance against the bacterial blight pathogen Xanthomonas ory-
zae pv. oryzae (Shen et al. 2010 ). Activation of OsMPK6 resulted in the formation
of lesion mimics and local resistance to X . oryzae pv. oryzae , accompanied by the
accumulation of SA and JA, and the induced expression of SA- and JA-signaling
genes. By contrast, the knock-out of OsMPK6 results in increased accumulation of
SA and enhanced resistance to X . oryzae pv. oryzae in systemic tissues. The knock-
out of OsMPK6 induces the expression of PR1a , the marker gene of systemic
acquired resistance (SAR) when challenged with the pathogen (Shen et al. 2010 ).
These results suggest that OsMPK6 positively regulates local resistance while it
negatively regulates systemic acquired resistance against X . oryzae pv. oryzae by
differentially modulating SA and JA signaling pathways.

7.2.7 Oilseed Rape BnMPK4 Positively Regulates JA-Mediated

Defense Responses

In oilseed rape ( Brassica napus ), overexpression of BnMPK4 enhances resistance to
Sclerotinia sclerotiorum (Wang et al. 2009 ). The transgenic plants inhibited growth