Introduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 7

recently reported (Luna et al. 2011 ). Thus, SA signal is transduced not only within
the plant tissues, but also transferred even to the next generations.
Plant hormones activate different signaling pathways inducing distinctly different
defense genes (Spoel et al. 2007 ; Zhang et al. 2007 ; Mitsuhara et al. 2008 ). These
signaling pathways are not simple linear and isolated cascades, but can crosstalk
with each other. Both antagonism and synergism between the signaling systems
have been reported. Cross-talk between defense signaling pathways is thought to
provide the plant with a powerful regulatory potential, which helps the plant to
“decide” which defensive strategy to follow, depending on the type of attacker it
is encountering (De Vos et al. 2005 ). It may also allow pathogens to manipulate
plants to their own benefi t by shutting down induced defense through infl uences on
the signaling network.

1.5 War Between Host Plants and Pathogens

and the Winner Is ------- ?

Plant innate immune systems have high potential to fi ght against viral, bacterial,
oomycete, and fungal pathogens and protect the crop plants against wide range of
diseases (Knecht et al. 2010 ; Lacombe et al. 2010 ; Hwang and Hwang 2011 ; Alkan
et al. 2012 ). However, potential pathogens produce several effectors to nullify the
defense responses induced by the innate immune system (Wu et al. 2011 ; Akimoto-
Tomiyama et al. 2012 ; Cheng et al. 2012 ). To avoid or suppress or delay the expres-
sion of the defense gene-activating signaling systems, the pathogens secrete several
effectors into the host cell (Göhre et al. 2008 ; Kim et al. 2010 ; Wu et al. 2011 ;
Cheng et al. 2012 ). Pathogens may also hijack some signaling systems to cause dis-
ease (de Torres-Zabala et al. 2007 ; Thatcher et al. 2009 ; El Rahman et al. 2012 ). It
has also been demonstrated that the virulent pathogen may suppress the particular
defense signaling system which induce the expression of specifi c defense genes
conferring resistance against the particular pathogen (van Verk et al. 2008 ;
Koornneef and Pieterse 2008 ; Makandar et al. 2010 ). Activation of some signaling
systems may induce susceptibility, rather than resistance (Atsumi et al. 2009 ;
Yazawa et al. 2012 ). To overcome antiviral RNA silencing immunity, plant viruses
express silencing-suppressor proteins which can counteract the host silencing-based
antiviral process (Qu and Morris 2005 ; Ding and Voinnet 2007 ; Lewsey et al. 2010 ).
The war between the plant and pathogen appears to be in fi ne-tuning the signaling
systems to cause disease or enhance host defense. Fast and strong activation of the
plant immune responses aids the host plants to win the war against the pathogens
(Großkinsky et al. 2011 ). Overexpression or suppression of some specifi c signaling
systems in the plant immune system has been shown to help the plants to win in the
arms race between plants and pathogens (Cheung et al. 2007 ; Zhang et al. 2008 ;
Hwang and Hwang 2010 , 2011 ; Wu et al. 2010 ).
Engineering durable nonspecifi c resistance to phytopathogens is one of the ultimate
goals of plant breeding. However, most of the attempts to reach this goal fail as a