Introduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 6

1997 ; Ori et al. 1997 ) have been shown to be involved in triggering ROS production.
ROS is a messenger in transmitting the PAMP signal. Nitric oxide (NO) has been
identifi ed as a gaseous second messenger (Besson-Bard et al. 2008 ; Bellin et al.
2013 ). NO is a diffusible molecular messenger that plays an important role in the
plant immune response signal transduction system (Grennan 2007 ). PAMPs trigger
NO burst within minutes in plant cells (Foissner et al. 2000 ; Lamotte et al. 2004 ;
Tischner et al. 2010 ). NO acts substantially in cellular signal transduction through
stimulus-coupled S-nitrosylation of cysteine residues (Benhar et al. 2008 ). It serves
as a key redox-active signal for the activation of various defense responses (Klessig
et al. 2000 ).

1.4 Plant Hormone Signals in Plant Immune

Signaling System

The plant hormones salicylic acid (Mukherjee et al. 2010 ; Dempsey et al. 2011 ; Liu
et al. 2011a , b ), jasmonate (Wang et al. 2009 ; Sheard et al. 2010 ; Bertoni 2012 ),
ethylene (Boutrot et al. 2010 ; Laluk et al. 2011 ; Nie et al. 2011 ; Nambeesan et al.
2012 ), abscisic acid (Yazawa et al. 2012 ), auxin (Fu and Wang 2011 ), cytokinin
(Choi et al. 2011 ), gibberellins (Qin et al. 2013 ), and brassinosteroids (De Vleeschauwer
et al. 2012 ) play important role in defense signaling against various pathogens.
It has been demonstrated that specifi c plant hormone signaling pathways should be
activated to confer resistance against specifi c pathogens. JA and SA signaling systems
may differentially contribute for resistance against specifi c pathogens. JA-mediated
pathway effectively confers resistance against necrotrophic fungal pathogens
(Berrocal-Lobo and Molina 2004 ; McGrath et al. 2005 ; Zheng et al. 2006 ), while
SA- mediated pathway confers resistance against biotrophic fungal pathogens and
also against virus and bacterial diseases in some plants (Thomma et al. 1998 , 2001 ;
Thaler and Bostock 2004 ; Nie 2006 ; De Vos et al. 2006 ; Spoel et al. 2007 ; Zheng
et al. 2006 , 2007 ). Two forms of induced resistance, systemic acquired resistance
(SAR) and induced systemic resistance (ISR), have been recognized based on the
induction of specifi c plant hormone signaling systems (Li et al. 2008 ). SAR refers
to a distinct signaling pathway mediated by SA (Oostendorp et al. 2001 ), while ISR
refers to the signaling pathway mediated by JA and ET. SA signaling system acti-
vates not only local resistance, but also systemic acquired resistance (SAR) observed
in distal (systemic) tissues. SAR is a SA-dependent heightened defense to a broad
spectrum of pathogens that is activated throughout a plant following local infection
(Liu et al. 2011a ). SAR is associated with priming of defense (Kohler et al. 2002 ;
Jung et al. 2009 ; Luna et al. 2011 ) and the priming results in a faster and stronger
induction of defense mechanisms after pathogen attack (Conrath 2011 ). The priming
can be inherited epigenetically from disease-exposed plants (Pastor et al. 2012 ) and
descendants of primed plants exhibit next-generation systemic acquired resistance
(Slaughter et al. 2012 ; Luna et al. 2011 ). The transgenerational SAR has been