Introduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 2

ethylene, abscisic acid, auxin, cytokinin, gibberellins, and brassinosteroids play
important role in immune response signaling. Plant hormones activate different
signaling pathways inducing distinctly different defense genes. These signaling
pathways can crosstalk with each other and this crosstalk helps the plant to “decide”
which defensive strategy to follow, depending on the type of attacker it is encounter-
ing. Potential pathogens produce several effectors to nullify the defense responses
induced by the innate immune system. Pathogens may also hijack some signaling
systems to cause disease. The war between the plant and pathogen appears to be in
fi ne-tuning the signaling systems to cause disease or to enhance host defense
response. Recent advances in our understanding of the molecular basis of plant
innate immunity have opened new era in developing potential tools in management
of crop diseases.

Keywords Pathogen-associated molecular patterns (PAMPs) • Microbe-associated
molecular patterns (MAMPs) • Plant pattern recognition receptors (PRRs) •
Endocytosis of PRR proteins • PAMP-triggered immunity (PTI) • PAMP-PRR
signaling complex

1.1 Classical PAMPs

Innate immunity is the fi rst line of defense against invading microorganisms in
vertebrates and the only line of defense in invertebrates and plants (Silipo et al.
2010 ; Zamioudis and Peterse 2012 ). Several elicitors of microbial origin have been
identifi ed as primary danger/alarm signal molecules to switch on the plant immune
systems culminating in activation of defense genes (Aziz et al. 2003 ; D'Ovidio et al.
2004 ; Cavalcanti et al. 2006 ; Vidhyasekaran 2007 ; Thomma et al. 2011 ). The classical
general elicitors reported in plant pathogens resemble the pathogen-associated
molecular patterns (PAMPs), the classical activators of innate immune responses in
mammals (Nürnberger and Brunner 2002 ; Nürnberger et al. 2004 ; Nürnberger and
Lipka 2005 ). These historically termed general elicitors have been renamed as
PAMPs (Jones and Dangl 2006 ; Bent and Mackey 2007 ). PAMPs are often vital for
microbial survival and are therefore not subject to mutational variation (Gust et al. 2007 ;
Zhang and Zhou 2010 ). PAMPs are defi ned as evolutionarily conserved building
blocks of microbial surfaces that directly bind to plant pattern recognition receptors
(PRRs) and induce defense responses (Nürnberger and Brunner 2002 ; Qutob et al. 2006 ;
Nicaise et al. 2009 ; Tsuda and Katagiri 2010 ; Thomma et al. 2011 ). The molecular
signatures in PAMPs are not present in the host and these are perceived as 'non-self'
by plant pattern recognition receptors (Mackey and McFall 2006 ).
PAMPs that trigger innate immune responses in various vertebrates and non-
vertebrate organisms include eubacterial fl agellin, elongation factors, lipopolysac-
charides (LPS) from gram-negative bacteria, viral and bacterial nucleic acids, fungal
cell wall-derived chitins, glucans, mannans, or proteins and peptidoglycans from
gram-positive bacteria (Zipfel and Felix 2005 ; Jones and Dangl 2006 ). Similar