PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 42

reported to be the elicitor of a hypersensitive response in tobacco (Fellers et al.
2002 ). The helicase domain of the TMV replicase proteins act as elicitors and
induce the defense response in tobacco (Erikson et al. 1998). A 29 K-movement
protein (MP) of Tobacco rattle virus is considered as an elicitor (Ghazala and
Varrelmann 2007 ). The P3 protein involved in virus multiplication has been found
to be an elicitor in Turnip mosaic virus (Jenner et al. 2002 , 2003 ) and in Soybean
mosaic virus (Hajimorad et al. 2005 ). Virion structural proteins have been shown to
act as elicitor in Caulifl ower mosaic virus (Love et al. 2005 ).

2.9.2 Viral Double-Stranded RNAs May Be PAMPs

Small RNA-directed RNA silencing is a major immune system targeting foreign
nucleic acids of invading pathogens (Ding and Voinnet 2007; Jaubert et al. 2011 ).
In small RNA-based antiviral immunity, viral double-stranded RNA (vdsRNA) has
been identifi ed as a PAMP (Ding 2010 ). The RNA silencing pathway in plants
presents a formidable defense against viral pathogens (Qu and Morris 2005 ). RNA
silencing is triggered by dsRNA which are commonly generated during plant virus
replication (Willmann et al. 2011 ). In case of single-stranded RNA (ssRNA) viruses,
the viral RNA-dependent RNA polymerase (RdRP) encoded by the plant copies a
plus-sense ssRNA generating a dsRNA molecule (Qi et al. 2009 ; Garcia-Ruiz et al.
2010 ; Wang et al. 2010b ). In case of other RNA viruses, the two strands do not anneal
but can fold into highly structured molecules that have dsRNA regions (Alvarado and
Scholthof 2009 ). In case of geminiviruses, the RNAs transcribed from their circular
genomes act as a source of dsRNA (Chellappan et al. 2005 ). Viroids form hairpin
structures, which contain intervals of dsRNA (Papaefthimiou et al. 2001 ).
The plant innate immune system (defense surveillance system) detects the
presence of dsRNA as aberrant RNA molecule (Wypijewski et al. 2009 ) and gen-
erates small RNAs. The generated small RNAs direct the antiviral machinery to
cleave and destroy the invading viral genome (Alvarado and Scholthof 2009 ). The
results suggest that the viral dsRNA may be a PAMP triggering in antiviral
immune response in plants.

2.10 Host-Associated Molecular Patterns as Endogenous

Elicitors

2.10.1 Oligogalacturonides as HAMPs

Besides elicitors of pathogen origin (PAMPs), several elicitors of host origin
(endogenous elicitors) have also been reported to trigger immune responses.
These endogenous elicitors are called host-associated molecular patterns