PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 96

2.20.13 Hierarchy of PAMP-Induced Signaling Systems

Several signaling systems induced by PAMPs have been described widely. Cross-talk
between these signaling systems has also been reported. However, the exact sequence
of these signaling events has not been well understood. The hierarchy of early
signaling events induced by a PAMP in Nicotiana benthamiana has been reported
(Segonzac et al. 2011 ). Two different calcium infl ux inhibitors suppressed the ROS
burst, activation of MAPKs, and PAMP-induced gene expression. The calcium burst
was unaffected in plants specifi cally silenced for components involved in ROS burst
generation, or for MAPKs activated by PAMP treatment. ROS burst still occurred in
plants silenced for the two MAPK genes, NbSIPK and NtWIPK or both genes simul-
taneously, demonstrating that these MAPKs are dispensable for ROS production.
NbSIPK silencing is suffi cient to prevent PAMP-induced gene expression but both
the MAPKs are required for bacterial immunity against two strains of P . syringae .
These results suggest that the PAMP-triggered calcium infl ux is upstream of separate
signaling branches, one leading to MAPK activation and thence gene expression
and the other to ROS production (Segonzac et al. 2011 ).

2.21 Different PAMPs and HAMPs May Induce Similar

Early Signaling Systems

Each pathogen may contain or secrete several PAMPs. For example, bacterial patho-
gens may generally possess the PAMPs fl g22, EF-Tu and lipopolysaccharides.
Several HAMPs have also been identifi ed in different plants. The multiple and
highly variable PAMPs and HAMPs have been reported to induce almost similar
early signaling systems. At 1 h, 2586 and 1672 genes had altered expression levels
after fl g22 or oligogalacturonides (OGs) treatment, respectively (Denoux et al.
2008 ). The transcriptome analysis revealed that both of them induced expression of
almost same genes involved in Ca 2+ -signaling, ROS signaling, NO signaling, MAPK
signaling, SA signaling, JA signaling, ET signaling, and ABA signaling systems in
Arabidopsis (Denoux et al. 2008 ).
The transcriptome changes observed in Arabidopsis 30 min after fl g22 and
60 min after elf26 treatments were highly correlated (Zipfel et al. 2006 ). A clear
overlap in the sets of genes with altered expression in response to fl g22 and pepti-
doglycan (PGN) was also observed (Gust et al. 2007 ). A large number of genes
(441 genes) were commonly upregulated in EF-Tu and chitin-treated cells (Wan
et al. 2008b ). Based on the comparative analysis of microarray data using
Arabidopsis supplied with fl g22, elf18, and chitin (Zipfel et al. 2004 , 2006 ; Wan
et al. 2008b ) it was concluded that fl g22, elf18, and chitin signaling share a
conserved downstream signaling pathway leading to basal resistance.
Gene expression in tobacco cultured cells was monitored after application of two
different PAMPs/MAMPs, PiE (the PAMP from the cell walls of Phytophthora infes-
tans ) and TvX (a xylanase MAMP from Trichoderma viride ) (Suzuki et al. 2007 ).
There was no substantial difference in the gene expression profi les between cells