PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 102

observed when Arabidopsis was challenged with two PAMPs (fl g22 + peptidoglycan
[PGN], fl g22 + oligogalacturonides [OGA] and elf18 + OGA) concurrently. Induction
of ROS by fl g22 was reduced by more than 90 %, whenfl g22 was applied along with
OGA (Aslam et al. 2009 ).

2.25 Amount of PAMP/HAMP Determines the Intensity

of Expression of Defense Signaling Genes

The intensity of PAMP-induced expression of various defense signaling systems
may depend on the amount of PAMP applied to activate the immune systems. The
intensities of calcium signatures induced by both cryptogein and OGs were shown
to depend on the PAMP/HAMP concentration (Lecourieux et al. 2002 ). Flg22
induced the transcription factor gene WRKY40 in Arabidopsis and the induction of
gene expression increased at increasing concentrations of the elicitor (Fig. 2.5 ;
Denoux et al. 2008 ). The relationship between increase in concentrations of PAMPs
and increased upregulation of expression of defense-related genes has been reported
in barley (Fujita et al. 2004 ), rice (Schaffrath et al. 1995 ), and parsley cells (Davis
and Hahlbrock 1987 ).
The HAMP ZmPep1 application induced increased expression of defense genes
and the induced magnitude of change in transcript abundance was found to be dose-
dependent. The leaves treated with Zm Pep1 displayed increased defense gene
expression with increasing amounts of Zm Pep1 peptide application (Huffaker et al.
2011 ). These results suggest that the expression of defense signaling depends on the
applied dosage of the PAMP.
Dosage required to trigger the plant immune response may differ among differ-
ent PAMPs. At the same concentration (1 M), fl g22 is more potent than elf18 to

induce the oxidative burst in Arabidopsis leaves (Zipfel et al. 2006 ) and elf26 pep-
tides from Agrobacterium tumefaciens and Erwinia amylovora are 50 times more
potent to induce alkalinization of Arabidopsis cell culture than elf26 from P . syringae
pv. tomato DC3000 (Kunze et al. 2004 ).

μ

70

60

50

40

30

20

10

0

200 mM 40 mM 8 mM 1.6 mM

Dose of Flg22 in mM

Fig. 2.5 Effect of different concentrations of Flg22 in inducing expression of the transcription
factor WRKY40 in Arabidopsis (Adapted from Denoux et al. 2008 )