PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 103

Fig. 2.6 Relationship
between intensity of
expression of the PAMP
harpin gene and plant defense
genes (Adapted from Peng
et al. 2004 )

50

45

40

35

30

25

Tobacco line 7
Tobacco line 15

20

15

10

5

0

Harpin gene
expression

NPR1 gene
expression

Chinase gene
expression

2.26 Amount of PAMP Available in the Infection Court May

Determine the Level of Induction of Immune Responses

It has been suggested that the amount of PAMP available in the infection court
may determine the intensity of induced defense gene expression (Lecourieux
et al. 2002 ; Denoux et al. 2008 ). For example, transgenic tobacco plants express-
ing the hpaG Xoo gene encoding the PAMP harpin Xoo were obtained by Peng et al.
( 2004 ). The harpin gene expression varied in the transgenic lines. The line 7
showed highest expression of the harpin gene while the line 15 showed lowest
expression of the gene. The line, which showed highest PAMP expression, showed
greatest expression of the regulatory gene NPR1 and the defense gene Chia5
encoding chitinase. By contrast, the line 15, which showed lowest PAMP expres-
sion, showed lowest expression of the defense gene Chia5 and the regulatory gene
NPR1 (Fig. 2.6 ; Peng et al. 2004 ). High and low expression of other defense genes
PR-1a and PR-1b was also correlated with the high and low expression of the
PAMP gene in the lines 7 and 15, respectively. The line 7 showed highest disease
resistance, while the line 15 showed lowest disease resistance. These results
suggest a link between higher level of PAMP in plants and higher level of induction
of immune responses.

2.27 PAMPs May Trigger Different Signaling Systems

2.27.1 Some PAMPs May Activate Only Specifi c

Signaling Systems

The PAMPs may activate plant innate immune responses triggering different signal-
ing pathways. Each PAMP/elicitor may regulate distinctly different signaling
pathway(s). The harpin (HrpN) elicitor derived from the soft rot pathogen Erwinia
carotovora subsp. carotovora induced both SA and JA/ET signaling pathways,