PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 114

2.32.4 Importance of miRNA-Directed RNA Silencing

Pathway in PAMP-Triggered Immunity (PTI)

The Argonaute proteins AGO1 and AGO7 and the Dicer-like protein DCL1 have
been shown to be involved in PTI responses induced by fl g22 (Li et al. 2010 ). ago1
and dcl1 mutants are compromised in PTI responses and fl g22-induced disease
resistance, indicating that overall AGO1 and DCL1 positively regulate PTI (Li et al.
2010 ). The ago1 and dcl1 mutants showed defects in one or more of the late
responses induced by PAMPs. However, these mutants displayed normal MAPK
activation and transient oxidative burst, the events that occur less than 5 min after
fl g22 treatment (Li et al. 2010 ). It suggests that the PAMP-induced miRNA- medi-
ated defense gene expression and callose deposition occur independent of MAPK
activation and oxidative burst (Li et al. 2010 ).
The transgenic plants overexpressing miR160a exhibited enhanced callose depo-
sition, suggesting that the PAMP triggers innate immune system by activating accu-
mulation of specifi c miRNA (Li et al. 2010 ). The importance of miRNA pathway
in PAMP-triggered immunity was assessed by employing miRNA-defi cient
Arabidopsis mutants (Navarro et al. 2008 ). The miRNA-defi cient mutants sustained
growth of non-pathogenic Pseudomonas fl uorescens and Escherichia coli strains.
These mutants also restored growth of a type III secretion-defective mutant of
Pseudomonas syringae . Some P . syringae effectors suppress transcriptional activa-
tion of some PAMP-responsive miRNAs, miRNA biogenesis, stability or activity
(Navarro et al. 2008 ). These results suggest that bacterial pathogens have evolved to
suppress PAMP-triggered RNA silencing to cause disease.

2.32.5 Small RNAs May Also Be Involved

in Effector-Triggered Immunity (ETI)

It has also been reported that inoculation of plants with incompatible strains
P . syringae pv. tomato DC3000 ( avrRpm1 ) and DC3000 ( avrRpt2 ) but not the com-
patible strain DC3000 represses miR398 levels (Jagadeeswaran et al. 2009 ). It sug-
gests that miR398 may also be involved in effector-triggered immunity (ETI). One
of the long siRNAs (IsiRNAs), AtlsiRNA-1, is specifi cally induced by the bacterial
pathogen Pseudomonas syringae pv. tomato carrying effector avrRpt2 (Katiyar-
Agarwal et al. 2007 ). P . syringae pv. tomato (avrRpt2)-mediated induction of
AtlsiRNA-1 specifi cally targets the AtRAP gene, which encodes a RNA-binding
protein containing a putative RNA-binding RAP domain (Katiyar-Agarwal et al.
2007 ; Katiyar-Agarwal and Jin 2010 ). Induction of AtlsiRNA-1 leads to down-reg-
ulation of its target, AtRAP mRNA. It silences AtRAP , which encodes a RAP-
domain protein involved in disease resistance (Katiyar-Agarwal et al. 2007 ). AtRAP
is a negative regulator of both PAMP-triggered immunity (PTI) and effector-triggered