PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 112

Small RNAs are generated from dsRNA precursors by the ribonuclease III
enzyme Dicer (Qu et al. 2008 ). Four paralogs of Dicer (Dicer-like, DCLs) have been
detected in Arabidopsis. DCL1 excises miRNAs from intergenic stem-loop tran-
scripts to promote cleavage of cellular transcripts carrying miRNA-complementary
sequences (Bartel 2004 ). DCL2 produces viral-derived siRNAs (Xie et al. 2004 )
and siRNAs from antisense overlapping transcripts (Borsani et al. 2005 ). DCL3
generates DNA repeat-associated siRNAs (Xie et al. 2004 ), whereas DCL4 synthe-
sizes trans-acting siRNAs and mediates RNA interference (Dunoyer et al. 2005 ; Xie
et al. 2005 ; Howell et al. 2007 ).
The generated small RNAs are subsequently incorporated into RNA-induced
silencing complexes (RISCs). The functions of RISCs are carried out in large part by
the activity of RNase H-like Argonaute (AGO) proteins (Voinnnet 2009; Jaubert et al.
2011 ). Once produced, the miRNAs and siRNAs are recruited by the AGO proteins
into RISCs to direct the cleavage or translational repression of homologous mRNAs
(Baulcombe 2004 ; Dunoyer et al. 2010 ). These small RNAs bind to Argonaute nucle-
ases and form base paired structures with their RNA targets. In many instances the
target RNA is simply degraded, presumably by exonucleases. However, some of the
targeted molecules, especially those that interact with two different small RNAs are
degraded through a more complex mechanism. The targeted RNA is fi rst copied into
dsRNA by an independent RNA polymerase and is then cleaved into siRNAs by a
Dicer nuclease. The secondary siRNAs are able to guide Argonaute nucleases in deg-
radation of RNA targets (Fagard et al. 2000 ; Narry Kim 2005 ; Szittya et al. 2008 ).
RNA silencing operated through the production of small RNAs is an important
antiviral plant immune system (Voinnet 2001 ; Szittya et al. 2008 ; Garcia-Ruiz et al.
2010 ). Certain endogenous small RNAs in plants, including miRNAs and siRNAs,
are induced or repressed in response to bacterial and fungal pathogen attack and
subsequently regulate the expression of genes involved in disease resistance and
defense responses by mediating transcriptional or post-transcriptional gene silenc-
ing (Katiyar-Agarwal et al. 2006 , 2007 ; Agorio and Vera 2007 ; Jin 2008 ; Navarro
et al. 2008 ; Li et al. 2010 ). Small RNA signaling system has been shown to be
involved in PAMP-triggered immune responses. In Arabidopsis , fl g22 triggered
rapid changes in transcript levels, including down-regulation of a gene subset,
potentially by posttranscriptional mechanisms (Navarro et al. 2004 ). The posttran-
scriptional mechanism involves RNA silencing, a sequence-specifi c mRNA degra-
dation process mediated by small RNAs. It suggests that the PAMP-triggered
down-regulation of genes depends on small RNA-mediated RNA silencing system.

2.32.2 Flg22 Triggers Accumulation of miRNAs, Which

Cleave and Down-Regulate Auxin Signaling Genes

Flg22 induced a two-fold increase in microRNA (miR393) accumulation in
Arabidopsis seedlings. The up-regulation of miR393 by fl g22 resulted from enhanced
transcription of At-miR393a gene. Flg22 elicited a subset of mRNAs, including TIR1