PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 111

In general, small RNAs are grouped into two major classes: microRNAs (miRNAS)
(Carthew and Sontheimer 2009; Katiyar-Agarwal and Jin 2010 ; Cuperus et al. 2010;
Kulcheski et al. 2011), and small-interfering RNAs (siRNAs) (Llave et al. 2002;
Song and Joshua-Tor 2006; Chellappan et al. 2010 ; Chen et al. 2010b ; Dunoyer
et al. 2010 ; Katiyar-Agarwal and Jin 2010 ). Small RNAs are classifi ed into miRNAs
and siRNAs based on their precursor structures and biogenesis pathways. The miR-
NAs are originated from hairpin-folded single-stranded RNAs transcribed from
miRNA genes (Bartel 2004 ; Mallory and Vaucheret 2006 ), while siRNAs are pro-
duced usually from long double-stranded RNAs (dsRNAs) (Hamilton et al. 2002 ;
Narry Kim 2005 ; Chapman and Carrington 2007 ).
Small RNAs are involved in a variety of phenomena that are essential for genome
stability, development, and adaptive responses in biotic and abiotic stresses (Mallory
and Vaucheret 2006 ; Vaucheret 2006 ; Chen 2009; Kulcheski et al. 2011). Small
RNA molecules act as mobile signals that direct mRNA cleavage and DNA meth-
ylation in recipient cells (Ciomperlik et al. 2011 ; Molnar et al. 2011). They are key
regulators of gene expression that guide both transcriptional and post-transcriptional
silencing mechanisms in eukaryotes (Kulcheski et al. 2011). They function by guiding
sequence-specifi c gene silencing at the transcriptional and/or post-transcriptional
level (Vaucheret et al. 2006 ; Chellappan et al. 2010 ; Havecker et al. 2010). They are
also big contributors to plant innate immunity (Chellappan et al. 2009 ; Dunoyer
et al. 2010 ; Mosher et al. 2010 ; Molnar et al. 2011).
RNA silencing refers to a number of related cellular processes that employ the small
RNAs to regulate the expression of genetic material in a sequence-specifi c manner (Qu
et al. 2008 ; Jaubert et al. 2011 ; Zhao et al. 2012). RNA silencing is a conserved mecha-
nism in plants that plays a role in various biological processes including regulation of
gene expression. RNA silencing also plays a role in genome stability and protects plants
against invading nucleic acids such as transgenes and viruses (Ellendorff et al. 2009 ).
RNA silencing is a type of plant immune system conferring resistance against viruses
and also against bacteria and fungi (Voinnet 2001 ; Katiyar-Agarwal and Jin 2010 ; Zhang
et al. 2011 ). Plants use RNA silencing as a surveillance mechanism to protect against
viral (Mlotshwa et al. 2002 ; Garcia-Ruiz et al. 2010 ), bacterial (Katiyar-Agarwal et al.
2006 ; Navarro et al. 2006 , 2008 ; Agorio and Vera 2007 ; Jin 2008 ; Li et al. 2010 ; Zhang
et al. 2011 ), and fungal pathogens (Lu et al. 2007 ; Ellendorff et al. 2009 ).
The important feature of RNA silencing is its ability to spread from cell to cell
(Chitwood and Timmermans 2010). RNA silencing is a non-cell-autonomous pro-
cess; it spreads both to neighboring cells and systemically over long distances
(Dunoyer et al. 2010 ). A RNA silencing signal has been shown to move through
plasmodesmata and the phloem (Molnar et al. 2010). Both exogenous and endoge-
nous siRNAs, as opposed to their precursor molecules, act as mobile silencing sig-
nals between plant cells (Dunoyer et al. 2010 ). The silencing signal may involve
siRNA duplexes, and not Argonaute1 (AGO1) - bound siRNA single strands. The
small RNA signaling system has been shown to play important role in host defense
responses against viral, bacterial, and fungal pathogens. As a counter-defense,
pathogens encode specifi c proteins that function as suppressors of small RNA-directed
RNA silencing (Alvarado and Scholthof 2009 ; Lewsey et al. 2010 ; Xie et al. 2010 ;
Burgyán and Havelda 2011 ; Shimura and Pantaleo 2011).