PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 113

(Transport Inhibitor Response 1) and two of its three functional paralogs, AFB2 and
AFB3 (auxin signaling F-box proteins 2 and 3). The F-box proteins TIR1, AFB2 and
AFB3 were specifi cally cleaved by miR393 in a DCL1-dependent manner. A two- to
threefold reduction in the levels of TIR1, AFB1, AFB2, and AFB3 was observed
30 min after fl g22 elicitation (Navarro et al. 2006 ). TIR 1 is part of the ubiquitin-
ligase complex SCF TIR1 that interacts with Aux/IAA proteins to promote their degra-
dation. Aux/IAA proteins repress auxin signaling through heterodimerization with
Auxin Response Factors (ARFs). These transcription factors ARFs bind to auxin-
responsive elements (AuxREs) in promoters of primary auxin response genes and
activate (or repress) transcription. The PAMP fl g22 triggered events that contributed
to rapid down-regulation of the primary auxin-response genes GH3-like , BDL/
IAA12 , and AXR3 / IAA17 (Navarro et al. 2006 ).
Transgenic Arabidopsis plants over expressing miR393a showed resistance to
the pathogen (Navarro et al. 2006 ). Augmenting auxin signaling through over-
expressing a TIR1 paralog that is partially refractory to miR393 enhanced suscepti-
bility to the bacterial pathogen, and conversely, repressing auxin signaling through
miR393 overexpression increased bacterial resistance. These results suggest that
down-regulation of auxin signaling, resulting in ARF inactivation, is part of a
PAMP-induced immune response (Navarro et al. 2006 ). It is known that auxin pro-
motes susceptibility to bacterial diseases (O'Donnell et al. 2003a , b ).
Li et al. ( 2010 ) showed fl g22-triggered accumulation of another microRNA
called miR160a besides miR393a already reported in Arabidopsis . Flg22 induces
miR160a accumulation and represses its target genes ARF16 and ARF17 (Li et al.
2010 ). ARF proteins bind auxin-responsive elements to activate or repress tran-
scription of primary auxin-response genes. Thus, multiple auxin pathway genes
may be regulated by miRNAs during PAMP-triggered innate immunity (PTI)
responses (Li et al. 2010 ).

2.32.3 Flg22 Suppresses Accumulation of Some miRNAs,

Which Have a Negative Role in PAMP-Triggered
Innate Immunity

Some of the AGO1-bound miRNAs play a negative role in PTI resistance,
although AGO1 overall positively regulates PTI resistance. Flg22 suppressed
miR398b and miR773 accumulation (Li et al. 2010 ). Consistent with this, fl g22
treatment enhanced the expression of their target genes COX5b.1 , CSD2 , and
MET1 . COX5b.1 is a mitochondrial cytochrome c oxidase, CSD2 is a copper and
zinc-containing superoxide dismutase enzyme that converts superoxide anion to
hydrogen peroxide and MET1 is a DNA methyltransferase. The miR398b and
miR773 overexpression plants were compromised in PTI defenses exemplifi ed
by reduced callose deposition and supported greater P . syringae pv. tomato
DC3000 and DC3000 hrcC - strains proliferation, indicating that miR398b and
miR773 negatively regulate plant disease resistance (Li et al. 2010 ).