Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 220

5.2 Action Mechanisms at the Gene Expression Level
SA has long been known to induce both local resistance to TMV and the accumu-
lation of pathogenesis-related (PR) proteins. Furthermore, SA treatment induces the
expression of the same set of genes as TMV and the high-level expression of these
genes correlates well with the onset of a resistant state (Ward et al. 1991 ). Several
proteins, members of different classes of PRs, were identified,each having different
functions.These included b-1,3-glucanase, chitinase, proteinase-inhibitor, endo-
proteinase, peroxidase, thaumatin-like, ribonuclease-like, defensin, thionin and
lipid-transfer protein (Van Loon and Van Strien 1999). The Arabidopsis PR-1 and
tobacco PR-1a promoters, which are used as model systems to study SA-induced
transcriptional regulation, each contain an activator sequence-1(as-1)-like element
that is important for SA-inducible gene expression (Lebel et al. 1998 ; Strompen
et al. 1998 ). Studies on the expression of PR genes also revealed the central role of
protein non-expressor of PR genes1 (NPR1). NPR1 interacts with TGA transcrip-
tion factors (a subgroup of the basic leucine zipper transcription factor family) that
can bind to the as-1-like elements of PR-1 genes (Durrant and Dong 2004 ; Katagiri
et al. 1989 ). The basic model of SA action is that SA accumulation stimulates the
translocation of NPR1 into the nucleus, where it interacts with members of the TGA
transcription factor family and enhances the binding of these factors to SA response
elements in the promoters of PR genes, thus ultimately affecting the transcription of
numerous genes in the SAR pathway. SA can modulate the redox state of TGA1
(reduction of its disulphide bridges) while NPR1 interacts specifically with the
reduced form of TGA1 (Després et al. 2003 ; Echardt 2003 ; Kesarwani et al. 2007 ).
Apart from the role of NPR in regulating SAR in the nucleus, the cytosolic function
of NPR1 in cross-communication between SA- and JA-dependent defence signal-
ling pathways has also been identified (Pieterse and Van Loon 2004 ). The existence
of crosstalk between gibberellins and SA signalling in Arabidopsis seeds was
suggested, because GA treatment or the overexpression of a GA-responsive gene
were able to increase not only endogenous levels of SA, but also the expression of
the ICS1 and NPR1genes (Alonso-Ramírez et al. 2009 ). Thus, NPR1 plays multiple
roles in regulating SA-mediated defence, acting as a positive SA signal transducer
and a positive and a negative regulator of SA accumulation (Wang et al. 2011 ).
A negative feedback loop was demonstrated, which involves NPR1 and regulates
SA accumulation, as SA accumulation after inoculattion with pathogen was higher
in npr1Arabidopsis mutants, than in wild plants (Lu 2009 ). The expression of
SID2 was also greater in pathogen-inoculated npr1 plants than in wild plants
(Wildermuth et al. 2001 ).
The expressions of OsBIERF1 to OsBIERF4 [Oryza sativa benzothiadiazole
(BTH)-induced ethylene responsive transcriptional factor (ERF)] rice genes were
analysed in response to rice diseases and under various abiotic stress conditions.
It was found that OsBIERF3 was able to bind specifically to the GCC box
sequence, while the expression of OsBIERF1, OsBIERF3 and OsBIERF4 was
induced by treatment with BTH and salicylic acid (Cao et al. 2006 ).