Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 221

Several TGA factors were reported to be essential activators of PR-1 expression
(Zhang et al. 2003 ; Kesarwani et al. 2007 ; Blanco et al. 2009 ). In addition to TGA,
the Myb protein has also been shown to bind to the PR-1a promoter in tobacco,
where the Myb1 protein is preferentially bound to the MBSII sequence in the PR-
1a promoter (Yang and Klessig 1996 ). The tomato Pti4 protein, which functions as
a transcription factor, specifically binds the GCC-box cis element, which is present
in the promoter region of many pathogenesis-related (PR) genes. The expression of
the Pti4 gene in tomato leaves was rapidly induced by ethylene and by infection
with Pseudomonas syringaepv.tomato, and this induction preceded the expression
of GCC-box-containing PR genes. Although SA also induced Pti4 gene expres-
sion, it did not induce GCC-box PR genes. In fact, SA antagonized the ethylene-
mediated expression of GCC-box PR genes (Gu et al. 2000 ).
The expression of the WRKY genes was strongly induced by TMV infection
(Ülker and Somssich 2004 ). Like that of PR-1a, the expression of the NtWRKY12
gene was strongly induced by SA treatment. Accumulating evidence indicate that
WRKY proteins are involved in differential responses to biotic stresses, either as
transcriptional activators or as repressors, and WRKY proteins bind to the W box
(TTGAC[C/T]) in promoters of various pathogen-responsive genes.This is in close
proximity to binding sites in the PR-1a promoter for transcription factors TGA1a
(as-1 box) and Myb1 (MBSII box) (van Verk et al. 2008 ). The results indicated
that NtWRKY12 acts synergistically with TGA1a in the SA-mediated and path-
ogen-associated molecular pattern (PAMP)-mediated expression of the PR-1a
gene. It was also demonstrated that signal transduction via the MAPK cascade
MEKK1-MKK4/MKK5-MPK3/MPK6 leads to the activation of downstream
WRKY22 and WRKY29 (Asai et al. 2002 ), while another MAPK cascade
(MEKK1-MEK1/MKK2-MAPK4), induced by challenge inoculation with Ps.
syringae or treatment with flg22 (a peptide corresponding to the most conserved
domain of flagellin), leads to the phosphorylation of MAPK substrate 1 (MKS1),
through which WRKY33 and possibly WRKY25 are bound to MAPK4. Upon
phosphorylation of MKS1, WRKY33 is released in the nucleus to initiate the
positive regulation of JA-induced defence genes and the negative regulation of
SA-related defence genes (Pandey and Somssich 2009 ).
Multiple forms of cytochrome P450-dependent monooxygenases catalyse the
in-chain hydroxylation, end-terminal hydroxylation and epoxidation of medium-
and long-chain fatty acids. In plants, fatty acid hydroxylases are particularly
important in the synthesis of plant cuticles and signalling molecules derived from
fatty acids. Some members of the Arabidopsis thaliana CYP86A and CYP94B
cytochrome P450 monooxygenase subfamilies have been functionally defined as
being fatty acid omega-hydroxylases. Due to this activity, these and other fatty
acid hydroxylases have a potential role in the synthesis of cutin, the production of
signalling molecules, and the prevention of the accumulation of toxic levels of free
fatty acids. The constitutive and stress-inducible patterns of the five Arabidopsis
CYP86A subfamily members have now been defined (Duan and Schuler 2005 ).
The differences observed in inducible expression suggest that, in addition to their
roles in normal growth and development, each of these P450 s has a particular role