Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 236

transcription factors bind tandem repeats of a cis -element within the promoters
called activation sequence-1 ( as-1 ), which contains a TGACG motif (Lebel et al.
1998 ; Després et al. 2000 ). Several TGA transcription factors have been shown to
regulate expression of defense-related genes (Kesarwani et al. 2007 ). This family of
transcription factors recognizes the TGACG/ as-1 elements found in the promoters
of a variety of plant genes, including those regulating the expression of Arabidopsis
and tobacco PR-1 and the Caulifl ower mosaic virus 35S promoter (Lebel et al.
1998 ; Kim and Delaney 2002 ). The as-1 elements are responsible for SA respon-
siveness of these promoters. SA treatment increases the TGACG/ as-1 binding activ-
ity. Thus, the TGA transcription factors may play an important role in SA signaling
system (Zhou et al. 2000 ). Several SA-responsive genes are regulated by bZIP tran-
scription factors of TGA family (Ndamukong et al. 2007 ). Some of the bZIP tran-
scription factors, such as TGA2 and TGA5 in Arabidopsis , interact with NPR1 and
recognize the as-1 cis element found within the promoter of several PR genes (Kim
and Delaney 2002 ).
Transgenic plants overexpressing different TGA transcription factor genes
have been generated to develop disease resistant plants (Kim and Delaney 2002 ;
Fitzgerald et al. 2005 ). Transgenic Arabidopsis plants containing sense or anti-
sense TGA5 gene constructs were developed by Kim and Delaney ( 2002 ). None
of the TGA5 sense lines showed an apparent increase in TGA5 transcript levels
compared to wild-type plants, whereas the TGA5 -antisense lines showed a large
increase in TGA5 transcript accumulation. Increased TGA5 accumulation in anti-
sense lines may be due to negative autoregulation of the TGA5 gene (Kim and
Delaney 2002 ). The transgenic TGA5 - antisense lines showed reduced induction
of SA-mediated expression of PR-1 gene by the oomycete pathogen
Hyaloperonospora parasitica . The transgenic antisense lines showed enhanced
resistance to H. parasitica (Kim and Delaney 2002 ). The induced resistance by
TGA5 to the pathogen has been suggested to act independent of SA signaling
system.
The rice TGA factor, rTGA2.1, has been shown to bind to defense gene promot-
ers (Chern et al. 2001 ). It binds to oligonucleotides containing the as-1 like elements
from the PR-1 gene promoter and to the promoter of the rice chitinase gene, RCH10
(Chern et al. 2001 ). It appears that rTGA2.1 negatively regulates a subset of rice
defense genes (Fitzgerald et al. 2005 ). Transgenic rice plants that have the endoge-
nous rTGA2.1 transcripts silenced via dsRNA-mediated silencing (Sl) were also
generated. The loss of rTGA2.1 activity in the Sl lines resulted in reduced disease
symptom development (Fitzgerald et al. 2005 ).
Several TGA proteins have been shown as CaM-binding proteins. TGA3, a
member of a family of basic leucine zipper (bZIP) transcription factors, has been
identifi ed as a CaM binding protein that binds the promoter of CaM3 (Jakoby
et al. 2002 ). Eighteen bZIP family members have been identifi ed as CaM binding
proteins in Arabidopsis (Popescu et al. 2007 ). An abscisic acid (ABA) - respon-
sive bZIP transcription factor, ABF2, has been shown to bind CaM (Popescu
et al. 2007 ).