sponse to the signals that arise upon pathogen attack. It is conceivable that this class of ciselements may
reside in the promoters of other PR genes whose expression is regulated similarly to PR-1.
B. The as-1–Type Elements Are Found in the Stress-Related
GlutathioneS-Transferase Gene PromotersGlutathioneS-transferases (GSTs) are enzymes catalyzing the conjugation of glutathione to a broad range
of hydrophobic, electrophilic metabolites and xenobiotics such as herbicides [17,18]. This class of en-
zymes plays important roles in protecting plants from oxidative stress, xenobiotic toxicity, and heavy
metal toxicity. In addition, GSTs serve as ligandins for nonenzymatic binding and intracellular transport
of auxins and antimicrobial compounds (for review, see Ref. 19). GSTs are encoded by a large gene fam-
ily in higher plants and their expression is strongly activated in response to stress. Most, if not all, GST
genes whose promoters have been analyzed possess one or more as-1–type elements in their promoter.
These include tobacco GNT1andGNT35[4,20];parA,parB, and parC[6,21–24]; soybean GH2/4(7);
wheatGstA1(8); and Arabidopsis GST6[25,26]. The inducibility of the GST gene promoters by various
stress conditions and stress-related signal molecules is tightly correlated with the as-1–type elements
within their promoters. All the evidence accumulated so far for GSTs demonstrates that as-1–type ele-
ments also play very important roles in activating defense genes against abiotic stresses. It is predicted
that other abiotic stress-related genes possess as-1–type elements in their promoters.
C. An as-1Element Is Found in the Promoter ofArabidopsis GSH1
Encoding the Rate-Limiting Enzyme, -Glutamylcysteine
Synthetase, for Glutathione BiosynthesisThe tripeptide glutathione (GSH) is enzymatically synthesized by two ATP-dependent reactions cat-
alyzed by -glutamylcysteine synthetase encoded by GSH1[27] and GSH synthetase encoded by GSH2
[28], respectively. GSH is a major antioxidant in higher plants and plays a pivotal role in protecting plants
from oxidative stress through the ascorbate-GSH cycle [29], xenobiotics and cytotoxic metabolites
through GST-catalyzed conjugation [19], and heavy metals through phytochelatins [30]. We have demon-
strated that all the genes involved in GSH synthesis and recycling (GSH disulfide reduction by GSH re-
ductase encoded by GR) are coordinately up-regulated in response to heavy metals and jasmonic acid (JA)
treatments [31]. Analysis of the GSH1promoter revealed that the transcription up-regulation by heavy
metals and JA is mediated by an as-1element. Removal of this element rendered the promoter nonre-
sponsive to heavy metals and JA (C. Xiang and D.J. Oliver, unpublished results). The as-1element in the
GSH1promoter provides further evidence that as-1–type elements are directly involved in the activation
of the genes against abiotic stress. It is conceivable that as-1–type elements may reside in the promoters
of other genes involved in GSH synthesis and recycling because of their coordinated expression.
D. The as-1–Type Elements Are Found in the Promoter of TGA bZIP
Transcription FactorsTGA factors are a subclass of bZIP transcription factors that specifically interact with and activate as-
1 –type elements (see later). The as-1–type elements are found in the promoter of the genes encoding TGA
bZIP transcription factors. Multiple as-1elements were found in the promoter of TGA3 [32] and TGA6
(C. Xiang and E. Lam, unpublished results). The promoter of tobacco TGA1a also contains several such
elements [33]. This suggests that TGA bZIP transcription factors may autoregulate their own expression
and play critical roles in regulating the expression of stress-related genes in higher plants.
E. The as-1–Type Elements Are Found in Other Defense-Related
Plant Gene PromotersLipoxygenases are a group of enzymes involved in the synthesis of stress signal molecules such as ab-
scisic acid, traumatic acid, and JA [34]. A short region responsive to JA was identified in the promoter of
barley lipoxygenase 1 gene [11]. Within this region, a 36-bp fragment contains inverted repeats of the
TGACG motif of the as-1element. Mutations within the motif abolished JA-responsive expression, there-
ACTIVATION SEQUENCE-1 COGNATE PROMOTER ELEMENTS 529