Handbook of Plant and Crop Physiology

(Steven Felgate) #1

parB,parC, and GNT35) are also highly responsive to JA, SA, and 2,4-D. The induction of gene expres-
sion by auxins was also reported for a soybean GST [44] and tobacco GSTs [5,22,24,43,45]. Activated
transcription is responsible for the induction of the expression of these genes as demonstrated using acti-
nomycin D, an inhibitor for RNA polymerase II, as shown in Figure 4. Actinomycin D treatment com-
pletely abolishes the accumulation of the transcripts.
Overwhelming evidence has demonstrated that as-1–type elements are responsive to the signals orig-
inated from diverse stress-related stimuli. These stress-related stimuli include auxin and xenobiotics, JA,
SA, heavy metals, H 2 O 2 and oxidative stress, and biotic and abiotic environmental stresses that may elicit
the production of stress signal molecules such as JA, SA, and H 2 O 2.
It has been well documented that xenobiotics such as herbicides and the synthetic auxins 2,4-D and
-naphthaleneacetic acid (NAA) induce the expression of GST genes whose products are directly in-
volved in the detoxification of these hydrophobic and electrophilic organic compounds (19). Auxins act
as phytohormones at low physiological concentrations, and normally their concentration is under tight
control. When present above physiological concentrations, plant cells may sense them as cytotoxins that
have to be detoxified. These compounds are structurally diverse. The only chemical similarity shared
among these compounds is that they all contain or are able to form, through metabolism, a “Michael ac-
ceptor” (carbon-carbon double bonds adjacent to an electrophilic group) [46]. This feature may provide
the common ground by which these structurally diverse chemicals activate the same suite of genes. It can
be speculated that a component(s) in the signal transduction pathway leading to the expression of these


ACTIVATION SEQUENCE-1 COGNATE PROMOTER ELEMENTS 531


Figure 3 Northern blot analysis of steady-state transcript levels of GST genes in response to JA, SA, and 2,4-
D treatments in tobacco seedlings. One-week-old tobacco plants grown on MS agar plates were treated with
100 M JA, SA, or 2,4-D for 6 hr. Total RNA was isolated and analyzed as previously described [31] using
cDNAs for parA, ParB, parC, and GNT35and 18S rRNA for equal loading.


Figure 4 GST genes are transcriptionally activated. Two-week-old Arabidopsisplants grown in liquid cul-
ture were pretreated with 0.5 mM actinomycin D for 1 hr before 100 M JA or Cd was added. The coincuba-
tion of actinomycin D with either JA or Cd was continued for indicated time periods (Act. D/JA and Act.
D/CdCl 2 ). Treatments with actinomycin D, JA, and CdCl 2 , respectively, were controls. Total RNA was isolated
and analyzed as previously described [31] using cDNA parA. The EtBr-stained RNA gel is shown for equal
loading.
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