VI. SIGNALS ORIGINATED FROM DIVERSE STRESS-RELATED
STIMULI CONVERGE ON A SINGLE CISELEMENTHow signals originated from diverse stress-related stimuli converge on a single ciselement is a very im-
portant and interesting biological question. Answering this question should contribute tremendously to
our understanding of the fundamental process of transcriptional activation and signal transduction path-
ways that biotic and abiotic stresses use to regulate the genes of defense systems.
A simplified scheme of signal transduction pathways leading to the activation of as-1–type elements
is illustrated in Figure 5. In this scheme, the production of endogenous stress signal molecules (JA, SA,
and H 2 O 2 ) is elicited upon pathogen attack, wounding by insect feeding, or adverse abiotic stress condi-
tions. These signals are amplified and transduced through corresponding signal transduction pathways
leading to the TGA factors bound to the as-1–type elements and finally activate their transcription in the
nucleus. Auxins and xenobiotics may directly activate a signaling component through the conjugation by
its Michael acceptor as discussed before.
Unfortunately, little is known about the signal transduction pathways. The genetic approach has been
playing a significant role in elucidating these cellular processes. Several JA-responsive mutants were iso-
lated in Arabidopsis[63–65]. Only one signaling component in the JA signaling pathway has been isolated
by position cloning of the coi 1locus. The predicted amino acid sequence of the COI1 has similarity to the
F-box proteins that appear to function by targeting repressor proteins for removal by ubiquitination [66].
Significant progress has been made in the field of plant-microbe interaction concerning the SA sig-
naling pathways. SAR induction requires the signal molecule SA. The isolation and characterization of
Arabidopsismutants nonresponsive to the inducers of SAR [67] in NPR1(also known as NIM1) started
off a series of exciting new discoveries in this field. NPR1has been cloned and shown to contain ankyrin
repeat domains known for protein-protein interaction [68,69]. To understand the biological function of
NPR1, Zhang et al. [15] performed a yeast two-hybrid screen using NPR1 as bait and isolated NPR-1–in-
teracting proteins that turned out to be TGA bZIP transcription factors. Interestingly, all three TGA fac-
tors AHBP-1B (TGA2), OBF5 (TGA5), and TGA6 with strong interaction with NPR1 fall into the same
subgroup as mentioned earlier. These results suggest that NPR1 regulates PR-1gene expression by inter-
ACTIVATION SEQUENCE-1 COGNATE PROMOTER ELEMENTS 533
Figure 5 A simplified model for the signal transduction pathways that originate from diverse stress-related
stimuli and eventually converge on a single ciselement. To illustrate the complexity of the signal transduction
pathways that originate from diverse external stress-related stimuli (e.g., pathogen attack, wounding and insect
feeding, abiotic stresses, and exposure to heavy metals and organic xenobiotics), a single nos-1element is
shown to drive the expression of the reporter gene uidAand to be the ultimate signal recipient. The nos-1ele-
ment may be bound by two TGA dimers with which NPR1 and/or OBP-1 can interact. As a result, the complex
assembled on the nos-1element is able to interact positively with the transcription initiation complex and acti-
vate transcription. However, how signals are transduced to NPR1 and OBP-1 remains largely unknown.