acting with TGA factors that bind to the as-1cognate element in the PR-1gene promoter. Indeed, NPR1
stimulates the binding of TGA factors to their binding sites in vitro [62]. It appears that NPR1 preferen-
tially interacts with TGA factors [70]. These exciting results were further confirmed and extended by the
work of Despres et al. [62]. It was found that NPR1 can also interact with TGA3 but not TGA1 or TGA4
(OBF4). This result coincides with the subgroups within this gene family, suggesting that NPR1 has pref-
erence for TGA factors. It should be noted that NPR1 is not the only protein factor that interacts with TGA
factors. An OBF-binding factor, OBP1, has been cloned by Zhang et al. [71] OBP-1 itself is a sequence-
specific DNA-binding protein that enhances the binding of OBF to ocselements. Li et al. [72] identified
and cloned a negative regulator of SAR, SN1, in a screen for npr1-1suppressor, indicating that the in-
duction of SAR involves activation and derepression. A new component of the SA signal transduction
pathway was also characterized [73].
It is interesting to speculate that the biological significance of the preferential interaction may play a
role in selecting target genes in response to different stress signals. It is even more attractive if consider-
ing the variant TGA factor binding sites in the promoters of target genes and the distinct DNA binding
properties of different TGA factors together with the preferential interaction of NPR1 with TGA factors.
It has been observed that the expression of abiotic stress-related genes such as GSTs was also altered in
npr1mutants in response to stress signal molecules other than SA. More interestingly, there are clear dif-
ferences in the expression of these genes in different npr1alleles (unpublished results). These observa-
tions indicate that NPR1 may be a converging point of different signaling pathways and/or a channel for
cross talks between signaling pathways.
VII. PERSPECTIVES
The signal perception and transduction processes in plant defense systems are very complex and still re-
main largely unknown. The interconnectedness among different pathways makes it even more compli-
cated (for review, see Refs. 74, 75). There is no doubt that more mutants are required for dissecting these
pathways and functional genomic tools should also prove extremely useful in analyzing the global regu-
lation by stress signals as well as identifying target genes of a particular signaling pathway.
To dissect stress signaling pathways completely, new strategies for screening for more mutants have
to be designed. A single as-1–type elements–driven reporter gene system has greater advantages in that
it responds to various stress signals and avoids the complications of using a full-length promoter. A large-
scale mutagenesis in the genetic background shown in Figure 2 has been carried out, and many putative
H 2 O 2 -responsive mutants have been isolated. Screens for JA-, SA-, and 2,4-D–responsive mutants are be-
ing conducted. Characterization of these mutants should reveal more signaling components and shed new
light on this exciting research field.
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