fore identifying this element as a JA-responsive element. The as-1–type elements are strongly responsive
to JA (see later). JA induces the expression of defense-related genes encoding the proteinase inhibitors
[35] phenylalanine ammonia lyase [36], lipoxygenases [37], a thionin with potent antifugal activity [38],
and a ribosomal-inactivating protein [39]. It is predictable that the as-1–type elements may reside in these
JA-responsive gene promoters.
The presence of as-1–type elements in the promoters of various defense-related genes indicates the
importance of this class of ciselements in plant defense mechanisms. It is now clear that as-1–type cog-
nate elements are one class of multiple stress-responsive elements that play critical roles in regulating the
expression of defense-related genes in response to biotic and abiotic stresses.
As the Arabidopsisgenome sequencing nears completion, it will soon be possible to locate all as-
1 –type elements in the whole genome of Arabidopsis. It is expected that most, if not all, defense-related
genes contain one or more such elements in their promoters. On the other hand, if a gene with unknown
functions contains as-1elements in its promoter, one may predict that the function of the gene is defense
related.
IV. THE as-1–TYPE ELEMENTS ARE STRONGLY RESPONSIVE TO
DIVERSE STRESS-RELATED STIMULITheas-1–type elements are responsive to a wide spectrum of stress-related stimuli. The as-1–type ele-
ments, when fused to a minimal CaMV 35S promoter, are responsive to stress stimuli in activating re-
porter gene expression [26,40–43]. Figure 2 shows that the nos-1element–drivenuidAreporter gene ex-
pression in transgenic Arabidopsisbehaves similarly to the endogenous GST6gene and is strongly
induced by JA, H 2 O 2 , the synthetic auxin 2,4-D, the heavy metal Cu, and SA. Interestingly, the PR-1gene
is also responsive to the synthetic auxin 2,4-D. Figure 3 shows that tobacco genes coding for GSTs (parA,
530 XIANG
Figure 2 Induction of the expression of nos-1element–driven reporter gene uidAand defense-related genes
withas-1–type elements in their promoter. (A) Illustration (not drawn to scale) of the GUS reporter gene (uidA)
under the control of the synthetic tetrameric nos-1elements (nos) and the minimal 35S promoter of CaMV.
Only the T-DNA region of this binary vector is shown. RB and LB stand for the right and left border sequences
of the T-DNA, Pnos for the promoter of nopaline synthase gene, hptfor hygromycin resistance gene, and 3C
and Ag7 for terminators. (B) Northern blot analysis of the steady-state transcript levels in response to JA, SA,
2,4-D, Cu, and H 2 O 2 treatments. Two-week-old Arabidopsisplants grown in liquid culture were treated with 5
mM H 2 O 2 or 100 M JA, SA, Cu, or 2,4-D for 6 hrs. Total RNA was isolated and analyzed as previously de-
scribed (31) using cDNAs for GST6andPR-1oruidADNA as probes. The ethidium bromide- (EtBr) stained
RNA gel is shown for equal loading.