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We also show that ABI5, ABI3, ABA2 and ABA3 are important positive regu-
lators of ABA-induced WRKY2 accumulation (Figure 7). The expression levels of
WRKY2 were elevated more drastically in the wild type than in abi5-1 and abi3-1
mutants after ABA treatment (Figure 7). This result indicates that these two genes
are positive regulators of ABA-induced WRKY2 accumulation.
Genes encoding the enzymes for most of the steps of the ABA biosynthesis
pathway have been cloned and their functions confirmed using ABA-deficient
mutants for ABA1 [46], ABA2 [47,48], ABA3 [44,49,50] and ABA4 [51,52]. We
analyzed the effect of ABA biosynthesis genes on WRKY2 transcripts using ABA-
deficient aba2-3 and aba3-1 mutants [43,44]. We shows that elevated expres-
sion of WRKY2 after ABA treatment requires ABA2 and ABA3, indicating that
these two genes are positive regulators of ABA-induced WRKY2 accumulation
(Figure 7).
Several studies have shown that ABI5 binds to the ABA-responsive DNA ele-
ments (ABREs) with an ACGT core in the promoter of Em1 and Em6, and acti-
vates their expression [15,16]. On the other hand, other reports have shown that
wild oat WRKY proteins (ABF1 and ABF2) bind to the box2/W-box of the GA-
regulated
α
-Amy2 promoter [41], and GaWRKY1 highly activated the CAD1-A
promoter by binding to W-box [34], and a barley WRKY gene, HvWRKY38,
and its rice (Oryza sativa) ortholog, OsWRKY71 act as a transcriptional repres-
sor of gibberellin-responsive genes in aleurone cells [42]. The promoter zone of
WRKY2 has an ABA-responsive DNA element (CACGTGGC) containing an
ACGT core, and the promoter zone of ABI5 has 6 W-box, whereas the promoter
of ABI3 has 2 W-box. This raises the possibility that WRKY2, ABI3 and ABI5 are
mutually regulated.
Conclusion
Transcription factors ABI5 is an important regulator of postgermination develop-
mental arrest mediated by ABA. Postgermination proteolytic degradation of the
essential ABI5 transcription factor is interrupted by perception of an increase in
ABA concentration, leading to ABI5 accumulation and reactivation of embryonic
genes. Here we report that wrky2-1 and wrky2-2 mutants are more sensitive to
ABA responses than the wild type during seed germination and postgermination
early seedling establishment. ABA hypersensitivity of the wrky2 mutants is attrib-
utable to elevated mRNA levels of ABI5, ABI3 and ABI5-induced Em1 and Em6
in the mutants. WRKY2-mediated ABA responses are independent of miR159
and its target genes MYB33 and MYB101. ABA-induced WRKY2 accumula-
tion during germination and postgermination early growth requires ABI5, ABI3,
ABA2 and ABA3, suggesting that they are important regulators of the transcripts
