Biology Reference
In-Depth Information
Discussion
The ability of exogenous ABA to arrest postgermination development has been
used extensively to identify genes involved in ABA signaling [45]. In the present
study, we discovered that wrky2-1 and wrky2-2 mutants were more sensitive than
wild type to ABA responses during seed germination and postgermination early
growth.
The wrky2 mutants are hypersensitive to ABA responses only within a short
development window during seed germination and early seedling growth. Dur-
ing seed germination and early growth, the transcription factors ABI3 and ABI5
are known to be important regulators of ABA-dependent growth arrest, and their
expression defines a narrow developmental checkpoint following germination
[14,15]. ABI5 is a key player in ABA-triggered postgermination growth arrest,
and the abi5 mutant seeds are insensitive to growth arrest by ABA, whereas seeds
of ABI5-overexpressing lines are hypersensitive to ABA [5]. ABI3 acts upstream of
ABI5 and is an important regulator of germination and postgermination growth
by ABA [15]. ABA-induced ABI5 also occupies the promoter of Em1 and Em6
and activates these two late embryogenesis-abundant genes [15,16]. Lopez-Mo-
lina et al. (2001) have shown that application of 5 µM ABA within 60 h post-
stratification in the wild type [5] (Ws) does not prevent germination, but arrest
the germinated embryos for several days, during which both ABI5 transcripts and
ABI5 proteins are detected at high levels. When applied outside the 60-h time
frame, however, ABA fails to arrest growth and prevent greening and ABI5 tran-
scripts and proteins are present at low levels [5]. Therefore, it is possible that the
ABA-induced growth-arrest of wrky2 mutants might be associated with the ex-
pression level of ABI5. To test this hypothesis, we compared wild type and wrky2
mutants for analyzed ABA-regulated expression levels of ABI5 and ABI5-related
important regulators during seed germination and postgermination development.
We found that wrky2 mutants had higher mRNA levels of ABI5, ABI3 and ABI5-
induced Em1 and Em6 than the wild type (Figure 6). The higher expression levels
of these genes in the wrky2 mutants may be partly responsible for the enhanced
growth arrest relative to that in the wild type in the presence of ABA (Figures 3
and 4).
With ABA treatment, miR159 accumulation requires ABI3 but is only par-
tially dependent on ABI5 [17]. MYB33 and MYB101, which are miR159 targets,
are positive regulators of ABA responses during germination [17]. Therefore we
examined whether WRKY2 affected the mRNA levels of these genes. We found
that the expression levels of these genes were not significantly different between
wrky2 mutants and the wild type (Figure 8). These observations indicate that the
response of WRKY2 to ABA during germination and early growth is independent
of miR159 and its target genes (MYB33 and MYB101).
