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The translation of AP2 mRNA is suppressed by micro RNAs (Chen, 2004) and AP2 protein down-regulates AP2 and AtEBP expression (Okamuro et al., 1997; this study), as indicated in grey. In the current study, it was demonstrated that AP2 expression was regulated through AtEBP and EIN2, and that AtEBP may contribute to floral development, as indicated by in
black.
The AP2 mRNA level did not change in the ein3-1 mutant. The position of EIN3 is a branch of the ethylene signalling pathway under EIN2. It is known that the sensitivity
of ein3 mutants to ethylene is weaker than ein2 mutants (Wang et al., 2002). Previous studies reported that both EIN3-dependent and independent pathways exist downstream of EIN2 (Binder et al., 2004; Seifert et al., 2004). Furthermore, AtEBP expression is independently regulated under EIN3 in ethylene signalling (Ogawa et al., 2005; this study). In this study, AP2 was not induced by ethylene despite increasing expression of AtEBP. We suggest that these signal transductions compete with one another.
AtEBP knockout plants exhibited a weak floral phenotype with a lower number of stamens. An evaluation was also made of AtERF1 and AP2 expression in AtEBP knockout plants having the same level of WT (data not shown). The ctr1 mutants showed an earlier-maturing phenotype in the gyneocium compared with the flower, and ein mutants affect the maturation of the gyneocium (Kieber et al., 1993). Interestingly, the ant mutants show a similar phenotype to the AtEBP knockout plants (Elliott et al., 1996; Klucher et al., 1996). ANT is a member of the AP2/ERF family containing the AP2/EREBP domains.
Over-expression of AtEBP caused up-regulation of AP2 in leaves. Despite the accumulation of AP2 mRNA in transgenic Arabidopsis plants over-expressing AtEBP, no abnormal flowers were observed. Chen (2004) reported that micro RNAs control transcriptional regulation of AP2 expression. That is, most transgenic Arabidopsis plants over-expressing AP2 had normal flowers and only a fraction exhibited the agamous-like phenotype. However, over-expression of AP2 mutated at the target site of micro RNAs demonstrated a more severe floral phenotype. Accumulation of AP2 protein was detected
only in transgenic plants over-expressing mutated AP2, not in normal AP2. Thus, we consider that accumulation of AP2 mRNA in Arabidopsis over-expressing AtEBP is not sufficient to change flower development.
This study has shown the mutual relationships between AP2 and AtEBP. AtEBP and functional EIN2 affected the transcriptional regulation of AP2. AtEBP contributed slightly to flower development, especially stamen development. Future reports in this series will focus
on the homeotic role of AtEBP.
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