Abstract
The transcription factors ABI3 and ABI5 have important roles for ABA responses in seed maturation and germination of Arabidopsis. In a transient experiment using Arabidopsis T87 protoplasts, the ABI3, ABI5, and AREB1proteins activated transcription of a GUS reporter gene driven by the rd29B containing ABREs, or the rd29A promoter containing DREs and an ABRE. In the 35S::ABI3 Arabidopsis plants, the expression level of rd29B was higher than that in control plants. On the other hand, the expression level of rd29B in the 35S::ABI5 Arabidopsis plants was similar to that in control plants, indicating the ABI5 might need a modification for activation. Amino-acid substitution (Ser/Thr to Ala) of putative target sites for CDPK in the conserved region of ABI5/AREB family resulted in suppression of ABA-dependent transactivation. The ABI5 fragment was phosphorylated by ABA-activated SnRK2 kinases. These results suggest that ABA-dependent phosphorylation regulates the ABI5 transactivation activities.
