Abstract
A bZIP transcription factor ABI5 is involved in ABA signaling during seed maturation and germination. Amino-acid substitution of putative target sites for Ser/Thr protein kinases resulted in suppression of ABA-dependent transactivation. The ABI5 polypeptide was phosphorylated by ABA-activated SnRK2 protein kinases, SRK2D, SRK2E, and SRK2I. These genes were expressed differentially during seed maturation and germination. A srk2d/srk2e/srk2i triple mutant showed stronger in ABA insensitive phenotypes in seed germination than srk2d/srk2i double mutant, but not srk2d or srk2i single mutants. Changes in seed dormancy consistent with ABA insensitivity were observed. Viviparous phenotype was also observed in the triple mutant under high humidity conditions. Microarray experiments indicate that many genes down-regulated in the abi5 seeds were suppressed in the srk2d/srk2e/srk2i seeds. These results demonstrate that three redundant SnRK2 protein kinases, SRK2D, SRK2E, and SRK2I, have important roles in ABA signaling during seed maturation and germination of Arabidopsis.
