The Abscisic Acid-Responsive Kinase PKABA1 Interacts with a Seed-Specific Abscisic Acid Response Element-Binding Factor, TaABF, and Phosphorylates TaABF Peptide Sequences

Abstract

The abscisic acid (ABA)-induced protein kinase PKABA1 is present in dormant seeds and is a component of the signal transduction pathway leading to ABA-suppressed gene expression in cereal grains. We have identified a member of the ABA response element-binding factor (ABF) family of basic leucine zipper transcription factors from wheat (Triticum aestivum) that is specifically bound by PKABA1. This protein (TaABF) has highest sequence similarity to the Arabidopsis ABA response protein ABI5. In two-hybrid assays TaABF bound only to PKABA1, but not to a mutant version of PKABA1 lacking the nucleotide binding domain, suggesting that binding of TaABF requires prior binding of ATP as would be expected for binding of a protein substrate by a protein kinase. TaABF mRNA accumulated together with PKABA1 mRNA during wheat grain maturation and dormancy acquisition and TaABF transcripts increased transiently during imbibition of dormant grains. In contrast to PKABA1 mRNA, TaABF mRNA is seed specific and did not accumulate in vegetative tissues in response to stress or ABA application. PKABA1 produced in transformed cell lines was able to phosphorylate synthetic peptides representing three specific regions of TaABF. These data suggest that TaABF may serve as a physiological substrate for PKABA1 in the ABA signal transduction pathway during grain maturation, dormancy expression, and ABA-suppressed gene expression.Abscisic acid (ABA) is required during seed development for the acquisition of desiccation tolerance and dormancy and plays an important role in mediating many plant responses to the environment (Busk and Pages, 1998). As ABA levels increase during the seed development program or in response to environmental stress, a number of ABA-induced genes are expressed. These include the LEA (late embryogenesis abundant) genes, which may serve to protect the developing seed from desiccation (Ried and Walker-Simmons, 1993), as well as genes such as KIN1, which encode proteins similar to antifreeze proteins (Wang and Cutler, 1995). In addition to stimulating the transcription of a suite of ABAinducible genes, ABA also suppresses the expression of GA-induced genes encoding ␣-amylase and Cys proteinase in aleurone layers of cereal grains (Bethke et al., 1997).Because many physiological responses to ABA are dependent upon ABA-mediated gene expression, the signal transduction pathway involved in this process has received considerable attention (Lovegrove and Hooley, 2000; Rock, 2000). Detailed studies of the promoters of ABA-induced genes have identified an ABA response complex that is necessary and sufficient for ABA-induced transcription (Shen et al., 1996). This complex consists of the ABA response element (ABRE) (T/C)ACGTGGC together with a coupling element (CE) containing the consensus sequence CGCGTG. A number of transcription factors involved in ABA-induced gene expression have also been identified. The VP1 protein is able to activate ABA-induced genes, and requires the presence of ABREs to do so. However, VP1 does not bind...

Abscisic acid-dependent multisite phosphorylation regulates the activity of a transcription activator AREB1

“…On the basis of recent reports that AREB1 and its homologues, such as ABI5, TRAB1, and TaABF, are phosphorylated in vitro or in vivo (7,(29)(30)(31), phosphorylation of AREB1 may be involved in the posttranscriptional modification. Because of the occurrence of highly conserved target sites for putative protein kinases in the conserved regions of AREB proteins, we analyzed ABA-dependent phosphorylation of these sites of AREB1 (Fig.…”

The Arabidopsis CDPK-SnRK Superfamily of Protein Kinases

“…An ABAresponse element binding factor (TaABF) has been shown by two-hybrid screening to specifically bind to PKABA1. TaABF is seed specific, suggesting the Ta-ABF may serve as a physiological substrate for PKABA1 in ABA-signaling processes in wheat seeds (Johnson et al, 2002). Expression of PKABA1 in Escherichia coli or Pichia pastoris yields inactive enzyme, however, the active form can be obtained by expression in insect cells (S.D.…”

The unique mode of action of a divergent member of the ABA-receptor protein family in ABA and stress signaling

“…SnRK2.2/2.3/2.6 are positive regulators of ABA signaling [17][18][19]. Activated SnRK2.2/2.3/2.6 can phosphorylate and activate transcription factors such as ABFs/ AREBs [20,21] to induce the expression of ABAresponsive genes such as RD29B [4]. In guard cells, the SnRK2s can also phosphorylate and activate the NADPH oxidase catalytic subunit RBOHF, leading to the production of reactive oxygen species (ROS) [17,22,23].…”