ABI5 Interacts with Abscisic Acid Signaling Effectors in Rice Protoplasts

Abstract

Abscisic acid (ABA) regulates seed maturation, germination, and adaptation of vegetative tissues to environmental stresses. The mechanisms of ABA action and the specificity conferred by signaling components in overlapping pathways are not completely understood. The ABI5 gene (ABA insensitive 5) of Arabidopsis encodes a basic leucine zipper factor required for ABA response in the seed and vegetative tissues. Using transient gene expression in rice protoplasts, we provide evidence for the functional interactions of ABI5 with ABA signaling effectors VP1 (viviparous 1) and ABI1 (ABA insensitive 1). Co-transformation experiments with ABI5 cDNA constructs resulted in specific transactivation of the ABA-inducible wheat Em, Arabidopsis AtEm6, bean ␤-Phaseolin, and barley HVA1 and HVA22 promoters. Furthermore, ABI5 interacted synergistically with ABA and co-expressed VP1, indicating that ABI5 is involved in ABA-regulated transcription mediated by VP1. ABI5-mediated transactivation was inhibited by overexpression of abi1-1, the dominant-negative allele of the protein phosphatase ABI1, and by 1-butanol, a competitive inhibitor of phospholipase D involved in ABA signaling. Lanthanum, a trivalent ion that acts as an agonist of ABA signaling, potentiated ABI5 transactivation. These results demonstrate that ABI5 is a key target of a conserved ABA signaling pathway in plants.Abscisic acid (ABA) 1 is one of the major plant hormones and functions in regulation of seed maturation, germination, and adaptation of vegetative tissues to environmental stresses (1, 2). ABA acts to effect changes on multiple physiological processes such as inducing the rapid closure of stomatal pores to limit transpiration and by triggering slower changes in gene expression (see Refs. 3-5 for reviews). Although these disparate processes share genetic elements (some ABA mutants affect both processes) and signaling intermediates such as phospholipases, cADP-ribose, inositol 1,4,5-trisphosphate, and calcium ions (6 -9), these secondary messengers are not specific to ABA pathways. Our knowledge of separate yet overlapping ABA and stress signal transduction pathways is fragmentary.Genetic analyses (10, 11) of germination processes in Arabidopsis have resulted in map-based cloning of the ABA-insensitive genes, ABI1-5 (12-19). The ABI1 and ABI2 genes encode homologous type 2C protein Ser/Thr phosphatases (PP2Cs) with partially redundant but distinct tissue-specific negative regulator functions in the regulation of ABA-, cold-, or droughtinducible genes and ion channels (20 -24). The original mutant alleles, abi1-1 and abi2-1, are both missense mutations of a conserved Gly-to-Asp mutation (G180D in abi1-1 and G168D in abi2-1) that results in a dominant phenotype in vivo and reduced phosphatase activity in vitro. The substrates for ABAregulatory protein phosphatases 2C are not known (15,16, 25).The ABI3, ABI4, and ABI5 genes encode proteins belonging to three distinct classes of transcription factors: the basic B3 domain, APETALA2 domain, and the basic leucine ...

The Transcription Factors HvABI5 and HvVP1 Are Required for the Abscisic Acid Induction of Gene Expression in Barley Aleurone Cells

“…However, a synergistic effect has been observed with the bZIP and ABA, or with the bZIP, VP1, and ABA (Hobo et al, 1999;Gampala et al, 2002). Our data also agree with these observations.…”

Regulation and Role of the ArabidopsisAbscisic Acid-Insensitive 5Gene in Abscisic Acid, Sugar, and Stress Response

“…Observation of additional crossregulation among these ABI loci led to the suggestion that these transcription factors act combinatorially to regulate subsets of ABA-inducible and/or seed-specific genes. Subsequent studies demonstrated direct physical interactions between ABI5 and ABI3 or their rice (Oryza sativa) homologs in yeast (Hobo et al, 1999;Nakamura et al, 2001), as well as synergistic interactions between ABI5 and VP1 or their rice homologs in mediating ABAactivated gene expression in transient expression assays in plant protoplasts (Hobo et al, 1999;Gampala et al, 2002). ABI5 overexpression has been shown to resemble ABI3 overexpression in that it confers hypersensitivity to ABA inhibition of root growth and promotes slightly more efficient water retention (Parcy et al, 1994;Parcy and Giraudat, 1997;LopezMolina et al, 2001).…”

S Phase Progression Is Required for Transcriptional Activation of the β-Phaseolin Promoter

“…It has also been shown by yeast two-hybrid screening that ABI5 interacts with ABI3 (40). Moreover, in transient assays, ABI5 has been demonstrated to trans-activate AtEM6 and phas promoters (38). The decrease in MAT and LEA gene expression (Fig.…”

“…ABI5, a b-zip class transcription factor, is a positive regulatory ABA signal effector (37) shown in transient assays to trans-activate phas promoter-driven GUS expression (38). It interacts with abscisic acid-response elements (39) and with ABI3 (40), and its debilitation leads to a severe reduction in LEA proteins (37).…”