Showing papers on "Pyrabactin published in 2007"

Identification of Two Protein Kinases Required for Abscisic Acid Regulation of Seed Germination, Root Growth, and Gene Expression in Arabidopsis

Abstract: Abscisic acid (ABA) is an important phytohormone regulating various plant processes, including seed germination. Although phosphorylation has been suggested to be important, the protein kinases required for ABA signaling during seed germination and seedling growth remain elusive. Here, we show that two protein kinases, SNF1-RELATED PROTEIN KINASE2.2 (SnRK2.2) and SnRK2.3, control responses to ABA in seed germination, dormancy, and seedling growth in Arabidopsis thaliana. A snrk2.2 snrk2.3 double mutant, but not snrk2.2 or snrk2.3 single mutants, showed strong ABA-insensitive phenotypes in seed germination and root growth inhibition. Changes in seed dormancy and ABA-induced Pro accumulation consistent with ABA insensitivity were also observed. The snrk2.2 snrk2.3 double mutant had a greatly reduced level of a 42-kD kinase activity capable of phosphorylating peptides from ABF (for ABA Response Element Binding Factor) transcription factors. ABA-induced expression of several genes whose promoters contain an ABA response element (ABRE) was reduced in snrk2.2 snrk2.3, suggesting that the mechanism of SnRK2.2 and SnRK2.3 action in ABA signaling involves the activation of ABRE-driven gene expression through the phosphorylation of ABFs. Together, these results demonstrate that SnRK2.2 and SnRK2.3 are redundant but key protein kinases that mediate a major part of ABA signaling in Arabidopsis.

A G Protein-Coupled Receptor Is a Plasma Membrane Receptor for the Plant Hormone Abscisic Acid

Abstract: The plant hormone abscisic acid (ABA) regulates many physiological and developmental processes in plants. The mechanism of ABA perception at the cell surface is not understood. Here, we report that a G protein-coupled receptor genetically and physically interacts with the G protein alpha subunit GPA1 to mediate all known ABA responses in Arabidopsis. Overexpressing this receptor results in an ABA-hypersensitive phenotype. This receptor binds ABA with high affinity at physiological concentration with expected kinetics and stereospecificity. The binding of ABA to the receptor leads to the dissociation of the receptor-GPA1 complex in yeast. Our results demonstrate that this G protein-coupled receptor is a plasma membrane ABA receptor.

ABA-Hypersensitive Germination1 encodes a protein phosphatase 2C, an essential component of abscisic acid signaling in Arabidopsis seed.

Abstract: The phytohormone abscisic acid (ABA) regulates physiologically important stress and developmental responses in plants. To reveal the mechanism of response to ABA, we isolated several novel ABA-hypersensitive Arabidopsis thaliana mutants, named ahg (ABA-hypersensitive germination). ahg1-1 mutants showed hypersensitivity to ABA, NaCl, KCl, mannitol, glucose and sucrose during germination and post-germination growth, but did not display any significant phenotypes in adult plants. ahg1-1 seeds accumulated slightly more ABA before stratification and showed increased seed dormancy. Map-based cloning of AHG1 revealed that ahg1-1 has a nonsense mutation in a gene encoding a novel protein phosphatase 2C (PP2C). We previously showed that the ahg3-1 mutant has a point mutation in the AtPP2CA gene, which encodes another PP2C that has a major role in the ABA response in seeds (Yoshida et al., 2006b). The levels of AHG1 mRNA were higher in dry seeds and increased during late seed maturation--an expression pattern similar to that of ABI5. Transcriptome analysis revealed that, in ABA-treated germinating seeds, many seed-specific genes and ABA-inducible genes were highly expressed in ahg1-1 and ahg3-1 mutants compared with the wild-type. Detailed analysis suggested differences between the functions of AHG1 and AHG3. Dozens of genes were expressed more strongly in the ahg1-1 mutant than in ahg3-1. Promoter-GUS analyses demonstrated both overlapping and distinct expression patterns in seed. In addition, the ahg1-1 ahg3-1 double mutant was more hypersensitive than either monogenic mutant. These results suggest that AHG1 has specific functions in seed development and germination, shared partly with AHG3.

Abscisic Acid (ABA)

Abstract: Abscisic acid (ABA) is a phytohormone that mediates responses to abiotic stress conditions and controls stomatal aperture. It is also involved in seed development and is associated with seed dormancy. ABA is biosynthesised in higher plants from carotenoid precursors and early steps take place in the chloroplast. ABA binds to PYRABACTIN RESISTANCE (PYR)/PYR1-like (PYL) receptor proteins that are localised in the cytosol and nucleus. The ABA–receptor complex binds to and inhibits specific PP2C protein phosphatases. Inhibition of these PP2Cs results in activation of SnRK2 protein kinases that in turn activate downstream signalling proteins resulting in both short-term changes in ion-channel activity and long-term effects on gene expression. ABA effects depend on influx and efflux carriers that facilitate movement between cells and through conductive tissue to coordinate responses in shoots and roots. Key Concepts ABA reduces water use and causes adaptive responses to water stress. One of the most important effects of ABA is to reduce stomatal aperture. ABA acts by binding to PYR/PYL/RCAR receptors located in the cytoplasm and nucleus. ABA causes rapid changes in ion-channel activity and longer term changes in transcription factor activity leading to changes in gene expression. Movement of ABA between cells and tissues is regulated by influx and efflux carriers on cell surfaces. ABA is synthesised from carotenoid precursors and early steps in the pathway occur in the chloroplast. Keywords: abiotic stress; drought; stomata; germination; dormancy; 9-cis-epoxycarotenoid dioxygenase; phytohormone