A protein phosphatase 2C involved in ABA signal transduction in Arabidopsis thaliana

Ferulate-5-hydroxylase from Arabidopsis thaliana defines a new family of cytochrome P450-dependent monooxygenases

The fah1 mutant of Arabidopsis is defective in the accumulation of sinapic acid-derived metabolites, including the guaiacyl-syringyl lignin typical of angiosperms. Earlier results indicated that the FAH1 locus encodes ferulate-5-hydroxylase (F5H), a cytochrome P450-dependent monooxygenase (P450) of the general phenylpropanoid pathway. We have cloned the gene encoding this P450 by T-DNA tagging and have confirmed the identity of the cloned gene by complementation of the mutant phenotype. F5H shows 34% amino acid sequence identity with the avocado ripening-induced P450 CYP71A1 and 32% identity with the flavonoid-3',5'-hydroxylases of Petunia hybrida. In contrast, it shares much less homology with cinnamate-4-hydroxylase, a P450 that catalyzes the hydroxylation of cinnamic acid three steps earlier in the general phenylpropanoid pathway. Since the highest degree of identity between F5H and previously sequenced P450s is only 34%, F5H identifies a new P450 subfamily that has been designated CYP84.

The protein phosphatase 2C (PP2C) superfamily: detection of bacterial homologues

A thorough sequence analysis of the various members of the eukaryotic protein serine/threonine phosphatase 2C (PP2C) family revealed the conservation of 11 motifs. These motifs could be identified in numerous other sequences, including fungal adenylate cyclases that are predicted to contain a functionally active PP2C domain, and a family of prokaryotic serine/threonine phosphatases including SpoIIE. Phylogenetic analysis of all the proteins indicates a widespread sequence family for which a considerable number of isoenzymes can be inferred.

THE MOLECULAR BASIS OF DEHYDRATION TOLERANCE IN PLANTS

Molecular studies of drought stress in plants use a variety of strategies and include different species subjected to a wide range of water deficits. Initial research has by necessity been largely descriptive, and relevant genes have been identified either by reference to physiological evidence or by differential screening. A large number of genes with a potential role in drought tolerance have been described, and major themes in the molecular response have been established. Particular areas of importance are sugar metabolism and late-embryogenesis-abundant (LEA) proteins. Studies have begun to examine mechanisms that control the gene expression, and putative regulatory pathways have been established. Recent attempts to understand gene function have utilized transgenic plants. These efforts are of clear agronomic importance.

PLANT PROTEIN PHOSPHATASES

Posttranslational modification of proteins by phosphorylation is a universal mechanism for regulating diverse biological functions. Recognition that many cellular proteins are reversibly phosphorylated in response to external stimuli or intracellular signals has generated an ongoing interest in identifying and characterizing plant protein kinases and protein phosphatases that modulate the phosphorylation status of proteins. This review discusses recent advances in our understanding of the structure, regulation, and function of plant protein phosphatases. Three major classes of enzymes have been reported in plants that are homologues of the mammalian type-1, -2A, and -2C protein serine/threonine phosphatases. Molecular genetic and biochemical studies reveal a role for some of these enzymes in signal transduction, cell cycle progression, and hormonal regulation. Studies also point to the presence of additional phosphatases in plants that are unrelated to these major classes.

Molecular responses to drought and cold stress

A variety of plant genes are induced by drought and cold stress, and they are thought to be involved in the stress tolerance of the plant. At least five signal transduction pathways control these genes: two are dependent on abscisic acid (ABA), and the others are ABA-independent. A novel cis-acting element involved in one of the ABA-independent signal transduction pathways has been identified. In addition, a number of genes for protein kinases and transcription factors thought to be involved in the stress signal transduction cascades have been shown to be induced by environmental stresses.

Novel plant Ca(2+)-binding protein expressed in response to abscisic acid and osmotic stress

A cDNA corresponding to an mRNA which accumulates in germinating rice seeds in response to the phytohormone abscisic acid was isolated by differential hybridization. Northern blotting indicated that the mRNA also accumulates in vegetative tissues in response to treatment with abscisic acid and to osmotic stress. Sequencing identified a major open reading frame encoding a novel protein of 27.4 kDa. The identity of the open reading frame was confirmed by comparing the translation products of cellular, hybrid-selected, and in vitro transcribed RNAs and by immunoprecipitation. Western blotting of cellular extracts indicated that the protein is associated with microsomal or membrane fractions. Data base searches indicated that it contains a conserved Ca(2+)-binding, EF-hand motif and that related proteins are similarly expressed in Arabidopsis thaliana. A fusion protein purified from Escherichia coli containing the putative EF-hand region was shown to bind Ca2+ in blot binding assays. These data identify a novel gene family encoding proteins involved in the response of plants to abscisic acid and osmotic stress.

Strong regulation of slow anion channels and abscisic acid signaling in guard cells by phosphorylation and dephosphorylation events

Recent evidence suggests that slow anion channels in guard cells need to be activated to trigger stomatal closing and efficiently inactivated during stomatal opening. The patch-clamp technique was employed here to determine mechanisms that produce strong regulation of slow anion channels in guard cells. MgATP in guard cells, serving as a donor for phosphorylation, leads to strong activation of slow anion channels. Slow anion-channel activity was almost completely abolished by removal of cytosolic ATP or by the kinase inhibitors K-252a and H7. Nonhydrolyzable ATP, GTP, and guanosine 5'-[gamma-thio]triphosphate did not replace the ATP requirement for anion-channel activation. In addition, down-regulation of slow anion channels by ATP removal was inhibited by the phosphatase inhibitor okadaic acid. Stomatal closures in leaves induced by the plant hormone abscisic acid (ABA) and malate were abolished by kinase inhibitors and/or enhanced by okadaic acid. These data suggest that ABA signal transduction may proceed by activation of protein kinases and inhibition of an okadaic acid-sensitive phosphatase. This modulation of ABA-induced stomatal closing correlated to the large dynamic range for up- and down-regulation of slow anion channels by opposing phosphorylation and dephosphorylation events in guard cells. The presented opposing regulation by kinase and phosphatase modulators could provide important mechanisms for signal transduction by ABA and other stimuli during stomatal movements.

Sensitivity to abscisic acid of guard-cell K+ channels is suppressed by abi1-1, a mutant Arabidopsis gene encoding a putative protein phosphatase

Abscisic acid (ABA) modulates the activities of three major classes of ion channels--inward- and outward-rectifying K+ channels (IK,in and IK,out, respectively) and anion channels--at the guard-cell plasma membrane to achieve a net efflux of osmotica and stomatal closure. Disruption of ABA sensitivity in wilty abi1-1 mutants of Arabidopsis and evidence that this gene encodes a protein phosphatase suggest that protein (de)-phosphorylation contributes to guard-cell transport control by ABA. To pinpoint the role of ABI1, the abi1-1 dominant mutant allele was stably transformed into Nicotiana benthamiana and its influence on IK,in, IK,out, and the anion channels was monitored in guard cells under voltage clamp. Compared with guard cells from wild-type and vector-transformed control plants, expression of the abi1-1 gene was associated with 2- to 6-fold reductions in IK,out and an insensitivity of both IK,in and IK,out to 20 microM ABA. In contrast, no differences between control and abi1-1 transgenic plants were observed in the anion current or its response to ABA. Parallel measurements of intracellular pH (pHi) using the fluorescent dye 2',7'-bis(2-carboxyethyl)-5-(and -6)-carboxyfluorescein (BCECF) in every case showed a 0.15- to 0.2-pH-unit alkalinization in ABA, demonstrating that the transgene was without effect on the pHi signal that mediates in ABA-evoked K+ channel control. In guard cells from the abi1-1 transformants, normal sensitivity of both K+ channels to and stomatal closure in ABA was recovered in the presence of 100 microM H7 and 0.5 microM staurosporine, both broad-range protein kinase antagonists. These results demonstrate an aberrant K+ channel behavior--including channel insensitivity to ABA-dependent alkalinization of pHi--as a major consequence of abi1-1 action and implicate AB11 as part of a phosphatase/kinase pathway that modulates the sensitivity of guard-cell K+ channels to ABA-evoked signal cascades.

Serine/threonine protein phosphatases

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The C. elegans sex-determining gene fem-2 encodes a putative protein phosphatase

The genetic and molecular analysis of genes involved in the regulation of sex determination in Caenorhabditis elegans suggests that the gene fem-2 plays an important role in regulating a pathway transducing a non-cell-autonomous signal to a nuclear transcription factor. The wild-type fem-2 gene was cloned by identifying sequences from the C. elegans physical map that could restore normal Fem-2 function to homozygous mutant fem-2 transgenic animals. cDNA sequences mapping to the minimal rescuing region correspond to an open reading frame with a sequence similar to protein phosphatase 2C enzymes from systems as diverse as yeast, humans, and plants, but the alignments suggest that FEM-2 falls into a separate class of proteins than the canonical homologues. Several fem-2 mutant alleles were sequenced, and the mutations are predicted to cause protein changes consistent with their observed phenotypes, such as missense mutations in conditional alleles, and a nonsense mutation in a predicted null allele. This is the first evidence implicating phosphorylation and/or dephosphorylation as a control mechanism in C. elegans sex determination.

Role of phosphorylation in elicitation of the oxidative burst in cultured soybean cells

The oxidative burst is likely the most rapid defense response mounted by a plant under pathogen attack, and the generated oxidant species may be essential to several subsequent defense responses. In our effort to characterize the signal-transduction pathways leading to rapid H2O2/O2- biosynthesis, we have examined the role of protein phosphorylation in this resistance mechanism. K-252a and staurosporine, two protein-kinase inhibitors, were found to block the oxidative burst in a concentration-dependent manner. When added during H2O2 generation, the burst was observed to rapidly terminate, suggesting that continuous phosphorylation was essential for its maintenance. Importantly, phosphatase inhibitors (calyculin A and okadaic acid) were found to induce the oxidative burst in the absence of any additional stimulus. This may suggest that certain kinases required for the burst are constitutively active and that stabilization of the phosphorylated forms of their substrates is all that is required for burst activity. In autoradiographs of elicited and unstimulated cells equilibrated with 32PO4(3-), several phosphorylated polypeptide bands were revealed that could represent proteins essential for the burst.

Use of Arabidopsis thaliana defense-related mutants to dissect the plant response to pathogens

The plant defense response to microbial pathogens had been studied primarily by using biochemical and physiological techniques. Recently, several laboratories have developed a variety of pathosystems utilizing Arabidopsis thaliana as a model host so that genetic analysis could also be used to study plant defense responses. Utilizing a pathosystem that involves the infection of Arabidopsis with pathogenic pseudomonads, we have cloned the Arabidopsis disease-resistance gene RPS2, which corresponds to the avirulence gene avrRpt2 in a gene-for-gene relationship. RPS2 encodes a 105-kDa protein containing a leucine zipper, a nucleotide binding site, and 14 imperfect leucine-rich repeats. The RPS2 protein is remarkably similar to the product of the tobacco N gene, which confers resistance to tobacco mosaic virus. We have also isolated a series of Arabidopsis mutants that synthesize decreased levels of an Arabidopsis phytoalexin called camalexin. Analysis of these mutants indicated that camalexin does not play a significant role in limiting growth of avirulent Pseudomonas syringae strains during the hypersensitive defense response but that it may play a role in limiting the growth of virulent strains. More generally, we have shown that we can utilize Arabidopsis to systematically dissect the defense response by isolation and characterization of appropriate defense-related mutants.

Characterization of a cDNA encoding the 55 kDa B regulatory subunit of Arabidopsis protein phosphatase 2A

Type 2A serine/threonine protein phosphatases (PP2A) are key components in the regulation of signal transduction and control of cell metabolism. The activity of these protein phosphatases is modulated by regulatory subunits. While PP2A activity has been characterized in plants, little is known about its regulation. We used the polymerase chain reaction to amplify a segment of a cDNA encoding the B regulatory subunit of PP2A from Arabidopsis. The amplified DNA fragment of 372 nucleotides was used as a probe to screen an Arabidopsis cDNA library and a full-length clone (AtB alpha) of 2.1 kbp was isolated. The predicted protein encoded by AtB alpha is 43 to 46% identical and 53 to 56% similar to its yeast and mammalian counterparts, and contains three unique regions of amino acid insertions not present in the animal B regulatory subunit. Genomic Southern blots indicate the Arabidopsis genome contains at least two genes encoding the B regulatory subunit. In addition, other plant species also contain DNA sequences homologous to the B regulatory subunit, indicating that regulation of PP2A activity by the 55 kDa B regulatory subunit is probably ubiquitous in plants. Northern blots indicate the AtB alpha mRNA accumulates in all Arabidopsis tissues examined, suggesting the protein product of the AtB alpha gene performs a basic housekeeping function in plant cells.

Abscisic acid signaling

The plant hormone abscisic acid (ABA) regulates the development and germination of seeds, as well as the adaptation of vegetative tissues to conditions of environmental stress. During the past year, considerable insights have been gained into the molecular nature of the complex signaling network that mediates the actions of ABA. Biophysical studies indicate that at least some of the effects of ABA in stomatal guard cells involve intracellular receptors. Also, increasing evidence supports the view that guard cells contain redundant ABA transduction pathways, and that cytoplasmic Ca2+ acts as a second messenger in at least one of these pathways. Finally, mutational analysis in Arabidopsis indicates that the multiple effects of ABA at the whole plant level are mediated by overlapping branches of a highly ramified signaling network. Two Arabidopsis loci that determine ABA sensitivity have already been cloned and found to encode a protein phosphatase and a transcriptional activator.

Chromosome landing: a paradigm for map-based gene cloning in plants with large genomes

The original concept behind map-based or positional cloning was to find a DNA marker linked to a gene of interest, and then to 'walk' to the gene via overlapping clones (e.g. cosmids or YACs). While chromosome walking is straightforward in organisms with small genomes, it is difficult to apply in most plant species, which typically have large, complex genomes. The strategy of chromosome walking is based on the assumption that it is difficult and time consuming to find DNA markers that are physically close to a gene of interest. Recent technological developments invalidate this assumption for many species. As a result, the mapping paradigm has now changed such that one first isolates one or more DNA marker(s) at a physical distance from the targeted gene that is less than the average insert size of the genomic library being used for clone isolation. The DNA marker is then used to screen the library and isolate (or 'land' on) the clone containing the gene, without any need for chromosome walking and its associated problems. Chromosome landing, together with the technology that has made it possible, is likely to become the main strategy by which map-based cloning is applied to isolate both major genes and genes underlying quantitative traits in plant species.

Current advances in abscisic acid action and signalling

Abscisic acid (ABA) participates in the control of diverse physiological processes. The characterization of deficient mutants has clarified the ABA biosynthetic pathway in higher plants. Deficient mutants also lead to a revaluation of the extent of ABA action during seed development and in the response of vegetative tissues to environmental stress. Although ABA receptor(s) have not yet been identified, considerable progress has been recently made in the characterization of more downstream elements of the ABA regulatory network. ABA controls stomatal aperture by rapidly regulating identified ion transporters in guard cells, and the details of the underlying signalling pathways start to emerge. ABA actions in other cell types involve modifications of gene expression. The promoter analysis of ABA-responsive genes has revealed a diversity of cis-acting elements and a few associated trans-acting factors have been isolated. Finally, characterization of mutants defective in ABA responsiveness, and molecular cloning of the corresponding loci, has proven to be a powerful approach to dissect the molecular nature of ABA signalling cascades.

Mapping in plants: progress and prospects

Comprehensive genetic maps are now available for all of the world's important crop species. Data show a remarkable conservation of order of markers over family-wide taxonomic groupings and illuminate species relationships and mechanisms of genome evolution. Comparison of genetic and physical maps has revealed differences in genetic distance throughout genomes with implications for genome organization, gene isolation and transformation.

Interaction of a protein phosphatase with an Arabidopsis serine-threonine receptor kinase

A protein phosphatase was cloned that interacts with a serine-threonine receptor-like kinase, RLK5, from Arabidopsis thaliana. The phosphatase, designated KAPP (kinase-associated protein phosphatase), is composed of three domains: an amino-terminal signal anchor, a kinase interaction (KI) domain, and a type 2C protein phosphatase catalytic region. Association of RLK5 with the KI domain is dependent on phosphorylation of RLK5 and can be abolished by dephosphorylation. KAPP may function as a signaling component in a pathway involving RLK5.

Plant regulators. Insensitivity is in the genes

The cloning of loci determining abscisic acid insensitivity in Arabidopsis has identified a phosphatase and a transcriptional activator that mediate responses to abscisic acid and so regulate plant growth and development.

The A. thaliana disease resistance gene RPS2 encodes a protein containing a nucleotide-binding site and leucine-rich repeats

In plants, resistance to a pathogen is frequently correlated with a genetically defined interaction between a plant resistance gene and a corresponding pathogen avirulence gene. A simple model explains these gene-for-gene interactions: avirulence gene products generate signals (ligands), and resistance genes encode cognate receptors. The A. thaliana RPS2 gene confers resistance to the bacterial pathogen P. syringae carrying the avirulence gene avrRpt2. A map-based positional cloning strategy was used to identify RPS2. The identification of RPS2 was verified using a newly developed transient assay for RPS2 function and by genetic complementation in transgenic plants. RPS2 encodes a novel 105 kDa protein containing a leucine zipper, a nucleotide-binding site, and 14 imperfect leucine-rich repeats.