Cloning of cDNAs from Arabidopsis thaliana that encode putative protein phosphatase 2C and a human Dr1-like protein by transformation of a fission yeast mutant

TaDrAp1 and TaDrAp2, Partner Genes of a Transcription Repressor, Coordinate Plant Development and Drought Tolerance in Spelt and Bread Wheat

Down-regulator associated protein, DrAp1, acts as a negative cofactor (NC2α) in a transcription repressor complex together with another subunit, down-regulator Dr1 (NC2β). In binding to promotors and regulating the initiation of transcription of various genes, DrAp1 plays a key role in plant transition to flowering and ultimately in seed production. TaDrAp1 and TaDrAp2 genes were identified, and their expression and genetic polymorphism were studied using bioinformatics, qPCR analyses, a 40K Single nucleotide polymorphism (SNP) microarray, and Amplifluor-like SNP genotyping in cultivars of bread wheat (Triticum aestivum L.) and breeding lines developed from a cross between spelt (T. spelta L.) and bread wheat. TaDrAp1 was highly expressed under non-stressed conditions, and at flowering, TaDrAp1 expression was negatively correlated with yield capacity. TaDrAp2 showed a consistently low level of mRNA production. Drought caused changes in the expression of both TaDrAp1 and TaDrAp2 genes in opposite directions, effectively increasing expression in lower yielding cultivars. The microarray 40K SNP assay and Amplifluor-like SNP marker, revealed clear scores and allele discriminations for TaDrAp1 and TaDrAp2 and TaRht-B1 genes. Alleles of two particular homeologs, TaDrAp1-B4 and TaDrAp2-B1, co-segregated with grain yield in nine selected breeding lines. This indicated an important regulatory role for both TaDrAp1 and TaDrAp2 genes in plant growth, ontogenesis, and drought tolerance in bread and spelt wheat.

The General Transcription Repressor TaDr1 Is Co-expressed With TaVrn1 and TaFT1 in Bread Wheat Under Drought

The general transcription repressor, TaDr1 gene, was identified during screening of a wheat SNP database using the Amplifluor-like SNP marker KATU-W62. Together with two genes described earlier, TaDr1A and TaDr1B, they represent a set of three homeologous genes in the wheat genome. Under drought, the total expression profiles of all three genes varied between different bread wheat cultivars. Plants of four high-yielding cultivars exposed to drought showed a 2.0-2.4-fold increase in TaDr1 expression compared to controls. Less strong, but significant 1.3-1.8-fold up-regulation of the TaDr1 transcript levels was observed in four low-yielding cultivars. TaVrn1 and TaFT1, which controls the transition to flowering, revealed similar profiles of expression as TaDr1. Expression levels of all three genes were in good correlation with grain yields of evaluated cultivars growing in the field under water-limited conditions. The results could indicate the involvement of all three genes in the same regulatory pathway, where the general transcription repressor TaDr1 may control expression of TaVrn1 and TaFT1 and, consequently, flowering time. The strength of these genes expression can lead to phenological changes that affect plant productivity and hence explain differences in the adaptation of the examined wheat cultivars to the dry environment of Northern and Central Kazakhstan. The Amplifluor-like SNP marker KATU-W62 used in this work can be applied to the identification of wheat cultivars differing in alleles at the TaDr1 locus and in screening hybrids.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

The role of ABA and MAPK signaling pathways in plant abiotic stress responses

As sessile organisms, plants have developed specific mechanisms that allow them to rapidly perceive and respond to stresses in the environment. Among the evolutionarily conserved pathways, the ABA (abscisic acid) signaling pathway has been identified as a central regulator of abiotic stress response in plants, triggering major changes in gene expression and adaptive physiological responses. ABA induces protein kinases of the SnRK family to mediate a number of its responses. Recently, MAPK (mitogen activated protein kinase) cascades have also been shown to be implicated in ABA signaling. Therefore, besides discussing the role of ABA in abiotic stress signaling, we will also summarize the evidence for a role of MAPKs in the context of abiotic stress and ABA signaling.

Molecular basis of the core regulatory network in ABA responses: sensing, signaling and transport

ABA is a major phytohormone that regulates a broad range of plant traits and is especially important for adaptation to environmental conditions. Our understanding of the molecular basis of ABA responses in plants improved dramatically in 2009 and 2010, banner years for ABA research. There are three major components; PYR/PYL/ RCAR (an ABA receptor), type 2C protein phosphatase (PP2C; a negative regulator) and SNF1-related protein kinase 2 (SnRK2; a positive regulator), and they offer a double negative regulatory system, [PYR/PYL/RCAR-| PP2C-| SnRK2]. In the absence of ABA, PP2C inactivates SnRK2 by direct dephosphorylation. In response to environmental or developmental cues, ABA promotes the interaction of PYR/PYL/RCAR and PP2C, resulting in PP2C inhibition and SnRK2 activation. This signaling complex can work in both the nucleus and cytosol, as it has been shown that SnRK2 phosphorylates basic-domain leucine zipper (bZIP) transcription factors or membrane proteins. Several structural analyses of PYR/PYL/RCAR have provided the mechanistic basis for this 'core signaling' model, by elucidating the mechanism of ABA binding of receptors, or the 'gate-latch-lock' mechanism of interaction with PP2C in inhibiting activity. On the other hand, intercellular ABA transport had remained a major issue, as had intracellular ABA signaling. Recently, two plasma membrane-type ABC transporters were identified and shed light on the influx/efflux system of ABA, resolving how ABA is transported from cell to cell in plants. Our knowledge of ABA responses in plants has been greatly expanded from intracellular signaling to intercellular transport of ABA.

Ectopic expression of a rice protein phosphatase 2C gene OsBIPP2C2 in tobacco improves disease resistance

Protein phosphatase 2Cs (PP2Cs) have been demonstrated to play critical roles in regulation of plant growth/development, abscisic acid signaling pathway and adaptation to environmental stresses. Here we report the cloning and molecular characterization of a novel rice protein phosphatase 2C gene, OsBIPP2C2 (Oryza sativa L. BTH-induced protein phosphatase 2C 2). OsBIPP2C2 has three alternatively spliced transcripts and the largest transcript OsBIPP2C2a encodes a 380 aa protein containing all 11 conserved catalytic subdomains of PP2Cs. Expression of OsBIPP2C2a was significantly induced by benzothiadiazole (BTH), one of defense-related signal molecules in plants. Expression of OsBIP2C2a was induced by infection with the blast fungus, Magnaporthe grisea, and the pathogen-induced expression of OsBIPP2C2a in BTH-treated rice seedlings was much earlier and stronger than those in water-treated seedlings. Overexpression of OsBIPP2C2a in transgenic tobacco plants resulted in increased disease resistance against tobacco mosaic virus and Phytophthora parasitica var. nicotianae. Importantly, the OsBIPP2C2a-overexpressing transgenic tobacco plants showed constitutive expression of defense-related genes. These results suggest that OsBIPP2C2a may play an important role in disease resistance through activation of defense response.

Microarray analysis of transcript accumulation during perithecium development in the filamentous fungus Gibberella zeae (anamorph Fusarium graminearum)

Gibberella zeae (anamorph Fusarium graminearum) is the causal agent of Fusarium head blight (FHB) of wheat and barley in the United States. Ascospores forcibly discharged from mature fruiting bodies, the perithecia, serve as the primary inoculum for FHB epidemics. To identify genes important for perithecium development and function, a cDNA microarray that covered 11% of the G. zeae genome was constructed. The microarray was used to measure changes in transcription levels of genes expressed during three successive stages of perithecium development. When compared with vegetative mycelia, 651 (31%) cDNA clones showed changes in transcript levels in at least one of the three developmental stages. During perithecium development, 263 (13%) cDNA clones showed temporal changes in transcript profiles. Transcripts that showed the greatest changes in levels in maturing perithecia belonged to genes in the FunCat main functional categories of cell rescue, metabolism, cell type differentiation, energy, and cellular transport. For genes related to metabolism and cell type differentiation, transcripts showed the highest levels in immature perithecia, whereas for cellular transport-related genes, transcripts showed the highest levels in mature perithecia. This study represents the first large-scale investigation of both spatial and temporal changes in transcript levels during perithecium development. It provides clear evidence that the sexual development in fungi is a complex, multigenic process and identifies genes involved in sexual development of this agriculturally important fungus.

ABA-hypersensitive germination3 encodes a protein phosphatase 2C (AtPP2CA) that strongly regulates abscisic acid signaling during germination among Arabidopsis protein phosphatase 2Cs

The phytohormone abscisic acid (ABA) regulates physiologically important developmental processes and stress responses. Previously, we reported on Arabidopsis (Arabidopsis thaliana) L. Heynh. ahg mutants, which are hypersensitive to ABA during germination and early growth. Among them, ABA-hypersensitive germination3 (ahg3) showed the strongest ABA hypersensitivity. In this study, we found that the AHG3 gene is identical to AtPP2CA, which encodes a protein phosphatase 2C (PP2C). Although AtPP2CA has been reported to be involved in the ABA response on the basis of results obtained by reverse-genetics approaches, its physiological relevance in the ABA response has not been clarified yet. We demonstrate in vitro and in vivo that the ahg3-1 missense mutation causes the loss of PP2C activity, providing concrete confirmation that this PP2C functions as a negative regulator in ABA signaling. Furthermore, we compared the effects of disruption mutations of eight structurally related PP2C genes of Arabidopsis, including ABI1, ABI2, HAB1, and HAB2, and found that the disruptant mutant of AHG3/AtPP2CA had the strongest ABA hypersensitivity during germination, but it did not display any significant phenotypes in adult plants. Northern-blot analysis clearly showed that AHG3/AtPP2CA is the most active among those PP2C genes in seeds. These results suggest that AHG3/AtPP2CA plays a major role among PP2Cs in the ABA response in seeds and that the functions of those PP2Cs overlap, but their unique tissue- or development-specific expression confers distinct and indispensable physiological functions in the ABA response.

Negative regulation of abscisic acid signaling by the Fagus sylvatica FsPP2C1 plays a role in seed dormancy regulation and promotion of seed germination

FsPP2C1 was previously isolated from beech (Fagus sylvatica) seeds as a functional protein phosphatase type-2C (PP2C) with all the conserved features of these enzymes and high homology to ABI1, ABI2, and PP2CA, PP2Cs identified as negative regulators of ABA signaling. The expression of FsPP2C1 was induced upon abscisic acid (ABA) treatment and was also up-regulated during early weeks of stratification. Furthermore, this gene was specifically expressed in ABA-treated seeds and was hardly detectable in vegetative tissues. In this report, to provide genetic evidence on FsPP2C1 function in seed dormancy and germination, we used an overexpression approach in Arabidopsis because transgenic work is not feasible in beech. Constitutive expression of FsPP2C1 under the cauliflower mosaic virus 35S promoter confers ABA insensitivity in Arabidopsis seeds and, consequently, a reduced degree of seed dormancy. Additionally, transgenic 35S:FsPP2C1 plants are able to germinate under unfavorable conditions, as inhibitory concentrations of mannitol, NaCl, or paclobutrazol. In vegetative tissues, Arabidopsis FsPP2C1 transgenic plants show ABA-resistant early root growth and diminished induction of the ABA-response genes RAB18 and KIN2, but no effect on stomatal closure regulation. Seed and vegetative phenotypes of Arabidopsis 35S:FsPP2C1 plants suggest that FsPP2C1 negatively regulates ABA signaling. The ABA inducibility of FsPP2C1 expression, together with the transcript accumulation mainly in seeds, suggest that it could play an important role modulating ABA signaling in beechnuts through a negative feedback loop. Finally, we suggest that negative regulation of ABA signaling by FsPP2C1 is a factor contributing to promote the transition from seed dormancy to germination during early weeks of stratification.

A novel transcription factor involved in plant defense endowed with protein phosphatase activity

In plants, expression of a disease-resistance character following perception of a pathogen involves massive deployment of transcription-dependent defenses. Thus, if rapid and effective defense responses have to be achieved, it is crucial that the pathogenic signal is transduced and amplified through pre-existing signaling pathways. Reversible phosphorylation of specific transcription factors, by a concerted action of protein kinases and phosphatases, may represent a mechanism for rapid and flexible regulation of selective gene expression by environmental stimuli. Here we identified a novel DNA-binding protein from tobacco plants, designated DBP1, with protein phosphatase activity, which binds in a sequence-specific manner to a cis- acting element of a defense-related gene and participates in its transcriptional regulation. This finding helps delineate a terminal event in a signaling pathway for the selective activation of early transcription-dependent defense responses in plants, and suggests that stimulus-dependent reversible phosphorylation of regulatory proteins may occur directly in a transcription protein-DNA complex.

Stress-induced protein phosphatase 2C is a negative regulator of a mitogen-activated protein kinase

Protein phosphatases of type 2C (PP2Cs) play important roles in eukaryotic signal transduction. In contrast to other eukaryotes, plants such as Arabidopsis have an unusually large group of 69 different PP2C genes. At present, little is known about the functions and substrates of plant PP2Cs. We have previously shown that MP2C, a wound-induced alfalfa PP2C, is a negative regulator of mitogen-activated protein kinase (MAPK) pathways in yeast and plants. In this report, we provide evidence that alfalfa salt stress-inducible MAPK (SIMK) and stress-activated MAPK (SAMK) are activated by wounding and that MP2C is a MAPK phosphatase that directly inactivates SIMK but not the wound-activated MAPK, SAMK. SIMK is inactivated through threonine dephosphorylation of the pTEpY motif, which is essential for MAPK activity. Mutant analysis indicated that inactivation of SIMK depends on the catalytic activity of MP2C. A comparison of MP2C with two other PP2Cs, ABI2 and AtP2CHA, revealed that although all three phosphatases have similar activities toward casein as a substrate, only MP2C is able to dephosphorylate and inactivate SIMK. In agreement with the notion that MP2C interacts directly with SIMK, the MAPK was identified as an interacting partner of MP2C in a yeast two-hybrid screen. MP2C can be immunoprecipitated with SIMK in a complex in vivo and shows direct binding to SIMK in vitro in protein interaction assays. Wound-induced MP2C expression correlates with the time window when SIMK is inactivated, corroborating the notion that MP2C is involved in resetting the SIMK signaling pathway.

Protein phosphatases in plants

Phosphorylation and dephosphorylation of a protein often serve as an "on-and-off" switch in the regulation of cellular activities. Recent studies demonstrate the involvement of protein phosphorylation in almost all signaling pathways in plants. A significant portion of the sequenced Arabidopsis genome encodes protein kinases and protein phosphatases that catalyze reversible phosphorylation. For optimal regulation, kinases and phosphatases must strike a balance in any given cell. Only a very small fraction of the thousands of protein kinases and phosphatases in plants has been studied experimentally. Nevertheless, the available results have demonstrated critical functions for these enzymes in plant growth and development. While serine/threonine phosphorylation is widely accepted as a predominant modification of plant proteins, the function of tyrosine phosphorylation, desptie its overwhelming importance in animal systems, had been largely neglected until recently when tyrosine phosphatases (PTPs) were characterized from plants. This review focuses on the structure, regulation, and function of protein phosphatases in higher plants.

Physical and functional interaction of the Arabidopsis K(+) channel AKT2 and phosphatase AtPP2CA

The AKT2 K(+) channel is endowed with unique functional properties, being the only weak inward rectifier characterized to date in Arabidopsis. The gene is expressed widely, mainly in the phloem but also at lower levels in leaf epiderm, mesophyll, and guard cells. The AKT2 mRNA level is upregulated by abscisic acid. By screening a two-hybrid cDNA library, we isolated a protein phosphatase 2C (AtPP2CA) involved in abscisic acid signaling as a putative partner of AKT2. We further confirmed the interaction by in vitro binding studies. The expression of AtPP2CA (beta-glucuronidase reporter gene) displayed a pattern largely overlapping that of AKT2 and was upregulated by abscisic acid. Coexpression of AtPP2CA with AKT2 in COS cells and Xenopus laevis oocytes was found to induce both an inhibition of the AKT2 current and an increase of the channel inward rectification. Site-directed mutagenesis and pharmacological analysis revealed that this functional interaction involves AtPP2CA phosphatase activity. Regulation of AKT2 activity by AtPP2CA in planta could allow the control of K(+) transport and membrane polarization during stress situations.

Functional dissection of a Rice Dr1/DrAp1 transcriptional repression complex

We characterized rice cDNA sequences for OsDr1 and OsDrAp1, which encode structural homologs of the eukaryotic general repressors Dr1 and DrAp1, respectively. OsDr1 and OsDrAp1 are nuclear proteins that interact with each other and with the TATA binding protein/DNA complex. In vitro and in vivo functional analyses showed that OsDrAp1 functions as a repressor, unlike its role in other eukaryotic systems, in which DrAp1 is a corepressor. OsDr1 and OsDrAp1 functioned together as a much stronger repressor than either one alone. Functional dissections revealed that the N-terminal histone-fold domains of OsDr1 and OsDrAp1 were necessary and sufficient for their repression and protein-protein interaction with each other. The unique glutamine- and proline-rich domain of OsDr1 had no repression activity. The basic amino acid-rich region and an arginine and glycine repeat domain of OsDrAp1 enhanced its repression activity. Thus, although OsDr1 and OsDrAp1 function as repressors, the functions of the two components are reversed compared with those of their nonplant counterparts.

A new protein phosphatase 2C (FsPP2C1) induced by abscisic acid is specifically expressed in dormant beechnut seeds

An abscisic acid (ABA)-induced cDNA fragment encoding a putative protein phosphatase 2C (PP2C) was obtained by means of differential reverse transcriptase-polymerase chain reaction approach. The full-length clone was isolated from a cDNA library constructed using mRNA from ABA-treated beechnut (Fagus sylvatica) seeds. This clone presents all the features of plant type PP2C and exhibits homology to members of this family such as AthPP2CA (61%), ABI1 (48%), or ABI2 (47%), therefore it was named FsPP2C1. The expression of FsPP2C1 is detected in dormant seeds and increases after ABA treatment, when seeds are maintained dormant, but it decreases and tends to disappear when dormancy is being released by stratification or under gibberellic acid treatment. Moreover, drought stress seems to have no effect on FsPP2C1 transcript accumulation. The FsPP2C1 transcript expression is tissue specific and was found to accumulate in ABA-treated seeds rather than in other ABA-treated vegetative tissues examined. These results suggest that the corresponding protein could be related to ABA-induced seed dormancy. By expressing FsPP2C1 in Escherichia coli as a histidine tag fusion protein, we have obtained direct biochemical evidence supporting Mg2+-dependent phosphatase activity of this protein.

The ABI1 and ABI2 protein phosphatases 2C act in a negative feedback regulatory loop of the abscisic acid signalling pathway

The Arabidopsis ABI1 and ABI2 genes encode two protein serine/threonine phosphatases 2C (PP2C). These genes have been originally identified by the dominant mutations abi1--1 and abi2--1, which reduce the plant's responsiveness to the hormone abscisic acid (ABA). However, recessive mutants of ABI1 were recently shown to be supersensitive to ABA, which demonstrated that the ABI1 phosphatase is a negative regulator of ABA signalling. We report here the isolation and characterisation of the first reduction-of-function allele of ABI2, abi2--1R1. The in vitro phosphatase activity of the abi2--1R1 protein is approximately 100-fold lower than that of the wild-type ABI2 protein. Abi2--1R1 plants displayed a wild-type ABA sensitivity. However, doubly mutant plants combining the abi2--1R1 allele and a loss-of-function allele at the ABI1 locus were more responsive to ABA than each of the parental single mutants. These data indicate that the wild-type ABI2 phosphatase is a negative regulator of ABA signalling, and that the ABI1 and ABI2 phosphatases have overlapping roles in controlling ABA action. Measurements of PP2C activity in plant extracts showed that the phosphatase activity of ABI1 and ABI2 increases in response to ABA. These results suggest that ABI1 and ABI2 act in a negative feedback regulatory loop of the ABA signalling pathway.

Oxidative stress, heat shock and drought differentially affect expression of a tobacco protein phosphatase 2C

A protein phosphatase 2C (PP2C)-homologous cDNA was isolated from Nicotiana tabacum (NtPP2C1). The deduced protein sequence of 416 amino acids showed the highest degree of similarity to the PP2C of Arabidopsis thaliana (AtPP2CA) implicated in abscisic acid signalling. The expression of NtPP2C1 was strongly induced by drought, but repressed by oxidative stress and heat shock. It is suggested that NtPP2C1 operates at the junction of drought, heat shock and oxidative stress.

Members of the Arabidopsis 14-3-3 gene family trans-complement two types of defects in fission yeast

14-3-3 proteins are highly conserved among eukaryotes and perform diverse biochemical activities. We isolated five types of Arabidopsis 14-3-3 cDNAs in a screen for clones that could block ectopic meiosis driven by the pat1 mutation in fission yeast. Overexpression of fission yeast rad24, which encodes a 14-3-3 protein, also suppressed pat1. All Arabidopsis clones isolated could rescue the deformed morphology and elevated UV sensitivity of the rad24 mutant. Thus, it appears that Arabidopsis 14-3-3 proteins can generally substitute for their fission yeast counterpart in function. Expression of an Arabidopsis 14-3-3 clone, GF14µ, was shown to be rather ubiquitous among plant organs.

A Cluster of ABA-Regulated Genes on Arabidopsis thaliana BAC T07M07

Arabidopsis thaliana BAC T07M07 encoding the abscisic acid-insensitive 4 (ABI4) locus has been sequenced completely. It contains a 95,713-bp insert and 24 predicted genes. Most putative genes were confirmed by gel-based RNA profiling and a cluster of ABA-regulated genes was identified. One of the 24 genes, designatedPP2C5, encodes a putative protein phosphatase 2C. The encoded protein was expressed in Escherichia coli, and its enzyme activity in vitro was confirmed.

[The sequence data described in this paper have been submitted to GenBank under accession no. AF085279.]

A protein phosphatase 2C gene, LjNPP2C1, from Lotus japonicus induced during root nodule development

Symbiotic interactions between legumes and compatible strains of rhizobia result in root nodule formation. This new plant organ provides the unique physiological environment required for symbiotic nitrogen fixation by the bacterial endosymbiont and assimilation of this nitrogen by the plant partner. We have isolated two related genes (LjNPP2C1 and LjPP2C2) from the model legume Lotus japonicus that encode protein phosphatase type 2C (PP2C). Expression of the LjNPP2C1 gene was found to be enhanced specifically in L. japonicus nodules, whereas the LjPP2C2 gene was expressed at a similar level in nodules and roots. A glutathione S-transferase-LjNPP2C1 fusion protein was shown to have Mg2+- or Mn2+-dependent and okadaic acid-insensitive PP2C activity in vitro. A chimeric construct containing the full-length LjNPP2C1 cDNA, under the control of the Saccharomyces cerevisiae alcohol dehydrogenase promoter, was found to be able to complement a yeast PP2C-deficient mutant (pct1Delta). The transcript level of the LjNPP2C1 gene was found to increase significantly in mature nodules, and its highest expression level occurred after leghemoglobin (lb) gene induction, a molecular marker for late developmental events in nodule organogenesis. Expression of the LjNPP2C1 gene was found to be drastically altered in specific L. japonicus lines carrying monogenic-recessive mutations in symbiosis-related loci, suggesting that the product of the LjNPP2C1 gene may function at both early and late stages of nodule development.

Protein phosphatase 2C (PP2C) function in higher plants

In the past few years, molecular cloning studies have revealed the primary structure of plant protein serine/threonine phosphatases. Two structurally distinct families, the PP1/PP2A family and the PP2C family, are present in plants as well as in animals. This review will focus on the plant PP2C family of protein phosphatases. Biochemical and molecular genetic studies in Arabidopsis have identified PP2C enzymes as key players in plant signal transduction processes. For instance, the ABI1/ABI2 PP2Cs are central components in abscisic acid (ABA) signal transduction. Arabidopsis mutants containing a single amino acid exchange in ABI1 or ABI2 show a reduced response to ABA. Another member of the PP2C family, kinase-associated protein phosphatase (KAPP), appears to be an important element in some receptor-like kinase (RLK) signalling pathways. Finally, an alfalfa PP2C acts as a negative regulator of a plant mitogen-activated protein kinase (MAPK) pathway. Thus, the plant PP2Cs function as regulators of various signal transduction pathways.

Molecular cloning in Arabidopsis thaliana of a new protein phosphatase 2C (PP2C) with homology to ABI1 and ABI2

We report the cloning of both the cDNA and the corresponding genomic sequence of a new PP2C from Arabidopsis thaliana, named AtP2C-HA (for homology to ABI1/ABI2). The AtP2C-HA cDNA contains an open reading frame of 1536 bp and encodes a putative protein of 511 amino acids with a predicted molecular mass of 55.7 kDa. The AtP2C-HA protein is composed of two domains, a C-terminal PP2C catalytic domain and a N-terminal extension of ca. 180 amino acid residues. The deduced amino acid sequence is 55% and 54% identical to ABI1 and ABI2, respectively. Comparison of the genomic structure of the ABI1, ABI2 and AtP2C-HA genes suggests that they belong to a multigene family. The expression of the AtP2C-HA gene is up-regulated by abscisic acid (ABA) treatment.

ABI1 of Arabidopsis is a protein serine/threonine phosphatase highly regulated by the proton and magnesium ion concentration

The plant hormone abscisic acid (ABA) mediates various responses such as stomatal closure, maintenance of seed dormancy, and inhibition of plant growth. All three responses are regulated by the ABI1 gene product. The ABI1 protein (ABI1p) has been characterized as a protein serine/threonine phosphatase of type 2C that is highly affected in its activity by changes in the proton and magnesium ion concentrations. In the ABA-insensitive mutant abi1 of Arabidopsis thaliana a single amino acid exchange in the primary structure results in both a dominant insensitive phenotype and a strongly reduced protein phosphatase activity in vitro by possibly impairing metal ion coordination.

Mutational analysis of protein phosphatase 2C involved in abscisic acid signal transduction in higher plants

Protein phosphatase 2C (PP2C) is a class of ubiquitous and evolutionarily conserved serine/threonine PP involved in stress responses in yeasts, mammals, and plants. Here, I present mutational analysis of two Arabidopsis thaliana PP2Cs, encoded by ABI1 and AtPP2C, involved in the plant stress hormone abscisic acid (ABA) signaling in maize mesophyll protoplasts. Consistent with the crystal structure of the human PP2C, the mutation of two conserved motifs in ABI1, predicted to be involved in metal binding and catalysis, abolished PP2C activity. Surprisingly, although the DGH177-179KLN mutant lost the ability to be a negative regulator in ABA signaling, the MED141-143IGH mutant still inhibited ABA-inducible transcription, perhaps through a dominant interfering effect. Moreover, two G to D mutations near the DGH motif eliminated PP2C activity but displayed opposite effects on ABA signaling. The G174D mutant had no effect but the G180D mutant showed strong inhibitory effect on ABA-inducible transcription. Based on the results that a constitutive PP2C blocks but constitutive Ca2+-dependent protein kinases (CDPKs) activate ABA responses, the MED141-143IGH and G180D dominant mutants are unlikely to impede the wild-type PP2C and cause hyperphosphorylation of substrates. In contrast, these dominant mutants could trap cellular targets and prevent phosphorylation by PKs required for ABA signaling. The equivalent mutations in AtPP2C showed similar effects on ABA responses. This study suggests a mechanism for the action of dominant PP2C mutants that could serve as valuable tools to understand protein-protein interactions mediating ABA signal transduction in higher plants.

[Protein phosphatases and protein kinases in higher plants]

The recent gain in knowledge concerning enzymes involved in signal transduction pathways is a direct consequence of the considerable advances made in molecular biology. Protein kinases and protein phosphatases, the two major enzymes implicated in post-translational modifications, have been studied in particular. The number of characterized plant genes and/or cDNAs encoding these enzymes is increasing everyday. Since 1991, 26 genes and cDNAs coding for plant protein phosphatases have been isolated and characterized. The huge number of protein kinases (estimated at several thousands) makes it impossible to give an exhaustive list of the genes already identified, but a classification of these enzymes, based on phylogenetic criteria, allows us to appreciate the range of functions this protein family may play in plants.

Construction of a Lotus japonicus late nodulin expressed sequence tag library and identification of novel nodule-specific genes

A range of novel expressed sequence tags (ESTs) associated with late developmental events during nodule organogenesis in the legume Lotus japonicus were identified using mRNA differential display; 110 differentially displayed polymerase chain reaction products were cloned and analyzed. Of 88 unique cDNAs obtained, 22 shared significant homology to DNA/protein sequences in the respective databases. This group comprises, among others, a nodule-specific homolog of protein phosphatase 2C, a peptide transporter protein, and a nodule-specific form of cytochrome P450. RNA gel-blot analysis of 16 differentially displayed ESTs confirmed their nodule-specific expression pattern. The kinetics of mRNA accumulation of the majority of the ESTs analyzed were found to resemble the expression pattern observed for the L. japonicus leghemoglobin gene. These results indicate that the newly isolated molecular markers correspond to genes induced during late developmental stages of L. japonicus nodule organogenesis and provide important, novel tools for the study of nodulation.

CCAAT binding NF-Y-TBP interactions: NF-YB and NF-YC require short domains adjacent to their histone fold motifs for association with TBP basic residues

Both the TATA and CCAAT boxes are widespread promoter elements and their binding proteins, TBP and NF-Y, are extremely conserved in evolution. NF-Y is composed of three subunits, NF-YA, NF-YB and NF-YC, all necessary for DNA binding. NF-YB and NF-YC contain a putative histone-like motif, a domain also present in TBP-associated factors (TAFIIs) and in the subunits of the transcriptional repressor NC2. Immunopurification of holo-TFIID with anti-TBP and anti-TAFII100 antibodies indicates that a fraction of NF-YB associates with TFIID in the absence of NF-YA. Sedimentation velocity centrifugation experiments confirm that two pools of NF-YB, and most likely NF-YC, exist: one associated with NF-YA and binding to the CCAAT box; another involved in high molecular weight complexes. We started to dissect NF-Y-TFIID interactions by showing that: (i) NF-YB and NF-YC interact with TBP in solution, both separately and once bound to each other; (ii) short stretches of both NF-YB and NF-YC located within the evolutionary conserved domains, adjacent to the putative histone fold motifs, are necessary for TBP binding; (iii) TBP single amino acid mutants in the HS2 helix, previously shown to be defective in NC2 binding, are also unable to bind NF-YB and NF-YC.

Characterization of the basal inhibitor of class II transcription NC2 from Saccharomyces cerevisiae

Human NC2 utilizes a unique mechanism of repression of transcription by associating with TBP and inhibition of preinitiation complex formation. Here we have cloned two genes from Saccharomyces cerevisiae and functionally characterized them as yeast NC2. We show that yeast NC2 binds to TBP as a heterodimer and represses RNA polymerase II transcription during assembly of the preinitiation complex. Yeast NC2 is highly homologous to its human counterpart within histone fold domains. C-Terminal regions previously discussed to be important for repression in man are in part not conserved. The human alpha but not the beta subunit efficiently heterodimerizes and represses transcription in combination with the corresponding yeast subunit. Yeast and human NC2 inhibit transcription in the presence of yeast and human TBP. However, repression is optimal within one species. The N-terminus of human TBP supports repression of transcription by human but not by yeast NC2.

Differentiation-dependent enhanced expression of protein phosphatase 2Cbeta in germ cells of mouse seminiferous tubules

The presence of five distinct isoforms of protein phosphatase 2Cbeta (PP2Cbeta-1 approximately -5) is known. In this study, we demonstrate that the mRNA levels of PP2Cbeta-3, -4 and -5 and PP2Cbeta protein level increased during the course of the first wave of spermatogenesis in neonatal mouse testis. Northern blot and in situ hybridization analyses revealed that PP2Cbeta-3, -4 and -5 were expressed predominantly in pachytene spermatocytes and in more highly differentiated germ cells. The substrate specificity of PP2Cbeta-4 determined with artificial substrates differed from those of PP2Cbeta-3 and -5, suggesting that the difference in the structure of PP2Cbeta-3, -4 and -5 reflect their unique physiological functions in testicular germ cells.

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.