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

Comparative transcriptome analysis reveals the potential mechanism of GA3-induced dormancy release in Suaeda glauca black seeds

Suaeda glauca Bunge produces dimorphic seeds on the same plant, with brown seeds displaying non-dormant characteristics and black seeds exhibiting intermediate physiological dormancy traits. Previous studies have shown that black seeds have a very low germination rate under natural conditions, but exogenous GA3 effectively enhanced the germination rate of black seeds. However, the physiological and molecular mechanisms underlying the effects of GA3 on S. glauca black seeds are still unclear. In this study, transcriptomic profiles of seeds at different germination stages with and without GA3 treatment were analyzed and compared, and the TTF, H2O2, O2 -, starch, and soluble sugar contents of the corresponding seed samples were determined. The results indicated that exogenous GA3 treatment significantly increased seed vigor, H2O2, and O2 - contents but decreased starch and soluble sugar contents of S. glauca black seeds during seed dormancy release. RNA-seq results showed that a total of 1136 DEGs were identified in three comparison groups and were involved mainly in plant hormone signal transduction, diterpenoid biosynthesis, flavonoid biosynthesis, phenylpropanoid biosynthesis, and carbohydrate metabolism pathway. Among them, the DEGs related to diterpenoid biosynthesis (SgGA3ox1, SgKAO and SgGA2ox8) and ABA signal transduction (SgPP2Cs) could play important roles during seed dormancy release. Most genes involved in phenylpropanoid biosynthesis were activated under GA3 treatment conditions, especially many SgPER genes encoding peroxidase. In addition, exogenous GA3 treatment also significantly enhanced the expression of genes involved in flavonoid synthesis, which might be beneficial to seed dormancy release. In accordance with the decline in starch and soluble sugar contents, 15 genes involved in carbohydrate metabolism were significantly up-regulated during GA3-induced dormancy release, such as SgBAM, SgHXK2, and SgAGLU, etc. In a word, exogenous GA3 effectively increased the germination rate and seed vigor of S. glauca black seeds by mediating the metabolic process or signal transduction of plant hormones, phenylpropanoid and flavonoid biosynthesis, and carbohydrate metabolism processes. Our results provide novel insights into the transcriptional regulation mechanism of exogenous GA3 on the dormancy release of S. glauca black seeds. The candidate genes identified in this study may be further studied and used to enrich our knowledge of seed dormancy and germination.

Re-localization of hormone effectors is associated with dormancy alleviation by temperature and after-ripening in sunflower seeds

Temperature is the primary factor that affects seed dormancy and germination. However, the molecular mechanism that underlies its effect on dormancy alleviation remained largely unknown. In this study, we investigate hormone involvement in temperature induced germination as compared to that caused by after-ripening. Dormant (D) sunflower seeds cannot germinate at 10 °C but fully germinate at 20 °C. After-ripened seeds become non-dormant (ND), i.e. able to germinate at 10 °C. Pharmacological experiments showed the importance of abscisic acid (ABA), gibberellins (GAs) and ethylene in temperature- and after-ripening-induced germination of sunflower seeds. Hormone quantification showed that after-ripening is mediated by a decline in both ABA content and sensitivity while ABA content is increased in D seeds treated at 10 or 20 °C, suggesting that ABA decrease is not a prerequisite for temperature induced dormancy alleviation. GAs and ethylene contents were in accordance with germination potential of the three conditions (GA₁ was higher in D 20 °C and ND 10 °C than in D 10 °C). Transcripts analysis showed that the major change concerns ABA and GAs metabolism genes, while ABA signalling gene expression was significantly unchanged. Moreover, another level of hormonal regulation at the subcellular localization has been revealed by immunocytolocalization study. Indeed, ABA, protein Abscisic acid-Insensitive 5 (ABI5), involved in ABA-regulated gene expression and DELLA protein RGL2, a repressor of the gibberellins signalling pathway, localized mainly in the nucleus in non-germinating seeds while they localized in the cytosol in germinating seeds. Furthermore, ACC-oxidase (ACO) protein, the key ethylene biosynthesis enzyme, was detected in the meristem only in germinating seeds. Our results reveal the importance of hormone actors trafficking in the cell and their regulation in specialized tissue such as the meristem in dormancy alleviation and germination.

Proteomic and Phosphoproteomic Analysis in Tobacco Mosaic Virus-Infected Tobacco (Nicotiana tabacum)

Tobacco mosaic virus (TMV) is a common source of biological stress that significantly affects plant growth and development. It is also useful as a model in studies designed to clarify the mechanisms involved in plant viral disease. Plant responses to abiotic stress were recently reported to be regulated by complex mechanisms at the post-translational modification (PTM) level. Protein phosphorylation is one of the most widespread and major PTMs in organisms. Using immobilized metal ion affinity chromatography (IMAC) enrichment, high-pH C18 chromatography fraction, and high-accuracy mass spectrometry (MS), a set of proteins and phosphopeptides in both TMV-infected tobacco and control tobacco were identified. A total of 4905 proteins and 3998 phosphopeptides with 3063 phosphorylation sites were identified. These 3998 phosphopeptides were assigned to 1311 phosphoproteins, as some proteins carried multiple phosphorylation sites. Among them, 530 proteins and 337 phosphopeptides corresponding to 277 phosphoproteins differed between the two groups. There were 43 upregulated phosphoproteins, including phosphoglycerate kinase, pyruvate phosphate dikinase, protein phosphatase 2C, and serine/threonine protein kinase. To the best of our knowledge, this is the first phosphoproteomic analysis of leaves from a tobacco cultivar, K326. The results of this study advance our understanding of tobacco development and TMV action at the protein phosphorylation level.

Functional analysis of a type 2C protein phosphatase gene from Ammopiptanthus mongolicus

In Arabidopsis and certain other plant species, the type 2C protein phosphatases (PP2Cs) of the clade A class have been demonstrated to act as negative regulators in ABA-induced stress responses, such as stomatal closure. The present study reports the identification of a PP2C ortholog from the ancient desert shrub Ammopiptanthus mongolicus (Maxim.) Cheng f. (AmPP2C), which is functionally conserved over its counterparts reported from other plant species. AmPP2C was primarily expressed in leaves, with strong transcriptional accumulation being observed in the guard cells. The expression of AmPP2C was induced in response to PEG or ABA treatments, implying the potential involvement in ABA-induced stress responses. The GFP-tagging observation revealed that AmPP2C was predominantly localized to the nuclei and partly to the cytoplasm. Furthermore, BiFC assays demonstrated an interaction between AmPP2C and the typical protein kinase SnRK2.6 (AmOST1). Overexpression of AmPP2C in Arabidopsis significantly overcame the inhibition of seed germination by ABA. The transgenic Arabidopsis lines exhibited larger stomatal apertures and significantly reduced sensitivity to ABA-induced stomatal closure, which subsequently led to greater water loss and decreased biomass under PEG-simulated drought stress treatments. Under limited nitrogen or potassium supplements, plants overexpressing AmPP2C obtained a superior capability of nitrogen (N) and potassium (K) acquisition in the green parts. Therefore, the impairment of ABA-induced stomatal closure rendered by the function of PP2C helped to identify a potential survival strategy in plants suffering persistent drought stress via the maintenance of the necessary mineral nutrient acquisition driven by transpirational solute flow.

Intra-specific variations in expression of stress-related genes in beech progenies are stronger than drought-induced responses

Rapidly decreasing water availability as a consequence of climate change is likely to endanger the range of long-lived tree species. A pressing question is, therefore, whether adaptation to drought exists in important temperate tree species like European beech (Fagus sylvatica L.), a wide-spread, dominant forest tree in Central Europe. Here, five beech stands were selected along a precipitation gradient from moist to dry conditions. Neutral genetic markers revealed strong variation within and little differentiation between the populations. Natural regeneration from these stands was transferred to a common garden and used to investigate the expression of genes for abscisic acid (ABA)-related drought signaling [9-cis-epoxy-dioxygenase (NCED), protein phosphatase 2C (PP2C), early responsive to dehydration (ERD)] and stress protection [ascorbate peroxidase (APX), superoxide dismutase (SOD), aldehyde dehydrogenase (ALDH), glutamine amidotransferase (GAT)] that are involved in drought acclimation. We hypothesized that progenies from dry sites exhibit constitutively higher expression levels of ABA- and stress-related genes and are less drought responsive than progenies from moist sites. Transcript levels and stress responses (leaf area loss, membrane integrity) of well-irrigated and drought-stressed plants were measured during the early, mid- and late growing season. Principal component (PC) analysis ordered the beech progenies according to the mean annual precipitation at tree origin by the transcript levels of SOD, ALDH, GAT and ERD as major loadings along PC1. PC2 separated moist and drought treatments with PP2C levels as important loading. These results suggest that phosphatase-mediated signaling is flexibly acclimated to the current requirements, whereas stress compensatory measures exhibited genotypic variation, apparently underlying climate selection. In contrast to expectation, the drought responses were less pronounced than the progeny-related differences and the transcript levels were constitutively lower in beeches from dry than from moist sites. These results imply that beeches from dry origins may have evolved mechanisms to avoid oxidative stress.

Type 2C phosphatase 1 of Artemisia annua L. is a negative regulator of ABA signaling

The phytohormone abscisic acid (ABA) plays an important role in plant development and environmental stress response. Additionally, ABA also regulates secondary metabolism such as artemisinin in the medicinal plant Artemisia annua L. Although an earlier study showed that ABA receptor, AaPYL9, plays a positive role in ABA-induced artemisinin content improvement, many components in the ABA signaling pathway remain to be elucidated in Artemisia annua L. To get insight of the function of AaPYL9, we isolated and characterized an AaPYL9-interacting partner, AaPP2C1. The coding sequence of AaPP2C1 encodes a deduced protein of 464 amino acids, with all the features of plant type clade A PP2C. Transcriptional analysis showed that the expression level of AaPP2C1 is increased after ABA, salt, and drought treatments. Yeast two-hybrid and bimolecular fluorescence complementation assays (BiFC) showed that AaPYL9 interacted with AaPP2C1. The P89S, H116A substitution in AaPYL9 as well as G199D substitution or deletion of the third phosphorylation site-like motif in AaPP2C1 abolished this interaction. Furthermore, constitutive expression of AaPP2C1 conferred ABA insensitivity compared with the wild type. In summary, our data reveals that AaPP2C1 is an AaPYL9-interacting partner and involved in the negative modulation of the ABA signaling pathway in A. annua L.

Interaction between abscisic acid receptor PYL3 and protein phosphatase type 2C in response to ABA signaling in maize

Abscisic acid (ABA) is a ubiquitous hormone that regulates plant growth, development and responses to environmental stresses. In recent researches, pyrabactin resistance 1-like protein (PYL) and protein phosphatase type 2C (PP2C) were identified as the direct receptor and the second component of ABA signaling pathway, respectively. However, a lot of PYL and PP2C members were found in Arabidopsis and several other plants. Some of them were found not to be involved in ABA signaling. Because of the complex diversity of the genome, few documents have been available on the molecular details of the ABA signal perception system in maize. In the present study, we conducted bioinformatics analysis to find out the candidates (ZmPYL3 and ZmPP2C16) of the PYL and PP2C members most probably involved in ABA signaling in maize, cloned their encoding genes (ZmPYL3 and ZmPP2C16), verified the interaction between these two proteins in response to exogenous ABA induction by yeast two-hybrid assay and bimolecular fluorescence complementation, and investigated the expression patterns of these two genes under the induction of exogenous ABA by real-time fluorescence quantitative PCR. The results indicated that the ZmPYL3 and ZmPP2C16 proteins interacted in vitro and in vivo in response to the induction of exogenous ABA. The downregulated expression of the ZmPYL3 gene and the upregulated expression of the ZmPP2C16 gene are responsive to the induction of exogenous ABA. The ZmPYL3 and ZmPP2C16 proteins are the most probable members of the receptors and the second components of ABA signaling pathway, respectively.

Action of natural abscisic acid precursors and catabolites on abscisic acid receptor complexes

The phytohormone abscisic acid (ABA) regulates stress responses and controls numerous aspects of plant growth and development. Biosynthetic precursors and catabolites of ABA have been shown to trigger ABA responses in physiological assays, but it is not clear whether these are intrinsically active or whether they are converted into ABA in planta. In this study, we analyzed the effect of ABA precursors, conjugates, and catabolites on hormone signaling in Arabidopsis (Arabidopsis thaliana). The compounds were also tested in vitro for their ability to regulate the phosphatase moiety of ABA receptor complexes consisting of the protein phosphatase 2C ABI2 and the coreceptors RCAR1/PYL9, RCAR3/PYL8, and RCAR11/PYR1. Using mutants defective in ABA biosynthesis, we show that the physiological activity associated with ABA precursors derives predominantly from their bioconversion to ABA. The ABA glucose ester conjugate, which is the most widespread storage form of ABA, showed weak ABA-like activity in germination assays and in triggering ABA signaling in protoplasts. The ABA conjugate and precursors showed negligible activity as a regulatory ligand of the ABI2/RCAR receptor complexes. The majority of ABA catabolites were inactive in our assays. To analyze the chemically unstable 8'- and 9'-hydroxylated ABA catabolites, we used stable tetralone derivatives of these compounds, which did trigger selective ABA responses. ABA synthetic analogs exhibited differential activity as regulatory ligands of different ABA receptor complexes in vitro. The data show that ABA precursors, catabolites, and conjugates have limited intrinsic bioactivity and that both natural and synthetic ABA-related compounds can be used to probe the structural requirements of ABA ligand-receptor interactions.

The nuclear interactor PYL8/RCAR3 of Fagus sylvatica FsPP2C1 is a positive regulator of abscisic acid signaling in seeds and stress

The functional protein phosphatase type 2C from beechnut (Fagus sylvatica; FsPP2C1) was a negative regulator of abscisic acid (ABA) signaling in seeds. In this report, to get deeper insight on FsPP2C1 function, we aim to identify PP2C-interacting partners. Two closely related members (PYL8/RCAR3 and PYL7/RCAR2) of the Arabidopsis (Arabidopsis thaliana) BetV I family were shown to bind FsPP2C1 in a yeast two-hybrid screening and in an ABA-independent manner. By transient expression of FsPP2C1 and PYL8/RCAR3 in epidermal onion (Allium cepa) cells and agroinfiltration in tobacco (Nicotiana benthamiana) as green fluorescent protein fusion proteins, we obtained evidence supporting the subcellular localization of both proteins mainly in the nucleus and in both the cytosol and the nucleus, respectively. The in planta interaction of both proteins in tobacco cells by bimolecular fluorescence complementation assays resulted in a specific nuclear colocalization of this interaction. Constitutive overexpression of PYL8/RCAR3 confers ABA hypersensitivity in Arabidopsis seeds and, consequently, an enhanced degree of seed dormancy. Additionally, transgenic 35S:PYL8/RCAR3 plants are unable to germinate under low concentrations of mannitol, NaCl, or paclobutrazol, which are not inhibiting conditions to the wild type. In vegetative tissues, Arabidopsis PYL8/RCAR3 transgenic plants show ABA-resistant drought response and a strong inhibition of early root growth. These phenotypes are strengthened at the molecular level with the enhanced induction of several ABA response genes. Both seed and vegetative phenotypes of Arabidopsis 35S:PYL8/RCAR3 plants are opposite those of 35S:FsPP2C1 plants. Finally, double transgenic plants confirm the role of PYL8/RCAR3 by antagonizing FsPP2C1 function and demonstrating that PYL8/RCAR3 positively regulates ABA signaling during germination and abiotic stress responses.

Seed dormancy and ABA signaling: the breakthrough goes on

The seed is an important organ of higher plants regarding plant survival and species dispersion. The transition between seed dormancy and germination represents a critical stage in the plant life cycle and it is an important ecological and commercial trait. A dynamic balance of synthesis and catabolism of two antagonistic hormones, abscisic acid (ABA) and giberellins (GAs), controls the equilibrium between seed dormancy and germination. Embryonic ABA plays a central role in induction and maintenance of seed dormancy, and also inhibits the transition from embryonic to germination growth. Therefore, the ABA metabolism must be highly regulated at both temporal and spatial levels during phase of dessication tolerance. On the other hand, the ABA levels do not depend exclusively on the seeds because sometimes it becomes a strong sink and imports it from the roots and rhizosphere through the xylem and/or phloem. All theses events are discussed in depth here. Likewise, the role of some recently characterized genes belonging to seeds of woody species and related to ABA signaling, are also included. Finally, although four possible ABA receptors have been reported, not much is known about how they mediate ABA signalling transduction. However, new publications seem to shown that almost all these receptors lack several properties to consider them as such.

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.

Two cDNA clones (FsDhn1 and FsClo1) up-regulated by ABA are involved in drought responses in Fagus sylvatica L. seeds

Two abscisic acid (ABA)-responsive clones (FsDhn1 and FsClo1) were isolated from a cDNA library of ABA-treated seeds of Fagus sylvatica L. FsDhn1 codes for type-II late embryogenesis abundant (LEA) proteins, also known as dehydrins. The corresponding transcripts were ABA-induced and expressed when seeds were artificially dried. FsClo1 codes for a caleosin, a calcium-binding protein. By expressing FsCLO1 as a fusion protein in Escherichia coli, we obtained biochemical evidence of its ability to bind calcium, a second messenger involved in ABA signaling. This cDNA clone is seed specific, shows a LEA expression pattern and is also induced by ABA and dehydration treatments. The relationship of both genes with seed responses to ABA is discussed.

Overexpression of a protein phosphatase 2C from beech seeds in Arabidopsis shows phenotypes related to abscisic acid responses and gibberellin biosynthesis

A functional abscisic acid (ABA)-induced protein phosphatase type 2C (PP2C) was previously isolated from beech (Fagus sylvatica) seeds (FsPP2C2). Because transgenic work is not possible in beech, in this study we overexpressed this gene in Arabidopsis (Arabidopsis thaliana) to provide genetic evidence on FsPP2C2 function in seed dormancy and other plant responses. In contrast with other PP2Cs described so far, constitutive expression of FsPP2C2 in Arabidopsis, under the cauliflower mosaic virus 35S promoter, produced enhanced sensitivity to ABA and abiotic stress in seeds and vegetative tissues, dwarf phenotype, and delayed flowering, and all these effects were reversed by gibberellic acid application. The levels of active gibberellins (GAs) were reduced in 35S:FsPP2C2 plants, although transcript levels of AtGA20ox1 and AtGA3ox1 increased, probably as a result of negative feedback regulation, whereas the expression of GASA1 was induced by GAs. Additionally, FsPP2C2-overexpressing plants showed a strong induction of the Responsive to ABA 18 (RAB18) gene. Interestingly, FsPP2C2 contains two nuclear targeting sequences, and transient expression assays revealed that ABA directed this protein to the nucleus. Whereas other plant PP2Cs have been shown to act as negative regulators, our results support the hypothesis that FsPP2C2 is a positive regulator of ABA. Moreover, our results indicate the existence of potential cross-talk between ABA signaling and GA biosynthesis.

Effects of chronic ozone exposure on gene expression in Arabidopsis thaliana ecotypes and in Thellungiella halophila

Arabidopsis thaliana (At) ecotypes Columbia-0 (Col-0), Wassilewskija (WS), Cape Verde Islands (Cvi-0) and a relative, Thellungiella halophila (Th), were exposed to 20-25% over ambient ozone [O3] in a free air concentration enrichment (FACE) experiment (http://www.soyFACE. uiuc.edu), mirroring increases expected in the near future. Col-0 and WS accelerated development and developed lesions within 10 d under increased ozone, while Cvi-0 and Th grew slowly. RNAs were used in microarray hybridizations (Col-0-based 26 000 elements, 70-mer oligonucleotides). A two-step analysis of variance (ANOVA) model, including comparison with values obtained under [O3], was used for analyses. WS showed the greatest number of changes in gene expression in response to ozone. Th showed the least changes, suggesting that its expression state at [O3] was sufficient for resistance at increased ozone. Patterns observed in ambient air controls for Cvi-0 and Col-0 were most similar, while Th showed the greatest number of differences compared with the other controls. Compared with Col-0, however, Cvi-0 showed higher levels of expression of chaperones, receptor kinase-like and photosynthesis-related genes in ambient air. Cvi-0 exhibited ozone-mediated changes in a pathway involving AtSR, a homologue of the mammalian NF kappa B family of redox-sensitive transcription factors, changes in chaperones, WRKY and C2H2 proteins and antioxidants. WS displayed ozone-mediated decreases in the expression of two AtSR/NF kappa B family members, C2-domain proteins and genes associated with cell wall growth and changes in the expression of marker genes for programmed cell death (PCD), among them RCD1, a key regulator in this pathway. Microarray data were verified by reverse transcriptase (RT)-PCR. We relate O3-response diversity across the four lines to different responses among signaling and transcriptional response networks and differences in gene expression at [O3] levels.

Characterization of a protein kinase (FsPK4) with an acidic domain, regulated by abscisic acid and specifically located in Fagus sylvatica L. seeds

An abscisic acid (ABA)-induced cDNA fragment encoding a putative serine/threonine protein kinase (PK) was obtained by means of differential reverse transcriptase-polymerase chain reaction (RT-PCR). The full-length clone (FsPK4) was isolated from a cDNA library constructed using mRNA from ABA-treated Fagus sylvatica L. seeds. This clone contained the 11 catalytic domains present in all PKs and a highly acidic domain in the C-terminus. By expressing FsPK4 in Escherichia coli as a His tag fusion protein, we obtained direct biochemical evidence supporting Ca2+-dependent kinase activity of this protein. The expression of FsPK4 increased after ABA treatment or warm pre-treatment, when seeds are maintained dormant, but decreased and tended to disappear when dormancy was released by stratification or under gibberellic acid (GA3) treatment, and when seeds were artificially dried. Further, FsPK4 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 expression of the corresponding protein could be more closely related with the maintenance of seed dormancy than with responses to drought stress mediated by ABA.

Evidence of a role for tyrosine dephosphorylation in the control of postgermination arrest of development by abscisic acid in Arabidopsis thaliana L

In the present paper evidence is presented indicating that tyrosine dephosphorylation is a key regulatory mechanism in postgermination arrest of Arabidopsis thaliana L. seed development mediated by abscisic acid (ABA). By using phenylarsine oxide (PAO), an inhibitor of tyrosine phosphatases, the sensitivity to the inhibitory effect of ABA on seed germination is enhanced. Consistent with this finding, we demonstrate that the ABA-responsive gene, RAB18, is hyperinduced in seeds imbibed in ABA plus PAO, compared with seeds imbibed only with ABA.

Gene expression analysis by cDNA-AFLP highlights a set of new signaling networks and translational control during seed dormancy breaking in Nicotiana plumbaginifolia

Seed dormancy in Nicotiana plumbaginifolia is characterized by an abscisic acid accumulation linked to a pronounced germination delay. Dormancy can be released by 1 year after-ripening treatment. Using a cDNA-amplified fragment length polymorphism (cDNA-AFLP) approach we compared the gene expression patterns of dormant and after-ripened seeds, air-dry or during one day imbibition and analyzed 15,000 cDNA fragments. Among them 1020 were found to be differentially regulated by dormancy. Of 412 sequenced cDNA fragments, 83 were assigned to a known function by search similarities to public databases. The functional categories of the identified dormancy maintenance and breaking responsive genes, give evidence that after-ripening turns in the air-dry seed to a new developmental program that modulates, at the RNA level, components of translational control, signaling networks, transcriptional control and regulated proteolysis.

Evidence of a cross-talk regulation of a GA 20-oxidase (FsGA20ox1) by gibberellins and ethylene during the breaking of dormancy in Fagus sylvatica seeds

Gibberellin 20-oxidase (GA 20-oxidase) is an enzyme that catalyses the last three steps in the synthesis of active GAs and is a potential control point in the regulation of GA biosynthesis. Reverse transcriptase-polymerase chain reaction with degenerated oligonucleotides conserved among GA 20-oxidases was used to isolate a cDNA clone for this enzyme in Fagus sylvatica L. seeds. This clone contains all the features and exhibits homology to GA 20 oxidases from several plant species. Expression of this clone, named FsGA20ox1, as a fusion protein expressed in Escherichia coli confirmed that it was able to metabolize [(14)C]GA(12) to [(14)C]GA(9) and [(14)C]GA(53) to [(14)C]GA(20). Analysis of FsGA20ox1 transcript levels showed similar low expression during stratification at 4 degrees C and in the presence of gibberellic acid or ethephon (compound that releases ethylene in solution), treatments proved to be efficient in breaking the dormancy of beech seeds. However, there was a drastic increase of FsGA20ox1 transcript levels in the presence of paclobutrazol (PCB), a well-known GAs biosynthesis inhibitor, or of 2-aminoxyacetic acid (AOA), an inhibitor of ethylene biosynthesis. Furthermore, the effect of AOA was reversed by the addition of GA(3) and that of PCB by ethephon. This indicates that the gene product is subjected to down-regulation by GA and ethylene, and further suggests a cross-talk gene regulation by these two hormones during the transition from seed dormancy to germination.

Gain-of-function and loss-of-function phenotypes of the protein phosphatase 2C HAB1 reveal its role as a negative regulator of abscisic acid signalling

HAB1 was originally cloned on the basis of sequence homology to ABI1 and ABI2, and indeed, a multiple sequence alignment of 32 Arabidopsis protein phosphatases type-2C (PP2Cs) reveals a cluster composed by the four closely related proteins, ABI1, ABI2, HAB1 and At1g17550 (here named HAB2). Characterisation of transgenic plants harbouring a transcriptional fusion ProHAB1: green fluorescent protein (GFP) indicates that HAB1 is broadly expressed within the plant, including key target sites of abscisic acid (ABA) action as guard cells or seeds. The expression of the HAB1 mRNA in vegetative tissues is strongly upregulated in response to exogenous ABA. In this work, we show that constitutive expression of HAB1 in Arabidopsis under a cauliflower mosaic virus (CaMV) 35S promoter led to reduced ABA sensitivity both in seeds and vegetative tissues, compared to wild-type plants. Thus, in the field of ABA signalling, this work represents an example of a stable phenotype in planta after sustained overexpression of a PP2C genes. Additionally, a recessive T-DNA insertion mutant of HAB1 was analysed in this work, whereas previous studies of recessive alleles of PP2C genes were carried out with intragenic revertants of the abi1-1 and abi2-1 mutants that carry missense mutations in conserved regions of the PP2C domain. In the presence of exogenous ABA, hab1-1 mutant shows ABA-hypersensitive inhibition of seed germination; however, its transpiration rate was similar to that of wild-type plants. The ABA-hypersensitive phenotype of hab1-1 seeds together with the reduced ABA sensitivity of 35S:HAB1 plants are consistent with a role of HAB1 as a negative regulator of ABA signalling. Finally, these results provide new genetic evidence on the function of a PP2C in ABA signalling.

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.

Molecular cloning of a functional protein phosphatase 2C (FsPP2C2) with unusual features and synergistically up-regulated by ABA and calcium in dormant seeds of Fagus sylvatica

Phosphorylation/dephosphorylation of proteins is a general mechanism of hormonal signal transduction, including ABA, and serine/threonine protein phosphatases 2C (PP2C, EC 3.1.3.16) have been suggested to play an important role in this process. By means of differential reverse transcriptase-polymerase chain reaction (RT-PCR) and further screening of a cDNA library made from mRNA of ABA-treated Fagus sylvatica L. seeds, a full-length cDNA clone (FsPP2C2) encoding a putative PP2C was obtained. Comparison to the databases revealed high homology to plant PP2C and most features of these enzymes, but unusual characteristics were found within the catalytic domain and the N-terminal region of the amino acid sequence. The coding region of FsPP2C2 was expressed in Escherichia coli as histidine tag fusion protein and shows Mg2+-dependent in vitro phosphatase activity. Transcription of the FsPP2C2 gene is low during seeds stratification at 4 degrees C or under gibberellic acid (GA3) treatment and clearly increases when seeds are treated with ABA and calcium (Ca2+) together, while the addition of calcium chelators (EGTA or TMB-8) decreases its expression. Furthermore, FsPP2C2 is only expressed in ABA-treated tissues, preferentially in seeds, which suggests that this PP2C is specifically induced by ABA in dormant seeds, in a Ca2+-dependent manner, and also in other ABA-treated tissues.

GA(3)-induced expression of a new functional AAA-ATPase (FsA1) is correlated with the onset of germination in Fagus sylvatica L. seeds (beechnuts)

A full-length cDNA clone, named FsA1, has been isolated from a cDNA library constructed using mRNA from Fagus sylvatica L. dormant seeds (beechnuts). This clone shows high identity with members of the AAA superfamily, for ATPases Associated with a variety of cellular Activities, encoding subunit 8 of the 26S proteasome or Tat binding proteins (TBPs). Direct biochemical evidence supporting Mg(2+)-dependent ATPase activity has been obtained by expressing FsA1 in Escherichia coli as histidine tag fusion protein and using the recombinant protein in the stimulation of ATP hydrolysis. Analysis of the expression of FsA1 transcripts during stratification shows an increase in the presence of gibberellic acid (GA(3)), a treatment that proved to be efficient in breaking dormancy and increasing germination percentages of these seeds, while the addition of paclobutrazol, a well-known GA biosynthesis inhibitor, greatly reduces the expression of the clone. A low level of expression was maintained in the stratification control in H(2)O, where dormancy is slowly released. These results show that this new member of the AAA-ATPase family is up-regulated by GAs and its expression correlated with the germination arise in Fagus sylvatica seeds. The possible function of this protein during the transition from dormancy to germination is discussed.