Sequence analysis of a functional member of the Em gene family from wheat

Modulation of wheat grain dormancy by introducing the recombinant abscisic acid-stimulated abscisic acid biosynthesis gene

Pre-harvest sprouting of cereals greatly reduces yield and quality of the grains. Abscisic acid (ABA) is an essential phytohormone for the induction and maintenance of seed dormancy. In this study, the ABA responsive promoter-driven ABA biosynthesis gene system was introduced to common wheat (Triticum aestivum L.) to enhance ABA production in the embryos and pre-harvest sprouting tolerance of the grains. This system consists of a wheat ABA responsive element containing Early-Methionine-labelled (EM) promoter and a sorghum 9-cis-epoxycarotenoid dioxygenase (SbNCED) gene which encodes an ABA biosynthesis rate-limiting enzyme. Twenty-three independent single-insertion lines were obtained, from which five homozygous lines showing various SbNCED expression levels were selected. Correlations were observed between SbNCED expression, ABA accumulation in the embryos and enhanced dormancy levels of the grains. The engineered wheat grains exhibited a few day-delay in germination, which should be effective in reducing pre-harvest sprouting damage. However, the increase in ABA levels in the recombinant grains was moderate, which explains why germination was not completely suppressed. Further analysis indicated a concomitant increase in the expression of the ABA catabolic enzyme gene TaABA8'OH1 and in the levels of isoleucine-conjugated jasmonic acid, implying the presence of possible negative feedback regulation in the innate system, which should be overcome for future technology development. These findings advance an understanding of the regulatory mechanisms of hormone metabolism in seeds and facilitate the development of pre-harvest sprouting tolerance in cereal grains.

Analysis of the phytochrome gene family in Ceratodon purpureus by gene targeting reveals the primary phytochrome responsible for photo- and polarotropism

Using gene targeting by homologous recombination in Ceratodon purpureus, we were able to knock out four phytochrome photoreceptor genes independently and to analyze their function with respect to red light dependent phototropism, polarotropism, and chlorophyll content. The strongest phenotype was found in knock-out lines of a newly described phytochrome gene termed CpPHY4 lacking photo- and polarotropic responses at moderate fluence rates. Eliminating the atypical phytochrome gene CpPHY1, which is the only known phytochrome-like gene containing a putative C-terminal tyrosine kinase-like domain, affects red light-induced chlorophyll accumulation. This result was surprising, since no light dependent function was ever allocated to this unusual gene.

The evolution of the abscisic acid-response in land plants: comparative analysis of group 1 LEA gene expression in moss and cereals

The moss Physcomitrella patens possesses a single copy of a Group 1 LEA gene, designated PpLEA-1. Sequence analysis of the PpLEA-1 gene reveals the gene to contain a single intron in a position conserved in all members of the Group 1 LEA gene family, but also to contain a premature termination codon within the first exon. Nevertheless, a PpLEA-1 transcript accumulates in moss tissues in response both to the imposition of osmotic stress, and to the plant growth regulator abscisic acid (ABA). This response appears to be mediated at the transcriptional level, and observation of the pattern of gene expression, reported by histochemical staining of plants expressing a PpLea-1::GUS transgene suggests that the promoter responds preferentially to ABA in protonemal filaments, whereas osmotic stress induces gene expression primarily in the gametophores. Quantitative analysis of promoter activity by transient expression in Physcomitrella protoplasts shows the PpLEA-1 promoter to be highly active in response to ABA and osmotic stress. ABA-mediated transgene expression from the PpLea-1 promoter occurs at a level similar to that driven by the highly active promoter of the wheat Group 1 LEA gene, E(m). Site-directed mutagenesis of the PpLEA-1 promoter indicates that ABA-inducibility is mediated via an ACGT-core motif similar to that seen in the ABA response elements of higher plant LEA genes. However, whereas the wheat E(m )promoter is active in moss tissues, the moss promoter is not reciprocally active in cereal cells: no activity, ABA-inducible or otherwise was detected in barley aleurone protoplasts transfected with the PpLEA-1::GUS construct. We propose that ABA activation of gene expression in moss cells represents an ancestral state, with only minimal requirements for promoter recognition, whereas cereal cells require the interaction of additional factors with ABA-responsive promoters.

The LEA-like protein HSP 12 in Saccharomyces cerevisiae has a plasma membrane location and protects membranes against desiccation and ethanol-induced stress

The LEA-like protein HSP 12 was identified as having a plasma membrane location in yeast. Gold particles, indicative of the presence of HSP 12, were observed on the external side of the plasma membrane when yeast grown to stationary phase were subjected to immunocytochemical analysis. Growth of yeast in the osmolyte mannitol resulted in an increased number of gold particles that were now observed to be present on both sides of the plasma membrane. No gold particles were observed using a mutant strain of the same yeast that did not express HSP 12. A model liposome system encapsulating the fluorescent dye calcein was used to investigate the protection by HSP 12 of membranes during desiccation. HSP 12 was found to act in an analogous manner to trehalose and protect liposomal membrane integrity against desiccation. The interaction between HSP 12 and the liposomal membrane was judged to be electrostatic as membrane protection was only observed with positively charged liposomes and not with either neutral or negatively charged liposomes. The ability of the wild-type and mutant yeast to grow in media containing ethanol was compared. It was found that yeast not expressing the HSP 12 protein were less able to grow in media containing ethanol. HSP 12 was shown to confer increased integrity on the liposomal membrane in the presence of ethanol. Ethanol, like mannitol, was found to induce HSP 12 protein synthesis. However, yeast grown in both ethanol and mannitol showed a decreased HSP 12 response compared with yeast grown in the presence of either osmolyte alone.

Transcriptional regulation of the human alpha1a-adrenergic receptor gene. Characterization Of the 5'-regulatory and promoter region

We recently cloned cDNAs encoding three subtypes of human alpha1-adrenergic receptors (alpha1ARs), alpha1a, alpha1b, and alpha1d (Schwinn, D. A., Johnston, G. L., Page, S. O., Mosley, M. J., Wilson, K. H., Worman, N. P., Campbell, S., Fidock, M. D., Furness, L. M., Parry-Smith, D. J., Peter, B., and Bailey, D. S. (1995) J. Pharmacol. Exp. Ther. 272, 134-142) and demonstrated predominance of alpha1aARs in many human tissues (Price, D. T., Lefkowitz, R. J., Caron, M. G., Berkowitz, D., and Schwinn, D. A. (1994) Mol. Pharmacol. 45, 171-175). Several lines of evidence indicate that alpha1aARs are important in clinical diseases such as myocardial hypertrophy and benign prostatic hyperplasia. Therefore, we initiated studies to understand mechanisms underlying regulation of alpha1aAR gene transcription. A genomic clone containing 6.2 kb of 5'-untranslated region of the human alpha1aAR gene was recently isolated. Ribonuclease protection and primer extension assays indicate that alpha1aAR gene transcription occurs at multiple initiation sites with the major site located 696 base pairs upstream of the ATG, where a classic initiator sequence is located. Transfection of luciferase reporter constructs containing varying amounts of 5'-untranslated region into human SK-N-MC neuroblastoma cells indicate that a region extending 125 base pairs upstream from the main transcription initiation site contains full alpha1aAR promoter activity. Furthermore, distinct activator and suppressor elements lie 2-3 and 3-5 kilobase pairs upstream, respectively. Although the alpha1aAR promoter contains neither TATA or CAAT elements, gel shift mobility assays targeting three GC boxes immediately upstream of the main transcription initiation site confirm binding of Sp1. Activity of the alpha1aAR promoter is cell-specific, demonstrating highest activity in cells endogenously expressing alpha1aARs. The human alpha1aAR gene also contains several cis regulatory elements, including several insulin and cAMP response elements. Consistent with these observations, we provide the first evidence that treatment of SK-N-MC cells with insulin and cAMP elevating agents leads to an increase in alpha1aAR expression. In conclusion, these data represent the first characterization of the alpha1aAR gene; our findings should facilitate further studies designed to understand mechanisms regulating alpha1AR subtype-specific expression in healthy and diseased human tissue.

PCR-based isolation and chromosome assignment of members of the Em gene family from wheat

Four members belonging to the wheat Em gene family were isolated by PCR, cloned and subsequently sequenced. One of the genes corresponds perfectly to a previous published cDNA sequence, the other three genes are new. The amplified sequences contain the entire coding region, which is interrupted by a short intron of variable length, and part of the 3' untranslated region. The chromosomal assignment of each of the four sequences and three extra, previously published, Em sequences was determined using PCR with sequence-specific primers on wheat aneuploid nullitetrasomic lines. Three sequences were shown to be encoded by the Em-A1 locus (on chromosome 1A), one by Em-B1 on chromosome 1B and two by Em-D1 on chromosome 1D. Hence, primer sets specific for each of the three homoeologous chromosomes of the group 1 are available. A lot of DNA sequence polymorphism exists among the sequences most of which is found in the non-coding parts and mainly in the introns. Sequence alignment groups the seven known Em sequences irrespective of their locus origin. The implication of these findings in relation to the organisation and evolution of the Em gene family are discussed.

Characterization of three rice basic/leucine zipper factors, including two inhibitors of EmBP-1 DNA binding activity

The promoter of the wheat Em gene contains elements with a CACGTG core sequence (G-boxes), which are recognized by EmBP-1, a wheat basic/leucine zipper (bZIP) protein. G-boxes are required for Em expression in response to the phytohormone abscisic acid and for transactivation by the Viviparous-1 protein (VP1) using transient expression systems. In order to identify other factors that are part of the transcriptional complex that associates with G-boxes, we have screened a rice (Oryza sativa) cDNA library with biotinylated EmBP-1. We have isolated osZIP-1a, a homolog of EmBP-1 and other plant G-box-binding factors. We show that EmBP-1 and osZIP-1a will preferentially heterodimerize in vitro. Overexpression of osZIP-1a in rice protoplasts can enhance expression from the Em promoter only in the presence of abscisic acid. Two other clones have been identified by screening with EmBP-1: osZIP-2a and osZIP-2b. These osZIP-2 factors represent a novel class of bZIP proteins with an unusual DNA-binding domain that does not recognize G-boxes. The osZIP-2 factors can heterodimerize with EmBP-1 and prevent it from binding to the Em promoter. Interestingly, osZIP-1a does not heterodimerize with the osZIP-2 factors and its DNA binding activity is unaffected by their presence. Thus, osZIP-2 factors may be involved in sequestering a particular group of G-box-binding factors into inactive heterodimers.

A conserved domain of the viviparous-1 gene product enhances the DNA binding activity of the bZIP protein EmBP-1 and other transcription factors

The maize VP1 protein is a seed-specific regulator of gene expression that effects the expression of a subset of abscisic acid (ABA)-regulated genes that are expressed during the maturation program of the seed. In addition, VP1 has pleiotropic effects on seed development that are not related to ABA. In transient expression assays, VP1 has been shown to transactivate gene expression through at least two distinct promoter elements: the G boxes from the ABA-inducible wheat Em gene and the SphI box from the maize C1 gene. We have investigated how VP1 can transactivate gene expression through diverse promoter elements by analyzing its association in vitro with EmBP-1, a factor that binds the Em promoter. We demonstrate that VP1 can greatly enhance the DNA binding activity of EmBP-1 in a gel retardation assay. This enhancing activity has also been observed on transcription factors as diverse as Opaque-2, Max, Sp1, and NF-kappaB. Deletion of a small but highly conserved region (BR2) in VP1 eliminates the enhancement in vitro as well as the ability of VP1 to transactivate Em gene expression in a transient expression assay. A 40-amino acid fragment from VP1 sandwiched between the maltose-binding protein and LacZ can confer the enhancement function to this fusion protein in vitro. A weak and relatively nonspecific interaction between BR2 and DNA is demonstrated by UV cross-linking. The in vitro properties we observe for VP1 might explain the regulatory effects of VP1 on a diverse set of genes and why mutations in the vp1 locus have pleiotropic effects.

Evolution of the Group 1 late embryogenesis abundant (Lea) genes: analysis of the Lea B19 gene family in barley

The highly conserved Group 1 late embryogenesis abundant (Lea) genes are present in the genome of most plants as a gene family. Family members are conserved along the entire coding region, especially within the extremely hydrophilic internal 20 amino acid motif, which may be repeated. Cloning of Lea Group 1 genes from barley resulted in the characterization of four family members named B19.1, B19.1b, B19.3 and B19.4 after the presence of this motif 1, 1, 3 and 4 times in each gene, respectively. We present here the results of comparative and evolutionary analyses of the barley Group 1 Lea gene family (B19). The most important findings resulting from this work are (1) the tandem clustering of B19.3 and B19.4, (2) the spatial conservation of putative regulatory elements between the four B19 gene promoters, (3) the determination of the relative 'age' of the gene family members and (4) the 'chimeric' nature of B19.3 and B19.4, reflecting a cross-over or gene-conversion event in their common ancestor. We also show evidence for the presence of one or two additional expressed B19 genes in the barley genome. Based on our results, we present a model for the evolution of the family in barley, including the 20 amino acid motif. Comparisons of the relatedness between the barley family and all other known Group 1 Lea genes using maximum parsimony (PAUP) analysis provide evidence for the time of divergence between the barley genes containing the internal motif as a single copy and as a repeat. The PAUP analyses also provide evidence for independent duplications of Group 1 genes containing the internal motif as a repeat in both monocots and dicots.

Another Lea B19 gene (Group1 Lea) from barley containing a single 20 amino acid hydrophilic motif

A new member of the Lea B19 gene family from barley, termed B19.1b, has been isolated and characterized. The coding region of B19.1b is highly similar to the other members of the B19 family (Espelund et al., Plant J 2 (1992) 241-252) and contains only one copy of the hydrophilic sequence found as a repeat in two other B19 genes. Like the other B19 genes, B19.1b is only expressed in embryos. The transcript appears during development at 25 days after anthesis and remains at a high level throughout embryogenesis. In immature embryos the B19.1b mRNA can be induced by abscisic acid, salt and mannitol.

Sequence analysis of two tandemly linked Em genes from wheat

DNA sequences are presented for two members of the wheat Em gene family. The sequences correspond to the two linked genes at the Xem-1AL locus. Comparisons of these sequences with that of another wheat Em gene and two Em cDNA clones reveals substantial homology within the protein-coding regions, and the presence in the 5'-flanking regions of the genomic sequences of motifs characteristic of ABA-responsive cis-acting elements.

Two different Em-like genes are expressed in Arabidopsis thaliana seeds during maturation

Using a radish cDNA probe, we have isolated and characterized two genomic clones from Arabidopsis thaliana (GEA1 and GEA6) encoding two different proteins that are homologous to the "Early methionine-labelled" (Em) protein of wheat. GEA1 differs from GEA6 and Em clones of wheat in that a sequence coding for 20 amino acid residues is tandemly repeated 4 times. These two genomic clones correspond to two genes named AtEm1 and AtEm6. Sequencing of several cDNA clones showed that both genes are expressed. The transcription start site was determined for both genes by RNase mapping. The site of polyadenylation is variable and there is no obvious consensus sequence for polyadenylation at the 3' ends of the genes. mRNA corresponding to GEA6 is present only in nearly dry and dry seeds, whereas the corresponding to GEA1 appears in immature seeds and is maximum in dry seeds. No expression of either gene could be detected in leaf, stem, or floral buds. Expression of both genes could be detected in immature seeds when the siliques were incubated with abscisic acid (ABA), demonstrating that both genes are ABA responsive. However, examination of the 5' upstream region does not reveal any extensive homology, suggesting that regulation of the two genes differs. In situ hybridization with a GEA1 probe demonstrated that the expression of this gene is essentially located in the provascular tissues of the cotyledons and axis of the dry seed as well as in the epiderm and outer layers of the cortex in the embryo axis.

Differential response of maize catalases to abscisic acid: Vp1 transcriptional activator is not required for abscisic acid-regulated Cat1 expression

In this paper we describe the distinctive responses of the maize catalases to the plant growth regulator abscisic acid (ABA). We analyzed RNA and enzyme accumulation in excised maize embryos and found that each catalase responded differently to exogenously applied ABA. Levels of Cat1 transcript and enzyme activity rapidly increased. In contrast, levels of Cat2 transcript and protein decreased, while Cat3 transcript levels were not affected. In developing kernels of the ABA-deficient/biosynthetic viviparous mutant vp5, lower levels of Cat1 RNA correlated with lower endogenous ABA levels when compared to measured levels in comparably aged wild-type siblings from the same ear. The maize vp1 mutant line is morphologically insensitive to normal endogenous levels of ABA. Analysis of the response of Cat1 to exogenously applied ABA in mutant and wild-type vp1 sibling embryos suggests that, unlike other ABA-responsive genes analyzed to date, the Vp1 gene product is not essential for the ABA-mediated regulation of Cat1. The significance of these responses to ABA in defining the roles of the various CATs in maize is discussed.

Late embryogenesis-abundant genes encoding proteins with different numbers of hydrophilic repeats are regulated differentially by abscisic acid and osmotic stress

The late embryogenesis-abundant (Lea) genes, which are suggested to act as desiccation protectants during seed desiccation and in water-stressed seedlings, can be induced by abscisic acid (ABA) and various kinds of water-related stress. Using cotton Lea cDNAs as probes it was found that several of the Lea genes are conserved at the mRNA level in dicots and monocots. By screening a barley cDNA library with a cotton Lea D19 cDNA a family of three members was isolated. The putative B19 proteins have strong similarities to the Em protein in wheat and to LEA proteins from several dicots. However, the middle part of the B19 proteins consists of a 20-amino acid motif repeated three and four times in B19.3 and B19.4, respectively, but only once in B19.1. The gene products are strongly hydrophilic, the internal 20-amino acid motif being the most hydrophilic part. This motif is found once in cotton Lea D19 but is repeated twice in cotton Lea D132, indicating that the repeats are universal among monocot and dicot B19-like genes. The B19 genes are regulated similarly during embryo development, but to very different levels. In contrast, they are differentially regulated by ABA and various types of osmotic stress. In immature embryos all three genes are responsive to ABA and mannitol. However, B19.1 is also responsive to salt. Cold stress does not induce B19 mRNAs; only a stabilization of the transcript levels is seen. These results suggest that the responses to salt stress and exogenous ABA operate through different pathways.

Mechanisms of action of abscisic acid at the cellular level

Abscisic acid (ABA) has been implicated in the control of a diverse range of physiological processes in higher plants. In this review, we focus on the events which constitute the cellular responses to ABA. Current evidence suggests that it is possible to classify the responses to ABA on the basis of whether they are rapid, involving ion fluxes (typified by the stomatal response), or slower and requiring alterations to gene expression (for example the response of cereal embryos to ABA). In our consideration of ABA stimulus response coupling pathways, we have chosen to highlight the role of the calcium ion in the rapid responses, while we have concentrated on the contribution of as-acting elements and trans-acting factors in the regulation of ABA-responsive genes. We also draw attention to the possibility that interaction may exist between these pathways. Additionally, we discuss the controls of ABA concentrations during development and in response to environmental stimuli. Factors which contribute to the controls of ABA sensitivity are also reviewed. In our conclusions, we suggest that a general role for ABA may be to prepare tissue for entry into a new and different physiological state, perhaps by resetting the direction of cellular metabolism. CONTENTS Summary 9 I. Introduction 10 II. Stimulus response coupling 17 Synopsis 27 Acknowledgements 28 References 28.

A plant leucine zipper protein that recognizes an abscisic acid response element

The mechanism by which phytohormones, like abscisic acid (ABA), regulate gene expression is unknown. An activity in nuclear extracts that interacts with the ABA response element (ABRE) from the 5' regulatory region of the wheat Em gene was identified. A complementary DNA clone was isolated whose product is a DNA binding protein (EmBP-1) that interacts specifically with an 8-base pair (bp) sequence (CACGTGGC) in the ABRE. A 2-bp mutation in this sequence prevented binding of EmBP-1. The same mutation reduced the ability of the ABRE to confer ABA responsiveness on a viral promoter in a transient assay. The 8-bp EmBP-1 target sequence was found to be conserved in several other ABA-responsive promoters and in promoters from plants that respond to signals other than ABA. Similar sequences are found in promoters from mammals, yeast, and in the major late promoter of adenovirus. The deduced amino acid sequence of EmBP-1 contains conserved basic and leucine zipper domains found in transcription factors in plants, yeast, and mammals. EmBP-1 may be a member of a highly conserved family of proteins that recognize a core sequence found in the regulatory regions of various genes that are integrated into a number of different response pathways.