Functional dissection of an abscisic acid (ABA)-inducible gene reveals two independent ABA-responsive complexes each containing a G-box and a novel cis-acting element

Module-specific regulation of the beta-phaseolin promoter during embryogenesis

The phas promoter displays stringent spatial regulation, being very highly expressed during embryogenesis and completely silent during all phases of vegetative development in bean, Phaseolus vulgaris. This pattern is maintained in transgenic tobacco and, as shown here, Arabidopsis. Dimethyl sulphate in vivo footprinting analyses revealed that over 20 cis-elements within the proximal 295 bp of the phas promoter are protected by factor binding in seed tissues whereas none are bound in leaves. The hypothesis that this complex profile represents a summation of several module (cotyledon, hypocotyl, and radicle)-specific factor-DNA interactions has been explored by the incorporation of site-directed substitution mutations into 10 locations within the -295phas promoter. Only 2.6% of -295phas promoter activity remained after mutation of the G-box; the CCAAAT box, the E-box and the RY elements were also found to mediate high levels of expression in embryos. Whereas the CACA element has dual positive and negative regulatory roles, the vicilin box was identified as a strong negative regulatory element. The proximal (-70 to -64) RY motif was found to bestow expression in the hypocotyl while all the RY elements contribute to expression in cotyledons but not to vascular tissue expression during embryogenesis. RY elements at positions -277 to -271, -260 to -254, and -237 to -231 were found to orchestrate radicle-specific repression. The G-box appears to be the functional abscisic acid responsive element and the E-site may be a coupling element. The results substantiate the concept that autarkical cis-element functions generate modular patterning during embryogenesis. They also reflect the existence of both redundancy and hierarchy in cis-element interactions. Importantly, the virtually identical expression patterns observed for the two distantly related plants studied argue strongly for the generality of function for the observed factor-element interactions.

Structure and expression of the rice class-I type histone deacetylase genes OsHDAC1-3: OsHDAC1 overexpression in transgenic plants leads to increased growth rate and altered architecture

Histone deacetylases (HDACs) modulate chromatin structure and transcription. HDACs have been studied as negative regulators in eukaryotic transcription. We isolated the rice OsHDAC1-3 genes for class I-type histone deacetylases, which are related to the RPD3 family. The OsHDAC1 gene encoded a protein of approximately 57 kDa that shared 73.5, 72.7, 79.9, and 57.1% amino acid sequence identity with the OsHDAC2, OsHDAC3, maize RPD3, and human HDAC1 proteins, respectively. Genomic structures and Southern blot analyses revealed that OsHDAC1-3 contained seven, six, and seven exons, respectively, and constituted a class I-type family in the rice genome. OsHDAC1 was expressed at similar levels in the leaves, roots, and callus cells, whereas OsHDAC2 and 3 were expressed in the roots and callus cells, but not in the leaves, exhibiting distinct tissue specificity. To explore the role of histone deacetylases in transgenic plants, we inserted the OsHDAC1 cDNA fragment into the expression vector Ai::OsHDAC1 under the control of the ABA-inducible promoter Ai, and transformed the construct into rice. Levels of mRNA, protein, and HDAC activity were significantly increased in Ai::OsHDAC1 callus cells. The amount of tetra-acetylated H4 in the transgenic cells was greatly reduced, and the reduction was abolished upon treatment with trichostatin A. These results demonstrate that OsHDAC1 overexpression in transgenic cells both yields enzymatically active HDAC complexes and induces changes in histone acetylation in vivo. The overexpression leads to a range of novel phenotypes, involving increased growth rate and altered plant architecture, suggesting that OsHDAC1 functions in the genome-wide programming of gene expression.

The transcription factors HvABI5 and HvVP1 are required for the abscisic acid induction of gene expression in barley aleurone cells

The abscisic acid (ABA) response promoter complexes (ABRCs) of the HVA1 and HVA22 genes have been shown to confer ABA-induced gene expression in cereals. A barley basic domain/Leu zipper (bZIP) transcription factor, HvABI5, is able to recognize ABRCs in vitro in a sequence-specific manner and to transactivate ABRC-beta-glucuronidase reporter genes when introduced to barley aleurone cells via particle bombardment. This transactivation is dependent on the presence of another transcription factor, HvVP1, and cannot be blocked by the negative regulator abi1-1. Using the double-stranded RNA interference technique, we show that HvABI5 and HvVP1 are necessary for the ABA induction of gene expression but have no effect on another hormone-regulated process, the gibberellin-induced and ABA-suppressed expression of alpha-amylase. Our work indicates that although other typical plant bZIP transcription factors may bind ABRCs in vitro, HvABI5 is related to a subfamily of bZIPs responsible for the ABA induction of gene expression. Furthermore, HvABI5 and HvVP1 are not involved in the ABA suppression of gene expression.

Abscisic acid-induced transcription is mediated by phosphorylation of an abscisic acid response element binding factor, TRAB1

The rice basic domain/Leu zipper factor TRAB1 binds to abscisic acid (ABA) response elements and mediates ABA signals to activate transcription. We show that TRAB1 is phosphorylated rapidly in an in vivo labeling experiment and by phosphatase-sensitive mobility shifts on SDS-polyacrylamide gels. We had shown previously that a chimeric promoter containing GAL4 binding sites became ABA inducible when a GAL4 binding domain-TRAB1 fusion protein was present. This expression system allowed us to assay the ABA response function of TRAB1. Using this system, we show that Ser-102 of TRAB1 is critical for this function. Because no ABA-induced mobility shift was observed when Ser-102 was replaced by Ala, we suggest that this Ser residue is phosphorylated in response to ABA. Cell fractionation experiments, as well as fluorescence microscopy observations of transiently expressed green fluorescent protein-TRAB1 fusion protein, indicated that TRAB1 was localized in the nucleus independently of ABA. Our results suggest that the terminal or nearly terminal event of the primary ABA signal transduction pathway is the phosphorylation in the nucleus of preexisting TRAB1.

Maize ABI4 binds coupling element1 in abscisic acid and sugar response genes

Significant progress has been made in elucidating the mechanism of abscisic acid (ABA)-regulated gene expression, including the characterization of an ABA-responsive element (ABRE), which is regulated by basic domain/Leu zipper transcription factors. In addition to the ABRE, a coupling element (CE1) has been demonstrated to be involved in ABA-induced expression. However, a trans factor that interacts with CE1 has yet to be characterized. We report the isolation of a seed-specific maize ABI4 homolog and demonstrate, using a PCR-based in vitro selection procedure, that the maize ABI4 protein binds to the CE-1 like sequence CACCG. Using electrophoretic mobility shift assays, we demonstrate that recombinant ZmABI4 protein binds to the CE1 element in a number of ABA-related genes. ZmABI4 also binds to the promoter of the sugar-responsive ADH1 gene, demonstrating the ability of this protein to regulate both ABA- and sugar-regulated pathways. ZmABI4 complements Arabidopsis ABI4 function, because abi4 mutant plants transformed with the ZmABI4 gene have an ABA- and sugar-sensitive phenotype. Identification of the maize ABI4 ortholog and the demonstration of its binding to a known ABA response element provide a link between ABA-mediated kernel development and the regulation of ABA response genes.

Transcriptional and posttranscriptional regulation of Arabidopsis TCH4 expression by diverse stimuli. Roles of cis regions and brassinosteroids

The Arabidopsis TCH4 gene is up-regulated in expression by diverse environmental and hormonal stimuli. Because TCH4 encodes a xyloglucan endotransglucosylase/hydrolase, this change in expression may reflect a recruitment of cell wall-modifying activity in response to environmental stress and growth. How diverse stimuli lead to the common response of TCH4 expression regulation is not known. Here, we show that induction of expression by the diverse stimuli of touch, darkness, cold, heat, and brassinosteroids (BRs) is conferred to reporter genes by the same 102-bp 5'-untranscribed TCH4 region; this result is consistent with the idea that shared regulatory elements are employed by diverse stimuli. Distal regions influence magnitude and kinetics of expression and likely harbor regulatory elements that are redundant with those located more proximal to the transcriptional start site. Substitution of the proximal regulatory region sequences in the context of distal elements does not disrupt inducible expression. TCH4 expression induction is transcriptional, at least in part because 5'-untranscribed sequences are sufficient to confer this regulation. However, 5'-untranslated sequences are necessary and sufficient to confer the marked transience of TCH4 expression, most likely through an effect on mRNA stability. Perception of BR is not necessary for TCH4::GUS induction by environmental stimuli because regulation is intact in the BR-insensitive mutant, bri1-2. The full response to auxin, however, requires the functioning of BRI1. Developmental expression of TCH4 is unlikely to be meditated by BR because TCH4::GUS is expressed in BR perception and biosynthetic mutants bri1-2 and det2-1, respectively.

Identification and mapping of a putative stress response regulator gene in barley

Plants respond to environmental stress with a number of physiological and developmental changes. Water deficit is one of the major factors limiting plant growth and development and crop productivity. One response of plants to water deficit is accumulation of abscisic acid (ABA). An increase of ABA is responsible for the induction of many genes, presumably some of which contribute to drought tolerance. Analysis of gene expression in barley seedling shoots by differential display reverse transcriptase polymerase chain reaction (DDRT-PCR) led to the isolation of several drought-, cold- and ABA-induced partial cDNA fragments. Here we extensively characterize one of these cDNAs, designated DD6. First, a larger cDNA was extended from DD6 by 5'-RACE (rapid amplification of cDNA ends). Subsequently, the corresponding gene was isolated by screening a barley BAC library, and the sequences of the transcribed and flanking regions were determined. The deduced amino acid sequence has similarity to an Arabidopsis hypothetical protein and to a human and mouse DNA-binding protein. The corresponding gene, named Srg6 (stress-responsive gene), was mapped in a barley doubled haploid mapping population to chromosome 7H between markers ABC455 and salfp76, within a region that previously has been linked to osmotic adaptation in barley and other grass genomes.

Repression of stress-responsive genes by FIERY2, a novel transcriptional regulator in Arabidopsis

Low temperature, drought, and high salinity induce the expression of many plant genes. To understand the mechanisms for the transcriptional activation of these genes, we conducted a reporter gene-aided genetic screen in Arabidopsis. Seven allelic mutations in the FIERY2 (FRY2) locus result in significant increases in the expression of stress-responsive genes with the DRE/CRT (drought-responsive/C-repeat) cis element but non-DRE/CRT type stress-responsive genes were less affected. The specific regulation of DRE/CRT class of genes by FRY2 appears to be caused by repression of stress induction of the upstream CBF/DREB transcription factor genes. fry2 mutants show increased tolerance to salt stress and to abscisic acid during seed germination but are more sensitive to freezing damage at the seedling stage. FRY2/CPL1 encodes a novel transcriptional repressor harboring two double-stranded RNA-binding domains and a region homologous to the catalytic domain of RNA polymerase II C-terminal domain phosphatases found in yeast and in animals that regulate gene transcription. These data indicate that FRY2 is an important negative regulator of stress gene transcription and suggest that structured RNA may regulate hormone and stress responses in plants as it does in animals.

Cloning of the promoter for a novel barley gene, Lem1, and its organ-specific promotion of Gfp expression in lemma and palea

The differential display method was used to identify a novel barley gene, Lem1, expressed primarily in the outer organs (lemma and palea) that enclose developing florets and seeds. The promoter was isolated from a BAC genomic clone and used in a translational fusion with a green fluorescent protein gene (Gfp) to produce a transient expression vector. After particle bombardment, Gfp was expressed only in lemmas, paleas and awns of developing spikelets. Lem1 did not promote Gfp expression in vegetative leaves or in mature spikes, although expression of co-bombarded uidA (GUS) occurred under the regulation of a ubiquitin promoter. This reproduced the developmentally regulated pattern of mRNA accumulation. Deletion studies showed that the promoter activity is confined to a cis element within 80 bp of the transcription start site. Upstream from this, the promoter contains putative auxin-, ethylene- and gibberellin-responsive elements or homologues. Lem1 was found to be a single intronless gene encoding an acidic 102 amino acid protein, possibly associated with membranes. In a two-rowed barley, Lem1 mRNA was absent in the lateral spikelets, which fail to develop, and present only in the developing median spikelets. This suggests that Lem1 may play a role in organ development.

Citrulline and DRIP-1 protein (ArgE homologue) in drought tolerance of wild watermelon

Drought-affected plants experience more than just desiccation of their organs due to water deficit. Plants transpire 1000 times more molecules of water than of CO2 fixed by photosynthesis in full sunlight. One effect of transpiration is to cool the leaves. Accordingly, drought brings about such multi-stresses as high temperatures, excess photoradiation and other factors that affect plant viability. Wild watermelon serves as a suitable model system to study drought responses of C3 plants, since this plant survives drought by maintaining its water content without any wilting of leaves or desiccation even under severe drought conditions. Under drought conditions in the presence of strong light, wild watermelon accumulates high concentrations of citrulline, glutamate and arginine in its leaves. The accumulation of citrulline and arginine may be related to the induction of DRIP-1, a homologue of ArgE in Escherichia coli, where it functions to incorporate the carbon skeleton of glutamate into the urea cycle. Immunogold electron microscopy reveals the enzyme to be confined exclusively to the cytosol. DRIP-1 is also induced by treating wild watermelon with 150 mM NaCl, but is not induced following treatment with 100 microM abscisic acid. The salt treatment causes the accumulation of gamma-aminobutyrate, glutamine and alanine, in addition to a smaller amount of citrulline. Citrulline may function as a potent hydroxyl radical scavenger.

Chromosome regions and stress-related sequences involved in resistance to abiotic stress in Triticeae

Drought, low temperature and salinity are the most important abiotic stress factors limiting crop productivity. A genomic map of major loci and QTLs affecting stress tolerance in Triticeae identified the crucial role of the group 5 chromosomes, where the highest concentration of QTLs and major loci controlling plant's adaptation to the environment (heading date, frost and salt tolerance) has been found. In addition, a conserved region with a major role in drought tolerance has been localized to the group 7 chromosomes. Extensive molecular biological studies have led to the cloning of many stress-related genes and responsive elements. The expression of some stress-related genes was shown to be linked to stress-tolerant QTLs, suggesting that these genes may represent the molecular basis of stress tolerance. The development of suitable genetic tools will allow the role of stress-related sequences and their relationship with stress-tolerant loci to be established in the near future.

Experimentally determined sequence requirement of ACGT-containing abscisic acid response element

The sequence requirement of the ACGT-containing abscisic acid response element (ABRE) was analyzed by systematically substituting the bases surrounding the ACGT-core of motif A, the principal ABRE of the rice gene, OSEM: This was done within the context of a 55-bp promoter fragment that minimally confers ABA-responsiveness to a heterologous promoter. Based on this analysis, the sequence requirement of the ACGT-containing ABRE was determined as ACGTG G/T C, which matched very well with the consensus derived from sequence comparison of ABA-responsive promoters.

Dissection of abscisic acid signal transduction pathways in barley aleurone layers

Abscisic acid (ABA) induces genes that are highly expressed during late embryogenesis, but suppresses gibberellin (GA)-responsive genes essential for seed germination and seedling growth. Promoter elements necessary and sufficient for ABA up- and down-regulation of gene expression have been previously defined in barley aleurone layers. We have studied the effect of a protein phosphatase 2C, ABI1, an ABA-inducible protein kinase, PKABA1, and a transcription factor, VP1, on ABA action in a barley aleurone transient expression system. The observations have allowed us to dissect ABA signal transduction pathways leading to either induction or suppression of gene expression. The ABA induction of embryogenesis genes is highly inhibited in the presence of a mutated protein phosphatase 2C, encoded by the abi1-1 dominant mutant gene that is known to block ABA responses in Arabidopsis. However, the abi1-1 gene product has no effect on the ABA suppression of a GA-responsive alpha-amylase gene. On the other hand, PKABA1 suppresses the expression of alpha-amylase genes, but has little effect on ABA up-regulated genes. Therefore, it appears that ABA induction and suppression follow two separate signal transduction pathways with the former inhibited by ABI1 and the latter modulated by PKABA1. The presence of VP1 enhances the ABA induction of late embryogenesis genes, but also suppresses germination specific genes. A schematic model based on these observations is presented to explain the effect of these regulatory proteins on ABA-mediated gene expression.

The Arabidopsis LOS5/ABA3 locus encodes a molybdenum cofactor sulfurase and modulates cold stress- and osmotic stress-responsive gene expression

To understand low temperature and osmotic stress signaling in plants, we isolated and characterized two allelic Arabidopsis mutants, los5-1 and los5-2, which are impaired in gene induction by cold and osmotic stresses. Expression of RD29A-LUC (the firefly luciferase reporter gene under the control of the stress-responsive RD29A promoter) in response to cold and salt/drought is reduced in the los5 mutants, but the response to abscisic acid (ABA) remains unaltered. RNA gel blot analysis indicates that the los5 mutation reduces the induction of several stress-responsive genes by cold and severely diminishes or even completely blocks the induction of RD29A, COR15, COR47, RD22, and P5CS by osmotic stresses. los5 mutant plants are compromised in their tolerance to freezing, salt, or drought stress. los5 plants are ABA deficient, as indicated by increased transpirational water loss and reduced accumulation of ABA under drought stress in the mutant. A comparison with another ABA-deficient mutant, aba1, reveals that the impaired low-temperature gene regulation is specific to the los5 mutation. Genetic tests suggest that los5 is allelic to aba3. Map-based cloning reveals that LOS5/ABA3 encodes a molybdenum cofactor (MoCo) sulfurase. MoCo sulfurase catalyzes the generation of the sulfurylated form of MoCo, a cofactor required by aldehyde oxidase that functions in the last step of ABA biosynthesis in plants. The LOS5/ABA3 gene is expressed ubiquitously in different plant parts, and the expression level increases in response to drought, salt, or ABA treatment. Our results show that LOS5/ABA3 is a key regulator of ABA biosynthesis, stress-responsive gene expression, and stress tolerance.

The Arabidopsis LOS5/ABA3 locus encodes a molybdenum cofactor sulfurase and modulates cold stress- and osmotic stress-responsive gene expression

To understand low temperature and osmotic stress signaling in plants, we isolated and characterized two allelic Arabidopsis mutants, los5-1 and los5-2, which are impaired in gene induction by cold and osmotic stresses. Expression of RD29A-LUC (the firefly luciferase reporter gene under the control of the stress-responsive RD29A promoter) in response to cold and salt/drought is reduced in the los5 mutants, but the response to abscisic acid (ABA) remains unaltered. RNA gel blot analysis indicates that the los5 mutation reduces the induction of several stress-responsive genes by cold and severely diminishes or even completely blocks the induction of RD29A, COR15, COR47, RD22, and P5CS by osmotic stresses. los5 mutant plants are compromised in their tolerance to freezing, salt, or drought stress. los5 plants are ABA deficient, as indicated by increased transpirational water loss and reduced accumulation of ABA under drought stress in the mutant. A comparison with another ABA-deficient mutant, aba1, reveals that the impaired low-temperature gene regulation is specific to the los5 mutation. Genetic tests suggest that los5 is allelic to aba3. Map-based cloning reveals that LOS5/ABA3 encodes a molybdenum cofactor (MoCo) sulfurase. MoCo sulfurase catalyzes the generation of the sulfurylated form of MoCo, a cofactor required by aldehyde oxidase that functions in the last step of ABA biosynthesis in plants. The LOS5/ABA3 gene is expressed ubiquitously in different plant parts, and the expression level increases in response to drought, salt, or ABA treatment. Our results show that LOS5/ABA3 is a key regulator of ABA biosynthesis, stress-responsive gene expression, and stress tolerance.

The Arabidopsis LOS5/ABA3 locus encodes a molybdenum cofactor sulfurase and modulates cold stress- and osmotic stress-responsive gene expression

To understand low temperature and osmotic stress signaling in plants, we isolated and characterized two allelic Arabidopsis mutants, los5-1 and los5-2, which are impaired in gene induction by cold and osmotic stresses. Expression of RD29A-LUC (the firefly luciferase reporter gene under the control of the stress-responsive RD29A promoter) in response to cold and salt/drought is reduced in the los5 mutants, but the response to abscisic acid (ABA) remains unaltered. RNA gel blot analysis indicates that the los5 mutation reduces the induction of several stress-responsive genes by cold and severely diminishes or even completely blocks the induction of RD29A, COR15, COR47, RD22, and P5CS by osmotic stresses. los5 mutant plants are compromised in their tolerance to freezing, salt, or drought stress. los5 plants are ABA deficient, as indicated by increased transpirational water loss and reduced accumulation of ABA under drought stress in the mutant. A comparison with another ABA-deficient mutant, aba1, reveals that the impaired low-temperature gene regulation is specific to the los5 mutation. Genetic tests suggest that los5 is allelic to aba3. Map-based cloning reveals that LOS5/ABA3 encodes a molybdenum cofactor (MoCo) sulfurase. MoCo sulfurase catalyzes the generation of the sulfurylated form of MoCo, a cofactor required by aldehyde oxidase that functions in the last step of ABA biosynthesis in plants. The LOS5/ABA3 gene is expressed ubiquitously in different plant parts, and the expression level increases in response to drought, salt, or ABA treatment. Our results show that LOS5/ABA3 is a key regulator of ABA biosynthesis, stress-responsive gene expression, and stress tolerance.

FIERY1 encoding an inositol polyphosphate 1-phosphatase is a negative regulator of abscisic acid and stress signaling in Arabidopsis

The plant hormone abscisic acid (ABA) plays a wide range of important roles in plant growth and development, including embryogenesis, seed dormancy, root and shoot growth, transpiration, and stress tolerance. ABA and various abiotic stresses also activate the expression of numerous plant genes through undefined signaling pathways. To gain insight into ABA and stress signal transduction, we conducted a genetic screen based on ABA- and stress-inducible gene transcription. Here we report the identification of an Arabidopsis mutation, fiery1 (fry1), which results in super-induction of ABA- and stress-responsive genes. Seed germination and postembryonic development of fry1 are more sensitive to ABA or stress inhibition. The mutant plants are also compromised in tolerance to freezing, drought, and salt stresses. Map-based cloning revealed that FRY1 encodes an inositol polyphosphate 1-phosphatase, which functions in the catabolism of inositol 1, 4, 5-trisphosphate (IP(3)). Upon ABA treatment, fry1 mutant plants accumulated more IP(3) than did the wild-type plants. These results provide the first genetic evidence indicating that phosphoinositols mediate ABA and stress signal transduction in plants and their turnover is critical for attenuating ABA and stress signaling.

FIERY1 encoding an inositol polyphosphate 1-phosphatase is a negative regulator of abscisic acid and stress signaling in Arabidopsis

The plant hormone abscisic acid (ABA) plays a wide range of important roles in plant growth and development, including embryogenesis, seed dormancy, root and shoot growth, transpiration, and stress tolerance. ABA and various abiotic stresses also activate the expression of numerous plant genes through undefined signaling pathways. To gain insight into ABA and stress signal transduction, we conducted a genetic screen based on ABA- and stress-inducible gene transcription. Here we report the identification of an Arabidopsis mutation, fiery1 (fry1), which results in super-induction of ABA- and stress-responsive genes. Seed germination and postembryonic development of fry1 are more sensitive to ABA or stress inhibition. The mutant plants are also compromised in tolerance to freezing, drought, and salt stresses. Map-based cloning revealed that FRY1 encodes an inositol polyphosphate 1-phosphatase, which functions in the catabolism of inositol 1, 4, 5-trisphosphate (IP(3)). Upon ABA treatment, fry1 mutant plants accumulated more IP(3) than did the wild-type plants. These results provide the first genetic evidence indicating that phosphoinositols mediate ABA and stress signal transduction in plants and their turnover is critical for attenuating ABA and stress signaling.

Arginine decarboxylase transgene expression and analysis of environmental stress tolerance in transgenic rice

Arginine decarboxylase (ADC) cDNA from oat (Avena sativa L.) was introduced into rice (Oryza sativa L.) genome by an Agrobacterium-mediated transformation method. Expression of the ADC transgene under the control of an ABA-inducible promoter led to stress-induced upregulation of ADC activity and polyamine accumulation in transgenic rice plants. Second-generation (Rl) transgenic rice plants showed an increase in biomass under salinity-stress conditions, as compared to the non-transformed control plants.

The stress- and abscisic acid-induced barley gene HVA22: developmental regulation and homologues in diverse organisms

Abscisic acid (ABA) induces the expression of a battery of genes in mediating plant responses to environmental stresses. Here we report one of the early ABA-inducible genes in barley (Hordeum vulgare L.), HVA22, which shares little homology with other ABA-responsive genes such as LEA (late embryogenesis-abundant) and RAB (responsive to ABA) genes. In grains, the expression of HVA22 gene appears to be correlated with the dormancy status. The level of HVA22 mRNA increases during grain development, and declines to an undetectable level within 12 h after imbibition of non-dormant grains. In contrast, the HVA22 mRNA level remains high in dormant grains even after five days of imbibition. Treatment of dormant grains with gibberellin (GA) effectively breaks dormancy with a concomitant decline of the level of HVA22 mRNA. The expression of HVA22 appears to be tissue-specific with the level of its mRNA readily detectable in aleurone layers and embryos, yet undetectable in the starchy endosperm. The expression of HVA22 in vegetative tissues can be induced by ABA and environmental stresses, such as cold and drought. Apparent homologues of this barley gene are found in phylogenetically divergent eukaryotic organisms, including cereals, Arabidopsis, Caenorhabditis elegans, man, mouse and yeast, but not in any prokaryotes. Interestingly, similar to barley HVA22, the yeast homologue is also stress-inducible. These observations suggest that the HVA22 and its homologues encode a highly conserved stress-inducible protein which may play an important role in protecting cells from damage under stress conditions in many eukaryotic organisms.

Homeodomain leucine zipper proteins bind to the phosphate response domain of the soybean VspB tripartite promoter

The soybean (Glycine max L. Merr. cv Williams 82) genes VspA and VspB encode vacuolar glycoprotein acid phosphatases that serve as vegetative storage proteins during seed fill and early stages of seedling growth. VspB expression is activated by jasmonates (JAs) and sugars and down-regulated by phosphate and auxin. Previous promoter studies demonstrated that VspB promoter sequences between -585 and -535 mediated responses to JA, and sequences between -535 and -401 mediated responses to sugars, phosphate, and auxin. In this study, the response domains were further delineated using transient expression of VspB promoter-beta-glucuronidase constructs in tobacco protoplasts. Sequences between -536 and -484 were identified as important for phosphate responses, whereas the region from -486 to -427 mediated sugar responses. Gel-shift and deoxyribonuclease-I footprinting assays revealed four DNA-binding sites between -611 and -451 of the soybean VspB promoter: one in the JA response domain, two in the phosphate response domain, and one binding site in the sugar response domain. The sequence CATTAATTAG present in the phosphate response domain binds soybean homeodomain leucine zipper proteins, suggesting a role for these transcription factors in phosphate-modulated gene expression.

Arabidopsis basic leucine zipper transcription factors involved in an abscisic acid-dependent signal transduction pathway under drought and high-salinity conditions

The induction of the dehydration-responsive Arabidopsis gene, rd29B, is mediated mainly by abscisic acid (ABA). Promoter analysis of rd29B indicated that two ABA-responsive elements (ABREs) are required for the dehydration-responsive expression of rd29B as cis-acting elements. Three cDNAs encoding basic leucine zipper (bZIP)-type ABRE-binding proteins were isolated by using the yeast one-hybrid system and were designated AREB1, AREB2, and AREB3 (ABA-responsive element binding protein). Transcription of the AREB1 and AREB2 genes is up-regulated by drought, NaCl, and ABA treatment in vegetative tissues. In a transient transactivation experiment using Arabidopsis leaf protoplasts, both the AREB1 and AREB2 proteins activated transcription of a reporter gene driven by ABRE. AREB1 and AREB2 required ABA for their activation, because their transactivation activities were repressed in aba2 and abi1 mutants and enhanced in an era1 mutant. Activation of AREBs by ABA was suppressed by protein kinase inhibitors. These results suggest that both AREB1 and AREB2 function as transcriptional activators in the ABA-inducible expression of rd29B, and further that ABA-dependent posttranscriptional activation of AREB1 and AREB2, probably by phosphorylation, is necessary for their maximum activation by ABA. Using cultured Arabidopsis cells, we demonstrated that a specific ABA-activated protein kinase of 42-kDa phosphorylated conserved N-terminal regions in the AREB proteins.

PLANTCELLULAR ANDMOLECULARRESPONSES TOHIGHSALINITY

Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to those that regulate osmotic stress responses of yeast. There is evidence also of signaling cascades that are not known to exist in the unicellular eukaryote, some that presumably function in intercellular coordination or regulation of effector genes in a cell-/tissue-specific context required for tolerance of plants. A complex set of stress-responsive transcription factors is emerging. The imminent availability of genomic DNA sequences and global and cell-specific transcript expression data, combined with determinant identification based on gain- and loss-of-function molecular genetics, will provide the infrastructure for functional physiological dissection of salt tolerance determinants in an organismal context. Furthermore, protein interaction analysis and evaluation of allelism, additivity, and epistasis allow determination of ordered relationships between stress signaling components. Finally, genetic activation and suppression screens will lead inevitably to an understanding of the interrelationships of the multiple signaling systems that control stress-adaptive responses in plants.

Alternative transcript initiation and novel post-transcriptional processing of a leucine-rich repeat receptor-like protein kinase gene that responds to short-day photoperiodic floral induction in morning glory (Ipomoea nil)

A gene (inrpk1) encoding a putative receptor-like protein kinase was isolated from the Japanese morning glory, Ipo-moea (Pharbitis) nil Roth. cv. Violet. The receptor-like portion of the largest derived polypeptide contains 26 direct leucine-rich repeats (LRRs) in a single block, and the catalytic portion has all the conserved amino acid residues characteristic of Ser/Thr protein kinases. RNA blot analysis detected multiple transcripts in cotyledons. The largest (4.4 kb) transcript encodes the predicted full length polypeptide (INRPK1), whereas a 1.6 kb transcript apparently originates from a secondary transcription initiation site within the gene and potentially encodes a protein kinase identical to INRPK1 but lacking most of the LRRs. Two transcripts (ca. 2.7 and 2.6 kb) are created by alternative 3'-splicing of a large (ca. 1.4-1.5 kb) cryptic intron in the LRR region, creating one transcript (2.6 kb) potentially encoding a small, secretable polypeptide. The larger transcript encoding a polypeptide identical to INRPK1, but lacking 21 LRRs, predominates in vegetative roots. Competitive PCR indicates that inrpk1 mRNA increases 20-fold in cotyledons in response to a previously given single floral-inducing short-day (SD). No differences of this magnitude were detected in any other organs examined from plants similarly treated. This pattern of expression and differential processing suggests a role for inrpk1 in some aspect of SD photoperiodic-induced flowering in morning glory.

Amino acid transporters in plants

Amino acid transporters are essential participants in the resource allocation processes that support plant growth and development. Recent results have identified several new transporters that contribute to a wide array of physiological activities, and detailed molecular analysis has provided fundamental insights into the structure, function and regulation of these integral membrane proteins.

Glucose modulates the abscisic acid-inducible Rab16A gene in cereal embryos

Glucose effects on the expression of the abscisic acid-inducible Rab16A gene were examined in rice and barley embryos. Glucose feeding to rice embryos negatively affects the endogenous abscisic acid content and represses the promoter activity of the Rab16A gene. Glucose repression of the Rab16A gene takes place both at a transcriptional and a post-transcriptional level. Modulation of the abscisic acid content in rice embryos triggered by glucose did not directly influence the expression of the rice alpha-amylase gene RAmy3D, which is known to be under glucose control. The possible interaction between the glucose and abscisic acid signaling pathway is discussed.

Isolation and characterization of the wheat prolyl isomerase FK506-binding protein (FKBP) 73 promoter

The wheat FK506-binding protein (FKBP) 73 is a member of the peptidyl prolyl cis-trans isomerase gene family, which catalyses the interconversion between the cis and trans forms of the peptide bond preceding proline residues in proteins. A 3.5 kb sequence 5' upstream of the ATG codon of the wheat FKBP73 was isolated from a wheat genomic library, and characterized by deletion analysis and transient expression in wheat embryos. The 1517 bp fragment is referred to as the full promoter due to the maximal activity of the fused luciferase reporter gene. Sequence analysis revealed the presence of three abscisic acid (ABA)-responsive elements (ABREs) proximal to coupling elements (CE1-like), a putative lectin box, two putative binding sites for the myb transcription factor and a 36 bp fragment which exhibits 100% identity to the pSau3A9 clone located in the centromeric region of wheat chromosomes. In a transient expression assay the promoter preserved the tissue specificity described in vivo, namely it is expressed only in germinating embryos and young shoots. The promoter was induced 1.9-fold by ABA, the minimal promoter was designated at -221 and the TATA box located at -137. The inducibility by ABA and the expression during germination may indicate that FKBP73 belongs to the group of genes induced by ABA upon germination.

S phase-specific DNA-binding proteins interacting with the Hex and Oct motifs in type I element of the wheat histone H3 promoter

The type I element (CCACGTCANCGATCCGCG), consisting of the Hex motif (CCACGTCA) and the reverse-oriented Oct motif (GATCCGCG), is necessary and sufficient to confer the S phase-specific transcription of the wheat histone H3 (TH012) gene. The transcriptional regulation via the type I element is thought to occur through interactions between transcription factors which bind specifically to the Hex and Oct motifs. Here we report S phase-specific DNA-binding proteins interacting with the type I element in partially synchronized wheat cultured cells. Hex motif-binding proteins found here resembled HBP-1a, as reported previously in terms of DNA-binding specificity. DNA-binding activities of the HBP-1a-like proteins were modulated by phosphorylation/dephosphorylation. In the electrophoretic mobility shift assay of the wheat nuclear extract, we also found three Oct motif-specific binding proteins, named OBRF (octamer-binding regulatory factor)-1, -2 and -3. One of the HBP-1a-like proteins and OBRF-1 appeared predominantly at the S phase. Thus, it was supposed that these two factors play a crucial role in the S phase-specific regulation of wheat histone gene expression.

A bZIP factor, TRAB1, interacts with VP1 and mediates abscisic acid-induced transcription

The transcription factor VP1 regulates maturation and dormancy in plant seeds by activating genes responsive to the stress hormone abscisic acid (ABA). Although activation involves ABA-responsive elements (ABREs), VP1 itself does not specifically bind ABREs. Instead, we have identified and cloned a basic region leucine zipper (bZIP) factor, TRAB1, that interacts with both VP1 and ABREs. Transcription from a chimeric promoter with GAL4-binding sites was ABA-inducible if cells expressed a GAL4 DNA-binding domain::TRAB1 fusion protein. Results indicate that TRAB1 is a true trans-acting factor involved in ABA-regulated transcription and reveal a molecular mechanism for the VP1-dependent, ABA-inducible transcription that controls maturation and dormancy in plant embryos.

The maize EmBP-1 orthologue differentially regulates opaque2-dependent gene expression in yeast and cultured maize endosperm cells

In addition to the bZIP protein Opaque2 (O2), there are other maize endosperm nuclear proteins that recognize the O2 box in 22 kDa zein gene promoters. In an effort to understand the effect of these factors on 22 kDa zein expression, we have cloned one of these and identified it as the putative maize (Zea mays L.) orthologue of the wheat bZIP protein EmBP-1 (mEmBP-1). The mEmBP-1 protein exhibits 52% sequence identity and 68% similarity with the wheat protein and recognizes a similar spectrum of DNA sequences, albeit with slightly altered specificity. The mEmBP-1 gene exists as duplicate loci in maize on chromosomes 7S (mEmBP-1a) and 2L (mEmBP-1b). The mEmBP-1 genes are expressed in endosperm, embryo, immature ears, tassel, roots, and seedling shoots at low levels. Although mEmBP-1 binds to the O2 box from the 22 kDa zein gene promoter as a homodimer, it is unable to heterodimerize with O2. The mEmBP-1 protein can activate transcription from a truncated promoter containing a pentamer of the O2 site in yeast cells; however, it inhibited regulated transcription of a 22 kDa zein promoter in a transient expression assay using cultured maize endosperm cells.

ACGT-containing abscisic acid response element (ABRE) and coupling element 3 (CE3) are functionally equivalent

ACGT-containing ABA response elements (ABREs) have been functionally identified in the promoters of various genes. In addition, single copies of ABRE have been found to require a cis-acting, coupling element to achieve ABA induction. A coupling element 3 (CE3) sequence, originally identified as such in the barley HVA1 promoter, is found approximately 30 bp downstream of motif A (ACGT-containing ABRE) in the promoter of the Osem gene. The relationship between these two elements was further defined by linker-scan analyses of a 55 bp fragment of the Osem promoter, which is sufficient for ABA-responsiveness and VP1 activation. The analyses revealed that both motif A and CE3 sequence were required not only for ABA-responsiveness but also for VP1 activation. Since the sequences of motif A and CE3 were found to be similar, motif-exchange experiments were carried out. The experiments demonstrated that motif A and CE3 were interchangeable by each other with respect to both ABA and VP1 regulation. In addition, both sequences were shown to be recognized by a VP1-interacting, ABA-responsive bZIP factor TRAB1. These results indicate that ACGT-containing ABREs and CE3 are functionally equivalent cis-acting elements. Furthermore, TRAB1 was shown to bind two other non-ACGT ABREs. Based on these results, all these ABREs including CE3 are proposed to be categorized into a single class of cis-acting elements.

Interaction between composite elements in the napA promoter: both the B-box ABA-responsive complex and the RY/G complex are necessary for seed-specific expression

During seed maturation, the transcriptional activity of napin genes is regulated by developmental signals involving the transcriptional activator ABI3 and abscisic acid (ABA). To localize cis elements involved in the seed-specific activity of the napin napA promoter, a systematic analysis was performed focusing on two major element complexes, the B-box and RY/G. Substitution mutation analysis using promoter-reporter gene fusions in stable transgenic tobacco showed synergistic interactions between elements within these complexes. The distal part of the B-box shows similarities to abscisic acid response elements and the proximal portion contains a CA-rich element. In vitro studies involving Exonuclease III protection and electrophoretic mobility shift assays revealed binding by nuclear proteins to elements within the B-box. The distal and proximal parts of the B-box were found to bind distinct nuclear protein complexes. By gain-of-function analysis with a tetramer of the B-box fused to a truncated (-46) cauliflower mosaic virus (CaMV) 35S minimal promoter, it was demonstrated that the B-box mediates strong activity in seeds. Further, it was shown that the elements in the B-box constitute an ABA-responsive complex, since the B-box tetramer mediates ABA-responsiveness in vegetative tissues to a construct containing the CaMV virus 35S enhancer (-343 to -90). Thus, the seed-specific activity of the napA promoter relies on the combinatorial interaction between the RY/G complex and the B-box ABA-responsive complex during the ABA response in seed development.

An abscisic acid-induced protein kinase, PKABA1, mediates abscisic acid-suppressed gene expression in barley aleurone layers

The phytohormone abscisic acid (ABA) induces genes-encoding proteins involved in desiccation tolerance and dormancy in seeds, but ABA also suppresses gibberellin (GA)-responsive genes encoding hydrolytic enzymes essential for postgermination growth. A unique serine/threonine protein kinase, PKABA1 mRNA, up-regulated by ABA in seeds, has been identified. In this report, the effect of PKABA1 on the signal transduction pathway mediating ABA induction and suppression of genes has been determined in aleurone layers of barley seeds. Two groups of gene constructs were introduced to barley aleurone layers by using particle bombardment: the reporter constructs containing the coding sequence of beta-glucuronidase gene linked to hormone-responsive promoters and the effector constructs containing the coding region of protein kinases linked to a constitutive promoter. Constitutive expression of PKABA1 drastically suppressed expression of low- and high-pI alpha-amylase and protease genes induced by GA. However, the presence of PKABA1 had only a small effect on the ABA induction of a gene encoding a late embryogenesis abundant protein, HVA1. Our results indicate that PKABA1 acts as a key intermediate in the signal transduction pathway leading to the suppression of GA-inducible gene expression in cereal aleurone layers.

cDNA and genomic cloning of sugar beet V-type H+-ATPase subunit A and c isoforms: evidence for coordinate expression during plant development and coordinate induction in response to high salinity

The plant V-type H+-ATPase (V-ATPase) does not only serve basic housekeeping functions but is also involved in stress-induced NaCl sequestration during salinity stress. To address the question whether the same isoforms conferring housekeeping functions are equally involved in the response to high salinity, we have isolated cDNA clones for subunits A and c, as representing the peripheral V1 complex and the membrane-integral V0 complex, respectively, from the halotolerant sugar beet (Beta vulgaris L., diploid variety). RNA blot analysis with gene-specific probes revealed a coordinate expression of the cloned subunit A and c isoforms during plant development and in response to high salinity. Also, in rapidly dividing suspension-cultured cells with 10-fold increased transcript amounts as compared to young leaf tissue, the ratio of transcripts for both genes was similar to the ratio found for transcripts in leaves of different age. We have then isolated partial genomic clones (BVA/70 for Beta V-ATPase 70 kDa subunit; BVA/16-1 for Beta V-ATPase 16 kDa subunit), including the promoter regions. Transcription start mapping revealed long 5'-UTR leader sequences (230 and 172 bases, respectively) for both genes. Both promoters contain putative G-box motifs in similar distance to the TATA boxes. For a quantitative comparison of relative promoter strength, the BVA/70 and BVA/16-1 promoters linked to the luciferase reporter gene (LUC) were delivered to sugar beet suspension-cultured cells by particle bombardment. The BVA/16-1 promoter showed a 1.7-fold higher activity as compared with the BVA/70 promoter. Salt treatment induced an increase of BVA/70 (+70%) and BVA/16-1 (+57%) promoter activities, concomitant with increased transcript amounts. The following sequences have been deposited at the EMBL database X98767: Beta vulgaris V-ATPase subunit A, cDNA clone; X98851, B. vulgaris V-ATPase subunit c isoform 1, cDNA clone; Y11038, B. vulgaris V-ATPase subunit A, partial genomic clone; Y11037, B. vulgaris V-ATPase subunit c isoform 1, partial genomic clone.

Identification of promoter elements in a low-temperature-responsive gene (blt4.9) from barley (Hordeum vulgare L.)

The blt4 barley gene family encodes non-specific lipid transfer proteins and has been shown, by in situ localisation, to be expressed in the epidermal cells of leaves. The transcriptionally controlled, low-temperature-responsive member of this gene family, blt4.9, is predominantly expressed in shoot meristems. The promoter region (1938 bp) of blt4.9 contains sequence motifs which have been implicated in responses to low temperature, abscisic acid and other environmental factors. Deletion analysis showed that a 42 bp sequence proximal to, but not including, the CAAT and TATA boxes, confers enhanced low-temperature response to a reporter gene in a barley shoot explant transient expression system. Other promoter regions were shown to contain negative and positive regulatory regions. Electrophoretic mobility shift analysis (EMSA) was used with nuclear proteins from either low-temperature- or control-temperature-treated plants to further investigate the blt4.9 promoter. Synthetic oligonucleotides were used to identify a hexanucleotide, CCGAAA, within the 42 bp, low-temperature-responsive promoter region, as the binding site of a low-mobility nuclear protein complex. This complex was present in nuclear extracts from both low-temperature-treated and control plants and was the only complex formed within this region. Mutation of the CCGAAA motif within the low-temperature-responsive 42 bp promoter sequence reduced low-temperature responsiveness to basal levels. A related upstream element, CCGAC, known to be a low-temperature-responsive element in other plants, did not bind to nuclear proteins in this study. It is proposed that the hexanucleotide CCGAAA, at -195 from the first ATG, is involved in the low-temperature response of blt4.9 in barley.

Overexpression of deltaEmBP, a truncated dominant negative version of the wheat G-box binding protein EmBP-1, alters vegetative development in transgenic tobacco

As a first step toward elucidating the in vivo function of plant bZIP proteins and their related G-box cis elements, we have introduced a dominant negative inhibitor of G-box-dependent transcriptional activation into tobacco plants by transforming them with a truncated EmBP-1 gene (deltaEmBP) containing the DNA binding and dimerization domains under the control of the CaMV 35S promoter. Five independent lines of transgenic plants expressing deltaEmBP were identified, as demonstrated by immunodetection of the transgenic protein in leaf extracts, and the ability of the protein to bind a target G-box DNA sequence. The transgenic plants exhibited an abnormal phenotype characterized by interveinal chlorosis, growth inhibition and weakening of stems and petioles, the severity of which positively correlated with deltaEmBP expression and G-box DNA binding capability. Furthermore, development of chlorosis and growth inhibition was dependent on growth irradiance. Low light promoted the development of interveinal chlorosis and growth inhibition in the transgenic plants, whereas high light conditions led to near-complete amelioration of the abnormal phenotype. Transgenic plants under both light regimes showed signs of impaired stem and petiole function which was not observed in wild-type tobacco. RhcS gene expression was not significantly altered by deltaEmBP expression, suggesting that down-regulation of this gene was not responsible for the altered phenotype. The results suggest that G-box elements specific for the EmBP-1 class of bZIP proteins have an important developmental function in vegetative plant tissues, and that the trans-dominant negative mutant approach is a useful tool for continued in vivo functional analysis of bZIP transcription factors and their corresponding cis elements in plants.

Dehydration-stress-regulated transgene expression in stably transformed rice plants

To confer abscisic acid (ABA) and/or stress-inducible gene expression, an ABA-response complex (ABRC1) from the barley (Hordeum vulgare L.) HVA22 gene was fused to four different lengths of the 5' region from the rice (Oryza sativa L.) Act1 gene. Transient assay of beta-glucuronidase (GUS) activity in barley aleurone cells shows that, coupled with ABRC1, the shortest minimal promoter (Act1-100P) gives both the greatest induction and the highest level of absolute activity following ABA treatment. Two plasmids with one or four copies of ABRC1 combined with the same Act1-100P and HVA22(I) of barley HVA22 were constructed and used for stable expression of uidA in transgenic rice plants. Three Southern blot-positive lines with the correct hybridization pattern for each construct were obtained. Northern analysis indicated that uidA expression is induced by ABA, water-deficit, and NaCl treatments. GUS activity assays in the transgenic plants confirmed that the induction of GUS activity varies from 3- to 8-fold with different treatments or in different rice tissues, and that transgenic rice plants harboring four copies of ABRC1 show 50% to 200% higher absolute GUS activity both before and after treatments than those with one copy of ABRC1.

ABSCISIC ACID SIGNAL TRANSDUCTION

The plant hormone abscisic acid (ABA) plays a major role in seed maturation and germination, as well as in adaptation to abiotic environmental stresses. ABA promotes stomatal closure by rapidly altering ion fluxes in guard cells. Other ABA actions involve modifications of gene expression, and the analysis of ABA-responsive promoters has revealed a diversity of potential cis-acting regulatory elements. The nature of the ABA receptor(s) remains unknown. In contrast, combined biophysical, genetic, and molecular approaches have led to considerable progress in the characterization of more downstream signaling elements. In particular, substantial evidence points to the importance of reversible protein phosphorylation and modifications of cytosolic calcium levels and pH as intermediates in ABA signal transduction. Exciting advances are being made in reassembling individual components into minimal ABA signaling cascades at the single-cell level.

Developmental control of H+/amino acid permease gene expression during seed development of Arabidopsis

Long distance transport of amino acids is mediated by several families of differentially expressed amino acid transporters. The two genes AAP1 and AAP2 encode broad specificity H(+)-amino acid co-transporters and are expressed to high levels in siliques of Arabidopsis, indicating a potential role in supplying the seeds with organic nitrogen. The expression of both genes is developmentally controlled and is strongly induced in siliques at heart stage of embryogenesis, shortly before induction of storage protein genes. Histochemical analysis of transgenic plants expressing promoter-GUS fusions shows that the genes have nonoverlapping expression patterns in siliques. AAP1 is expressed in the endosperm and the cotyledons whereas AAP2 is expressed in the vascular strands of siliques and in funiculi. The endosperm expression of AAP1 during early stages of seed development indicates that the endosperm serves as a transient storage tissue for organic nitrogen. Amino acids are transported in both xylem and phloem but during seed filling are imported only via the phloem. AAP2, which is expressed in the phloem of stems and in the veins supplying seeds, may function in uptake of amino acids assimilated in the green silique tissue, in the retrieval of amino acids leaking passively out of the phloem and in xylem-to-phloem transfer along the path. The promoters provide excellent tools to study developmental, hormonal and metabolic control of nitrogen nutrition during development and may help to manipulate the timing and composition of amino acid import into seeds.

Characterization of a gene encoding an abscisic acid-inducible type-2 lipid transfer protein from rice

The cloning and sequence analysis of a novel gene that encodes a type 2 non-specific lipid transfer-like protein (LTP) from rice is reported. Sequence analysis revealed an ORF encoding a protein showing characteristics of the LTP proteins. However, rice LTP2 is more similar to heterologous LTPs than to rice LTP1, supporting the existence of two distinct families of plant LTPs. Ltp2 mRNA is accumulated only in mature seeds. In vegetative tissues, mRNA was only detected after treatment with abscisic acid (ABA), mannitol or NaCl. Transient expression experiments that the 61 nucleotides upstream of the TATA box, containing two ACGT boxes and the motif I, are sufficient for ABA responsiveness of the Ltp gene.

Three drought-responsive members of the nonspecific lipid-transfer protein gene family in Lycopersicon pennellii show different developmental patterns of expression

Genomic clones of two nonspecific lipid-transfer protein genes from a drought-tolerant wild species of tomato (Lycopersicon pennellii Corr.) were isolated using as a probe a drought- and abscisic acid (ABA)-induced cDNA clone (pLE16) from cultivated tomato (Lycopersicon esculentum Mill.). Both genes (LpLtp1 and LpLtp2) were sequenced and their corresponding mRNAs were characterized; they are both interrupted by a single intron at identical positions and predict basic proteins of 114 amino acid residues. Genomic Southern data indicated that these genes are members of a small gene family in Lycopersicon spp. The 3'-untranslated regions from LpLtp1 and LpLtp2, as well as a polymerase chain reaction-amplified 3'-untranslated region from pLE16 (cross-hybridizing to a third gene in L. pennellii, namely LpLtp3), were used as gene-specific probes to describe expression in L. pennellii through northern-blot analyses. All LpLtp genes were exclusively expressed in the aerial tissues of the plant and all were drought and ABA inducible. Each gene had a different pattern of expression in fruit, and LpLtp1 and LpLtp2, unlike LpLtp3, were both primarily developmentally regulated in leaf tissue. Putative ABA-responsive elements were found in the proximal promoter regions of LpLtp1 and LpLtp2.

Conserved Ser residues in the basic region of the bZIP-type transcription factor HBP-1a(17): importance in DNA binding and possible targets for phosphorylation

HBP-1a(17) is representative of a group of plant bZIP-type transcription factors which includes HBP-1a proteins and G-box-binding factors. We found kinase activity in wheat nuclear extract that phosphorylated HBP-1a(17). Experiments using recombinant HBP-1a(17) derivatives as substrates revealed that all three of the Ser residues in the basic region, Ser-261, Ser-265, and Ser-269, were phosphorylated in a Ca(2+)-stimulated manner. DNA-binding analysis of mutants with a Ser-to-Glu change, prepared to mimic the phosphorylated proteins, indicated that introduction of a negative charge at position 265 or 269 prevents HBP-1a(17) from binding DNA not only in the homodimer of mutants but also in heterodimers with a wild-type protein. It is therefore suggested that the phosphorylation regulates the function of HBP-1a(17) at least at the level of DNA binding. Since Ser-265 and Ser-269 are highly conserved among the plant bZIP-type factors known to date, a common Ca(2+)-mediated regulatory mechanism may exert an effect on the bZIP-type factors through phosphorylation of these conserved Ser residues.

Protein binding to the abscisic acid-responsive element is independent of VIVIPAROUS1 in vivo

The plant hormone abscisic acid and the transcriptional activator VIVIPAROUS1 have a synergistic effect on transcription during embryo development. An abscisic acid-responsive element (ABRE) mediates induction by abscisic acid and VIVIPAROUS1, but the mechanism involved has not been determined. In this study, we explore the interaction between abscisic acid and VIVIPAROUS1 and its effect on the ABRE from the maize rab28 gene. In transient transformation experiments, abscisic acid stimulated transcription via several elements, whereas activation by VIVIPAROUS1 was mediated exclusively through the ABRE. In vivo footprinting showed only minor differences in binding to the ABRE between wild-type and VIVIPAROUS1-deficient embryos, suggesting that VIVIPAROUS1 stimulates transcription through the ABRE without major changes in protein-DNA interactions. A factor that bound to the ABRE in electrophoretic mobility shift assays was present at the same developmental stages as rab28 mRNA and had binding characteristics similar to those observed by in vivo footprinting. This suggests that the factor binds to the ABRE in the rab28 promoter in vivo. We discuss the constraints that our results put on the possible mechanism for action of VIVIPAROUS1 in vivo.

Genetic analysis of osmotic and cold stress signal transduction in Arabidopsis: interactions and convergence of abscisic acid-dependent and abscisic acid-independent pathways

To dissect genetically the complex network of osmotic and cold stress signaling, we constructed lines of Arabidopsis plants displaying bioluminescence in response to low temperature, drought, salinity, and the phytohormone abscisic acid (ABA). This was achieved by introducing into Arabidopsis plants a chimeric gene construct consisting of the firefly luciferase coding sequence (LUC) under the control of the stress-responsive RD29A promoter. LUC activity in the transgenic plants, as assessed by using in vivo luminescence imaging, faithfully reports the expression of the endogenous RD29A gene. A large number of cos (for constitutive expression of osmotically responsive genes), los (for low expression of osmotically responsive genes), and hos (for high expression of osmotically responsive genes) mutants were identified by using a high-throughput luminescence imaging system. The los and hos mutants were grouped into 14 classes according to defects in their responses to one or a combination of stress and ABA signals. Based on the classes of mutants recovered, we propose a model for stress signaling in higher plants. Contrary to the current belief that ABA-dependent and ABA-independent stress signaling pathways act in a parallel manner, our data reveal that these pathways cross-talk and converge to activate stress gene expression.

Molecular cloning of GA-suppressed G2 pea genes by cDNA RDA

GA-treated and non-treated G2 pea cDNAs were compared using a newly developed method called cDNA representational difference analysis (cDNA-RDA), and several GA-suppressed mRNAs were found. After cloning of the larger fragments PGAS1-3 (pea GA-suppressed cDNA 1-3), they were demonstrated to be expressed only in pea tissue not treated with GA3 through Northern analysis. Compared with subtractive hybridization and differential display techniques, this method not only can be easily manipulated but also has a relatively low rate of false positive and is highly repetitive. It is the major progress in molecular cloning techniques.