Isolation of a novel class of bZIP transcription factors that interact with ABA-responsive and embryo-specification elements in the Dc3 promoter using a modified yeast one-hybrid system

Structure and expression of the Arabidopsis thaliana homeobox gene Athb-12

We have isolated the Arabidopsis thaliana homeobox gene Athb-12, determined its structure and activation domain, demonstrated that its promoter is inducible in response to abscisic acid (ABA) treatment, and characterized the cellular distribution of its transcripts. The single intron of the gene interrupted the leucine-zipper domain region. The 5' regulatory region of Athb-12 can drive beta-glucuronidase (GUS) expression in tobacco transgenic plants. Athb-12 gene expression was further examined using in situ hybridization to determine the cellular distribution of Athb-12 transcripts during ABA induction. A complex pattern of Athb-12 expression was observed, often associated with regions of developing vascular tissues. Analysis of chimeras constructed from Athb-12 and the DNA-binding domain of the Saccharomyces cerevisiae transcription factor GAL4 revealed that the activation domain of Athb-12 lies in the C-terminal region (amino acids 180 to 235). Taken together, our data suggest that Athb-12 is a transcriptional activator important in regulating certain developmental processes as well as in the plant's response to water stress involving ABA-mediated gene expression.

Abiotic stress signal transduction in plants: Molecular and genetic perspectives

Low temperature, drought and salinity are major adverse environmental factors that limit plant productivity. Understanding the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses is essential for engineering stress-tolerant crop plants. Molecular and biochemical studies suggest that abiotic stress signaling in plants involves receptor-coupled phosphorelay, phosphoinositol-induced Ca2+ changes, mitogen-activated protein kinase cascades and transcriptional activation of stress-responsive genes. In addition, protein posttranslational modifications and adapter or scaffold-mediated protein-protein interactions are also important in abiotic stress signal transduction. Most of these signaling modules, however, have not been genetically established to function in plant abiotic stress signal transduction. To overcome the scarcity of abiotic stress-specific phenotypes for conventional genetic screens, molecular genetic analysis using stress-responsive promoter-driven reporter is suggested as an alternative approach to genetically dissect abiotic stress signaling networks in plants.

Physical interactions between ABA response loci of Arabidopsis

Genetic and physiological studies have shown that the Arabidopsis thaliana abscisic acid-insensitive (ABI) loci interact to regulate seed-specific and/or ABA-inducible gene expression. We have used the yeast two-hybrid assay to determine whether any of these genetic interactions reflect direct physical interactions. By this criterion, only ABI3 and ABI5 physically interact with each other, and ABI5 can form homodimers. The B1 domain of ABI3 is essential for this interaction; this is the first specific function ascribed to this domain of the ABI3/VP1 family. The ABI5 domains required for interaction with ABI3 include two conserved charged domains in the amino-terminal half of the protein. An additional conserved charged domain appears to have intrinsic transcription activation function in this assay. Yeast one-hybrid assays with a lacZ reporter gene under control of the late embryogenesis-abundant AtEm6 promoter show that only ABI5 binds directly to this promoter fragment.

A postgermination developmental arrest checkpoint is mediated by abscisic acid and requires the ABI5 transcription factor in Arabidopsis

Seed dormancy is a trait of considerable adaptive significance because it maximizes seedling survival by preventing premature germination under unfavorable conditions. Understanding how seeds break dormancy and initiate growth is also of great agricultural and biotechnological interest. Abscisic acid (ABA) plays primary regulatory roles in the initiation and maintenance of seed dormancy. Here we report that the basic leucine zipper transcription factor ABI5 confers an enhanced response to exogenous ABA during germination, and seedling establishment, as well as subsequent vegetative growth. These responses correlate with total ABI5 levels. We show that ABI5 expression defines a narrow developmental window following germination, during which plants monitor the environmental osmotic status before initiating vegetative growth. ABI5 is necessary to maintain germinated embryos in a quiescent state thereby protecting plants from drought. As expected for a key player in ABA-triggered processes, ABI5 protein accumulation, phosphorylation, stability, and activity are highly regulated by ABA during germination and early seedling growth.

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.

Differential screening indicates a dramatic change in mRNA profiles during grape berry ripening. Cloning and characterization of cDNAs encoding putative cell wall and stress response proteins

We used differential screening to isolate ripening-associated cDNAs from a Shiraz grape (Vitis vinifera L.) berry cDNA library. A rapid increase in the mRNA levels of a number of cDNAs not present in unripe fruit occurred in grape berries at the onset of ripening. The putative translation products of some of these clones had homologs in other species that are involved in cell wall structure. These included four proline-rich proteins, a small protein that is similar to the non-catalytic, N-terminal domain of some pectin methylesterases, and two other glutamate-rich proteins. The remainder of the clones encoded putative stress response proteins. These included two thaumatin-like proteins, a metallothionein, a transcription factor, a cytochrome P450 enzyme, and proteins induced by water, sugar, and/or cold stress in other species. Many of the homologs of the grape cDNAs thought to be involved in cell wall structure or stress-related responses also accumulate in a developmental manner in other plants. This may indicate that the grape mRNAs accumulate in response to stresses such as the storage of high concentrations of sugars and rapid cell expansion, or they may accumulate as part of the ripening developmental program.

ABFs, a family of ABA-responsive element binding factors

Abscisic acid (ABA) plays an important role in environmental stress responses of higher plants during vegetative growth. One of the ABA-mediated responses is the induced expression of a large number of genes, which is mediated by cis-regulatory elements known as abscisic acid-responsive elements (ABREs). Although a number of ABRE binding transcription factors have been known, they are not specifically from vegetative tissues under induced conditions. Considering the tissue specificity of ABA signaling pathways, factors mediating ABA-dependent stress responses during vegetative growth phase may thus have been unidentified so far. Here, we report a family of ABRE binding factors isolated from young Arabidopsis plants under stress conditions. The factors, isolated by a yeast one-hybrid system using a prototypical ABRE and named as ABFs (ABRE binding factors) belong to a distinct subfamily of bZIP proteins. Binding site selection assay performed with one ABF showed that its preferred binding site is the strong ABRE, CACGTGGC. ABFs can transactivate an ABRE-containing reporter gene in yeast. Expression of ABFs is induced by ABA and various stress treatments, whereas their induction patterns are different from one another. Thus, a new family of ABRE binding factors indeed exists that have the potential to activate a large number of ABA/stress-responsive genes in Arabidopsis.

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.

Footprinting in vivo reveals changing profiles of multiple factor interactions with the beta-phaseolin promoter during embryogenesis

Whereas in vitro techniques are essentially limited to the analysis of interactions with a single or limited number of cis-elements, in vivo footprinting techniques can be used to assess the total profile of factor interactions with a promoter. By probing with dimethylsulphate and using sensitive ligation-mediated PCR analytical techniques, the in vivo status of the phas promoter was determined in transcriptionally active (embryo) and inactive (leaf) tissues. Changes in factor occupancy were detected during embryogenesis, and the greatest complexity seen (at mid-maturation) was in accordance with the many potential binding sites predicted on the basis of sequence comparison. Evidence was obtained that several cis-elements not previously shown to be used for factor binding in plant promoters are occupied. The great complexity of footprints may represent the need for multiple factor interaction to achieve high levels of transcription. Alternatively, it is possible that the differential levels of expression in individual regions of the embryo evident from histochemical analysis of the GUS reporter result from the interaction of relatively few factors, with the overall footprinting pattern representing a summation of patterns from various tissues.

Identification of a new sea urchin ets protein, SpEts4, by yeast one-hybrid screening with the hatching enzyme promoter

We report the use of a yeast one-hybrid system to isolate a transcriptional regulator of the sea urchin embryo hatching enzyme gene, SpHE. This gene is asymmetrically expressed along the animal-vegetal axis of sea urchin embryos under the cell-autonomous control of maternal regulatory activities and therefore provides an excellent entry point for understanding the mechanism that establishes animal-vegetal developmental polarity. To search for transcriptional regulators, we used a fragment of the SpHE promoter containing several individual elements instead of the conventional bait that contains a multimerized cis element. This screen yielded a number of positive clones that encode a new member of the Ets family, named SpEts4. This protein contains transcriptional activation activity, since expression of reporter genes in yeast does not depend on the presence of the yeast GAL4 activation domain. Sequences in the N-terminal region of SpEts4 mediate the activation activity, as shown by deletion or domain-swapping experiments. The newly identified DNA binding protein binds with a high degree of specificity to a SpHE promoter Ets element and forms a complex with a mobility identical to that obtained with 9-h sea urchin embryo nuclear extracts. SpEts4 positively regulates SpHE transcription, since mutation of the SpEts4 site in SpHE promoter transgenes reduces promoter activity in vivo while SpEts4 mRNA coinjection increases its output. As expected for a positive SpHE transcriptional regulator, the timing of SpEts4 gene expression precedes the transient expression of SpHE in the very early sea urchin blastula.

Identification of the cis-acting endoplasmic reticulum stress response element responsible for transcriptional induction of mammalian glucose-regulated proteins. Involvement of basic leucine zipper transcription factors

When unfolded proteins accumulate in the endoplasmic reticulum (ER), transcription of glucose-regulated proteins (GRPs) representing ER-resident molecular chaperones is markedly induced via the unfolded protein response (UPR) pathway. In contrast to recent progress in the analysis of yeast UPR, both cis-acting elements and transactivators responsible for mammalian UPR have remained obscure. Here, we analyzed the promoter regions of human GRP78, GRP94, and calreticulin genes and identified a novel element designated the ER stress response element (ERSE). ERSE, with a consensus of CCAATN9CCACG, was shown to be necessary and sufficient for induction of these GRPs. Using yeast one-hybrid screening, we isolated a human cDNA encoding a basic leucine zipper (bZIP) protein, ATF6, as a putative ERSE-binding protein. When overexpressed in HeLa cells, ATF6 enhanced transcription of GRP genes in an ERSE-dependent manner, whereas CREB-RP, another bZIP protein closely related to ATF6, specifically inhibited GRP induction. Endogenous ATF6 constitutively expressed as a 90-kDa protein was converted to a 50-kDa protein in ER-stressed cells, which appeared to be important for the cellular response to ER stress. These results suggest that, as in yeast, bZIP proteins are involved in mammalian UPR, acting through newly defined ERSE.

Abscisic acid-dependent and -independent expression of the carrot late-embryogenesis-abundant-class gene Dc3 in transgenic tobacco seedlings

We studied the expression of three promoter 5' deletion constructs (-218, -599, and -1312) of the LEA (late embryogenesis abundant)-class gene Dc3 fused to beta-glucuronidase (GUS), where each construct value refers to the number of base pairs upstream of the transcription start site at which the deletion occurred. The Dc3 gene is noted for its induction by abscisic acid (ABA), but its response to other plant hormones and various environmental stresses has not been reported previously for vegetative cells. Fourteen-day-old transgenic tobacco (Nicotiana tabacum L.) seedlings were exposed to dehydration, hypoxia, salinity, exogenous ethylene, or exogenous methyl jasmonate (MeJa). GUS activity was quantified fluorimetrically and expression was observed by histochemical staining of the seedlings. An increase in GUS activity was observed in plants with constructs -599 and -1312 in response to dehydration and salinity within 6 h of stress, and at 12 h in response to hypoxia. No increase in endogenous ABA was found in any of the three lines, even after 72 h of hypoxia. An ABA-independent increase in GUS activity was observed when endogenous ABA biosynthesis was blocked by fluridone and plants were exposed to 5 &mgr;L L-1 ethylene in air or 100 &mgr;M MeJa. Virtually no expression was observed in construct -218 in response to dehydration, salinity, or MeJa, but there was a moderate response to ethylene and hypoxia. This suggests that the region between -218 and -599 is necessary for ABA (dehydration and salinity)- and MeJa-dependent expression, whereas ethylene-mediated expression does not require this region of the promoter.

ABA signal transduction

Recent advances in the study of abscisic acid signal transduction include the identification of cyclic ADP-ribose as a central mediator of abscisic acid responses. The characterisation of type 2C protein phosphatases, ABI1 and ABI2, implicates negative control and redundant action on the signal pathway of this hormone. In addition, abscisic acid-mediated inhibition of gibberellin-stimulated responses seems to depend on the activation of a phospholipase D during induction of alpha-amylase in barley aleurone cells as well as on a putative acetyltransferase involved in elongation growth.

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.

Ectopic expression of a novel MYB gene modifies the architecture of the Arabidopsis inflorescence

The Arabidopsis thaliana mutants fus3, lec1 and abi3 have pleiotropic defects during late embryogenesis. Mutant embryos fail to enter the maturation programme and initiate a vegetative germination pathway instead. Screening for genes which are differentially expressed in the fus3 mutant of Arabidopsis resulted in the isolation of several members of the MYB family. MYB domain proteins in plants represent an extended gene family of transcription factors, suggesting their participation in a variety of plant specific cellular functions. Here, the authors describe one of these genes, designated AtMYB13, representing a novel member of the MYB gene family. The structure of the gene as well as its genomic organisation and localisation are reported. The expression of the gene is regulated by dehydration, exogenous abscisic acid, light and wounding. A chimeric AtMYB13 promoter/GUS gene is tissue-specifically expressed in transgenic Arabidopsis plants. The GUS staining was predominantly detected in the shoot apex zone and at the basis of developing flowers. In addition, the AtMYB13 gene promoter is active at branching points of the inflorescence. Furthermore, ectopic expression of the AtMYB13 gene has a characteristic impact on the architecture of the inflorescence leading to peculiar hook structures at pedicel branching points. In addition, some transgenic plants exhibit a reversed order of first flowers and axillary buds. These data suggest a function of the AtMYB13 gene product in linking shoot morphogenic activity with environmental as well as intrinsic signals.