Heat-inducible hygromycin resistance in transgenic tobacco

Reduced Dormancy5 encodes a protein phosphatase 2C that is required for seed dormancy in Arabidopsis

Seed dormancy determines germination timing and contributes to crop production and the adaptation of natural populations to their environment. Our knowledge about its regulation is limited. In a mutagenesis screen of a highly dormant Arabidopsis thaliana line, the reduced dormancy5 (rdo5) mutant was isolated based on its strongly reduced seed dormancy. Cloning of RDO5 showed that it encodes a PP2C phosphatase. Several PP2C phosphatases belonging to clade A are involved in abscisic acid signaling and control seed dormancy. However, RDO5 does not cluster with clade A phosphatases, and abscisic acid levels and sensitivity are unaltered in the rdo5 mutant. RDO5 transcript could only be detected in seeds and was most abundant in dry seeds. RDO5 was found in cells throughout the embryo and is located in the nucleus. A transcriptome analysis revealed that several genes belonging to the conserved PUF family of RNA binding proteins, in particular Arabidopsis PUMILIO9 (APUM9) and APUM11, showed strongly enhanced transcript levels in rdo5 during seed imbibition. Further transgenic analyses indicated that APUM9 reduces seed dormancy. Interestingly, reduction of APUM transcripts by RNA interference complemented the reduced dormancy phenotype of rdo5, indicating that RDO5 functions by suppressing APUM transcript levels.

Cloning and characterization of a novel dehydrin gene, SiDhn2, from Saussurea involucrata Kar. et Kir

Saussurea involucrata Kar. et Kir. is a hardy dicotyledonous plant capable of tolerating severe abiotic stress conditions. In a previous study, we created a cDNA library to determine what factors are associated with the cold acclimation response in S. involucrata. From this, a full-length cDNA of a dehydrin-like gene (SiDhn2) was obtained by RT-PCR. The SiDhn2 gene was characterized in this study. The full-length SiDhn2 cDNA comprised 693 bp containing an open reading frame of 345 bp specifying a protein of 115 amino acids. An alignment of the deduced amino acid sequence showed that SiDhn2 shared 55 % identity with two Brassica dehydrins. Agrobacterium tumefaciens was used to transform RD29A:SiDhn2 and 35S:SiDhn2 constructs into tobacco to investigate the germination and resistance to freezing and drought stress of transgenic plants. The RD29A:SiDhn2 transgenic plants showed greater resistance to freezing and drought stress than 35S:SiDhn2 transgenic plants or the wild-type. This study demonstrates that SiDhn2 confers cold hardiness and drought resistance, and may be a candidate resistance gene for genetic improvement of crops to increase stress resistance.

Integrating long-day flowering signals: a LEAFY binding site is essential for proper photoperiodic activation of APETALA1

The transition to flowering in Arabidopsis is characterized by the sharp and localized upregulation of APETALA1 (AP1) transcription in the newly formed floral primordia. Both the flower meristem-identity gene LEAFY (LFY) and the photoperiod pathway involving the FLOWERING LOCUS T (FT) and FD genes contribute to this upregulation. These pathways have been proposed to act independently but their respective contributions and mode of interaction have remained elusive. To address these questions, we studied the AP1 regulatory region. Combining in vitro and in vivo approaches, we identified which of the three putative LFY binding sites present in the AP1 promoter is essential for its activation by LFY. Interestingly, we found that this site is also important for the correct photoperiodic-dependent upregulation of AP1. In contrast, a previously proposed putative FD-binding site appears dispensable and unable to bind FD and we found no evidence for FD binding to other sites in the AP1 promoter, suggesting that the FT/FD-dependent activation of AP1 might be indirect. Altogether, our data give new insight into the interaction between the FT and LFY pathways in the upregulation of AP1 transcription under long-day conditions.

Precocious flowering in trees: the FLOWERING LOCUS T gene as a research and breeding tool in Populus

Expression of FLOWERING LOCUS T (FT) and its homologues has been shown to accelerate the onset of flowering in a number of plant species, including poplar (Populus spp.). The application of FT should be of particular use in forest trees, as it could greatly accelerate and enable new kinds of breeding and research. Recent evidence showing the extent to which FT is effective in promoting flowering in trees is discussed, and its effectiveness in poplar is reported. Results using one FT gene from Arabidopsis and two from poplar, all driven by a heat-inducible promoter, transformed into two poplar genotypes are also described. Substantial variation in flowering response was observed depending on the FT gene and genetic background. Heat-induced plants shorter than 30 cm failed to flower as well as taller plants. Plants exposed to daily heat treatments lasting 3 weeks tended to produce fewer abnormal flowers than those in heat treatments of shorter durations; increasing the inductive temperature from 37 degrees C to 40 degrees C produced similar benefits. Using optimal induction conditions, approximately 90% of transgenic plants could be induced to flower. When induced FT rootstocks were grafted with scions that lacked FT, flowering was only observed in rootstocks. The results suggest that a considerable amount of species- or genotype-specific adaptation will be required to develop FT into a reliable means for shortening the generation cycle for breeding in poplar.

Functional analysis of transcription factors in Arabidopsis

Transcription factors (TFs) regulate the expression of genes at the transcriptional level. Modification of TF activity dynamically alters the transcriptome, which leads to metabolic and phenotypic changes. Thus, functional analysis of TFs using 'omics-based' methodologies is one of the most important areas of the post-genome era. In this mini-review, we present an overview of Arabidopsis TFs and introduce strategies for the functional analysis of plant TFs, which include both traditional and recently developed technologies. These strategies can be assigned to five categories: bioinformatic analysis; analysis of molecular function; expression analysis; phenotype analysis; and network analysis for the description of entire transcriptional regulatory networks.

Applications of EcR gene switch technology in functional genomics

Genetic engineering of plants using transgenic technology is targeted to enhance agronomic performance or improved quality traits in a wide variety of plant species, and has become a fundamental tool for basic research in plant biotechnology. Constitutive promoters are presently the primary means used to express transgenes in plants. However, inducible gene regulation systems based on specific chemicals have many potential applications in agriculture and for enhancing the basic understanding of gene function. As a result, several gene switches have been developed. The ecdysone receptor gene switch is one of the best inducible gene regulation systems available, because the chemical, methoxyfenozide, required for its regulation is registered for field use. An EcR gene switch with a potential for use in large-scale field applications has been developed by adopting a two-hybrid format. In a two-hybrid switch format, the GAL4 DNA binding domain (GAL4 DBD) was fused to the ligand binding domain (LBD) of the Choristoneura fumiferana ecdysone receptor (CfEcR); and, the VP16 activation domain (VP16 AD) was fused to the LBD of Locust migratoria retinoid X receptor (LmRXR). The sensitivity of the CfEcR gene switch was improved from micromolar to nanomolar concentrations of ligand by using the CfEcR:LmRXR two-hybrid switch. In this report, we demonstrate the utility of CfEcR:LmRXR two-hybrid gene switch in functional genomics applications for regulating the expression of a Superman-like single zinc finger protein 11 (ZFP11) gene in both Arabidopsis and tobacco transgenic plants.

Evaluation of CRE-mediated excision approaches in Arabidopsis thaliana

The ability of the CRE recombinase to catalyze excision of a DNA fragment flanked by directly repeated lox sites has been exploited to modify gene expression and proved to function well in particular case studies. However, very often variability in CRE expression and differences in efficiency of CRE-mediated recombination are observed. Here, various approaches were investigated to reproducibly obtain optimal CRE activity. CRE recombination was analyzed either by transforming the CRE T-DNA into plants containing a lox-flanked fragment or by transforming a T-DNA harboring a lox-flanked fragment into plants producing the CRE recombinase. Although somatic CRE-mediated excision of a lox-flanked fragment was obtained in all transformants, a variable amount of germline-transmitted deletions was found among different independent transformants, irrespective of the orientation of transformation. Also, the efficiency of CRE-mediated excision correlated well with the CRE mRNA level. In addition, CRE-mediated fragment excision was compared after floral dip and after root tissue transformation when transforming in a CRE-expressing background. Importantly, less CRE activity was needed to excise the lox-flanked fragment from the transferred T-DNA after root tissue transformation than after floral dip transformation. We hypothesize that this is correlated with the lower T-DNA copy number inserted during root transformation as compared to floral dip transformation.

Development of a methoxyfenozide-responsive gene switch for applications in plants

The ecdysone receptor (EcR) has been used to develop gene switches for conditional regulation of transgene expression in plants and humans. All EcR-based gene switches developed to date for use in plants are monopartate and require micromolar concentrations of ligand for activation of the transgene; this has limited the use of these gene switches. We have developed a Choristoneura fumiferana ecdysone receptor (CfEcR)-based two-hybrid gene switch that works through the formation of a functional heterodimer between EcR and the retinoid X receptor (RXR) upon application of the chemical ligand methoxyfenozide. Methoxyfenozide is already registered for field use with an excellent safety profile, and it has potential as a gene switch ligand for applications in the field. The receptor constructs were prepared by fusing DEF domains (hinge region plus ligand-binding domain) of CfEcR to the GAL4 DNA-binding domain and EF domains (ligand-binding domain) of ultraspiracle from Choristoneura fumiferana (CfUSP) or RXR from Locusta migratoria (LmRXR), Mus musculus (MmRXR) or Homo sapiens (HsRXR) to the VP16 activation domain. These receptor constructs were tested for their ability to induce expression of the luciferase gene placed under the control of 5x GAL4 response elements and -46 35S minimal promoter in tobacco, corn and soybean protoplasts and in transgenic Arabidopsis and tobacco plants. By adopting the two-hybrid format, the sensitivity of the CfEcR gene switch has been improved from micromolar to nanomolar concentrations of methoxyfenozide. The sensitivity of the CfEcR + LmRXR two-hybrid switch was 25 to 625 times greater than the monopartate gene switch, depending on the plant species tested.

Controlled expression of recombinant proteins in Physcomitrella patens by a conditional heat-shock promoter: a tool for plant research and biotechnology

The ability to express tightly controlled amounts of endogenous and recombinant proteins in plant cells is an essential tool for research and biotechnology. Here, the inducibility of the soybean heat-shock Gmhsp17.3B promoter was addressed in the moss Physcomitrella patens, using beta-glucuronidase (GUS) and an F-actin marker (GFP-talin) as reporter proteins. In stably transformed moss lines, Gmhsp17.3B-driven GUS expression was extremely low at 25 degrees C. In contrast, a short non-damaging heat-treatment at 38 degrees C rapidly induced reporter expression over three orders of magnitude, enabling GUS accumulation and the labelling of F-actin cytoskeleton in all cell types and tissues. Induction levels were tightly proportional to the temperature and duration of the heat treatment, allowing fine-tuning of protein expression. Repeated heating/cooling cycles led to the massive GUS accumulation, up to 2.3% of the total soluble proteins. The anti-inflammatory drug acetyl salicylic acid (ASA) and the membrane-fluidiser benzyl alcohol (BA) also induced GUS expression at 25 degrees C, allowing the production of recombinant proteins without heat-treatment. The Gmhsp17.3B promoter thus provides a reliable versatile conditional promoter for the controlled expression of recombinant proteins in the moss P. patens.

The mRNA of the Arabidopsis Gene FT Moves from Leaf to Shoot Apex and Induces Flowering

Day length controls flowering time in many plants. The day-length signal is perceived in the leaf, but how this signal is transduced to the shoot apex, where floral initiation occurs, is not known. In Arabidopsis, the day-length response depends on the induction of the FLOWERING LOCUS T (FT) gene. We show here that local induction of FT in a single Arabidopsis leaf is sufficient to trigger flowering. The FT messenger RNA is transported to the shoot apex, where downstream genes are activated. These data suggest that the FT mRNA is an important component of the elusive "florigen" signal that moves from leaf to shoot apex.

A chimeric ecdysone receptor facilitates methoxyfenozide-dependent restoration of male fertility in ms45 maize

A mutation in the maize Ms45 gene results in abortion of microspore development and a male-sterile phenotype. MS45 protein has been localized to the tapetum and maximally expressed in anthers at the early vacuolate stage of microspore development. Molecular complementation analysis determined that a transformed copy of the gene fully restored fertility to ms45 maize. In this report, using phenotypic complementation as an assay, chimeric transcriptional activators were expressed to regulate a gal:MS45 gene and test the ability of a multi-component system to restore male fertility. A high frequency of phenotypic complementation was observed when either C1-GAL4 or VP16-GAL4 activators were transcribed by promoters that expressed at a stage of anther development that precedes the early vacuolate stage of microsporogenesis. For the conditional regulation of male fertility, these transcriptional activators were modified by the addition of regions that include the ligand-binding domain from the European corn borer ecdysone receptor to generate the nuclear receptors C1-GAL4-EcR (CGEcR) and VP16-GAL4-EcR (VGEcR). These chimeric receptors were introduced with the gal:MS45 gene into ms45 maize, and in the absence of ligand, these plants were male sterile. In contrast, application of the ecdysone agonist, methoxyfenozide, to plants containing either a constitutive (Ubiquitin1) or anther-specific (maize 5126) VGEcR resulted in the restoration of fertility to ms45 plants grown in either the greenhouse or in the field.

Technical advance: stringent control of transgene expression in Arabidopsis thaliana using the Top10 promoter system

We show that the tightly regulated tetracycline-sensitive Top10 promoter system (Weinmann et al. Plant J. 1994, 5, 559-569) is functional in Arabidopsis thaliana. A pure breeding A. thaliana line (JL-tTA/8) was generated which expressed a chimeric fusion of the tetracycline repressor and the activation domain of Herpes simplex virus (tTA), from a single transgenic locus. Plants from this line were crossed with transgenics carrying the ER-targeted green fluorescent protein coding sequence (mGFP5) under control of the Top10 promoter sequence. Progeny from this cross displayed ER-targeted GFP fluorescence throughout the plant, indicating that the tTA-Top10 promoter interaction was functional in A. thaliana. GFP expression was repressed by 100 ng ml-1 tetracycline, an order of magnitude lower than the concentration used previously to repress expression in Nicotiana tabacum. Moreover, the level of GFP expression was controlled by varying the concentration of tetracycline in the medium, allowing a titred regulation of transgenic activity that was previously unavailable in A. thaliana. The kinetics of GFP activity were determined following de-repression of the Top10:mGFP5 transgene, with a visible ER-targeted GFP signal appearing from 24 to 48 h after de-repression.

Developmental regulation and tissue-specific differences of heat shock gene expression in transgenic tobacco and Arabidopsis plants

The heat shock (hs) response during plant growth and development was analyzed in tobacco and Arabidopsis using chimaeric beta-glucuronidase reporter genes (hs-Gus) driven by a soybean hs promoter. Fluorimetric measurements and histochemical staining revealed high Gus activities in leaves, roots, and flowers exclusively after heat stress. The highest levels of heat-inducible expression were found in the vascular tissues. Without heat stress, a developmental induction of hs-Gus was indicated by the accumulation of high levels of Gus in transgenic tobacco seeds. There was no developmental induction of hs-Gus in Arabidopsis seeds. In situ hybridization to the RNA of the small heat shock protein gene Athsp17.6 in tissue sections revealed an expression in heat-shocked leaves but no expression in control leaves of Arabidopsis. However, a high level of constitutive expression of hs genes was detected in meristematic and provascular tissues of the Arabidopsis embryo. The developmental and tissue-specific regulation of the hs response is discussed.

Arabidopsis heat shock factor: isolation and characterization of the gene and the recombinant protein

We have isolated the Arabidopsis heat shock factor gene Athsf1 as genomic and corresponding cDNA sequences via cross-hybridization with tomato clones. Sequence analysis indicates only a partial homology with the HSFs from tomato and other organisms which is confined to the DNA-binding and the oligomerization domains. The gene is constitutively expressed but the level of mRNA for Athsf1 increases two-fold upon heat shock. However, the putative promoter region lacks the canonical heat shock elements. After expression in Escherichia coli the recombinant Athsf1 protein binds specifically to a synthetic oligonucleotide containing five heat shock elements. The native size of recombinant ATHSF1 in vitro is consistent with a trimer as demonstrated by chemical cross-linking and pore exclusion limit analysis.

Isolation and characterization of cDNA clones for RNA species induced by substituted benzenesulfonamides in corn

A search of compounds capable of inducing specific gene expression in plants without affecting growth and development led to the examination of changes in the pattern of gene expression in corn after treatment with substituted benzenesulfonamide herbicide safeners. Following hydroponic treatment of corn with the safener N-(aminocarbonyl)-2-chlorobenzenesulfonamide (2-CBSU), the specific induction of new translatable mRNA species was observed. Replicate copies of a cDNA library made using RNA from 2-CBSU-treated corn roots were differentially screened with cDNA probes made from either the same mRNA fraction used for library construction or mRNA isolated from roots treated with 2-chlorobenzenesulfonamide (2-CBSA), an inactive analog of the safener. Colonies showing hybridization only with the probe made using mRNA from 2-CBSU-treated roots were further characterized to assess the specificity of the induction and decay of the corresponding induced RNA species. RNA blot analyses showed two clones, designated In2-1 and In2-2, contained plasmids that hybridized to RNAs that were induced from an undetectable background in corn roots within 30 minutes after treatment with 2-CBSU. Leaf and meristem tissues showed similar inductions of the In2-1 and In2-2 RNA species after a delay of several hours. In addition, both RNA species were induced in corn by foliar application of 2-CBSU. In contrast, neither RNA species was induced following stress treatments of plants. These results indicate a substituted benzenesulfonamide safener might be used with the promoters from the In2-1 and In2-2 genes to develop a new inducible gene expression system for plants.