Promoters from kin1 and cor6.6, two Arabidopsis thaliana low-temperature- and ABA-inducible genes, direct strong beta-glucuronidase expression in guard cells, pollen and young developing seeds

CRISPR/Cas9 edited HSFA6a and HSFA6b of Arabidopsis thaliana offers ABA and osmotic stress insensitivity by modulation of ROS homeostasis

The role of Heat Shock Transcription Factor 6 (HSFA6a & HSFA6b) in response to abiotic stresses such as ABA, drought, salinity, drought, and osmotic stress is individually well established. Unfortunately, the functional redundancy between the HSFA6a and HSFA6b as well as the consequences of simultaneous editing of both in response to aforementioned stresses remains elusive. Therefore, this study was designed with the aim of addressing whether there is any functional redundancy between HSFA6a and HSFA6b as well as to decipher their role in abiotic stresses tolerance in Arabidopsis thaliana, by using the CRISPR-Cas9. We have generated the single (hsfa6a and hsfa6b) as well as double mutants (hsfa6a/hsfa6b-1 and hsfa6a/hsfa6b-2) of HSFA6a and HSFA6b with higher frequencies of deletion, insertion, and substitution. The phenotypic characterization of generated double and single mutants under abiotic stresses such as ABA, mannitol, and NaCl identified double mutants more tolerant to subjected abiotic stresses than those of their single mutants. It warrants mentioning that we have identified that HSFA6a and HSFA6b also involved in other major ABA responses, including ABA-inhibited seed germination, stomatal movement, and water loss. In addition to the above, the simultaneous editing of HSFA6a and HSFA6b lead to a reduced ROS accumulation, accompanied by increased expression of much abiotic stress and ABA-responsive genes, including involved in regulation of ROS level. In conclusion, these results suggest that HSFA6a and HSFA6b may offer abiotic stress tolerance by regulating the ROS homeostasis in plants.

Over-expression of AtGSTU19 provides tolerance to salt, drought and methyl viologen stresses in Arabidopsis

The plant-specific tau class of glutathione S-transferases (GSTs) is often highly stress-inducible and expressed in a tissue-specific manner, thereby suggesting its important protective roles. Although activities associated with the binding and transport of reactive metabolites have been proposed, little is known about the regulatory functions of GSTs. Expression of AtGSTU19 is induced by several stimuli, but the function of this GST remains unknown. In this study, we demonstrated that transgenic over-expressing (OE) plants showed enhanced tolerance to different abiotic stresses and increased percentage of seed germination and cotyledon emergence. Transgenic plants exhibited an increased level of proline and activities of antioxidant enzymes, along with decreased malonyldialdehyde level under stress conditions. Real-time polymerase chain reaction (PCR) analyses revealed that the expression levels of several stress-regulated genes were altered in AtGSTU19 OE plants. These results indicate that AtGSTU19 plays an important role in tolerance to salt/drought/methyl viologen stress in Arabidopsis.

Comparative analysis of alternative splicing, alternative polyadenylation and the expression of the two KIN genes from cytoplasmic male sterility cabbage (Brassica oleracea L. var. capitata L.)

The KIN genes are crucial members of the cold-regulated gene family. They play exclusive roles during the developmental processes of many organs and respond to various abiotic stresses in plants. However, little is known about the regulation of KIN gene expression in cytoplasmic male sterility (CMS) cabbages (Brassica oleracea L. var. capitata L.). We carried out a genome-wide analysis to identify the KIN genes in the CMS cabbage. Two non-redundant KIN genes, named BoKIN1 (Bol021262) and BoKIN2 (Bol030498), were identified. Reverse transcriptase PCR detected alternative splicing (AS) products of BoKIN1 (four AS products) and BoKIN2 (three AS products). In addition, alternative polyadenylation (APA) was observed for BoKIN1 and BoKIN2 in the CMS cabbage, resulting in variable 3'UTRs in their transcripts. Furthermore, the transcription levels of BoKIN1-0 and BoKIN2-0, the introns of which were spliced completely, were analyzed in various organs and young leaves treated by abiotic stresses. Our data indicated that BoKIN1-0 is highly expressed in various organs, whereas BoKIN2-0 is expressed exclusively in the stamen. Our study also suggested that BoKIN1-0 was upregulated significantly in young leaves of plants exposed to abscisic acid treatment, and cold and heat stress. BoKIN1 and BoKIN2 had differential AS and APA patterns in pre-mRNA processing, and showed differences in their expression patterns and transcript levels. BoKIN1 participates widely in organ development and responds to diverse abiotic stresses, whereas BoKIN2 plays a main role in stamen development in the CMS cabbage.

Comparative analysis of the variable 3' UTR and gene expression of the KIN and KIN-homologous LEA genes in Capsella bursa-pastoris

As the crucial members of the cold-regulated (COR) gene family, KIN genes are involved in diverse abiotic stress responses in plants. In the present study, KIN genes from the widespread plant Capsella bursa-pastoris were identified and analyzed to better understand the powerful adaptation of this species. Two KIN genes were cloned and sequenced by 3' RACE. As some COR genes are homologous to LEA genes, three KIN-homologous LEA genes were also identified. We deduced the amino acid sequences of the five proteins to estimate their phylogenetic relationships, and grouped them into three subfamilies (CI, CII, and CIII). Variable 3' UTRs were found in CI, CII, and CIII genes. Using qPCR, we evaluated the transcriptional levels of the five genes in different organs and embryonic stages. Two CI genes were exclusively expressed in early embryos and flowers. The CII and CIII genes showed obvious up-regulation in young leaves after heat stress, cold stress, and ABA treatment. Two of the CI genes, however, rarely responded to those stresses in young leaves. In contrast, all five genes showed differential responses in flowers when C. bursa-pastoris plants were sprayed with ABA. Furthermore, the expression of these genes in C. bursa-pastoris was compared to that of the corresponding Arabidopsis genes, and similar gene expression profiles were found in both species. Our findings suggest that these five genes play different roles in development and the responses to abiotic stresses in C. bursa-pastoris. Key message We characterized two KIN and three KIN-homologous LEA genes, and analyzed their variable 3'UTR and organ-specific, embryo-developmental, stress-induced gene expression in Capsella bursa-pastoris.

Characterization of the variable 3' UTR and expression of the two intron-containing KIN transcripts from Capsella bursa-pastoris

KIN genes are crucial members of the cold-regulated (COR) gene family, and are exclusively involved in normal developmental processes in many organs and respond to a variety of abiotic stresses in plants. Here, we cloned and sequenced not only two completely-spliced KIN transcripts (CbKIN1-S and CbKIN2-S), but also two intron-containing KIN transcripts (CbKIN1-U and CbKIN2-U), from Capsella bursa-pastoris, a widespread plant of the Brassicaceae family. The CbKIN1-U and CbKIN2-U transcripts each contained one additional intron in the coding region compared to the corresponding CbKIN1-S and CbKIN2-S transcripts. In addition, the two intron-containing KIN transcripts were found by rapid amplification of cDNA 3' ends (3' RACE) analysis with specific primers to have variable 3' untranslated regions (3' UTRs). We also analyzed CbKIN1-U and CbKIN2-U levels in different organs and embryonic stages by quantitative polymerase chain reaction (qPCR). They were found to be expressed in middle-stage embryos and flowers. After abscisic acid (ABA) treatment, CbKIN1-U and CbKIN2-U showed strong responses in young leaves and weak responses in flowers. Levels of the two intron-containing KIN transcripts were markedly increased in young leaves when plants were exposed to cold and heat stress. Both of them showed stronger responses to ABA treatment and cold stress than that to heat stress. CbKIN1-U and CbKIN2-U share similar gene expression profiles in development and in response to exposure to different stresses, suggesting that they probably play similar biological roles in C. bursa-pastoris.

Genetic analysis reveals a complex regulatory network modulating CBF gene expression and Arabidopsis response to abiotic stress

Arabidopsis CBF genes (CBF1-CBF3) encode transcription factors having a major role in cold acclimation, the adaptive process whereby certain plants increase their freezing tolerance in response to low non-freezing temperatures. Under these conditions, the CBF genes are induced and their corresponding proteins stimulate the expression of target genes configuring low-temperature transcriptome and conditioning Arabidopsis freezing tolerance. CBF2 seems to be the most determinant of the CBFs since it also regulates CBF1 and CBF3 expression. Despite the relevance of CBF genes in cold acclimation, little is known about the molecular components that control their expression. To uncover factors acting upstream of CBF2, mutagenized Arabidopsis containing the luciferase reporter gene under the control of the CBF2 promoter were screened for plants with de-regulated CBF2 expression. Here, the identification and characterization of five of these mutants, named acex (altered CBF2 expression), is presented. Three mutants show increased levels of cold-induced CBF2 transcripts compared with wild-type plants, the other two exhibiting reduced levels. Some mutants are also affected in cold induction of CBF1 and CBF3. Furthermore, the mutants characterized display unique phenotypes for tolerance to abiotic stresses, including freezing, dehydration, and high salt. These results demonstrate that cold induction of CBF2 is subjected to both positive and negative regulation through different signal transduction pathways, some of them also mediating the expression of other CBF genes as well as Arabidopsis responses to abiotic stresses.

Analysis of transcriptional and upstream regulatory sequence activity of two environmental stress-inducible genes, NBS-Str1 and BLEC-Str8, of rice

Two abiotic stress-inducible upstream regulatory sequences (URSs) from rice have been identified and functionally characterized in rice. NBS-Str1 and BLEC-Str8 genes have been identified, by analysing the transcriptome data of cold, salt and desiccation stress-treated 7-day-old rice (Oryza sativa L. var. IR64) seedling, to be preferentially responsive to desiccation and salt stress, respectively. NBS-Str1 and BLEC-Str8 genes code for putative NBS (nucleotide binding site)-LRR (leucine rich repeat) and β-lectin domain protein, respectively. NBS-Str1 URS is induced in root tissue, preferentially in vascular bundle, during 3 and 24 h of desiccation stress condition in transgenic 7-day-old rice seedling. In mature transgenic plants, this URS shows induction in root and shoot tissue under desiccation stress as well as under prolonged (1 and 2 day) salt stress. BLEC-Str8 URS shows basal activity under un-stressed condition, however, it is inducible under salt stress condition in both root and leaf tissues in young seedling and mature plants. Activity of BLEC-Str8 URS has been found to be vascular tissue preferential, however, under salt stress condition its activity is also found in the mesophyll tissue. NBS-Str1 and BLEC-Str8 URSs are inducible by heavy metal, copper and manganese. Interestingly, both the URSs have been found to be non responsive to ABA treatment, implying them to be part of ABA-independent abiotic stress response pathway. These URSs could prove useful for expressing a transgene in a stress responsive manner for development of stress tolerant transgenic systems.

Modulation of transcription factor and metabolic pathway genes in response to water-deficit stress in rice

Water-deficit stress is detrimental for rice growth, development, and yield. Transcriptome analysis of 1-week-old rice (Oryza sativa L. var. IR64) seedling under water-deficit stress condition using Affymetrix 57 K GeneChip® has revealed 1,563 and 1,746 genes to be up- and downregulated, respectively. In an effort to amalgamate data across laboratories, we identified 5,611 differentially expressing genes under varying extrinsic water-deficit stress conditions in six vegetative and one reproductive stage of development in rice. Transcription factors (TFs) involved in ABA-dependent and ABA-independent pathways have been found to be upregulated during water-deficit stress. Members of zinc-finger TFs namely, C₂H₂, C₂C₂, C₃H, LIM, PHD, WRKY, ZF-HD, and ZIM, along with TF families like GeBP, jumonji, MBF1 and ULT express differentially under water-deficit conditions. NAC (NAM, ATAF and CUC) TF family emerges to be a potential key regulator of multiple abiotic stresses. Among the 12 TF genes that are co-upregulated under water-deficit, salt and cold stress conditions, five belong to the NAC TF family. We identified water-deficit stress-responsive genes encoding key enzymes involved in biosynthesis of osmoprotectants like polyols and sugars; amino acid and quaternary ammonium compounds; cell wall loosening and structural components; cholesterol and very long chain fatty acid; cytokinin and secondary metabolites. Comparison of genes responsive to water-deficit stress conditions with genes preferentially expressed during panicle and seed development revealed a significant overlap of transcriptome alteration and pathways.

ABO3, a WRKY transcription factor, mediates plant responses to abscisic acid and drought tolerance in Arabidopsis

The biological functions of WRKY transcription factors in plants have been widely studied, but their roles in abiotic stress are still not well understood. We isolated an ABA overly sensitive mutant, abo3, which is disrupted by a T-DNA insertion in At1g66600 encoding a WRKY transcription factor AtWRKY63. The mutant was hypersensitive to ABA in both seedling establishment and seedling growth. However, stomatal closure was less sensitive to ABA, and the abo3 mutant was less drought tolerant than the wild type. Northern blot analysis indicated that the expression of the ABA-responsive transcription factor ABF2/AREB1 was markedly lower in the abo3 mutant than in the wild type. The abo3 mutation also reduced the expression of stress-inducible genes RD29A and COR47, especially early during ABA treatment. ABO3 is able to bind the W-box in the promoter of ABF2in vitro. These results uncover an important role for a WRKY transcription factor in plant responses to ABA and drought stress.

Identification and expression profiling analysis of TIFY family genes involved in stress and phytohormone responses in rice

The TIFY family is a novel plant-specific gene family involved in the regulation of diverse plant-specific biologic processes, such as development and responses to phytohormones, in Arabidopsis. However, there is limited information about this family in monocot species. This report identifies 20 TIFY genes in rice, the model monocot species. Sequence analysis indicated that rice TIFY proteins have conserved motifs beyond the TIFY domain as was previously shown in Arabidopsis. On the basis of their protein structures, members of the TIFY family can be divided into two groups. Transcript level analysis of OsTIFY genes in tissues and organs revealed different tempo-spatial expression patterns, suggesting that expression and function vary by stage of plant growth and development. Most of the OsTIFY genes were predominantly expressed in leaf. Nine OsTIFY genes were responsive to jasmonic acid and wounding treatments. Interestingly, almost all the OsTIFY genes were responsive to one or more abiotic stresses including drought, salinity, and low temperature. Over-expression of OsTIFY11a, one of the stress-inducible genes, resulted in significantly increased tolerance to salt and dehydration stresses. These results suggest that the OsTIFY family may have important roles in response to abiotic stresses. The data presented in this report provide important clues for further elucidating the functions of the genes in the OsTIFY family.

Over-expression of a LEA gene in rice improves drought resistance under the field conditions

Late embryogenesis abundant (LEA) proteins have been implicated in many stress responses of plants. In this report, a LEA protein gene OsLEA3-1 was identified and over-expressed in rice to test the drought resistance of transgenic lines under the field conditions. OsLEA3-1 is induced by drought, salt and abscisic acid (ABA), but not by cold stress. The promoter of OsLEA3-1 isolated from the upland rice IRAT109 exhibits strong activity under drought- and salt-stress conditions. Three expression constructs consisting of the full-length cDNA driven by the drought-inducible promoter of OsLEA3-1 (OsLEA3-H), the CaMV 35S promoter (OsLEA3-S), and the rice Actin1 promoter (OsLEA3-A) were transformed into the drought-sensitive japonica rice Zhonghua 11. Drought resistance pre-screening of T(1) families at anthesis stage revealed that the over-expressing families with OsLEA3-S and OsLEA3-H constructs had significantly higher relative yield (yield under drought stress treatment/yield under normal growth conditions) than the wild type under drought stress conditions, although a yield penalty existed in T(1) families under normal growth conditions. Nine homozygous families, exhibiting over-expression of a single-copy of the transgene and relatively low yield penalty in the T(1) generation, were tested in the field for drought resistance in the T(2) and T(3) generations and in the PVC pipes for drought tolerance in the T(2) generation. Except for two families (transformed with OsLEA3-A), all the other families (transformed with OsLEA3-S and OsLEA3-H constructs) had higher grain yield than the wild type under drought stress in both the field and the PVC pipes conditions. No significant yield penalty was detected for these T(2 )and T(3) families. These results indicate that transgenic rice with significantly enhanced drought resistance and without yield penalty can be generated by over-expressing OsLEA3-1 gene with appropriate promoters and following a bipartite (stress and non-stress) in-field screening protocol.

Over-expression of a LEA gene in rice improves drought resistance under the field conditions

Late embryogenesis abundant (LEA) proteins have been implicated in many stress responses of plants. In this report, a LEA protein gene OsLEA3-1 was identified and over-expressed in rice to test the drought resistance of transgenic lines under the field conditions. OsLEA3-1 is induced by drought, salt and abscisic acid (ABA), but not by cold stress. The promoter of OsLEA3-1 isolated from the upland rice IRAT109 exhibits strong activity under drought- and salt-stress conditions. Three expression constructs consisting of the full-length cDNA driven by the drought-inducible promoter of OsLEA3-1 (OsLEA3-H), the CaMV 35S promoter (OsLEA3-S), and the rice Actin1 promoter (OsLEA3-A) were transformed into the drought-sensitive japonica rice Zhonghua 11. Drought resistance pre-screening of T(1) families at anthesis stage revealed that the over-expressing families with OsLEA3-S and OsLEA3-H constructs had significantly higher relative yield (yield under drought stress treatment/yield under normal growth conditions) than the wild type under drought stress conditions, although a yield penalty existed in T(1) families under normal growth conditions. Nine homozygous families, exhibiting over-expression of a single-copy of the transgene and relatively low yield penalty in the T(1) generation, were tested in the field for drought resistance in the T(2) and T(3) generations and in the PVC pipes for drought tolerance in the T(2) generation. Except for two families (transformed with OsLEA3-A), all the other families (transformed with OsLEA3-S and OsLEA3-H constructs) had higher grain yield than the wild type under drought stress in both the field and the PVC pipes conditions. No significant yield penalty was detected for these T(2 )and T(3) families. These results indicate that transgenic rice with significantly enhanced drought resistance and without yield penalty can be generated by over-expressing OsLEA3-1 gene with appropriate promoters and following a bipartite (stress and non-stress) in-field screening protocol.

Bacteriophage T7 RNA polymerase-directed, inducible and tissue-specific over-expression of foreign genes in transgenic plants

A widely applicable bacteriophage T7 RNA polymerase-directed, tissue-specific and inducible over-expression of foreign genes in transgenic plants was developed. This was achieved through the simultaneous transformation of a modified T7 RNA polymerase to specifically transcribe the foreign gene placed under the control of T7 expression signals. The T7 RNA polymerase recognized the chimeric uidA gene integrated randomly into tobacco and rice genomes. Results from the use of six different promoters with different tissue specificities indicated that the recombinant protein was expressed at a several-fold (3-10-fold) higher level when compared with transgenes expressed directly under the control of these tissue-specific promoters. An important feature of the T7 system in plants was the near-uniform expression in the independently transformed plants, in contrast with the large variations observed in transgene expression under the direct control of plant promoters. In addition, our results demonstrated the application of the T7 system in the regulation of transgene expression through chemically inducible mechanisms. This versatility of controlled and regulated expression offers a powerful tool that could be used in various programmes in plant biotechnology and genomic studies.

The abscisic acid-responsive kinase PKABA1 interacts with a seed-specific abscisic acid response element-binding factor, TaABF, and phosphorylates TaABF peptide sequences

The abscisic acid (ABA)-induced protein kinase PKABA1 is present in dormant seeds and is a component of the signal transduction pathway leading to ABA-suppressed gene expression in cereal grains. We have identified a member of the ABA response element-binding factor (ABF) family of basic leucine zipper transcription factors from wheat (Triticum aestivum) that is specifically bound by PKABA1. This protein (TaABF) has highest sequence similarity to the Arabidopsis ABA response protein ABI5. In two-hybrid assays TaABF bound only to PKABA1, but not to a mutant version of PKABA1 lacking the nucleotide binding domain, suggesting that binding of TaABF requires prior binding of ATP as would be expected for binding of a protein substrate by a protein kinase. TaABF mRNA accumulated together with PKABA1 mRNA during wheat grain maturation and dormancy acquisition and TaABF transcripts increased transiently during imbibition of dormant grains. In contrast to PKABA1 mRNA, TaABF mRNA is seed specific and did not accumulate in vegetative tissues in response to stress or ABA application. PKABA1 produced in transformed cell lines was able to phosphorylate synthetic peptides representing three specific regions of TaABF. These data suggest that TaABF may serve as a physiological substrate for PKABA1 in the ABA signal transduction pathway during grain maturation, dormancy expression, and ABA-suppressed gene expression.

A novel cold-inducible gene from Arabidopsis, RCI3, encodes a peroxidase that constitutes a component for stress tolerance

A cDNA from Arabidopsis corresponding to a new cold-inducible gene, RCI3 (for Rare Cold Inducible gene 3), was isolated. Isoelectric focusing electrophoresis and staining of peroxidase activity demonstrated that RCI3 encodes an active cationic peroxidase. RNA-blot analysis revealed that RCI3 expression in response to low temperature is negatively regulated by light, as RCI3 transcripts were exclusively detected in etiolated seedlings and roots of adult plants. RCI3 expression was also induced in etiolated seedlings, but not in roots, exposed to dehydration, salt stress or ABA, indicating that it is subjected to a complex regulation through different signaling pathways. Analysis of transgenic plants containing RCI3::GUS fusions established that this regulation occurs at the transcriptional level during plant development, and that cold-induced RCI3 expression in roots is mainly restricted to the endodermis. Plants overexpressing RCI3 showed an increase in dehydration and salt tolerance, while antisense suppression of RCI3 expression gave dehydration- and salt-sensitive phenotypes. These results indicate that RCI3 is involved in the tolerance to both stresses in Arabidopsis, and illustrate that manipulation of RCI3 has a potential with regard to plant improvement of stress tolerance.

Developmentally regulated dual-specificity kinase from peanut that is induced by abiotic stresses

Tyrosine (Tyr) phosphorylation represents an important biochemical mechanism to regulate many cellular processes. No Tyr kinase has been cloned so far in plants. Dual-specificity kinases are reported in plants and the function of these kinases remains unknown. A 1.7-kb cDNA that encodes serine/threonine/Tyr (STY) kinase was isolated by screening peanut (Arachis hypogaea) expression library using the anti-phospho-Tyr antibody. The histidine-tagged recombinant kinase histidine-6-STY predominantly autophosphorylated on Tyr and phosphorylated the histone primarily on threonine. Genomic DNA gel-blot analysis revealed that STY kinase is a member of a small multigene family. The transcript of STY kinase is accumulated in the mid-maturation stage of seed development, suggesting a role in the signaling of storage of seed reserves. The STY kinase mRNA expression, as well as kinase activity, markedly increased in response to cold and salt treatments; however, no change in the protein level was observed, suggesting a posttranslational activation mechanism. The activation of the STY kinase is detected after 12 to 48 h of cold and salt treatments, which indicates that the kinase may not participate in the initial response to abiotic stresses, but may play a possible role in the adaptive process to adverse conditions. The transcript levels and kinase activity were unaltered with abscisic acid treatment, suggesting an abscisic acid-independent cold and salt signaling pathway. Here, we report the first identification of a non-MAP kinase cascade dual-specificity kinase involved in abiotic stress and seed development.

Specific association of transcripts of tbzF and tbz17, tobacco genes encoding basic region leucine zipper-type transcriptional activators, with guard cells of senescing leaves and/or flowers

Induction by low temperature is a common feature of the lip19 subfamily members of the basic region leucine zipper gene family in plants. Here, we characterize two tobacco (Nicotiana tabacum) genes, tbzF and tbz17, belonging to the lip19 subfamily, whose gene products, TBZF and TBZ17, show 73% identity and are located in nuclei. They preferentially bind to DNA fragments spanning A-box/G-box and C-box/G-box hybrid motifs and show transactivation activity in cobombarded tobacco BY-2 cells, indicating they function as transcriptional activators. Transcripts of tbzF were detected at a high level in senescing leaves and flowers. In contrast, tbz17 transcripts could be shown to accumulate in aged leaves but not in flowers. In situ hybridization analysis revealed transcripts of tbzF and tbz17 to be predominantly located in guard cells and vascular tissues of senescing leaves. These results suggest that TBZF and TBZ17 are both involved in controlling gene transcription related to functions of guard cells in senescing leaves and that TBZF bifunctionally acts in floral development.

Developmental and stress regulation of RCI2A and RCI2B, two cold-inducible genes of arabidopsis encoding highly conserved hydrophobic proteins

The capability of most higher plants to tolerate environmental conditions strongly depends on their developmental stage. In addition, environmental factors have pleiotropic effects on many developmental processes. The interaction between plant development and environmental conditions implies that some genes must be regulated by both environmental factors and developmental cues. To understand their developmental regulation and obtain possible clues on their functions, we have isolated genomic clones for RCI2A and RCI2B, two genes from Arabidopsis ecotype Columbia (Col), whose expression is induced in response to low temperature, dehydration, salt stress, and abscisic acid. The promoters of RCI2A and RCI2B were fused to the uidA (GUS)-coding sequence and the resulting constructs used to transform Arabidopsis. GUS activity was analyzed in transgenic plants during development under both stressed and unstressed conditions. Transgenic plants with either the RCI2A or RCI2B promoter showed strong GUS expression during the first stages of seed development and germination, in vascular bundles, pollen, and most interestingly in guard cells. When transgenic plants were exposed to low temperature, dehydration, salt stress, or abscisic acid, reporter gene expression was induced in most tissues. These results indicate that RCI2A and RCI2B are regulated at transcriptional level during plant development and in response to different environmental stimuli and treatments. The potential role of RCI2A and RCI2B in plant development and stress response is discussed.

Glutathione biosynthesis in Arabidopsis trichome cells

In Arabidopsis thaliana, trichome cells are specialized unicellular structures with uncertain functions. Based on earlier observations that one of the genes involved in cysteine biosynthesis (Atcys-3A) is highly expressed in trichomes, we have extended our studies in trichome cells to determine their capacity for glutathione (GSH) biosynthesis. First, we have analyzed by in situ hybridization the tissue-specific expression of the genes Atcys-3A and sat5, which encode O-acetylserine(thio)lyase (OASTL) and serine acetyltransferase (SAT), respectively, as well as gsh1 and gsh2, which encode gamma-glutamylcysteine synthetase and glutathione synthetase, respectively. The four genes are highly expressed in leaf trichomes of Arabidopsis, and their mRNA accumulate to high levels. Second, we have directly measured cytoplasmic GSH concentration in intact cells by laser-scanning microscopy after labeling with monochlorobimane as a GSH-specific probe. From these measurements, cytosolic GSH concentrations of 238+/-25, 80+/-2, and 144+/-19 microM were estimated for trichome, basement, and epidermal cells, respectively. Taking into account the volume of the cells measured using stereological techniques, the trichomes have a total GSH content more than 300-fold higher than the basement and epidermal cells. Third, after NaCl treatment, GSH biosynthesis is markedly decreased in trichomes. Atcys-3A, sat5, gsh1, and gsh2 mRNA levels show a decrease in transcript abundance, and [GSH](cyt) is reduced to 47+/-5 microM. These results suggest the important physiological significance of trichome cells related to GSH biosynthesis and their possible role as a sink during detoxification processes.

An Arabidopsis mutant with deregulated ABA gene expression: implications for negative regulator function

The physiological acclimation of plants to osmotic stresses involves a complex programme of gene regulation. In one signalling pathway, elevated levels of abscisic acid (ABA) activate a subset of stress genes. Because ABA responses lack a definable morphological phenotype, we have screened for mutants that exhibit deregulated ABA-responsive gene expression. To monitor this ABA response, a line of Arabidopsis thaliana carrying a transgene composed of the ABA-responsive Arabidopsis kin2 promoter fused to the coding sequence for the firefly luciferase gene, kin2::luc, was generated. Patterns of ABA-responsive luciferase activity were monitored by photon counting. In contrast to wild-type plants which display a transient activation of kin2::luc, an ABA deregulated gene expression mutant (ade1) exhibits both sustained and enhanced levels of transgene activity. Levels of kin2, cor47 and rab18 expression in ade1 plants are also enhanced and prolonged indicating that the molecular mechanism(s) altered in ade1 plants affects the regulation of other ABA-responsive genes. The mutant phenotype is specific for the ABA response as cold-inducible kin2 expression is unaltered in ade1 plants. Genetic analyses indicate that the ade1 mutant is a monogenic recessive trait. A role for negative regulator function in ABA signalling is discussed.

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 phenotypic plasticity: where internal programming meets the external environment

Developmental plasticity has long been the focus of research in both evolutionary ecology and molecular genetics. Recently, the concept of ontogenetic contingency has been proposed to indicate the dependence of plastic responses on the timing and sequence of developmental events. Also, the idea of the developmental reaction norm has been put forward to indicate the complex interactions among development, phenotypic plasticity, and allometry of different structures. Finally, for the first time, studies ranging from the ecological to the molecular aspects of the same plastic response are available on insect and flowering plant model systems.

Structure of the dehydrin tas14 gene of tomato and its developmental and environmental regulation in transgenic tobacco

We have isolated a genomic clone encoding tomato TAS14, a dehydrin that accumulates in response to mannitol, NaCl or abscisic acid (ABA) treatment. A fragment of tas14 gene containing the region from -2591 to +162 fused to beta-glucuronidase gene drives ABA- and osmotic stress-induced GUS expression in transgenic tobacco. Histochemical analysis of salt-, mannitol- and ABA-treated plants showed GUS activity mainly localized to vascular tissues, outer cortex and adventitious root meristems, coinciding with the previously observed distribution of TAS14 protein in salt-stressed tomato plants. In addition, GUS activity was also observed in guard cells, trichomes and leaf axils. Developmentally regulated gus expression was studied in unstressed plants and found to occur not only in embryos, but also in flowers and pollen. Tas14 expression in floral organs was confirmed by northern blots of tomato flowers.