Mutational analysis of a plant heat shock element

Temporally gene knockout using heat shock-inducible genome-editing system in plants

Clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9) has emerged as a powerful tool to generate targeted loss-of-function mutations for functional genomic studies. As a next step, tools to generate genome modifications in a spatially and temporally precise manner will enable researchers to further dissect gene function. Here, we present two heat shock-inducible genome-editing (IGE) systems that efficiently edit target genes when the system is induced, thus allowing us to target specific developmental stages. For this conditional editing system, we chose the natural heat-inducible promoter from heat-shock protein 18.2 (HSP18.2) from Arabidopsis thaliana and the synthetic heat-inducible promoter heat shock-response element HSE-COR15A to drive the expression of Cas9. We tested these two IGE systems in Arabidopsis using cyclic or continuous heat-shock treatments at the seedling and bolting stages. A real-time quantitative polymerase chain reaction analysis revealed that the HSP18.2 IGE system exhibited higher Cas9 expression levels than the HSE-COR15A IGE system upon both cyclic and continuous treatments. By targeting brassinosteroid-insensitive 1 (BRI1) and phytoene desaturase (PDS), we demonstrate that both cyclic and continuous heat inductions successfully activated the HSP18.2 IGE system at the two developmental stages, resulting in highly efficient targeted mutagenesis and clear phenotypic outcomes. By contrast, the HSE-COR15A IGE system was only induced at the seedling stage and was less effective than the HSP18.2 IGE system in terms of mutagenesis frequencies. The presented heat shock-IGE systems can be conditionally induced to efficiently inactivate genes at any developmental stage and are uniquely suited for the dissection and systematic characterization of essential genes.

A non-tandem CCCH-type zinc-finger protein, IbC3H18, functions as a nuclear transcriptional activator and enhances abiotic stress tolerance in sweet potato

CCCH-type zinc-finger proteins play essential roles in regulating plant development and stress responses. However, the molecular and functional properties of non-tandem CCCH-type zinc-finger (non-TZF) proteins have been rarely characterized in plants. Here, we report the biological and molecular characterization of a sweet potato non-TZF gene, IbC3H18. We show that IbC3H18 exhibits tissue- and abiotic stress-specific expression, and could be effectively induced by abiotic stresses, including NaCl, polyethylene glycol (PEG) 6000, H2 O2 and abscisic acid (ABA) in sweet potato. Accordingly, overexpression of IbC3H18 led to increased, whereas knock-down of IbC3H18 resulted in decreased tolerance of sweet potato to salt, drought and oxidation stresses. In addition, IbC3H18 functions as a nuclear transcriptional activator and regulates the expression of a range of abiotic stress-responsive genes involved in reactive oxygen species (ROS) scavenging, ABA signaling, photosynthesis and ion transport pathways. Moreover, our data demonstrate that IbC3H18 physically interacts with IbPR5, and that overexpression of IbPR5 enhances salt and drought tolerance in transgenic tobacco plants. Collectively, our data indicate that IbC3H18 functions in enhancing abiotic stress tolerance in sweet potato, which may serve as a candidate gene for use in improving abiotic stress resistance in crops.

The ATAF1 transcription factor is a key regulator of aldehyde dehydrogenase 7B4 (ALDH7B4) gene expression in Arabidopsis thaliana

ALDH7B4 expression contributes to abiotic stress tolerance. The NAC transcription factor ATAF1 is a main regulator of expression of the ALDH7B4 gene in Arabidopsis thaliana as shown by ATAF1 mutants. The aldehyde dehydrogenase 7B4 (ALDH7B4) protein has important roles in detoxification of excessive aldehydes, elimination of reactive oxygen species (ROS) and inhibition of lipid peroxidation when plants are exposed to abiotic stress. However, the regulation of the expression of the ALDH7B4 gene under stress is largely unknown. Promoter studies revealed crucial cis-elements in the ALDH7B4 promoter in response to heat and stress combinations. Using a yeast one-hybrid assay, several NAC transcription factors, including ATAF1 were isolated. These transcription factors play an important role in plant adaptation to abiotic stress. ATAF1 activates the expression of the ALDH7B4 gene by directly binding to the promoter. Overexpression of ATAF1 in Arabidopsis plants results in elevated expression of ALDH7B4 in seeds, seedlings, and mature plants, whereas ATAF1 knock-out mutant plants abolished the expression of ALDH7B4. This study implies that ATAF1 may confer stress tolerance by up-regulating the target gene ALDH7B4.

Td4IN2: A drought-responsive durum wheat (Triticum durum Desf.) gene coding for a resistance like protein with serine/threonine protein kinase, nucleotide binding site and leucine rich domains

Wheat, the main food source for a third of world population, appears strongly under threat because of predicted increasing temperatures coupled to drought. Plant complex molecular response to drought stress relies on the gene network controlling cell reactions to abiotic stress. In the natural environment, plants are subjected to the combination of abiotic and biotic stresses. Also the response of plants to biotic stress, to cope with pathogens, involves the activation of a molecular network. Investigations on combination of abiotic and biotic stresses indicate the existence of cross-talk between the two networks and a kind of overlapping can be hypothesized. In this work we describe the isolation and characterization of a drought-related durum wheat (Triticum durum Desf.) gene, identified in a previous study, coding for a protein combining features of NBS-LRR type resistance protein with a S/TPK domain, involved in drought stress response. This is one of the few examples reported where all three domains are present in a single protein and, to our knowledge, it is the first report on a gene specifically induced by drought stress and drought-related conditions, with this particular structure.

Design of an optimal promoter involved in the heat-induced transcriptional pathway in Arabidopsis, soybean, rice and maize

Interactions between heat shock (HS) factors (HSFs) and heat shock response elements (HSEs) are important during the heat shock response (HSR) of flora and fauna. Moreover, plant HSFs that are involved in heat stress are also involved in abiotic stresses such as dehydration and cold as well as development, cell differentiation and proliferation. Because the specific combination of HSFs and HSEs involved in plants under heat stress remains unclear, the mechanism of their interaction has not yet been utilized in molecular breeding of plants for climate change. For the study reported herein, we compared the sequences of HS-inducible genes and their promoters in Arabidopsis, soybean, rice and maize and then designed an optimal HS-inducible promoter. Our analyses suggest that, for the four species, the abscisic acid-independent, HSE/HSF-dependent transcriptional pathway plays a major role in HS-inducible gene expression. We found that an 18-bp sequence that includes the HSE has an important role in the HSR, and that those sequences could be classified as representative of monocotyledons or dicotyledons. With the HS-inducible promoter designed based on our bioinformatic predictions, we were able to develop an optimal HS-specific inducible promoter for seedlings or single cells in roots. These findings demonstrate the utility of our HS-specific inducible promoter, which we expect will contribute to molecular breeding efforts and cell-targeted gene expression in specific plant tissues.

Arabidopsis ensemble reverse-engineered gene regulatory network discloses interconnected transcription factors in oxidative stress

The abiotic stress response in plants is complex and tightly controlled by gene regulation. We present an abiotic stress gene regulatory network of 200,014 interactions for 11,938 target genes by integrating four complementary reverse-engineering solutions through average rank aggregation on an Arabidopsis thaliana microarray expression compendium. This ensemble performed the most robustly in benchmarking and greatly expands upon the availability of interactions currently reported. Besides recovering 1182 known regulatory interactions, cis-regulatory motifs and coherent functionalities of target genes corresponded with the predicted transcription factors. We provide a valuable resource of 572 abiotic stress modules of coregulated genes with functional and regulatory information, from which we deduced functional relationships for 1966 uncharacterized genes and many regulators. Using gain- and loss-of-function mutants of seven transcription factors grown under control and salt stress conditions, we experimentally validated 141 out of 271 predictions (52% precision) for 102 selected genes and mapped 148 additional transcription factor-gene regulatory interactions (49% recall). We identified an intricate core oxidative stress regulatory network where NAC13, NAC053, ERF6, WRKY6, and NAC032 transcription factors interconnect and function in detoxification. Our work shows that ensemble reverse-engineering can generate robust biological hypotheses of gene regulation in a multicellular eukaryote that can be tested by medium-throughput experimental validation.

Genome-wide characterization of phenylalanine ammonia-lyase gene family in watermelon (Citrullus lanatus)

Phenylalanine ammonia-lyase (PAL), the first enzyme in the phenylpropanoid pathway, plays a critical role in plant growth, development, and adaptation. PAL enzymes are encoded by a gene family in plants. Here, we report a genome-wide search for PAL genes in watermelon. A total of 12 PAL genes, designated ClPAL1-12, are identified . Nine are arranged in tandem in two duplication blocks located on chromosomes 4 and 7, and the other three ClPAL genes are distributed as single copies on chromosomes 2, 3, and 8. Both the cDNA and protein sequences of ClPALs share an overall high identity with each other. A phylogenetic analysis places 11 of the ClPALs into a separate cucurbit subclade, whereas ClPAL2, which belongs to neither monocots nor dicots, may serve as an ancestral PAL in plants. In the cucurbit subclade, seven ClPALs form homologous pairs with their counterparts from cucumber. Expression profiling reveals that 11 of the ClPAL genes are expressed and show preferential expression in the stems and male and female flowers. Six of the 12 ClPALs are moderately or strongly expressed in the fruits, particularly in the pulp, suggesting the potential roles of PAL in the development of fruit color and flavor. A promoter motif analysis of the ClPAL genes implies redundant but distinctive cis-regulatory structures for stress responsiveness. Finally, duplication events during the evolution and expansion of the ClPAL gene family are discussed, and the relationships between the ClPAL genes and their cucumber orthologs are estimated.

Ectopic overexpression of SlHsfA3, a heat stress transcription factor from tomato, confers increased thermotolerance and salt hypersensitivity in germination in transgenic Arabidopsis

Plant heat stress transcription factors (Hsfs) are the critical components involved in mediating responses to various environmental stressors. However, the detailed roles of many plant Hsfs are far from fully understood. In this study, an Hsf (SlHsfA3) was isolated from the cultivated tomato (Solanum lycopersicum, Sl) and functionally characterized at the genetic and developmental levels. The nucleus-localized SlHsfA3 was basally and ubiquitously expressed in different plant organs. The expression of SlHsfA3 was induced dramatically by heat stress, moderately by high salinity, and slightly by drought, but was not induced by abscisic acid (ABA). The ectopic overexpression of SlHsfA3 conferred increased thermotolerance and late flowering phenotype to transgenic Arabidopsis plants. Moreover, SlHsfA3 played a negative role in controlling seed germination under salt stress. RNA-sequencing data demonstrated that a number of heat shock proteins (Hsps) and stress-associated genes were induced in Arabidopsis plants overexpressing SlHsfA3. A gel shift experiment and transient expression assays in Nicotiana benthamiana leaves demonstrated that SlHsfA3 directly activates the expression of SlHsp26.1-P and SlHsp21.5-ER. Taken together, our results suggest that SlHsfA3 behaves as a typical Hsf to contribute to plant thermotolerance. The late flowering and seed germination phenotypes and the RNA-seq data derived from SlHsfA3 overexpression lines lend more credence to the hypothesis that plant Hsfs participate in diverse physiological and biochemical processes related to adverse conditions.

Analysis of gene sequences indicates that quantity not quality of chloroplast small HSPs improves thermotolerance in C4 and CAM plants

Chloroplast-localized small heat-shock proteins (Cp-sHSP) protect Photosystem II and thylakoid membranes during heat and other stresses, and Cp-sHSP production levels are related to plant thermotolerance. However, to date, a paucity of Cp-sHSP sequences from C4 or CAM species, or from other extremely heat-tolerant species, has precluded an examination to determine if Cp-sHSP genes or proteins might differ among plants with photosynthetic pathways or between heat-sensitive and heat-tolerant species. To investigate this, we isolated and characterized novel Cp-sHSP genes in four plant species: two moderately heat-tolerant C4 species, Spartina alterniflora (monocot) and Amaranthus retroflexus (eudicot), and two very heat-tolerant CAM species, Agave americana (monocot) and Ferocactus wislizenii (eudicot) (respective genes: SasHSP27.12, ArsHSP26.43, AasHSP26.85 and FwsHSP27.52) by PCR-based genome walking and cDNA RACE. Analysis of these Cp-sHSPs has confirmed the presence of conserved domains common to previously examined species. As expected, the transit peptide was found to be the most variable part of these proteins. Promoter analysis of these genes revealed differences in CAM versus C3 and C4 species that were independent of a general difference between monocots and eudicots observed for the entire protein. Heat-induced gene and protein expression indicated that Cp-sHSP protein levels were correlated with thermotolerance of photosynthetic electron transport, and that in most cases protein and transcript levels were correlated. Thus, available evidence indicates little variation in the amino acid sequence of Cp-sHSP mature proteins between heat-sensitive and -tolerant species, but that variation in Cp-sHSP protein production is related to heat tolerance or photosynthetic pathway (CAM vs. C3 and C4) and is driven by promoter differences. Key message We isolated and characterized four novel Cp-sHSP genes with promoters from wild plants, analysis has shown qualitative and quantitative interspecific variations in Cp-sHSPs of C3, C4, and CAM plant thermotolerance.

Transcription factor OsHsfC1b regulates salt tolerance and development in Oryza sativa ssp. japonica

BACKGROUND AND AIMS

Salt stress leads to attenuated growth and productivity in rice. Transcription factors like heat shock factors (HSFs) represent central regulators of stress adaptation. Heat shock factors of the classes A and B are well established as regulators of thermal and non-thermal stress responses in plants; however, the role of class C HSFs is unknown. Here we characterized the function of the OsHsfC1b (Os01g53220) transcription factor from rice.

METHODOLOGY

We analysed the expression of OsHsfC1b in the rice japonica cultivars Dongjin and Nipponbare exposed to salt stress as well as after mannitol, abscisic acid (ABA) and H(2)O(2) treatment. For functional characterization of OsHsfC1b, we analysed the physiological response of a T-DNA insertion line (hsfc1b) and two artificial micro-RNA (amiRNA) knock-down lines to salt, mannitol and ABA treatment. In addition, we quantified the expression of small Heat Shock Protein (sHSP) genes and those related to signalling and ion homeostasis by quantitative real-time polymerase chain reaction in roots exposed to salt. The subcellular localization of OsHsfC1b protein fused to green fluorescent protein (GFP) was determined in Arabidopsis mesophyll cell protoplasts.

PRINCIPAL RESULTS

Expression of OsHsfC1b was induced by salt, mannitol and ABA, but not by H(2)O(2). Impaired function of OsHsfC1b in the hsfc1b mutant and the amiRNA lines led to decreased salt and osmotic stress tolerance, increased sensitivity to ABA, and temporal misregulation of salt-responsive genes involved in signalling and ion homeostasis. Furthermore, sHSP genes showed enhanced expression in knock-down plants under salt stress. We observed retarded growth of hsfc1b and knock-down lines in comparison with control plants under non-stress conditions. Transient expression of OsHsfC1b fused to GFP in protoplasts revealed nuclear localization of the transcription factor.

CONCLUSIONS

OsHsfC1b plays a role in ABA-mediated salt stress tolerance in rice. Furthermore, OsHsfC1b is involved in the response to osmotic stress and is required for plant growth under non-stress conditions.

Isolation of heat shock-induced Nicotiana tabacum transcription promoters and their potential as a tool for plant research and biotechnology

Transcription promoters of heat shock protein (HSP) genes have been used to control the expression of heterologous proteins in plants and plant cells. To obtain a strong HSP promoter that is functionally active in Nicotiana tabacum BY-2 cells, we set out to identify a promoter of an endogenous gene showing high activation of expression by heat. An N. tabacum BY-2 cell culture was treated for 8 h at 37°C and the cell protein extract analyzed by two-dimensional electrophoresis. A major spot was identified by mass spectrometry as belonging to the small HSP family. The promoter regions and the 5' and 3' untranslated regions of two genes, NtHSP3A and NtHSP3B, with sequences fitting the protein identified were cloned and fused to a hybrid reporter gene coding for β-glucuronidase (GUS) and a yellow fluorescent protein. These constructs were introduced into N. tabacum BY2 cells by Agrobacterium tumefaciens-mediated transformation. Both promoters conferred similar heat-induced GUS expression. In the best heat shock condition, GUS activity was increased 200 fold and reached 285 pmol min(-1) μg protein(-1). Up-scaling in a 4-l bioreactor resulted in similar heat-induced expression. The NtHSP3A promoter was then used to drive the expression of NtPDR1, a plasma membrane transporter belonging to the pleiotropic drug resistance family. No expression was observed at 25°C, while, at 37°C, expression was similar to that obtained using a strong constitutive promoter. These data show that the HSP promoters isolated are useful for high heat-induced expression in N. tabacum BY-2 cells.

Two sunflower 17.6HSP genes, arranged in tandem and highly homologous, are induced differently by various elicitors

Plants respond to environmental stimuli, such as heat shock, by re-programming cellular activity through differential gene expression, mainly controlled at the transcription level. The current study refers to two sunflower small heat shock protein (sHSP) genes arranged in tandem in head-to-head orientation and linked by a 3809 bp region. These genes exhibit only slight structural differences in the coding portion. They code for cytosolic class I sHSPs and are named HaHSP17.6a and HaHSP17.6b according to the molecular weight of the putative proteins. The genomic organization of these genes is consistent with the idea that many HSP genes originate from duplication events; in this case, probably an inversion and duplication occurred. The HaHSP17.6a and HaHSP17.6b genes are characterized by different expression levels under various heat stress conditions; moreover, their expression is differently induced by various elicitors. The differential regulation observed for HaHSP17.6a and HaHSP17.6b genes differs from previous observations on duplicated sHSP genes in plants.

Expression of rice heat stress transcription factor OsHsfA2e enhances tolerance to environmental stresses in transgenic Arabidopsis

Plant growth and crop yields are limited by high-temperature stresses. In this study, we attempted to isolate the rice genes responsible for high-temperature stress tolerance using a transformed Arabidopsis population expressing a full-length cDNA library of rice. From approximately 20,000 lines of transgenic Arabidopsis, we isolated a thermotolerant line, R04333, that could survive transient heat stress at the cotyledon stage. The rice cDNA inserted in R04333 encodes OsHsfA2e, a member of the heat stress transcription factors. The thermotolerant phenotype was observed in newly constructed transgenic Arabidopsis plants expressing OsHsfA2e. Among 5 A2-type HSF genes encoded in the rice genome, four genes, including OsHsfA2e, are induced by high temperatures in rice seedlings. The OsHsfA2e protein was localized to the nuclear region and exhibited transcription activation activity in the C-terminal region. Microarray analysis demonstrated that under unstressed conditions transgenic Arabidopsis overexpressing OsHsfA2e highly expressed certain stress-associated genes, including several classes of heat-shock proteins. The thermotolerant phenotype was observed not only in the cotyledons but also in rosette leaves, inflorescence stems and seeds. In addition, transgenic Arabidopsis exhibited tolerance to high-salinity stress. These observations suggest that the OsHsfA2e may be useful in molecular breeding designed to improve the environmental stress tolerance of crops.

Global impact: elucidating plant responses to viral infection

Viruses induce a variety of responses in host cells that are mediated by perturbation of different signaling pathways. Advances in our understanding of the functions of viral proteins, plant biology in general, as well as technologies for profiling gene expression have converged in recent years to provide new insight into the events occurring inside susceptible and resistant host cells in response to virus infection. These effects range from nonspecific changes in gene expression due to the general accumulation of viral proteins to those responses that are initiated by the specific interactions between virus and host proteins. Here, we discuss a variety of expression profiling methods and approaches that have been used to study the effects of viruses on host transcriptomes. These studies have identified distinct sets of genes that have altered expression profiles in response to viruses, including stress- and defense-related genes. The activities of viral RNA silencing suppressors and interference with hormone signaling or biogenesis also influence plant gene expression and lead to developmental abnormalities.

Arabidopsis heat shock transcription factor A2 as a key regulator in response to several types of environmental stress

We isolated 76 high-light and heat-shock (HL + HS) stress-inducible genes, including a putative heat-shock transcription factor (HsfA2), by suppression-subtractive hybridization from Arabidopsis. The transcript level of HsfA2 was significantly increased under the several stress conditions or by the H(2)O(2) treatment. Furthermore, the induction of HsfA2 expression was highest among those of other class A HSFs in response to HL + HS stress conditions. The promoter assay revealed that HsfA2 is induced mainly in rosette leaves under HL + HS stress conditions. In the HsfA2-overexpressing Arabidopsis (Pro(35S):HsfA2) plants, 46 genes, including a large number of heat-shock proteins, ascorbate peroxidase 2 and galactinol synthase 1 and 2, were highly expressed compared with those in the wild-type plants. The transcript levels of the HsfA2 target genes are highly correlated with those of HsfA2 in the Pro(35S):HsfA2 plants. The transcript levels of the HsfA2 target genes, as well as HsfA2 transcripts, were induced by treating with exogenous H(2)O(2). In the knockout HsfA2 Arabidopsis plants, the induction of 26 HsfA2 target genes was strongly reduced for up to 2 h under HL + HS stress conditions. Furthermore, the Pro(35S):HsfA2 plants showed increased tolerance to combined environmental stresses. Our present results indicate that HsfA2 is a key regulator in the induction of the defence system under several types of environmental stress.

Isolation and characterization of class A4 heat shock transcription factor from alfalfa

Plant heat shock transcription factors (HSFs) regulate transcription of heat shock (HS) genes. In Arabidopsis thaliana, 21 HSFs have been classified into groups A-C. Members of class A act as typical transcriptional activators, whereas B HSFs function as coactivators or repressors depending on promoter context. The function of class C HSFs is still unclear. Here, we present the isolation and characterization of the first HSF from alfalfa (Medicago sativa L.) and designate it MsHSFA4 based on amino acid sequence analysis. The MsHSFA4 gene was determined to be single copy and was detected at two separate genetic loci in the tetraploid Medicago sativa. Overexpression of MsHSFA4 in tobacco mesophyll protoplasts resulted in weak transcriptional activity, similar to that exhibited by Arabidopsis AtHSFA4a. The MsHSFA4 proximal promoter contains three putative HSE elements, and the gene itself is activated both by heat and cold stress.

Diverse RNA viruses elicit the expression of common sets of genes in susceptible Arabidopsis thaliana plants

Systemic infections of plants by viruses require that viruses modify host cells in order to facilitate infections. These modifications include induction of host factors required for replication, propagation and movement, and suppression of host defense responses, which are likely to be associated with changes in host gene expression. Past studies of the effects of viral infection on gene expression in susceptible hosts have been limited to only a handful of genes. To gain broader insight into the responses elicited by viruses in susceptible hosts, high-density oligonucleotide probe microarray technology was used. Arabidopsis leaves were either mock inoculated or inoculated with cucumber mosaic cucumovirus, oil seed rape tobamovirus, turnip vein clearing tobamovirus, potato virus X potexvirus, or turnip mosaic potyvirus. Inoculated leaves were collected at 1, 2, 4, and 5 days after inoculation, total RNA was isolated, and samples were hybridized to Arabidopsis GeneChip microarrays (Affymetrix). Microarray hybridization revealed co-ordinated changes in gene expression in response to infection by diverse viruses. These changes include virus-general and virus-specific alterations in the expression of genes associated with distinct defense or stress responses. Analyses of the promoters of these genes further suggest that diverse RNA viruses elicit common responses in susceptible plant hosts through signaling pathways that have not been previously characterized.

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.

Small heat shock proteins and stress tolerance in plants

Small heat shock proteins (sHsps) are produced ubiquitously in prokaryotic and eukaryotic cells upon heat. The special importance of sHsps in plants is suggested by unusual abundance and diversity. Six classes of sHsps have been identified in plants based on their intracellular localization and sequence relatedness. In addition to heat stress, plant sHsps are also produced under other stress conditions and at certain developmental stages. Induction of sHsp gene expression and protein accumulation upon environmental stresses point to the hypothesis that these proteins play an important role in stress tolerance. The function of sHsps as molecular chaperones is supported by in vitro and in vivo assays. This review summarizes recent knowledge about plant sHsp gene expression, protein structure and functions.

Combinatorial interaction of cis elements specifies the expression of the Arabidopsis AtHsp90-1 gene

The promoter region of the Arabidopsis AtHsp90-1 gene is congested with heat shock elements and stress response elements, as well as with other potential transcriptional binding sites (activating protein 1, CCAAT/enhancer-binding protein element, and metal regulatory element). To determine how the expression of this bona fide AtHsp90-1 gene is regulated, a comprehensive quantitative and qualitative promoter deletion analysis was conducted under various environmental conditions and during development. The promoter induces gene expression at high levels after heat shock and arsenite treatment. However, our results show that the two stress responses may involve common but not necessarily the same regulatory elements. Whereas for heat induction, heat shock elements and stress response elements act cooperatively to promote high levels of gene expression, arsenite induction seems to require the involvement of activating protein 1 regulatory sequences. In stressed transgenic plants harboring the full-length promoter, beta-glucuronidase activity was prominent in all tissues. Nevertheless, progressive deletion of the promoter decreases the level of expression under heat shock and restricts it predominantly in the two meristems of the plant. In contrast, under arsenite induction, proximal sequences induce AtHsp90-1 gene expression only in the shoot meristem. Distally located elements negatively regulate AtHsp90-1 gene expression under unstressed conditions, whereas flower-specific regulated expression in mature pollen grains suggests the prominent role of the AtHsp90-1 in pollen development. The results show that the regulation of developmental expression, suppression, or stress induction is mainly due to combinatorial contribution of the cis elements in the promoter region of the AtHsp90-1 gene.

Heat stress- and heat shock transcription factor-dependent expression and activity of ascorbate peroxidase in Arabidopsis

To find evidence for a connection between heat stress response, oxidative stress, and common stress tolerance, we studied the effects of elevated growth temperatures and heat stress on the activity and expression of ascorbate peroxidase (APX). We compared wild-type Arabidopsis with transgenic plants overexpressing heat shock transcription factor 3 (HSF3), which synthesize heat shock proteins and are improved in basal thermotolerance. Following heat stress, APX activity was positively affected in transgenic plants and correlated with a new thermostable isoform, APX(S). This enzyme was present in addition to thermolabile cytosolic APX1, the prevalent isoform in unstressed cells. In HSF3-transgenic plants, APX(S) activity was detectable at normal temperature and persisted after severe heat stress at 44 degrees C. In nontransgenic plants, APX(S) was undetectable at normal temperature, but could be induced by moderate heat stress. The mRNA expression profiles of known and three new Apx genes were determined using real-time PCR. Apx1 and Apx2 genes encoding cytosolic APX were heat stress and HSF dependently expressed, but only the representations of Apx2 mRNA met the criteria that suggest identity between APX(S) and APX2: not expressed at normal temperature in wild type, strong induction by heat stress, and HSF3-dependent expression in transgenic plants. Our data suggest that Apx2 is a novel heat shock gene and that the enzymatic activity of APX2/APX(S) is required to compensate heat stress-dependent decline of APX1 activity in the cytosol. The functional roles of modulations of APX expression and the interdependence of heat stress and oxidative stress response and signaling mechanisms are discussed.

In vivo analysis of plant promoters and transcription factors by agroinfiltration of tobacco leaves

A convenient, Agrobacterium-mediated transient expression assay has been evaluated for rapid analysis of plant promoters and transcription factors in vivo. By simple infiltration of Agrobacterium cells carrying appropriate plasmid constructs into tobacco leaves in planta, reproducible expression assays could be conducted in as little as 2-3 days without using expensive equipment (e.g. biolistic gun or electroporation apparatus) or complicated procedures (e.g. preparation of protoplasts). Biotic and abiotic treatments could be applied to the intact plant to examine their influence on promoter activity and gene expression. Using this method, we have tested the stress-responsive as-1 and heat shock elements, yeast GAL4 transactivation system, two promoters of pathogenesis-related (PR) genes as well as a heat shock promoter. Through deletion analyses of tobacco PR1a promoter and a novel myb1 promoter, we have also successfully identified the cis-regulatory regions in these promoters that are responsive to salicylic acid treatment or tobacco mosaic virus infection. Together, our results demonstrate that Agrobacterium-mediated transient expression is a simple and efficient method for in vivo assays of plant promoters and transcription factors.

Transcriptional activation of a heat shock gene promoter in sunflower embryos: synergism between ABI3 and heat shock factors

Transient expression analyses in sunflower embryos demonstrated that ABI3, a seed-specific transcription factor from Arabidopsis, activated chimaeric genes with the Ha hsp17.7 G4 promoter. Nucleotide substitutions at crucial positions of heat shock cis-elements established that they are required for the transcriptional activation involving ABI3. Trans-activation with Lp-HSFA1, a heat shock factor from tomato, reproduced the activation patterns of wild-type and mutant promoters observed with ABI3. In addition, ABI3 and Lp-HSFA1 synergistically activated the Ha hsp17. 7 G4 promoter, but only when it contained the intact proximal and distal heat shock cis-elements. The activation domain of Lp-HSFA1 was necessary for promoter activation. An amino terminal deletion of ABI3 had dominant negative effects on activation by Lp-HSFA1. We failed to detect a substantial transcriptional activation by ABI3 in the absence of either functional heat shock factors or heat shock elements (HSEs). Furthermore, the wild-type, but not the mutant HSEs (from - 136 to - 49 in Ha hsp17.7 G4) were sufficient, in the context of a - 46 CaMV 35S promoter, to support activation by Lp-HSFA1, or Lp-HSFA1 and ABI3. These results demonstrate, for the first time, transcriptional activation of a heat shock protein promoter by ABI3. We also suggest that ABI3 functions as a transcriptional co-activator through heat shock factors.

An imperfect heat shock element and different upstream sequences are required for the seed-specific expression of a small heat shock protein gene

Chimeric constructs containing the promoter and upstream sequences of Ha hsp17.6 G1, a small heat shock protein gene, reproduced in transgenic tobacco (Nicotiana tabacum) its unique seed-specific expression patterns previously reported in sunflower. These constructs did not respond to heat shock, but were expressed without exogenous stress during late zygotic embryogenesis coincident with seed desiccation. Site-directed mutagenesis of its distal and imperfect heat shock element strongly impaired in vitro heat shock transcription factor binding and transgene expression in seeds. Deletion analyses of upstream sequences indicated the contribution of additional cis-acting elements with either positive or negative effects on transgene expression. These results show differences in the transcriptional activation through the heat shock element of small heat shock protein gene promoters in seeds compared with the heat shock response. In addition, they suggest that heat shock transcription factors and other distinct trans-acting factors cooperate in the regulation of Ha hsp17.6 G1 during seed desiccation.

The heat-shock element is a functional component of the Arabidopsis APX1 gene promoter

Ascorbate peroxidases are important enzymes that detoxify hydrogen peroxide within the cytosol and chloroplasts of plant cells. To better understand their role in oxidative stress tolerance, the transcriptional regulation of the apx1 gene from Arabidopsis was studied. The apx1 gene was expressed in all tested organs of Arabidopsis; mRNA levels were low in roots, leaves, and stems and high in flowers. Steady-state mRNA levels in leaves or cell suspensions increased after treatment with methyl viologen, ethephon, high temperature, and illumination of etiolated seedlings. A putative heat-shock cis element found in the apx1 promoter was shown to be recognized by the tomato (Lycopersicon esculentum) heat-shock factor in vitro and to be responsible for the in vivo heat-shock induction of the gene. The heat-shock cis element also contributed partially to the induction of the gene by oxidative stress. By using in vivo dimethyl sulfate footprinting, we showed that proteins interacted with a G/C-rich element found in the apx1 promoter.

Dual regulation of a heat shock promoter during embryogenesis: stage-dependent role of heat shock elements

Transgenic tobacco expression was analysed of chimeric genes with point mutations in the heat shock element (HSE) arrays of a small heat shock protein (sHSP) gene from sunflower: Ha hsp17.7 G4. The promoter was developmentally regulated during zygotic embryogenesis and responded to heat stress in vegetative tissues. Mutations in the HSE affected nucleotides crucial for human heat shock transcription factor 1 (HSF1) binding. They abolished the heat shock response of Ha hsp17.7 G4 and produced expression changes that demonstrated dual regulation of this promoter during embryogenesis. Thus, whereas activation of the chimeric genes during early maturation stages did not require intact HSE, expression at later desiccation stages was reduced by mutations in both the proximal (-57 to -89) and distal (-99 to -121) HSE. In contrast, two point mutations in the proximal HSE that did not severely affect gene expression during zygotic embryogenesis, eliminated the heat shock response of the same chimeric gene in vegetative organs. Therefore, by site-directed mutagenesis, it was possible to separate the heat shock response of Ha hsp17.7 G4 from its developmental regulation. The results indicate the co-existence, in a single promoter, of HSF-dependent and -independent regulation mechanisms that would control sHSP gene expression at different stages during plant embryogenesis.

A plant small heat shock protein gene expressed during zygotic embryogenesis but noninducible by heat stress

A small heat shock protein (sHSP) gene from sunflower, Ha hsp17.6 G1, showed expression patterns that differ from what is known for members of this gene family. The mRNAs of this gene accumulated in seeds during late desiccation stages of zygotic embryogenesis but not in response to heat shock in vegetative tissues. The failure to respond to heat shock was independent of the developmental stage after germination and shock temperature. Nuclear run-on analyses demonstrated that transcription from the Ha hsp17.6 G1 promoter is not induced by heat shock. This agrees with the presence, in this promoter, of sequences with little similarity to heat shock elements. Our results show an evolutionary divergence, in the regulation of plant sHSP genes, which has originated stress-responsive genes and nonresponsive members within this gene family. We discuss implications for mechanisms controlling the developmental regulation of sHSP genes in plants.

Differential regulation of small heat-shock genes in plants: analysis of a water-stress-inducible and developmentally activated sunflower promoter

We have isolated two sunflower genes, Ha hsp18.6 G2 and Ha hsp17.7 G4, that encode small heat shock proteins (sHSPs). RNAse A protection experiments, carried out with RNA probes transcribed from each gene and hybridized to sunflower total RNA, allowed us to distinguish their mRNA accumulation patterns. In sunflower, Ha hsp17.7 G4 mRNAs accumulated during zygotic embryogenesis at 25 degrees C. In vegetative tissues, these mRNAs accumulated in response to either heat shock (42 degrees C), abscisic acid (ABA), or mild water stress treatments. In all cases, the mRNAs were transcribed from the same initiation site. In contrast, Ha hsp 18.6 G2 mRNAs accumulated only in response to heat-shock. This result demonstrates differential regulation of these two sHSP genes. The complex regulation depicted by the Ha hsp 17.7 G4 promoter has been further analyzed in transgenic tobacco, using G4::GUS translational fusions. Developmental induction of Ha hsp 17.7 G4 during zygotic embryogenesis was faithfully reproduced in the transgenic plants. 5'-distal sequences (between -1132 and -395) were required to confer a preferential spatial expression of GUS activity in the cotyledons. More proximal sequences (from -83 to +163) conferred to the chimeric genes most of the developmental regulation, and the responses to ABA and heat shock characteristic of the Ha hsp17.7 G4 promoter. The water stress response of this gene was not reproduced in transgenic tobacco and, thus, could be uncoupled from its regulation during embryogenesis.