Two cDNAs for tomato heat stress transcription factors

Characterization of high-temperature stress-tolerant tomato (Solanum lycopersicum L.) genotypes by biochemical analysis and expression profiling of heat-responsive genes

High-temperature stress severely impacts both yield and quality of tomato fruits, and therefore, it is required to develop stress-tolerant cultivars. In the present study, two tomato genotypes, H88-78-1 and CLN-1621, identified through preliminary phenotypic screening were characterized by analysis of molecular, physiological, and biochemical traits in comparison with a susceptible genotype Punjab Chhuhara. Phenotypic stress tolerance of both the genotypes was validated at biochemical level as they showed higher amount of relative water content, photosynthetic pigments, free cellular proline, and antioxidant molecules while less amount of H₂O₂ and electrolyte leakage. Expression analysis of 67 genes including heat shock factors, heat shock proteins, and other stress-responsive genes showed significant up-regulation of many of the genes such as 17.4 kDa class III heat shock protein, HSF A-4a, HSF30, HSF B-2a, HSF24, HSF B-3 like, 18.1 kDa class I HSP like, and HSP17.4 in H88-78-1 and CLN-1621 after exposure to high-temperature stress. These candidate genes can be transferred to cultivated varieties by developing gene-based markers and marker-assisted breeding. This confirms the rapid response of these genotypes to high-temperature stress. All these traits are characteristics of a stress-tolerance and establish them as candidate high-temperature stress-tolerant genotypes that can be effectively utilized in stress tolerance improvement programs.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-020-02587-6.

Transcriptomic Responses of Dove Tree (Davidia involucrata Baill.) to Heat Stress at the Seedling Stage

The dove tree (Davidia involucrata Baill.), a tertiary relic species, is adapted to cool climates. With the progression of global warming, high-temperature stress has become the primary environmental factor restricting geographic distribution, ex situ conservation, and landscape application for D. involucrata resources. However, the detailed molecular events underlying D. involucrata responses to heat stress are poorly understood. Here, we conducted RNA-Seq-based gene expression profiling in D. involucrata seedlings during the time course of a 42 °C heat treatment (0, 1, 6, and 12 h). After de novo assembly, we obtained 138,923 unigenes, of which 69,743 were annotated in public databases. Furthermore, 19,532, 20,497 and 27,716 differentially expressed genes (DEGs) were identified after 1 h (HS1), 6 h (HS6), and 12 h (HS12) of heat treatment in comparison to 0 h (HS0), respectively. Based on a KEGG enrichment analysis, the two pathways “protein processing in endoplasmic reticulum” and “plant hormone signal transduction” are hypothesized to play vital roles during heat response in D. involucrata, and their potential interactions during heat stress are also discussed. In addition, 32 genes encoding putative heat shock transcription factors (Hsfs) were found to be associated with the response of D. involucrata to heat stress. Finally, the expression patterns of eight heat-responsive genes derived from qRT-PCR were in agreement with their transcript level alterations, as determined by a transcriptome analysis. Taken together, our transcriptomic data provide the first comprehensive transcriptional profile affected by heat stress in D. involucrata, which will facilitate further studies on the improvement of heat tolerance in this rare and endangered species.

Overall picture of expressed Heat Shock Factors in Glycine max, Lotus japonicus and Medicago truncatula

Heat shock (HS) leads to the activation of molecular mechanisms, known as HS-response, that prevent damage and enhance survival under stress. Plants have a flexible and specialized network of Heat Shock Factors (HSFs), which are transcription factors that induce the expression of heat shock proteins. The present work aimed to identify and characterize the Glycine max HSF repertory in the Soybean Genome Project (GENOSOJA platform), comparing them with other legumes (Medicago truncatula and Lotus japonicus) in view of current knowledge of Arabidopsis thaliana. The HSF characterization in leguminous plants led to the identification of 25, 19 and 21 candidate ESTs in soybean, Lotus and Medicago, respectively. A search in the SuperSAGE libraries revealed 68 tags distributed in seven HSF gene types. From the total number of obtained tags, more than 70% were related to root tissues (water deficit stress libraries vs. controls), indicating their role in abiotic stress responses, since the root is the first tissue to sense and respond to abiotic stress. Moreover, as heat stress is related to the pressure of dryness, a higher HSF expression was expected at the water deficit libraries. On the other hand, expressive HSF candidates were obtained from the library inoculated with Asian Soybean Rust, inferring crosstalk among genes associated with abiotic and biotic stresses. Evolutionary relationships among sequences were consistent with different HSF classes and subclasses. Expression profiling indicated that regulation of specific genes is associated with the stage of plant development and also with stimuli from other abiotic stresses pointing to the maintenance of HSF expression at a basal level in soybean, favoring its activation under heat-stress conditions.

Transcriptional profiling of maturing tomato (Solanum lycopersicum L.) microspores reveals the involvement of heat shock proteins, ROS scavengers, hormones, and sugars in the heat stress response

Above-optimal temperatures reduce yield in tomato largely because of the high heat stress (HS) sensitivity of the developing pollen grains. The high temperature response, especially at this most HS-sensitive stage of the plant, is poorly understood. To obtain an overview of molecular mechanisms underlying the HS response (HSR) of microspores, a detailed transcriptomic analysis of heat-stressed maturing tomato microspores was carried out using a combination of Affymetrix Tomato Genome Array and cDNA-amplified fragment length polymorphism (AFLP) techniques. The results were corroborated by reverse transcription-PCR (RT-PCR) and immunoblot analyses. The data obtained reveal the involvement of specific members of the small heat shock protein (HSP) gene family, HSP70 and HSP90, in addition to the HS transcription factors A2 (HSFA2) and HSFA3, as well as factors other than the classical HS-responsive genes. The results also indicate HS regulation of reactive oxygen species (ROS) scavengers, sugars, plant hormones, and regulatory genes that were previously implicated in other types of stress. The use of cDNA-AFLP enabled the detection of genes representing pollen-specific functions that are missing from the tomato Affymetrix chip, such as those involved in vesicle-mediated transport and a pollen-specific, calcium-dependent protein kinase (CDPK2). For several genes, including LeHSFA2, LeHSP17.4-CII, as well as homologues of LeHSP90 and AtVAMP725, higher basal expression levels were detected in microspores of cv. Hazera 3042 (a heat-tolerant cultivar) compared with microspores of cv. Hazera 3017 (a heat-sensitive cultivar), marking these genes as candidates for taking part in microspore thermotolerance. This work provides a comprehensive analysis of the molecular events underlying the HSR of maturing microspores of a crop plant, tomato.

In the complex family of heat stress transcription factors, HsfA1 has a unique role as master regulator of thermotolerance in tomato

We generated transgenic tomato plants with altered expression of heat stress transcription factor HsfA1. Plants with 10-fold overexpression of HsfA1 (OE plants) were characterized by a single HsfA1 transgene cassette, whereas plants harboring a tandem inverted repeat of the cassette showed cosuppression (CS plants) by posttranscriptional silencing of the HsfA1 gene connected with formation of small interfering RNAs. Under normal growth conditions, major developmental parameters were similar for wild-type (WT), OE, and CS plants. However, CS plants and fruits were extremely sensitive to elevated temperatures, because heat stress-induced synthesis of chaperones and Hsfs was strongly reduced or lacking. Despite the complexity of the plant Hsf family with at least 17 members in tomato, HsfA1 has a unique function as master regulator for induced thermotolerance. Using transient reporter assays with mesophyll protoplasts from WT tomato, we demonstrated that plasmid-encoded HsfA1 and HsfA2 were well expressed. However, in CS protoplasts the cosuppression phenomenon was faithfully reproduced. Only transformation with HsfA2 expression plasmid led to normal expression of the transcription factor and reporter gene activation, whereas even high amounts of HsfA1 expression plasmids were silenced. Thermotolerance in CS protoplasts was restored by plasmid-borne HsfA2, resulting in expression of chaperones, thermoprotection of firefly luciferase, and assembly of heat stress granules.

A rice spotted leaf gene, Spl7, encodes a heat stress transcription factor protein

A rice spotted leaf (lesion-mimic) gene, Spl7, was identified by map-based cloning. High-resolution mapping with cleaved amplified polymorphic sequence markers enabled us to define a genomic region of 3 kb as a candidate for Spl7. We found one ORF that showed high similarity to a heat stress transcription factor (HSF). Transgenic analysis verified the function of the candidate gene for Spl7: leaf spot development was suppressed in spl7 mutants with a wild-type Spl7 transgene. Thus, we conclude that Spl7 encodes the HSF protein. The transcript of spl7 was observed in mutant plants. The levels of mRNAs (Spl7 in wild type and spl7 in mutant) increased under heat stress. Sequence analysis revealed only one base substitution in the HSF DNA-binding domain of the mutant allele, causing a change from tryptophan to cysteine.

Stress-specific activation and repression of heat shock factors 1 and 2

Vertebrate cells express a family of heat shock transcription factors (HSF1 to HSF4) that coordinate the inducible regulation of heat shock genes in response to diverse signals. HSF1 is potent and activated rapidly though transiently by heat shock, whereas HSF2 is a less active transcriptional regulator but can retain its DNA binding properties for extended periods. Consequently, the differential activation of HSF1 and HSF2 by various stresses may be critical for cells to survive repeated and diverse stress challenges and to provide a mechanism for more precise regulation of heat shock gene expression. Here we show, using a novel DNA binding and detection assay, that HSF1 and HSF2 are coactivated to different levels in response to a range of conditions that cause cell stress. Above a low basal activity of both HSFs, heat shock preferentially activates HSF1, whereas the amino acid analogue azetidine or the proteasome inhibitor MG132 coactivates both HSFs to different levels and hemin preferentially induces HSF2. Unexpectedly, we also found that heat shock has dramatic adverse effects on HSF2 that lead to its reversible inactivation coincident with relocalization from the nucleus. The reversible inactivation of HSF2 is specific to heat shock and does not occur with other stressors or in cells expressing high levels of heat shock proteins. These results reveal that HSF2 activity is negatively regulated by heat and suggest a role for heat shock proteins in the positive regulation of HSF2.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the plant heat stress transcription factor (Hsf) family. By structural characteristics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, and soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA motifs). There is intriguing flexibility of size and sequence in certain parts of the otherwise strongly conserved N-terminal half of these Hsfs. We have speculated about possible exon-intron borders in this region in the ancient precursor gene of plant Hsfs, similar to the exon-intron structure of the present mammalian Hsf-encoding genes.

Isolation and characterization of HsfA3, a new heat stress transcription factor of Lycopersicon peruvianum

Stress-induced transcription of heat shock proteins (Hsps) in eukaryotes is mediated by a conserved class of transcription factors called heat stress transcription factors (Hsfs). Here we report the isolation and functional characterization of HsfA3, a new member of the Hsf family. HsfA3 was cloned from a tomato heat stress cDNA library by yeast two-hybrid screening, using HsfA1 as a bait. HsfA3 is a single-copy gene with all the conserved sequence elements characteristic of a heat stress transcription factor. The constitutively expressed HsfA3 is mainly found in the cytoplasm under control conditions and in the nucleus under heat stress conditions. Functionally, HsfA3 behaves similarly to the already known members of tomato Hsf family. It is able to substitute yeast Hsf for viability functions and is a strong activator of Hsf-dependent reporter constructs both in tobacco protoplasts and yeast. Finally, similar to the AHA motifs in HsfA1 and HsfA2, the activator function depends on four short peptide motifs with a central tryptophan residue found in the C-terminal domain of HsfA3.

The tomato Hsf system: HsfA2 needs interaction with HsfA1 for efficient nuclear import and may be localized in cytoplasmic heat stress granules

In heat-stressed (HS) tomato (Lycopersicon peruvianum) cell cultures, the constitutively expressed HS transcription factor HsfA1 is complemented by two HS-inducible forms, HsfA2 and HsfB1. Because of its stability, HsfA2 accumulates to fairly high levels in the course of a prolonged HS and recovery regimen. Using immunofluorescence and cell fractionation experiments, we identified three states of HsfA2: (i) a soluble, cytoplasmic form in preinduced cultures maintained at 25 degrees C, (ii) a salt-resistant, nuclear form found in HS cells, and (iii) a stored form of HsfA2 in cytoplasmic HS granules. The efficient nuclear transport of HsfA2 evidently requires interaction with HsfA1. When expressed in tobacco protoplasts by use of a transient-expression system, HsfA2 is mainly retained in the cytoplasm unless it is coexpressed with HsfA1. The essential parts for the interaction and nuclear cotransport of the two Hsfs are the homologous oligomerization domain (HR-A/B region of the A-type Hsfs) and functional nuclear localization signal motifs of both partners. Direct physical interaction of the two Hsfs with formation of relatively stabile hetero-oligomers was shown by a two-hybrid test in Saccharomyces cerevisiae as well as by coimmunoprecipitation using tomato and tobacco whole-cell lysates.

Solution structure of the DNA-binding domain of the tomato heat-stress transcription factor HSF24

Two-dimensional-NMR and three-dimensional-NMR experiments were performed to determine the solution structure of the DNA-binding domain of the tomato heat-stress transcription factor HSF24. Samples of uniformly 15N-labeled and 15N, 13C-labeled recombinant proteins were used in the investigation. A near-complete assignment of the backbone 1H, 15N, and 13C resonances was obtained by three-dimensional triple-resonance experiments, whereas three-dimensional 15N-TOCSY-heteronuclear-single-quantum-correlation-spectroscopy, HCCH-COSY and HCCH-TOCSY spectra were recorded for side-chain assignments, 885 non-redundant distance constraints from two-dimensional-homonuclear and three-dimensional-15N-edited and 13C-edited NOESY spectra and 40 hydrogen-bond constraints from exchange experiments were used for structure calculations. The resulting three-dimensional structure contains a three-helix bundle and a small four-stranded antiparallel beta-sheet that forms a hydrophobic core. The two C-terminal helices are parts of a highly conserved helix-turn-helix motif that is probably involved in DNA recognition and binding. In contrast to heat-stress factors from yeast and animals, the plant heat-stress factors lack a loop of 11 amino acid residues inserted between beta3 and beta4. This leads to a tight turn between these beta-strands.

Isolation and characterization of six heat shock transcription factor cDNA clones from soybean

Thermal stress in soybean seedlings causes the activation of pre-existing heat shock transcription factor proteins (HSFs). Activation results in the induction of DNA binding activity which leads to the transcription of heat shock genes. From a soybean cDNA library we have isolated cDNA clones corresponding to six HSF genes. Two HSF genes are expressed constitutively at the transcriptional level, and the remaining four are heat-inducible. Two of the heat inducible genes are also responsive to cadmium stress. Comparative analysis of HSF sequences indicated higher conservation of the DNA binding domain among plant HSFs than those from yeast or other higher eukaryotes. The putative plant HSF oligomerization domain contains hydrophobic heptapeptide repeats characteristic of coiled coils and seems to exist in two structural variants. The carboxy-terminal domains are reduced in size and the C-terminal heptad repeat is degenerate.

Promoter specificity and deletion analysis of three heat stress transcription factors of tomato

Transient expression assays in transformed tobacco (Nicotiana plumbaginifolia) mesophyll protoplasts were used to test the activity of three tomato heat stress transcription factors, HSF24, HSF8 and HSF30, in a trans-activation and a trans-repression assay. The results document differences between the three HSFs with respect to their response to the configuration of heat stress promoter elements (HSEs) in the reporter construct (promoter specificity) and to the stress regime used for activation. Analysis of C-terminal deletions identified acidic sequence elements with a central tryptophan residue, which are important for HSF activity control. Surprisingly, heterologous HSFs from Drosophila and human cells, but not from yeast, were also functional as heat stress-induced transcription factors in this tobacco protoplast system.