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

Crosstalk between endoplasmic reticulum and cytosolic unfolded protein response in tomato

Conditions that cause proteotoxicity like high temperature trigger the activation of unfolded protein response (UPR). The cytosolic (CPR) and endoplasmic reticulum (ER) UPR rely on heat stress transcription factor (HSF) and two members of the basic leucine zipper (bZIP) gene family, respectively. In tomato, HsfA1a is the master regulator of CPR. Here, we identified the core players of tomato ER-UPR including the two central transcriptional regulators, namely bZIP28 and bZIP60. Interestingly, the induction of ER-UPR genes and the activation of bZIP60 are altered in transgenic plants where HsfA1a is either overexpressed (A1aOE) or suppressed (A1CS), indicating an interplay between CPR and ER-UPR systems. Several ER-UPR genes are differentially expressed in the HsfA1a transgenic lines either exposed to heat stress or to the ER stress elicitor tunicamycin (TUN). The ectopic expression of HsfA1a is associated with higher tolerance against TUN. On the example of the ER-resident Hsp70 chaperone BIP3, we show that the presence of cis-elements required for HSF and bZIP regulation serves as a putative platform for the co-regulation of these genes by both CPR and ER-UPR mechanisms, in the case of BIP3 in a stimulatory manner under high temperatures. In addition, we show that the accumulation of HsfA1a results in higher levels of three ATG genes and a more sensitized induction of autophagy in response to ER stress which also supports the increased tolerance to ER stress of the A1aOE line. These findings provide a basis for the coordination of protein homeostasis in different cellular compartments under stress conditions.

Design of an Arabidopsis thaliana reporter line to detect heat-sensing and signaling mutants

BACKGROUND

Global warming is a major challenge for plant survival and growth. Understanding the molecular mechanisms by which higher plants sense and adapt to upsurges in the ambient temperature is essential for developing strategies to enhance plant tolerance to heat stress. Here, we designed a heat-responsive Arabidopsis thaliana reporter line that allows an in-depth investigation of the mechanisms underlying the accumulation of protective heat-shock proteins (HSPs) in response to high temperature.

METHODS

A transgenic Arabidopsis thaliana reporter line named "Heat-Inducible Bioluminescence And Toxicity" (HIBAT) was designed to express from a conditional heat-inducible promoter, a fusion gene encoding for nanoluciferase and D-amino acid oxidase, whose expression is toxic in the presence of D-valine. HIBAT seedlings were exposed to different heat treatments in presence or absence of D-valine and analyzed for survival rate, bioluminescence and HSP gene expression.

RESULTS

Whereas at 22 °C, HIBAT seedlings grew unaffected by D-valine, and all survived iterative heat treatments without D-valine, 98% died following heat treatments on D-valine. The HSP17.3B promoter was highly specific to heat as it remained unresponsive to various plant hormones, Flagellin, H₂O₂, osmotic stress and high salt. RNAseq analysis of heat-treated HIBAT seedlings showed a strong correlation with expression profiles of two wild type lines, confirming that HIBAT does not significantly differ from its Col-0 parent. Using HIBAT, a forward genetic screen revealed candidate loss-of-function mutants, apparently defective either at accumulating HSPs at high temperature or at repressing HSP accumulation at non-heat-shock temperatures.

CONCLUSION

HIBAT is a valuable candidate tool to identify Arabidopsis mutants defective in the response to high temperature stress. It opens new avenues for future research on the regulation of HSP expression and for understanding the mechanisms of plant acquired thermotolerance.

Characterization of Transcription Regulatory Domains of OsMADS29: Identification of Proximal Auxin-Responsive Domains and a Strong Distal Negative Element

OsMADS29 (M29) is a seed-specific MADS-box transcription factor involved in programmed cell death of nucellar tissue and maintaining auxin:cytokinin homeostasis. It affects embryo and endosperm development and starch filling during seed development in rice. Its expression seems to be tightly regulated by developmental, spatial, and temporal cues; however, cis- and trans-regulatory factors that affect its expression are largely unknown. In silico analysis of the 1.7 kb upstream regulatory region (URR) consisting of 1,290 bp promoter and 425 bp 5'-UTR regions revealed several auxin-responsive and seed-specific cis-regulatory elements distributed across the URR. In this study, the analysis of four URR deletions fused to a downstream β-glucuronidase (GUS) reporter in transgenic rice has revealed the presence of several proximal positive elements and a strong distal negative element (NE). The promoter regions containing auxin-responsive elements responded positively to the exogenous application of auxins to transgenic seedlings. The proximal positive elements are capable of driving reporter expression in both vegetative and reproductive tissues. In contrast, the NE strongly suppresses reporter gene expression in both vegetative and reproductive tissues. In a transient onion peel assay system, the NE could reduce the efficacy of a 2x CaMV 35S promoter by ∼90%. Our results indicate the existence of a complex array of positive and negative regulatory regions along with auxin-responsive elements guiding the development-dependent and spatial expression of M29.

HsfA7 coordinates the transition from mild to strong heat stress response by controlling the activity of the master regulator HsfA1a in tomato

Plants respond to higher temperatures by the action of heat stress (HS) transcription factors (Hsfs), which control the onset, early response, and long-term acclimation to HS. Members of the HsfA1 subfamily, such as tomato HsfA1a, are the central regulators of HS response, and their activity is fine-tuned by other Hsfs. We identify tomato HsfA7 as capacitor of HsfA1a during the early HS response. Upon a mild temperature increase, HsfA7 is induced in an HsfA1a-dependent manner. The subsequent interaction of the two Hsfs prevents the stabilization of HsfA1a resulting in a negative feedback mechanism. Under prolonged or severe HS, HsfA1a and HsfA7 complexes stimulate the induction of genes required for thermotolerance. Therefore, HsfA7 exhibits a co-repressor mode at mild HS by regulating HsfA1a abundance to moderate the upregulation of HS-responsive genes. HsfA7 undergoes a temperature-dependent transition toward a co-activator of HsfA1a to enhance the acquired thermotolerance capacity of tomato plants.

SlMYB14 promotes flavonoids accumulation and confers higher tolerance to 2,4,6-trichlorophenol in tomato

Flavonoids are small molecular secondary metabolites, which have a variety of biological functions. Transcriptional regulations of key enzyme genes play critical roles in the flavonoid biosynthesis. In this study, an R2R3-MYB transcription factor gene, SlMYB14, was isolated from tomato and characterized. The nucleus-localized SlMYB14 functions as a transcriptional activator in yeast. The expression of SlMYB14 could be induced by methyl jasmonic acid, wounding and ABA. SlMYB14 works downstream of SlMYC2 in the jasmonate signaling pathway. Overexpression of SlMYB14 under the control of CaMV35S promoter in tomato led to increased accumulation of flavonoids. RNA-sequencing analysis revealed that the transcript levels of several structural genes associated with flavonoid biosynthesis were up-regulated in transgenic tomato plants. Gel-shift assays confirmed that SlMYB14 protein could bind to the promoter regions of SlPAL genes. It was also found that overexpression of SlMYB14 improved the tolerance of transgenic plants to 2,4,6-trichlorophenol (2,4,6-TCP), an environmental organic pollutant which could cause serious oxidative damage to plant. These results suggest that SlMYB14 participates in the regulation of flavonoid biosynthesis and might play a role in maintaining reactive oxygen species homeostasis in plant. SlMYB14 gene also has the potential to contribute to the phytoremediation of 2,4,6-TCP-contaminated soils.

Genome-wide identification and transcript profiles of walnut heat stress transcription factor involved in abiotic stress

BACKGROUND

Walnut (Juglans regia) is an important tree cultivated worldwide and is exposed to a series of both abiotic and biotic stress during their life-cycles. The heat stress transcription factors (HSFs) play a crucial role in plant response to various stresses by regulating the expression of stress-responsive genes. HSF genes are classified into 3 classes: HSFA, HSFB, and HSFC. HSFA gene has transcriptional activation function and is the main regulator of high temperature-induced gene expression. HSFB gene negatively regulates plant resistance to drought and NaCl. And HSFC gene may be involved in plant response to various stresses. There are some reports about the HSF family in herbaceous plants, however, there are no reports about the HSFs in walnut.

RESULT

In this study, based on the complete genome sequencing of walnut, the bioinformatics method was used and 29 HSF genes were identified. These HSFs covered 18 HSFA, 9 HSFB, and 2 HSFC genes. Phylogenetic analysis of these HSF proteins along with those from Arabidopsis thaliana showed that the HSFs in the two species are closely related to each other and have different evolutionary processes. The distribution of conserved motifs and the sequence analysis of HSF genes family indicated that the members of the walnut HSFs are highly conserved. Quantitative Real-Time PCR (qRT-PCR) analysis revealed that the most of walnut HSFs were expressed in the walnut varieties of 'Qingxiang' and 'Xiangling' under high temperature (HT), high salt and drought stress, and some JrHSFs expression pattern are different between the two varieties.

CONCLUSION

The complex HSF genes family from walnut was confirmed by genome-wide identification, evolutionary exploration, sequence characterization and expression analysis. This research provides useful information for future studies on the function of the HSF genes and molecular mechanism in plant stress response.

Natural variation in HsfA2 pre-mRNA splicing is associated with changes in thermotolerance during tomato domestication

Wild relatives of crops thrive in habitats where environmental conditions can be restrictive for productivity and survival of cultivated species. The genetic basis of this variability, particularly for tolerance to high temperatures, is not well understood. We examined the capacity of wild and cultivated accessions to acclimate to rapid temperature elevations that cause heat stress (HS). We investigated genotypic variation in thermotolerance of seedlings of wild and cultivated accessions. The contribution of polymorphisms associated with thermotolerance variation was examined regarding alterations in function of the identified gene. We show that tomato germplasm underwent a progressive loss of acclimation to strong temperature elevations. Sensitivity is associated with intronic polymorphisms in the HS transcription factor HsfA2 which affect the splicing efficiency of its pre-mRNA. Intron splicing in wild species results in increased synthesis of isoform HsfA2-II, implicated in the early stress response, at the expense of HsfA2-I which is involved in establishing short-term acclimation and thermotolerance. We propose that the selection for modern HsfA2 haplotypes reduced the ability of cultivated tomatoes to rapidly acclimate to temperature elevations, but enhanced their short-term acclimation capacity. Hence, we provide evidence that alternative splicing has a central role in the definition of plant fitness plasticity to stressful conditions.

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.

Functional diversification of tomato HsfA1 factors is based on DNA binding domain properties

In all eukaryotes, the response to heat stress (HS) is dependent on the activity of HS transcription factors (Hsfs). Plants contain a large number of Hsfs, however, only members of the HsfA1 subfamily are considered as master regulators of stress response and thermotolerance. In Solanum lycopersicum, among the four HsfA1 members, only HsfA1a has been proposed to possess a master regulator function. We performed a comparative analysis of HsfA1a, HsfA1b, HsfA1c and HsfA1e at different levels of regulation and function. HsfA1a is constitutively expressed under control and stress conditions, while the other members are induced in specific tissues and stages of HS response. Despite that all members are localized in the nucleus when expressed in protoplasts, only HsfA1a shows a wide range of basal activity on several HS-induced genes. In contrast, HsfA1b, HsfA1c, and HsfA1e show only high activity for specific subsets of genes. Domain swapping mutants between HsfA1a and HsfA1c revealed that the variation in that transcriptional transactivation activity is due to differences in the DNA binding domain (DBD). Specifically, we identified a conserved arginine (R107) residue in the turn of β3 and β4 sheet in the C-terminus of the DBD of HsfA1a that is highly conserved in plant HsfA1 proteins, but is replaced by leucine and cysteine in tomato HsfA1c and HsfA1e, respectively. Although not directly involved in DNA interaction, R107 contributes to DNA binding and consequently the activity of HsfA1a. Thus, we demonstrate that this variation in DBD in part explains the functional diversification of tomato HsfA1 members.

Identification and Characterization of a Thermotolerant TILLING Allele of Heat Shock Binding Protein 1 in Tomato

The identification of heat stress (HS)-resilient germplasm is important to ensure food security under less favorable environmental conditions. For that, germplasm with an altered activity of factors regulating the HS response is an important genetic tool for crop improvement. Heat shock binding protein (HSBP) is one of the main negative regulators of HS response, acting as a repressor of the activity of HS transcription factors. We identified a TILLING allele of Solanum lycopersicum (tomato) HSBP1. We examined the effects of the mutation on the functionality of the protein in tomato protoplasts, and compared the thermotolerance capacity of lines carrying the wild-type and mutant alleles of HSBP1. The methionine-to-isoleucine mutation in the central heptad repeats of HSBP1 leads to a partial loss of protein function, thereby reducing the inhibitory effect on Hsf activity. Mutant seedlings show enhanced basal thermotolerance, while mature plants exhibit increased resilience in repeated HS treatments, as shown by several physiological parameters. Importantly, plants that are homozygous for the wild-type or mutant HSBP1 alleles showed no significant differences under non-stressed conditions. Altogether, these results indicate that the identified mutant HSBP1 allele can be used as a genetic tool in breeding, aiming to improve the thermotolerance of tomato varieties.

The repressor and co-activator HsfB1 regulates the major heat stress transcription factors in tomato

Plants code for a multitude of heat stress transcription factors (Hsfs). Three of them act as central regulators of heat stress (HS) response in tomato (Solanum lycopersicum). HsfA1a regulates the initial response, and HsfA2 controls acquired thermotolerance. HsfB1 is a transcriptional repressor but can also act as co-activator of HsfA1a. Currently, the mode of action and the relevance of the dual function of HsfB1 remain elusive. We examined this in HsfB1 overexpression or suppression transgenic tomato lines. Proteome analysis revealed that HsfB1 overexpression stimulates the co-activator function of HsfB1 and consequently the accumulation of HS-related proteins under non-stress conditions. Plants with enhanced levels of HsfB1 show aberrant growth and development but enhanced thermotolerance. HsfB1 suppression has no significant effect prior to stress. Upon HS, HsfB1 suppression strongly enhances the induction of heat shock proteins due to the higher activity of other HS-induced Hsfs, resulting in increased thermotolerance compared with wild-type. Thereby, HsfB1 acts as co-activator of HsfA1a for several Hsps, but as a transcriptional repressor on other Hsfs, including HsfA1b and HsfA2. The dual function explains the activation of chaperones to enhance protection and regulate the balance between growth and stress response upon deviations from the homeostatic levels of HsfB1.

Heat Shock-Induced Resistance Against Pseudomonas syringae pv. tomato (Okabe) Young et al. via Heat Shock Transcription Factors in Tomato

*Abstract: We investigated the role of heat shock transcription factors (Hsfs) during induction of defense response by heat-shock treatment (HST) in tomato. Leaf disease symptoms were significantly reduced at 12 and 24 h after HST, consistent with upregulation of pathogenesis-related (PR) genes PR1a2 and PR1b1 peaking at 24 h after treatment. These genes were upregulated at the treatment application site, but not in untreated leaves. In contrast to HST, inoculation of the first leaf induced systemic upregulation of acidic PR genes in uninoculated second leaves. Furthermore, heat shock element motifs were found in upstream regions of PR1a2, PR1b1, Chitinase 3, Chitinase 9, Glucanase A, and Glucanase B genes. Upregulation of HsfA2 and HsfB1 peaked at 6 h after HST, 6 h earlier than salicylic acid accumulation. Foliar spray of heat shock protein 90 (Hsp90) inhibitor geldanamycin (GDA) induced PR gene expression comparable to that after HST. PR gene expression and defense response against Pseudomonas syringae pv. tomato (Pst) decreased when combining HST with Hsfs inhibitor KRIBB11. The Hsfs and PR gene expression induced by heat or GDA, together with the suppression of heat shock-induced resistance (HSIR) against Pst by KRIBB11, suggested a direct contribution of Hsfs to HSIR regulation in tomato.

A tomato ERF transcription factor, SlERF84, confers enhanced tolerance to drought and salt stress but negatively regulates immunity against Pseudomonas syringae pv. tomato DC3000

ERF proteins are plant-specific transcription factors that play significant roles in plant defense against various stresses. However, only little information regarding stress-related ERF genes is available in tomato (Solanum lycopersicum, Sl). In this study, a tomato ERF gene, SlERF84, was cloned and functionally characterized. The nucleus localization of SlERF84-sGFP was confirmed through a transient expression assay. Transactivation assays in yeast demonstrated that SlERF84 functions as a transcriptional activator. Real-time PCR analysis revealed that SlERF84 could be markedly induced by drought, salt and by several phytohormones (ABA, MeJA and ACC). Overexpression of SlERF84 in Arabidopsis endows transgenic plants with ABA hypersensitivity and enhanced tolerance to drought and salt stress. Histochemical staining assay showed that SlERF84 renders transgenic plants better ROS-scavenging capability. Pathogen inoculation assay revealed that SlERF84 might negatively modulate plant defense response to Pseudomonas syringae pv. tomato DC3000. Moreover, the transcript levels of pathogenesis-related genes AtPR1 and AtPR3 were compromised in transgenic Arabidopsis, as compared to that in Col-0 plants when inoculated with Pseudomonas syringae pv. tomato DC3000. These results suggest that SlERF84 functions as a stress-responsive transcription factor in differentially modulation of abiotic and biotic stress tolerance, and may have applications in the engineering of economically important crops.

Plant transcription factors from the homeodomain-leucine zipper family I. Role in development and stress responses

In front of stressful conditions plants display adaptation mechanisms leading to changes in their morphology, physiology, development and molecular composition. Transcription factors (TFs) play crucial roles in these complex adaptation processes. This work is focused in the homeodomain-leucine zipper I (HD-Zip I) family of TFs, unique to plants. First discovered in 1991, they were identified and isolated from monocotyledonous and dicotyledonous plants showing high structural similarity and diversified functions. These TFs have, besides the homeodomain and leucine zipper, conserved motifs in their carboxy-termini allowing the interaction with the basal machinery and with other regulatory proteins. The model dicotyledonous plant Arabidopsis thaliana has 17 HD-Zip I members; most of them regulated by external stimuli and hormones. These TFs are involved in key developmental processes like root and stem elongation, rosette leaves morphology determination, inflorescence stem branching, flowering and pollen hydration. Moreover, they are key players in responses to environmental stresses and illumination conditions. Several HD-Zip I encoding genes from different species were protected in patents because their overexpression or mutation generates improved agronomical phenotypes. Here we discuss many aspects about these TFs including structural features, biological functions and their utilization as biotechnological tools to improve crops. © 2017 IUBMB Life, 69(5):280-289, 2017.

DNA-binding and repressor function are prerequisites for the turnover of the tomato heat stress transcription factor HsfB1

HsfB1 is a central regulator of heat stress (HS) response and functions dually as a transcriptional co-activator of HsfA1a and a general repressor in tomato. HsfB1 is efficiently synthesized during the onset of HS and rapidly removed in the course of attenuation during the recovery phase. Initial results point to a complex regime modulating HsfB1 abundance involving the molecular chaperone Hsp90. However, the molecular determinants affecting HsfB1 stability needed to be established. We provide experimental evidence that DNA-bound HsfB1 is efficiently targeted for degradation when active as a transcriptional repressor. Manipulation of the DNA-binding affinity by mutating the HsfB1 DNA-binding domain directly influences the stability of the transcription factor. During HS, HsfB1 is stabilized, probably due to co-activator complex formation with HsfA1a. The process of HsfB1 degradation involves nuclear localized Hsp90. The molecular determinants of HsfB1 turnover identified in here are so far seemingly unique. A mutational switch of the R/KLFGV repressor motif's arginine and lysine implies that the abundance of other R/KLFGV type Hsfs, if not other transcription factors as well, might be modulated by a comparable mechanism. Thus, we propose a versatile mechanism for strict abundance control of the stress-induced transcription factor HsfB1 for the recovery phase, and this mechanism constitutes a form of transcription factor removal from promoters by degradation inside the nucleus.

Identification of Heat Shock Transcription Factor Genes Involved in Thermotolerance of Octoploid Cultivated Strawberry

Heat shock transcription factors (HSFs) are mainly involved in the activation of genes in response to heat stress as well as other abiotic and biotic stresses. The growth, development, reproduction, and yield of strawberry are strongly limited by extreme temperatures and droughts. In this study, we used Illumina sequencing and obtained transcriptome data set from Fragaria × ananassa Duchessne cv. Toyonoka. Six contigs and three unigenes were confirmed to encode HSF proteins (FaTHSFs). Subsequently, we characterized the biological functions of two particularly selected unigenes, FaTHSFA2a and FaTHSFB1a, which were classified into class A2 and B HSFs, respectively. Expression assays revealed that FaTHSFA2a and FaTHSFB1a expression was induced by heat shock and correlated well with elevated ambient temperatures. Overexpression of FaTHSFA2a and FaTHSFB1a resulted in the activation of their downstream stress-associated genes, and notably enhanced the thermotolerance of transgenic Arabidopsis plants. Besides, both FaTHSFA2a and FaTHSFB1a fusion proteins localized in the nucleus, indicating their similar subcellular distributions as transcription factors. Our yeast one-hybrid assay suggested that FaTHSFA2a has trans-activation activity, whereas FaTHSFB1a expresses trans-repression function. Altogether, our annotated transcriptome sequences provide a beneficial resource for identifying most genes expressed in octoploid strawberry. Furthermore, HSF studies revealed the possible insights into the molecular mechanisms of thermotolerance, thus rendering valuable molecular breeding to improve the tolerance of strawberry in response to high-temperature stress.

A matter of quantity: Common features in the drought response of transgenic plants overexpressing HD-Zip I transcription factors

Plant responses to water deficit involve complex molecular mechanisms in which transcription factors have key roles. Previous reports ectopically overexpressed a few members of the homeodomain-leucine zipper I (HD-Zip I) family of transcription factors from different species, and the obtained transgenic plants exhibited drought tolerance which extent depended on the level of overexpression, triggering diverse molecular and physiological pathways. Here we show that most HD-Zip I genes are regulated by drought in the vegetative and/or reproductive stages. Moreover, uncharacterized members of this family were expressed as transgenes both in Col-0 and rdr6-12 backgrounds and were able to enhance drought tolerance in host plants. The extent of such tolerance depended on the expression level of the transgene and was significantly higher in transgenic rdr6-12 than in Col-0. Comparative transcriptome analyses of Arabidopsis thaliana plants overexpressing HD-Zip I proteins indicated that many members have common targets. Moreover, the water deficit tolerance exhibited by these plants is likely due to the induction and repression of certain of these common HD-Zip I-regulated genes. However, each HD-Zip I member regulates other pathways, which, in some cases, generate differential and potentially undesirable traits in addition to drought tolerance. In conclusion, only a few members of this family could become valuable tools to improve drought-tolerance.

Heat shock factors in tomatoes: genome-wide identification, phylogenetic analysis and expression profiling under development and heat stress

The HSF (heat shock factor) gene family contains highly conserved plant-specific transcription factors that play an important role in plant high-temperature stress responses. The present study aimed to characterize the HSF transcription factor genes in tomato (Solanum lycopersicum), which is an important vegetable crop worldwide and the model plant for fruit development studies. Twenty-six SlyHSF genes were identified in tomato, and the phylogenetic analysis showed the possible evolution profile of subgroups among in the plant kingdom. A new group O was identified that involved HSF genes in primitive plant species, like in the green algae, mosses and lycophytes. The gene structure and motifs of each SlyHSF were comprehensively analyzed. We identified orthologous, co-orthologous and paralogous HSF gene pairs in tomato, Arabidopsis and rice, and constructed a complex interaction network among these genes. The SlyHSF genes were expressed differentially in different species and at a higher level in mature fruits. The qPCR analysis was performed and showed SlyHSF genes greatly participate in plant heat tolerant pathways. Our comprehensive genome-wide analysis provided insights into the HSF gene family of tomatoes.

Identification, classification, and expression profiles of heat shock transcription factors in tea plant (Camellia sinensis) under temperature stress

In vascular plants, heat shock transcription factors (Hsfs) regulate heat stress response by regulating the expression of heat shock proteins. This study systematically and comprehensively analyzed the Hsf family in tea plant [Camellia sinensis (L.) O. Kuntze]. A total of 16 CsHsfs were identified from the transcriptome database of tea plant and analyzed for their phylogenetic relationships, motifs, and physicochemical characteristics. On the basis of the phylogenetic comparison of tea plant with Arabidopsis thaliana, Populus trichocarpa, Theobroma cacao, and Oryza sativa, the CsHsfs were classified into three classes, namely, A (56.25%), B (37.50%), and C (6.25%). Heat mapping showed that the expression profiles of CsHsf genes under non-stress conditions varied among four tea plant cultivars, namely, 'Yunnanshilixiang', 'Chawansanhao', 'Ruchengmaoyecha', and 'Anjibaicha'. Six CsHsf genes (CsHsfA1a, CsHsfA1b, CsHsfA6, CsHsfB1, CsHsfB2b, and CsHsfC1) were selected from classes A, B, and C to analyze the expression profiles of CsHsf genes through quantitative real-time PCR in 'Yingshuang', 'Anjibaicha', and 'Yunnanshilixiang' under high (38 °C) or low (4 °C) temperature stress. Temperature stress positively or negatively regulated all of the selected CsHsf genes, and the expression levels evidently varied even among CsHsf genes belonging to the same class. This study provided a relatively detailed summary of Hsfs in tea plant and may serve as a reference for further studies on the mechanism of temperature stress regulation by CsHsfs.

An alternatively spliced heat shock transcription factor, OsHSFA2dI, functions in the heat stress-induced unfolded protein response in rice

As sessile organisms, plants have evolved a wide range of defence pathways to cope with environmental stress such as heat shock. However, the molecular mechanism of these defence pathways remains unclear in rice. In this study, we found that OsHSFA2d, a heat shock transcriptional factor, encodes two main splice variant proteins, OsHSFA2dI and OsHSFA2dII in rice. Under normal conditions, OsHSFA2dII is the dominant but transcriptionally inactive spliced form. However, when the plant suffers heat stress, OsHSFA2d is alternatively spliced into a transcriptionally active form, OsHSFA2dI, which participates in the heat stress response (HSR). Further study found that this alternative splicing was induced by heat shock rather than photoperiod. We found that OsHSFA2dI is localised to the nucleus, whereas OsHSFA2dII is localised to the nucleus and cytoplasm. Moreover, expression of the unfolded protein response (UNFOLDED PROTEIN RESPONSE) sensors, OsIRE1, OsbZIP39/OsbZIP60 and the UNFOLDED PROTEIN RESPONSE marker OsBiP1, was up-regulated. Interestingly, OsbZIP50 was also alternatively spliced under heat stress, indicating that UNFOLDED PROTEIN RESPONSE signalling pathways were activated by heat stress to re-establish cellular protein homeostasis. We further demonstrated that OsHSFA2dI participated in the unfolded protein response by regulating expression of OsBiP1.

LlHSFA1, a novel heat stress transcription factor in lily (Lilium longiflorum), can interact with LlHSFA2 and enhance the thermotolerance of transgenic Arabidopsis thaliana

A heat stress transcription factor LlHSFA1 in lily and its relationship with LlHSFA2 was investigated, and its function in enhancing thermotolerance was confirmed by analyzing transgenic Arabidopsis thaliana overexpressed LlHSFA1. A large family of heat stress transcription factors that are involved in the heat stress response in plants can induce the expression of multiple genes related to thermotolerance including heat-shock proteins. In this study, a novel class A1 HSF named LlHSFA1 was isolated from leaves of lily (Lilium longiflorum cv. 'White Heaven') using the rapid amplification of cDNA ends technique. Analysis of the deduced amino acid sequence and construction of a phylogenetic tree showed that LlHSFA1 contained five critical domains and motifs and belonged to the A1 family of HSFs. Following the heat treatment of lily leaves, transcription of LlHSFA1 was induced to a varying extent, related to the time of measurement. The induced expression peak of LlHSFA1 occurred prior to that of LlHSFA2, during the early phase of heat stress. Following transient expression of LlHSFA1 in Nicotiana benthamiana, LlHSFA1 was found to be localized in both the nucleus and the cytoplasm. Analysis using bimolecular fluorescence complementation and a yeast two-hybrid assay demonstrated that LlHSFA1 could interact with LlHSFA2. Use of a yeast one-hybrid assay confirmed that LlHSFA1 had transcriptional activation activity. In transgenic Arabidopsis lines overexpressing LlHSFA1 under unstressed conditions, the expression of some putative target genes was up-regulated, in comparison with expression in wild-type plants, and furthermore, the thermotolerance of the transgenic lines was enhanced. Overall, LlHSFA1 was demonstrated to play an important role in the heat stress response of lily and to be a novel candidate gene for application in lily breeding, using genetic modification approaches.

Plant homeodomain-leucine zipper I transcription factors exhibit different functional AHA motifs that selectively interact with TBP or/and TFIIB

Different members of the HD-Zip I family of transcription factors exhibit differential AHA-like activation motifs, able to interact with proteins of the basal transcriptional machinery. Homeodomain-leucine zipper proteins are transcription factors unique to plants, classified in four subfamilies. Subfamily I members have been mainly associated to abiotic stress responses. Several ones have been characterized using knockout or overexpressors plants, indicating that they take part in different signal transduction pathways even when their expression patterns are similar and they bind the same DNA sequence. A bioinformatic analysis has revealed the existence of conserved motifs outside the HD-Zip domain, including transactivation AHA motifs. Here, we demonstrate that these putative activation motifs are functional. Four members of the Arabidopsis family were chosen: AtHB1, AtHB7, AtHB12 and AtHB13. All of them exhibited activation activity in yeast and in plants but with different degrees. The protein segment necessary for such activation was different for these four transcription factors as well as the role of the tryptophans they present. When interaction with components of the basal transcription machinery was tested, AtHB1 was able to interact with TBP, AtHB12 interacted with TFIIB, AtHB7 interacted with both, TBP and TFIIB while AtHB13 showed weak interactions with any of them, in yeast two-hybrid as well as in pull-down assays. Transient transformation of Arabidopsis seedlings confirmed the activation capacity and specificity of these transcription factors and showed some differences with the results obtained in yeast. In conclusion, the differential activation functionality of these transcription factors adds an important level of functional divergence of these proteins, and together with their expression patterns, these differences could explain, at least in part, their functional divergence.

Functional characterization of Arabidopsis HsfA6a as a heat-shock transcription factor under high salinity and dehydration conditions

Although heat-shock transcription factors are well characterized in the heat stress-related pathway, they are poorly understood in other stress responses. Here, we functionally characterized AtHsfA6a in the presence of exogenous abscisic acid (ABA) and under high salinity and dehydration conditions. AtHsfA6a expression under normal conditions is very low, but was highly induced by exogenous ABA, NaCl and drought. Unexpectedly, the levels of AtHsfA6a transcript were not significantly altered under heat and cold stresses. Electrophoretic mobility shift assays and transient transactivation assays indicated that AtHsfA6a is transcriptionally regulated by ABA-responsive element binding factor/ABA-responsive element binding protein, which are key regulators of the ABA signalling pathway. Additionally, fractionation and protoplast transient assays showed that AtHsfA6a was in cytoplasm and nucleus simultaneously; however, under conditions of high salinity the majority of AtHsfA6A was in the nucleus. Furthermore, at both seed germination and seedlings stage, plants overexpressing AtHsfA6a were hypersensitive to ABA and exhibited enhanced tolerance against salt and drought stresses. Finally, the microarray and qRT-PCR analyses revealed that many stress-responsive genes were up-regulated in the plants overexpressing AtHsfA6a. Taken together, the data strongly suggest that AtHsfA6a acts as a transcriptional activator of stress-responsive genes via the ABA-dependent signalling pathway.

Dissecting the heat stress response in Chlamydomonas by pharmaceutical and RNAi approaches reveals conserved and novel aspects

To study how conserved fundamental concepts of the heat stress response (HSR) are in photosynthetic eukaryotes, we applied pharmaceutical and antisense/amiRNA approaches to the unicellular green alga Chlamydomonas reinhardtii. The Chlamydomonas HSR appears to be triggered by the accumulation of unfolded proteins, as it was induced at ambient temperatures by feeding cells with the arginine analog canavanine. The protein kinase inhibitor staurosporine strongly retarded the HSR, demonstrating the importance of phosphorylation during activation of the HSR also in Chlamydomonas. While the removal of extracellular calcium by the application of EGTA and BAPTA inhibited the HSR in moss and higher plants, only the addition of BAPTA, but not of EGTA, retarded the HSR and impaired thermotolerance in Chlamydomonas. The addition of cycloheximide, an inhibitor of cytosolic protein synthesis, abolished the attenuation of the HSR, indicating that protein synthesis is necessary to restore proteostasis. HSP90 inhibitors induced a stress response when added at ambient conditions and retarded attenuation of the HSR at elevated temperatures. In addition, we detected a direct physical interaction between cytosolic HSP90A/HSP70A and heat shock factor 1, but surprisingly this interaction persisted after the onset of stress. Finally, the expression of antisense constructs targeting chloroplast HSP70B resulted in a delay of the cell's entire HSR, thus suggesting the existence of a retrograde stress signaling cascade that is desensitized in HSP70B-antisense strains.

Ectopic overexpression of SsCBF1, a CRT/DRE-binding factor from the nightshade plant Solanum lycopersicoides, confers freezing and salt tolerance in transgenic Arabidopsis

The C-repeat (CRT)/dehydration-responsive element (DRE) binding factor (CBF/DREB1) transcription factors play a key role in cold response. However, the detailed roles of many plant CBFs are far from fully understood. A CBF gene (SsCBF1) was isolated from the cold-hardy plant Solanum lycopersicoides. A subcellular localization study using GFP fusion protein indicated that SsCBF1 is localized in the nucleus. We delimited the SsCBF1 transcriptional activation domain to the C-terminal segment comprising amino acid residues 193–228 (SsCBF1^193–228). The expression of SsCBF1 could be dramatically induced by cold, drought and high salinity. Transactivation assays in tobacco leaves revealed that SsCBF1 could specifically bind to the CRT cis-elements in vivo to activate the expression of downstream reporter genes. The ectopic overexpression of SsCBF1 conferred increased freezing and high-salinity tolerance and late flowering phenotype to transgenic Arabidopsis. RNA-sequencing data exhibited that a set of cold and salt stress responsive genes were up-regulated in transgenic Arabidopsis. Our results suggest that SsCBF1 behaves as a typical CBF to contribute to plant freezing tolerance. Increased resistance to high-salinity and late flowering phenotype derived from SsCBF1 OE lines lend more credence to the hypothesis that plant CBFs participate in diverse physiological and biochemical processes related to adverse conditions.

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.

The plant heat stress transcription factor (Hsf) family: structure, function and evolution

Ten years after the first overview of a complete plant Hsf family was presented for Arabidopsis thaliana by Nover et al. [1], we compiled data for 252 Hsfs from nine plant species (five eudicots and four monocots) with complete or almost complete genome sequences. The new data set provides interesting insights into phylogenetic relationships within the Hsf family in plants and allows the refinement of their classification into distinct groups. Numerous publications over the last decade document the diversification and functional interaction of Hsfs as well as their integration into the complex stress signaling and response networks of plants. This article is part of a Special Issue entitled: Plant gene regulation in response to abiotic stress.

Crosstalk between Hsp90 and Hsp70 chaperones and heat stress transcription factors in tomato

Heat stress transcription factors (Hsfs) regulate gene expression in response to environmental stress. The Hsf network in plants is controlled at the transcriptional level by cooperation of distinct Hsf members and by interaction with chaperones. We found two general mechanisms of Hsf regulation by chaperones while analyzing the three major Hsfs, A1, A2, and B1, in tomato (Solanum lycopersicum). First, Hsp70 and Hsp90 regulate Hsf function by direct interactions. Hsp70 represses the activity of HsfA1, including its DNA binding, and the coactivator function of HsfB1 in the complex with HsfA2, while the DNA binding activity of HsfB1 is stimulated by Hsp90. Second, Hsp90 affects the abundance of HsfA2 and HsfB1 by modulating hsfA2 transcript degradation involved in regulation of the timing of HsfA2 synthesis. By contrast, HsfB1 binding to Hsp90 and to DNA are prerequisites for targeting this Hsf for proteasomal degradation, which also depends on a sequence element in its carboxyl-terminal domain. Thus, HsfB1 represents an Hsp90 client protein that, by interacting with the chaperone, is targeted for, rather than protected from, degradation. Based on these findings, we propose a versatile regulatory regime involving Hsp90, Hsp70, and the three Hsfs in the control of heat stress response.

Specific interaction between tomato HsfA1 and HsfA2 creates hetero-oligomeric superactivator complexes for synergistic activation of heat stress gene expression

In plants, a family of more than 20 heat stress transcription factors (Hsf) controls the expression of heat stress (hs) genes. There is increasing evidence for the functional diversification between individual members of the Hsf family fulfilling distinct roles in response to various environmental stress conditions and developmental signals. In response to hs, accumulation of both heat stress proteins (Hsp) and Hsfs is induced. In tomato, the physical interaction between the constitutively expressed HsfA1 and the hs-inducible HsfA2 results in synergistic transcriptional activation (superactivation) of hs gene expression. Here, we show that the interaction is strikingly specific and not observed with other class A Hsfs. Hetero-oligomerization of the two-component Hsfs is preferred to homo-oligomerization, and each Hsf in the HsfA1/HsfA2 hetero-oligomeric complex has its characteristic contribution to its function as superactivator. Distinct regions of the oligomerization domain are responsible for specific homo- and hetero-oligomeric interactions leading to the formation of hexameric complexes. The results are summarized in a model of assembly and function of HsfA1/A2 superactivator complexes in hs gene regulation.

Plant stress granules and mRNA processing bodies are distinct from heat stress granules

Similar to the situation in mammalian cells and yeast, messenger ribonucleo protein (mRNP) homeostasis in plant cells depends on rapid transitions between three functional states, i.e. translated mRNPs in polysomes, stored mRNPs and mRNPs under degradation. Studies in mammalian cells showed that whenever the dynamic exchange of the components between these states is disrupted, stalled mRNPs accumulate in cytoplasmic aggregates, such as stress granules (SGs) or processing bodies (PBs). We identified PBs and SGs in plant cells by detection of DCP1, DCP2 and XRN4, as marker proteins for the 5'-->3' mRNA degradation pathway, and eIF4E, as well as the RNA binding proteins RBP47 and UBP1, as marker proteins for stored mRNPs in SGs. Cycloheximide-inhibited translation, stress treatments and mutants defective in mRNP homeostasis were used to study the dynamic transitions of mRNPs between SGs and PBs. SGs and PBs can be clearly discriminated from the previously described heat stress granules (HSGs), which evidently do not contain mRNPs. Thus, the role of HSGs as putative mRNP storage sites must be revised.

Expression of the recombinant bacterial outer surface protein A in tobacco chloroplasts leads to thylakoid localization and loss of photosynthesis

Bacterial lipoproteins play crucial roles in host-pathogen interactions and pathogenesis and are important targets for the immune system. A prominent example is the outer surface protein A (OspA) of Borrelia burgdorferi, which has been efficiently used as a vaccine for the prevention of Lyme disease. In a previous study, OspA could be produced in tobacco chloroplasts in a lipidated and immunogenic form. To further explore the potential of chloroplasts for the production of bacterial lipoproteins, the role of the N-terminal leader sequence was investigated. The amount of recombinant OspA could be increased up to ten-fold by the variation of the insertion site in the chloroplast genome. Analysis of OspA mutants revealed that replacement of the invariant cysteine residue as well as deletion of the leader sequence abolishes palmitolyation of OspA. Also, decoration of OspA with an N-terminal eukaryotic lipidation motif does not lead to palmitoylation in chloroplasts. Strikingly, the bacterial signal peptide of OspA efficiently targets the protein to thylakoids, and causes a mutant phenotype. Plants accumulating OspA at 10% total soluble protein could not grow without exogenously supplied sugars and rapidly died after transfer to soil under greenhouse conditions. The plants were found to be strongly affected in photosystem II, as revealed by the analyses of temporal and spatial dynamics of photosynthetic activity by chlorophyll fluorescence imaging. Thus, overexpression of OspA in chloroplasts is limited by its concentration-dependent interference with essential functions of chloroplastic membranes required for primary metabolism.

Role of heat stress transcription factor HsfA5 as specific repressor of HsfA4

Unlike other eukaryotes, plants possess a complex family of heat stress transcription factors (Hsfs) with usually more than 20 members. Among them, Hsfs A4 and A5 form a group distinguished from other Hsfs by structural features of their oligomerization domains and by a number of conserved signature sequences. We show that A4 Hsfs are potent activators of heat stress gene expression, whereas A5 Hsfs act as specific repressors of HsfA4 activity. The oligomerization domain of HsfA5 alone is necessary and sufficient to exert this effect. Due to the high specificity of the oligomerization domains, other class A Hsfs are not affected. Pull-down assay and yeast two-hybrid interaction tests demonstrate that the tendency to form HsfA4/A5 heterooligomers is stronger than the formation of homooligomers. The specificity of interaction between Hsfs A4 and A5 was confirmed by bimolecular fluorescence complementation experiments. The major role of the representatives of the HsfA4/A5 group, which are not involved in the conventional heat stress response, may reside in cell type-specific functions connected with the control of cell death triggered by pathogen infection and/or reactive oxygen species.

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.

Isolation and characterization of a cDNA encoding two novel heat-shock factor OsHSF6 and OsHSF12 in Oryza sativa L

As a crucial transcription factor family, heat-shock factors were mainly analyzed and characterized in tomato and Arabidopsis. In this study, we isolated two putative heatshock factors OsHSF6 and OsHSF12 that interact specifically with heat-shock element (HSE) from Oryza sativa L by yeast one-hybrid method. The full-length cDNA of OsHSF6 and OsHSF12 have 1074bp and 920bp open reading frame (ORF), respectively. Analysis of the deduced amino acid sequences revealed that OsHSF6 was a class A heat shock factor (HSF) with all the conserved sequence elements characteristic of heat stress transcription factor, while OsHSF12 was a class B HSF with C-terminal domain (CTD) lacking of AHA motif. Bioinformatic analysis showed that the sequences and structures of two HSFs' DNA binding domain (DBD) had a high similarity with LpHSF24. The results of RT-PCR indicated OsHSF6 gene was expressed immediately after rice plants exposure to heat stress, and the transcription of OsHSF6 gene accumulated primarily in immature seeds, roots and leaves. However, we did not find the transcription of OsHSF12 gene in different organs and growth periods. Our results implied that OsHSF6 might be function as a HSF regulating early expression of stress genes in response to heat shock, and OsHSF12 might be act as a synergistic factor to regulate the expression of down-stream genes.

Heat stress response in plants: a complex game with chaperones and more than twenty heat stress transcription factors

Compared to the overall multiplicity of more than 20 plant Hsfs, detailed analyses are mainly restricted to tomato and Arabidopsis and to three important representatives of the family (Hsfs A1, A2 and B1). The three Hsfs represent examples of striking functional diversification specialized for the three phases of the heat stress (hs) response (triggering, maintenance and recovery). This is best illustrated for the tomato Hsf system: (i) HsfA1a is the master regulator responsible for hs-induced gene expression including synthesis of HsfA2 and HsfB1. It is indispensible for the development of thermotolerance. (ii) Although functionally equivalent to HsfA1a, HsfA2 is exclusively found after hs induction and represents the dominant Hsf, the "working horse" of the hs response in plants subjected to repeated cycles of hs and recovery in a hot summer period. Tomato HsfA2 is tightly integrated into a network of interacting proteins (HsfA1a, Hsp17-CII, Hsp17-CI) influencing its activity and intracellular distribution. (iii) Because of structural peculiarities, HsfB1 acts as coregulator enhancing the activity of HsfA1a and/or HsfA2. But in addition, it cooperates with yet to be identified other transcription factors in maintaining and/or restoring housekeeping gene expression.

Plant class B HSFs inhibit transcription and exhibit affinity for TFIIB and TBP

Plant heat shock transcription factors (HSFs) are capable of transcriptional activation (class A HSFs) or both, activation and repression (class B HSFs). However, the details of mechanism still remain unclear. It is likely, that the regulation occurs through interactions of HSFs with general transcription factors (GTFs), as has been described for numerous other transcription factors. Here, we show that class A HSFs may activate transcription through direct contacts with TATA-binding protein (TBP). Class A HSFs can also interact weakly with TFIIB. Conversely, class B HSFs inhibit promoter activity through an active mechanism of repression that involves the C-terminal regulatory region (CTR) of class B HSFs. Deletion analysis revealed two sites in the CTR of soybean GmHSFB1 potentially involved in protein-protein interactions with GTFs: one is the repressor domain (RD) located in the N-terminal half of the CTR, and the other is a TFIIB binding domain (BD) that shows affinity for TFIIB and is located C-terminally from the RD. A Gal4 DNA binding domain-RD fusion repressed activity of LexA-activators, while Gal4-BD proteins synergistically activated strong and weak transcriptional activators. In vitro binding studies were consistent with this pattern of activity since the BD region alone interacted strongly with TFIIB, and the presence of RD had an inhibitory effect on TFIIB binding and transcriptional activation.

Characterization of C-terminal domains of Arabidopsis heat stress transcription factors (Hsfs) and identification of a new signature combination of plant class A Hsfs with AHA and NES motifs essential for activator function and intracellular localization

Heat stress transcription factors (Hsfs) are the major regulators of the plant heat stress (hs) response. Sequencing of the Arabidopsis genome revealed the existence of 21 open-reading frames (ORFs) encoding putative Hsfs assigned to classes A-C. Here we present results of a functional genomics approach to the Arabidopsis Hsf family focused on the analysis of their C-terminal domains (CTDs) harboring conserved modules for their function as transcription factors and their intracellular localization. Using reporter assays in tobacco protoplasts and yeast as well as glutathione-S-transferase (GST) pull-down assays, we demonstrate that short peptide motifs enriched with aromatic and large hydrophobic amino acid (aa) residues embedded in an acidic surrounding (AHA motifs) are essential for transcriptional activity of class A Hsfs. In contrast to this, class B and C Hsfs lack AHA motifs and have no activator function on their own. We also provide evidence for the function of a leucine (Leu)-rich region centered around a conserved QMGPhiL motif at the very C-terminus as a nuclear export signal (NES) of class A Hsfs. Sequence comparison indicates that the combination of a C-terminal AHA motif with the consensus sequence FWxxF/L,F/I/L as well as the adjacent NES represents a signature domain for plant class A Hsfs, which allowed to identify more than 60 new Hsfs from the expressed sequence tag (EST) database.

Role of Hsp17.4-CII as coregulator and cytoplasmic retention factor of tomato heat stress transcription factor HsfA2

HsfA2 is a heat stress (hs)-induced Hsf in peruvian tomato (Lycopersicon peruvianum) and the cultivated form Lycopersicon esculentum. Due to the high activator potential and the continued accumulation during repeated cycles of heat stress and recovery, HsfA2 becomes a dominant Hsf in thermotolerant cells. The formation of heterooligomeric complexes with HsfA1 leads to nuclear retention and enhanced transcriptional activity of HsfA2. This effect seems to represent one part of potential molecular mechanisms involved in its activity control. As shown in this paper, the activity of HsfA2 is also controlled by a network of nucleocytoplasmic small Hsps influencing its solubility, intracellular localization and activator function. By yeast two-hybrid interaction and transient coexpression studies in tobacco (Nicotiana plumbaginifolia) mesophyll protoplasts, we found that tomato (Lycopersicon esculentum) Hsp17.4-CII acts as corepressor of HsfA2. Given appropriate conditions, both proteins together formed large cytosolic aggregates which could be solubilized in presence of class CI sHsps. However, independent of the formation of aggregates or of the nucleocytoplasmic distribution of HsfA2, its transcriptional activity was specifically repressed by interaction of Hsp17.4-CII with the C-terminal activator domain. Although not identical in all aspects, the situation with the highly expressed, heat stress-inducible Arabidopsis HsfA2 was found to be principally similar. In corresponding reporter assays its activity was repressed in presence of AtHsp17.7-CII but not of AtHsp17.6-CII or LpHsp17.4-CII.

Tomato heat stress transcription factor HsfB1 represents a novel type of general transcription coactivator with a histone-like motif interacting with the plant CREB binding protein ortholog HAC1

In contrast with the class A heat stress transcription factors (HSFs) of plants, a considerable number of HSFs assigned to classes B and C have no evident function as transcription activators on their own. However, in the following article, we provide evidence that tomato (Lycopersicon peruvianum) HsfB1 represents a novel type of coactivator cooperating with class A HSFs (e.g., with tomato HsfA1). Provided the appropriate promoter architecture, the two HSFs assemble into an enhanceosome-like complex, resulting in strong synergistic activation of reporter gene expression. Moreover, HsfB1 also cooperates in a similar manner with other activators, for example, with the ASF1/2 enhancer binding proteins of the 35S promoter of Cauliflower mosaic virus or with yet unidentified activators controlling housekeeping gene expression. By these effects, HsfB1 may help to maintain and/or restore expression of certain viral or housekeeping genes during ongoing heat stress. The coactivator function of HsfB1 depends on a histone-like motif in its C-terminal domain with an indispensable Lys residue in the center (GRGKMMK). This motif is required for recruitment of the plant CREB binding protein (CBP) ortholog HAC1. HsfA1, HsfB1, and HAC1/CBP form ternary complexes in vitro and in vivo with markedly enhanced efficiency in promoter recognition and transcription activation in plant and mammalian (COS7) cells. Using small interfering RNA-mediated knock down of HAC1 expression in Arabidopsis thaliana mesophyll protoplasts, the crucial role for the coactivator function of HsfB1 was confirmed.

A heat-activated MAP kinase in tomato: a possible regulator of the heat stress response

Adaptation to elevated temperatures is of major importance for the survival of plants. The role of kinases in heat stress response was studied in tomato by in gel and in solution kinase assays using myelin basic protein as substrate. The application of heat stress in a naturally occurring temperature range resulted in a fast and transient activation of a 50 kDa mitogen-activated protein (MAP) kinase both in a photoautotrophic cell suspension culture and in leaves of mature plants. The heat activation of the MAP kinase was shown to be calcium-dependent. The specific phosphorylation of tomato heat stress transcription factor HsfA3 by a partially purified preparation of the heat-activated MAP kinase supports a physiological role of the identified kinase activity in transducing the heat stress signal.

Selective activation of the developmentally regulated Ha hsp17.6 G1 promoter by heat stress transcription factors

Using two well-characterized heat stress transcription factors (Hsfs) from tomato (Lycopersicon peruvianum; LpHsfA1 and LpHsfA2), we analyzed the transcriptional activation of the Ha hsp17.6 G1 promoter in sunflower (Helianthus annuus) embryos. In this system, we observed transient promoter activation only with LpHsfA2. In contrast, both factors were able to activate mutant versions of the promoter with improved consensus Hsf-binding sites. Exclusive activation by LpHsfA2 was also observed in yeast (Saccharomyces cerevisiae) without other Hsfs and with a minimal Cyc1 promoter fused to the Ha hsp17.6 G1 heat stress cis-element. Furthermore, the same promoter mutations reproduced the loss of activation selectivity, as observed in sunflower embryos. The results of in vitro binding experiments rule out differential DNA binding of the two factors as the explanation for the observed differential activation capacity. We conclude that the specific sequence of this heat stress cis-element is crucial for Hsf promoter selectivity, and that this selectivity could involve preferential transcriptional activation following DNA binding. In sunflower embryos, we also observed synergistic transcriptional activation by co-expression of LpHsfA1 and LpHsfA2. Mutational analyses of the Ha hsp17.6 G1 promoter, combined with in vitro binding assays, suggest that mixed oligomers of the two factors may be involved in promoter activation. We discuss the relevance of our observations for mechanisms of developmental regulation of plant heat stress protein genes.

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.

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.

The balance of nuclear import and export determines the intracellular distribution and function of tomato heat stress transcription factor HsfA2

Tomato heat stress transcription factor HsfA2 is a shuttling protein with dominant cytoplasmic localization as a result of a nuclear import combined with an efficient export. Besides the nuclear localization signal (NLS) adjacent to the oligomerization domain, a C-terminal leucine-rich motif functions as a nuclear export signal (NES). Mutant forms of HsfA2 with a defective or an absent NES are nuclear proteins. The same is true for the wild-type HsfA2 if coexpressed with HsfA1 or in the presence of export inhibitor leptomycin B (LMB). Fusion of the NES domain of HsfA2 to HsfB1, which is a nuclear protein, caused export of the HsfB1-A2NES hybrid protein, and this effect was reversed by the addition of LMB. Due to the lack of background problems, Chinese hamster ovary (CHO) cells represent an excellent system for expression and functional analysis of tomato Hsfs. The results faithfully reflect the situation found in plant cells (tobacco protoplasts). The intriguing role of NLS and NES accessibility for the intracellular distribution of HsfA2 is underlined by the results of heat stress treatments of CHO cells (41 degrees C). Despite the fact that nuclear import and export are not markedly affected, HsfA2 remains completely cytoplasmic at 41 degrees C even in the presence of LMB. The temperature-dependent conformational transition of HsfA2 with shielding of the NLS evidently needs intramolecular interaction between the internal HR-A/B and the C-terminal HR-C regions. It is not observed with the HR oligomerization domain (HR-A/B region) deletion form of HsfA2 or in HsfA2-HsfA1 hetero-oligomers.