Arabidopsis thaliana Hsp100 proteins: kith and kin

Insights into genomic variations in rice Hsp100 genes across diverse rice accessions

The Hsp101 gene is present across all sequenced rice genomes. However, as against Japonica rice, Hsp101 protein of most indica and aus rice contain insertion of glutamic acid at 907th position. The understanding of the heat stress response of rice plants is important for worldwide food security. We examined the presence/absence variations (PAVs) of heat shock proteins (Hsps)/heat shock transcription factor (Hsf) genes in cultivated rice accessions. While 53 Hsps/Hsfs genes showed variable extent of PAVs, 194 genes were the core genes present in all the rice accessions. ClpB1/Hsp101 gene, which is critically important for thermotolerance in plants, showed 100% distribution across the rice types. Within the ClpB1 gene sequence, 40 variation sites consisting of nucleotide polymorphisms (SNPs) and short insertion/deletions (InDels) were discerned. An InDel in ClpB1 leading to an in-frame insertion of 3 nucleotides (TCC) thereby an additional amino acid (glutamic acid) at 907th amino acid position was noted in most of the indica and aus as against japonica rice types. Three rice types namely Moroberekan (japonica), IR64 (indica) and N22 (aus) were further analyzed to address the question of ClpB1 genomic variations and its protein levels with the heat tolerance phenotype. The growth profiling analysis in the post heat stress (HS) period showed that N22 seedlings were most tolerant, IR64 moderately tolerant and Moroberekan highly sensitive. Importantly, the ClpB1 protein sequences of these three rice types showed distinct differences in terms of SNPs. As the ClpB1 protein levels accumulated post HS were generally higher in Moroberekan than N22 seedlings in our study, it is proposed that some additional gene loci in conjunction with ClpB1 regulate the overall rice heat stress response.

Heat and drought induced transcriptomic changes in barley varieties with contrasting stress response phenotypes

Drought and heat stress substantially impact plant growth and productivity. When subjected to drought or heat stress, plants exhibit reduction in growth resulting in yield losses. The occurrence of these two stresses together intensifies their negative effects. Unraveling the molecular changes in response to combined abiotic stress is essential to breed climate-resilient crops. In this study, transcriptome profiles were compared between stress-tolerant (Otis), and stress-sensitive (Golden Promise) barley genotypes subjected to drought, heat, and combined heat and drought stress for five days during heading stage. The major differences that emerged from the transcriptome analysis were the overall number of differentially expressed genes was relatively higher in Golden Promise (GP) compared to Otis. The differential expression of more than 900 transcription factors in GP and Otis may aid this transcriptional reprogramming in response to abiotic stress. Secondly, combined heat and water deficit stress results in a unique and massive transcriptomic response that cannot be predicted from individual stress responses. Enrichment analyses of gene ontology terms revealed unique and stress type-specific adjustments of gene expression. Weighted Gene Co-expression Network Analysis identified genes associated with RNA metabolism and Hsp70 chaperone components as hub genes that can be useful for engineering tolerance to multiple abiotic stresses. Comparison of the transcriptomes of unstressed Otis and GP plants identified several genes associated with biosynthesis of antioxidants and osmolytes were higher in the former that maybe providing innate tolerance capabilities to effectively combat hostile conditions. Lines with different repertoire of innate tolerance mechanisms can be effectively leveraged in breeding programs for developing climate-resilient barley varieties with superior end-use traits.

Identification of heat responsive genes in pea stipules and anthers through transcriptional profiling

Field pea (Pisum sativum L.), a cool-season legume crop, is known for poor heat tolerance. Our previous work identified PR11-2 and PR11-90 as heat tolerant and susceptible lines in a recombinant inbred population. CDC Amarillo, a Canadian elite pea variety, was considered as another heat tolerant variety based on its similar field performance as PR11-2. This study aimed to characterize the differential transcription. Plants of these three varieties were stressed for 3 h at 38°C prior to self-pollination, and RNAs from heat stressed anthers and stipules on the same flowering node were extracted and sequenced via the Illumina NovaSeq platform for the characterization of heat responsive genes. In silico results were further validated by qPCR assay. Differentially expressed genes (DEGs) were identified at log2 |fold change (FC)| ≥ 2 between high temperature and control temperature, the three varieties shared 588 DEGs which were up-regulated and 220 genes which were down-regulated in anthers when subjected to heat treatment. In stipules, 879 DEGs (463/416 upregulation/downregulation) were consistent among varieties. The above heat-induced genes of the two plant organs were related to several biological processes i.e., response to heat, protein folding and DNA templated transcription. Ten gene ontology (GO) terms were over-represented in the consistently down-regulated DEGs of the two organs, and these terms were mainly related to cell wall macromolecule metabolism, lipid transport, lipid localization, and lipid metabolic processes. GO enrichment analysis on distinct DEGs of individual pea varieties suggested that heat affected biological processes were dynamic, and variety distinct responses provide insight into molecular mechanisms of heat-tolerance response. Several biological processes, e.g., cellular response to DNA damage stimulus in stipule, electron transport chain in anther that were only observed in heat induced PR11-2 and CDC Amarillo, and their relevance to field pea heat tolerance is worth further validation.

Cloning, expression analysis and In silico characterization of HSP101: a potential player conferring heat stress in Aegilops speltoides (Tausch) Gren

Heat shock protein (HSP101) function as molecular chaperones and confer thermotolerance to plants. In the present investigation, identification, comprehensive expression analysis, phylogeny and protein modelling of HSP101 gene has been done in Aegilops speltoides accession Pau3583. In the present study, we cloned and in silico characterized a HSP101C gene designated as AsHSP101C-Pau3583. AsHSP101C-Pau3583 is 4180 bp long with seven exons and six introns and encoded a polypeptide of 910 amino acids predicted by FGENESH. We have identified 58 SNPs between the AsHSP101C-Pau3583 and reference gene sequence extracted from Ae. speltoides TGAC assembly. Real-time RT-PCR analysis of expression levels of HSP101 gene in two wheat genotypes under heat stress revealed that gene namely HSP101C was up-regulated in Aegilops speltoides acc. Pau3583 by > fourfold in comparison to Triticum aestivum cv. PBW343 under heat stress signifies that it plays a role in conferring heat tolerance. Sequence comparison and phylogenetic analysis of AsHSP101C-Pau3583 with seven wheat homologs Triticum aestivum, Aegilops speltoides (TGAC), Triticum durum cv Cappelli, Triticum durum cv Strongfield, Triticum monococcum, Aegilops tauschii and Triticum urartu showed significant similarities with highly conserved coding regions and functional domains (AAA, AAA + 2, ClpB domains), suggesting the conserved function of HSP101C in different species. The illustration of the protein models of HSP101C in homologs provided information for the ATP-binding motifs within the nucleotide binding domains (NBD), specific for the chaperone activity. These findings are important and identified SNPs could be used for designing markers for ensuring the transfer of AsHSP101C-Pau3583 gene into hexaploid wheat and its role in heat tolerance.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01005-2.

An overview on molecular chaperones enhancing solubility of expressed recombinant proteins with correct folding

The majority of research topics declared that most of the recombinant proteins have been expressed by Escherichia coli in basic investigations. But the majority of high expressed proteins formed as inactive recombinant proteins that are called inclusion body. To overcome this problem, several methods have been used including suitable promoter, environmental factors, ladder tag to secretion of proteins into the periplasm, gene protein optimization, chemical chaperones and molecular chaperones sets. Co-expression of the interest protein with molecular chaperones is one of the common methods The chaperones are a group of proteins, which are involved in making correct folding of recombinant proteins. Chaperones are divided two groups including; cytoplasmic and periplasmic chaperones. Moreover, periplasmic chaperones and proteases can be manipulated to increase the yields of secreted proteins. In this article, we attempted to review cytoplasmic chaperones such as Hsp families and periplasmic chaperones including; generic chaperones, specialized chaperones, PPIases, and proteins involved in disulfide bond formation.

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

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

Allyl-isothiocyanate treatment induces a complex transcriptional reprogramming including heat stress, oxidative stress and plant defence responses in Arabidopsis thaliana

Background

Isothiocyanates (ITCs) are degradation products of the plant secondary metabolites glucosinolates (GSLs) and are known to affect human health as well as plant herbivores and pathogens. To investigate the processes engaged in plants upon exposure to isothiocyanate we performed a genome scale transcriptional profiling of Arabidopsis thaliana at different time points in response to an exogenous treatment with allyl-isothiocyanate.

Results

The treatment triggered a substantial response with the expression of 431 genes affected (P < 0.05 and log₂ ≥ 1 or ≤ -1) already after 30 min and that of 3915 genes affected after 9 h of exposure, most of the affected genes being upregulated. These are involved in a considerable number of different biological processes, some of which are described in detail: glucosinolate metabolism, sulphate uptake and assimilation, heat stress response, oxidative stress response, elicitor perception, plant defence and cell death mechanisms.

Conclusion

Exposure of Arabidopsis thaliana to vapours of allyl-isothiocyanate triggered a rapid and substantial transcriptional response affecting numerous biological processes. These include multiple stress stimuli such as heat stress response and oxidative stress response, cell death and sulphur secondary defence metabolism. Hence, effects of isothiocyanates on plants previously reported in the literature were found to be regulated at the gene expression level. This opens some avenues for further investigations to decipher the molecular mechanisms underlying the effects of isothiocyanates on plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3039-x) contains supplementary material, which is available to authorized users.

Prospects of engineering thermotolerance in crops through modulation of heat stress transcription factor and heat shock protein networks

Cell survival under high temperature conditions involves the activation of heat stress response (HSR), which in principle is highly conserved among different organisms, but shows remarkable complexity and unique features in plant systems. The transcriptional reprogramming at higher temperatures is controlled by the activity of the heat stress transcription factors (Hsfs). Hsfs allow the transcriptional activation of HSR genes, among which heat shock proteins (Hsps) are best characterized. Hsps belong to multigene families encoding for molecular chaperones involved in various processes including maintenance of protein homeostasis as a requisite for optimal development and survival under stress conditions. Hsfs form complex networks to activate downstream responses, but are concomitantly subjected to cell-type-dependent feedback regulation through factor-specific physical and functional interactions with chaperones belonging to Hsp90, Hsp70 and small Hsp families. There is increasing evidence that the originally assumed specialized function of Hsf/chaperone networks in the HSR turns out to be a complex central stress response system that is involved in the regulation of a broad variety of other stress responses and may also have substantial impact on various developmental processes. Understanding in detail the function of such regulatory networks is prerequisite for sustained improvement of thermotolerance in important agricultural crops.

Chaperone network composition in Solanum lycopersicum explored by transcriptome profiling and microarray meta-analysis

Heat shock proteins (Hsps) are molecular chaperones primarily involved in maintenance of protein homeostasis. Their function has been best characterized in heat stress (HS) response during which Hsps are transcriptionally controlled by HS transcription factors (Hsfs). The role of Hsfs and Hsps in HS response in tomato was initially examined by transcriptome analysis using the massive analysis of cDNA ends (MACE) method. Approximately 9.6% of all genes expressed in leaves are enhanced in response to HS, including a subset of Hsfs and Hsps. The underlying Hsp-Hsf networks with potential functions in stress responses or developmental processes were further explored by meta-analysis of existing microarray datasets. We identified clusters with differential transcript profiles with respect to abiotic stresses, plant organs and developmental stages. The composition of two clusters points towards two major chaperone networks. One cluster consisted of constitutively expressed plastidial chaperones and other genes involved in chloroplast protein homeostasis. The second cluster represents genes strongly induced by heat, drought and salinity stress, including HsfA2 and many stress-inducible chaperones, but also potential targets of HsfA2 not related to protein homeostasis. This observation attributes a central regulatory role to HsfA2 in controlling different aspects of abiotic stress response and tolerance in tomato.

Intergenic sequence between Arabidopsis caseinolytic protease B-cytoplasmic/heat shock protein100 and choline kinase genes functions as a heat-inducible bidirectional promoter

In Arabidopsis (Arabidopsis thaliana), the At1g74310 locus encodes for caseinolytic protease B-cytoplasmic (ClpB-C)/heat shock protein100 protein (AtClpB-C), which is critical for the acquisition of thermotolerance, and At1g74320 encodes for choline kinase (AtCK2) that catalyzes the first reaction in the Kennedy pathway for phosphatidylcholine biosynthesis. Previous work has established that the knockout mutants of these genes display heat-sensitive phenotypes. While analyzing the AtClpB-C promoter and upstream genomic regions in this study, we noted that AtClpB-C and AtCK2 genes are head-to-head oriented on chromosome 1 of the Arabidopsis genome. Expression analysis showed that transcripts of these genes are rapidly induced in response to heat stress treatment. In stably transformed Arabidopsis plants harboring this intergenic sequence between head-to-head oriented green fluorescent protein and β-glucuronidase reporter genes, both transcripts and proteins of the two reporters were up-regulated upon heat stress. Four heat shock elements were noted in the intergenic region by in silico analysis. In the homozygous transfer DNA insertion mutant Salk_014505, 4,393-bp transfer DNA is inserted at position -517 upstream of ATG of the AtClpB-C gene. As a result, AtCk2 loses proximity to three of the four heat shock elements in the mutant line. Heat-inducible expression of the AtCK2 transcript was completely lost, whereas the expression of AtClpB-C was not affected in the mutant plants. Our results suggest that the 1,329-bp intergenic fragment functions as a heat-inducible bidirectional promoter and the region governing the heat inducibility is possibly shared between the two genes. We propose a model in which AtClpB-C shares its regulatory region with heat-induced choline kinase, which has a possible role in heat signaling.

Use of heat stress responsive gene expression levels for early selection of heat tolerant cabbage (Brassica oleracea L.)

Cabbage is a relatively robust vegetable at low temperatures. However, at high temperatures, cabbage has disadvantages, such as reduced disease tolerance and lower yields. Thus, selection of heat-tolerant cabbage is an important goal in cabbage breeding. Easier or faster selection of superior varieties of cabbage, which are tolerant to heat and disease and have improved taste and quality, can be achieved with molecular and biological methods. We compared heat-responsive gene expression between a heat-tolerant cabbage line (HTCL), "HO", and a heat-sensitive cabbage line (HSCL), "JK", by Genechip assay. Expression levels of specific heat stress-related genes were increased in response to high-temperature stress, according to Genechip assays. We performed quantitative RT-PCR (qRT-PCR) to compare expression levels of these heat stress-related genes in four HTCLs and four HSCLs. Transcript levels for heat shock protein BoHsp70 and transcription factor BoGRAS (SCL13) were more strongly expressed only in all HTCLs compared to all HSCLs, showing much lower level expressions at the young plant stage under heat stress (HS). Thus, we suggest that expression levels of these genes may be early selection markers for HTCLs in cabbage breeding. In addition, several genes that are involved in the secondary metabolite pathway were differentially regulated in HTCL and HSCL exposed to heat stress.

Dynamic changes in the leaf proteome of a C3 xerophyte, Citrullus lanatus (wild watermelon), in response to water deficit

Wild watermelon (Citrullus lanatus) is a xerophyte native to the Kalahari Desert, Africa. To better understand the molecular mechanisms of drought resistance in this plant, we examined changes in the proteome in response to water deficit. Wild watermelon leaves showed decreased transpiration and a concomitant increase in leaf temperature under water deficit conditions. Comparison of the proteome of stressed plants with that of unstressed plants by two-dimensional gel electrophoresis revealed that the intensity of 40 spots increased in response to the stress, and the intensity of 11 spots decreased. We positively identified 23 stress-induced and 6 stress-repressed proteins by mass spectrometry and database analyses. Interestingly, 15 out of the 23 up-regulated proteins (65% of annotated up-regulated proteins) were heat shock proteins (HSPs). Especially, 10 out of the 15 up-regulated HSPs belonged to the small heat shock protein (sHSP) family. Other stress-induced proteins included those related to antioxidative defense and carbohydrate metabolism. Fifteen distinct cDNA sequences encoding the sHSP were characterized from wild watermelon. Quantitative real-time PCR analysis of the representative sHSP genes revealed strong transcriptional up-regulation in the leaves under water deficit. Moreover, immunoblot analysis confirmed that protein abundance of sHSPs was massively increased under water deficit. Overall, these observations suggest that the defense response of wild watermelon may involve orchestrated regulation of a diverse array of functional proteins related to cellular defense and metabolism, of which HSPs may play a pivotal role on the protection of the plant under water deficit in the presence of strong light.

Proteomics analysis reveals post-translational mechanisms for cold-induced metabolic changes in Arabidopsis

Cold-induced changes of gene expression and metabolism are critical for plants to survive freezing. Largely by changing gene expression, exposure to a period of non-freezing low temperatures increases plant tolerance to freezing-a phenomenon known as cold acclimation. Cold also induces rapid metabolic changes, which provide instant protection before temperature drops below freezing point. The molecular mechanisms for such rapid metabolic responses to cold remain largely unknown. Here, we use two-dimensional difference gel electrophoresis (2-D DIGE) analysis of sub-cellular fractions of Arabidopsis thaliana proteome coupled with spot identification by tandem mass spectrometry to identify early cold-responsive proteins in Arabidopsis. These proteins include four enzymes involved in starch degradation, three HSP100 proteins, several proteins in the tricarboxylic acid cycle, and sucrose metabolism. Upon cold treatment, the Disproportionating Enzyme 2 (DPE2), a cytosolic transglucosidase metabolizing maltose to glucose, increased rapidly in the centrifugation pellet fraction and decreased in the soluble fraction. Consistent with cold-induced inactivation of DPE2 enzymatic activity, the dpe2 mutant showed increased freezing tolerance without affecting the C-repeat binding transcription factor (CBF) transcriptional pathway. These results support a model that cold-induced inactivation of DPE2 leads to rapid accumulation of maltose, which is a cold-induced compatible solute that protects cells from freezing damage. This study provides evidence for a key role of rapid post-translational regulation of carbohydrate metabolic enzymes in plant protection against sudden temperature drop.

Plant Hsp100/ClpB-like proteins: poorly-analyzed cousins of yeast ClpB machine

ClpB/Hsp100 proteins act as chaperones, mediating disaggregation of denatured proteins. Recent work shows that apart from cytoplasm, these proteins are localized to nuclei, chloroplasts, mitochondria and plasma membrane. While ClpB/Hsp100 genes are essentially stress-induced (mainly heat stress) in vegetative organs of the plant body, expression of ClpB/Hsp100 proteins is noted to be constitutive in plant reproductive structures like pollen grains, developing embryos, seeds etc. With global warming looming large on the horizon, ways to genetically engineer plants against high temperature stress are urgently needed. Yeast mutants unable to synthesize active ClpB/Hsp100 protein show a clear thermosensitive phenotype. ClpB/Hsp100 proteins are implicated in high temperature stress tolerance in plants. We herein highlight the selected important facets of this protein family in plants.

Genome-wide survey and expression profiling of heat shock proteins and heat shock factors revealed overlapped and stress specific response under abiotic stresses in rice

Heat shock proteins (Hsps) are molecular chaperons, which function in protein folding and assembly, protein intracellular localization and secretion, and degradation of misfolded and truncated proteins. Heat shock factors (Hsfs) are the transcriptional activators of Hsps. It has been reported that Hsps and Hsfs are widely involved in response to various abiotic stresses such as heat, drought, salinity and cold. To elucidate the function and regulation of rice Hsp and Hsf genes, we examined a global expression profiling with heat stressed rice seedling, and then compared our results with the previous rice data under cold, drought and salt stresses. The comparison revealed that, while most Hsfs and Hsps had highly similar and overlapped response and regulation patterns under different stresses, some of those genes showed significantly specific response to distinct stress. We also found that heat-responsive gene profiling differed largely from those under cold/drought/salt stresses, and that drought treatment was more effective to up-regulate Hsf expression in rice than in Arabidopsis. Overall, our data suggests that Hsps and Hsfs might be important elements in cross-talk of different stress signal transduction networks.

Proteomics applied on plant abiotic stresses: role of heat shock proteins (HSP)

The most crucial function of plant cell is to respond against stress induced for self-defence. This defence is brought about by alteration in the pattern of gene expression: qualitative and quantitative changes in proteins are the result, leading to modulation of certain metabolic and defensive pathways. Abiotic stresses usually cause protein dysfunction. They have an ability to alter the levels of a number of proteins which may be soluble or structural in nature. Nowadays, in higher plants high-throughput protein identification has been made possible along with improved protein extraction, purification protocols and the development of genomic sequence databases for peptide mass matches. Thus, recent proteome analysis performed in the vegetal Kingdom has provided new dimensions to assess the changes in protein types and their expression levels under abiotic stress. As reported in this review, specific and novel proteins, protein-protein interactions and post-translational modifications have been identified, which play a role in signal transduction, anti-oxidative defence, anti-freezing, heat shock, metal binding etc. However, beside specific proteins production, plants respond to various stresses in a similar manner by producing heat shock proteins (HSPs), indicating a similarity in the plant's adaptive mechanisms; in plants, more than in animals, HSPs protect cells against many stresses. A relationship between ROS and HSP also seems to exist, corroborating the hypothesis that during the course of evolution, plants were able to achieve a high degree of control over ROS toxicity and are now using ROS as signalling molecules to induce HSPs.

Genetic engineering for heat tolerance in plants

High temperature tolerance has been genetically engineered in plants mainly by over-expressing the heat shock protein genes or indirectly by altering levels of heat shock transcription factor proteins. Apart from heat shock proteins, thermotolerance has also been altered by elevating levels of osmolytes, increasing levels of cell detoxification enzymes and through altering membrane fluidity. It is suggested that Hsps may be directly implicated in thermotolerance as agents that minimize damage to cell proteins. The other three above approaches leading to thermotolerance in transgenic experiments though operate in their own specific ways but indirectly might be aiding in creation of more reductive and energy-rich cellular environment, thereby minimizing the accumulation of damaged proteins. Intervention in protein metabolism such that accumulation of damaged proteins is minimized thus appears to be the main target for genetically-engineering crops against high temperature stress.

Complexity of rice Hsp100 gene family: lessons from rice genome sequence data

Elucidation of genome sequence provides an excellent platform to understand detailed complexity of the various gene families. Hsp100 is an important family of chaperones in diverse living systems. There are eight putative gene loci encoding for Hsp100 proteins in Arabidopsis genome. In rice, two full-length Hsp100 cDNAs have been isolated and sequenced so far. Analysis of rice genomic sequence by in silico approach showed that two isolated rice Hsp100 cDNAs correspond to Os05g44340 and Os02g32520 genes in the rice genome database. There appears to be three additional proteins (encoded by Os03g31300, Os04g32560 and Os04g33210 gene loci) that are variably homologous to Os05g44340 and Os02g32520 throughout the entire amino acid sequence. The above five rice Hsp100 genes show significant similarities in the signature sequences known to be conserved among Hsp100 proteins. While Os05g44340 encodes cytoplasmic Hsp100 protein, those encoded by the other four genes are predicted to have chloroplast transit peptides.

Nuclear-mitochondrial cross-talk during heat shock in Arabidopsis cell culture

Apart from energy generation, mitochondria perform a signalling function determining the life and death of a cell under stress exposure. In the present study we have explored patterns of heat-induced synthesis of Hsp101, Hsp70, Hsp17.6 (class I), Hsp17.6 (class II) and Hsp60, and the development of induced thermotolerance in Arabidopsis thaliana cell culture under conditions of mitochondrial dysfunction. It was shown that treatment by mitochondrial inhibitors and uncouplers at the time of mild heat shock downregulates HSP synthesis, which is important for induced thermotolerance in plants. The exposure to elevated temperature induced an increase in cell oxygen consumption and hyperpolarization of the inner mitochondrial membrane. Taken together, these facts suggest that mitochondrial functions are essential for heat-induced HSP synthesis and development of induced thermotolerance in A. thaliana cell culture, suggesting that mitochondrial-nuclear cross-talk is activated under stress conditions. Treatment of Arabidopsis cell culture at 50 degrees C initiates a programmed cell death determined by the time course of viability decrease, DNA fragmentation and cytochrome c release from mitochondria. As treatment at 37 degrees C protected Arabidopsis cells from heat-induced cell death, it may be suggested that Hsp101, Hsp70 and small heat-shock proteins, the synthesis of which is induced under these conditions, are playing an anti-apoptotic role in the plant cell. On the other hand, drastic heat shock upregulated mitochondrial Hsp60 synthesis and induced its release from mitochondria to the cytosol, indicating a pro-apoptotic role of plant Hsp60.

Four members of the HSP101 gene family are differently regulated in Triticum durum Desf

Heat shock proteins play an essential role in preventing deleterious effects of high temperatures. In many plants, HSP101 has a central role in heat stress survival. We report the isolation and characterization of four cDNAs corresponding to different members of the durum wheat HSP101 gene family. Expression analysis revealed differences in their induction. Accordingly, durum wheat HSP101 genes are differently regulated, therefore having distinct roles in stress response and thermotolerance acquisition. These findings are important for further dissection of the molecular mechanisms underlying the stress response and for understanding the functions of the HSP101 family members. This information could be important for the exploitation of specific alleles in marker assisted selection for abiotic stress resistance.

Transcriptional profiling of Arabidopsis heat shock proteins and transcription factors reveals extensive overlap between heat and non-heat stress response pathways

BACKGROUND

The heat shock response of Arabidopsis thaliana is dependent upon a complex regulatory network involving twenty-one known transcription factors and four heat shock protein families. It is known that heat shock proteins (Hsps) and transcription factors (Hsfs) are involved in cellular response to various forms of stress besides heat. However, the role of Hsps and Hsfs under cold and non-thermal stress conditions is not well understood, and it is unclear which types of stress interact least and most strongly with Hsp and Hsf response pathways. To address this issue, we have analyzed transcriptional response profiles of Arabidopsis Hsfs and Hsps to a range of abiotic and biotic stress treatments (heat, cold, osmotic stress, salt, drought, genotoxic stress, ultraviolet light, oxidative stress, wounding, and pathogen infection) in both above and below-ground plant tissues.

RESULTS

All stress treatments interact with Hsf and Hsp response pathways to varying extents, suggesting considerable cross-talk between heat and non-heat stress regulatory networks. In general, Hsf and Hsp expression was strongly induced by heat, cold, salt, and osmotic stress, while other types of stress exhibited family or tissue-specific response patterns. With respect to the Hsp20 protein family, for instance, large expression responses occurred under all types of stress, with striking similarity among expression response profiles. Several genes belonging to the Hsp20, Hsp70 and Hsp100 families were specifically upregulated twelve hours after wounding in root tissue, and exhibited a parallel expression response pattern during recovery from heat stress. Among all Hsf and Hsp families, large expression responses occurred under ultraviolet-B light stress in aerial tissue (shoots) but not subterranean tissue (roots).

CONCLUSION

Our findings show that Hsf and Hsp family member genes represent an interaction point between multiple stress response pathways, and therefore warrant functional analysis under conditions apart from heat shock treatment. In addition, our analysis revealed several family and tissue-specific heat shock gene expression patterns that have not been previously described. These results have implications regarding the molecular basis of cross-tolerance in plant species, and raise new questions to be pursued in future experimental studies of the Arabidopsis heat shock response network.

Transcriptional profiling of Arabidopsis heat shock proteins and transcription factors reveals extensive overlap between heat and non-heat stress response pathways

Background

The heat shock response of Arabidopsis thaliana is dependent upon a complex regulatory network involving twenty-one known transcription factors and four heat shock protein families. It is known that heat shock proteins (Hsps) and transcription factors (Hsfs) are involved in cellular response to various forms of stress besides heat. However, the role of Hsps and Hsfs under cold and non-thermal stress conditions is not well understood, and it is unclear which types of stress interact least and most strongly with Hsp and Hsf response pathways. To address this issue, we have analyzed transcriptional response profiles of Arabidopsis Hsfs and Hsps to a range of abiotic and biotic stress treatments (heat, cold, osmotic stress, salt, drought, genotoxic stress, ultraviolet light, oxidative stress, wounding, and pathogen infection) in both above and below-ground plant tissues.

Results

All stress treatments interact with Hsf and Hsp response pathways to varying extents, suggesting considerable cross-talk between heat and non-heat stress regulatory networks. In general, Hsf and Hsp expression was strongly induced by heat, cold, salt, and osmotic stress, while other types of stress exhibited family or tissue-specific response patterns. With respect to the Hsp20 protein family, for instance, large expression responses occurred under all types of stress, with striking similarity among expression response profiles. Several genes belonging to the Hsp20, Hsp70 and Hsp100 families were specifically upregulated twelve hours after wounding in root tissue, and exhibited a parallel expression response pattern during recovery from heat stress. Among all Hsf and Hsp families, large expression responses occurred under ultraviolet-B light stress in aerial tissue (shoots) but not subterranean tissue (roots).

Conclusion

Our findings show that Hsf and Hsp family member genes represent an interaction point between multiple stress response pathways, and therefore warrant functional analysis under conditions apart from heat shock treatment. In addition, our analysis revealed several family and tissue-specific heat shock gene expression patterns that have not been previously described. These results have implications regarding the molecular basis of cross-tolerance in plant species, and raise new questions to be pursued in future experimental studies of the Arabidopsis heat shock response network.

Expression and variability of molecular chaperones in the sugarcane expressome

Molecular chaperones perform folding assistance in newly synthesized polypeptides preventing aggregation processes, recovering proteins from aggregates, among other important cellular functions. Thus their study presents great biotechnological importance. The present work discusses the mining for chaperone-related sequences within the sugarcane EST genome project database, which resulted in approximately 300 different sequences. Since molecular chaperones are highly conserved in most organisms studied so far, the number of sequences related to these proteins in sugarcane was very similar to the number found in the Arabidopsis thaliana genome. The Hsp70 family was the main molecular chaperone system present in the sugarcane expressome. However, many other relevant molecular chaperones systems were also present. A digital RNA blot analysis showed that 5'ESTs from all molecular chaperones were found in every sugarcane library, despite their heterogeneous expression profiles. The results presented here suggest the importance of molecular chaperones to polypeptide metabolism in sugarcane cells, based on their abundance and variability. Finally, these data have being used to guide more in deep analysis, permitting the choice of specific targets to study.

The Arabidopsis ClpB/Hsp100 family of proteins: chaperones for stress and chloroplast development

The Casein lytic proteinase/heat shock protein 100 (Clp/Hsp100) proteins are chaperones that act to remodel/disassemble protein complexes and/or aggregates using the energy of ATP. In plants, one of the best-studied proteins from this family is cytosolic ClpB1 (At1g74310), better known in Arabidopsis as AtHsp101, which is a heat shock protein required for acclimation to high temperatures. Three other ClpB homologues have been identified in the Arabidopsis genome (ClpB2, ClpB3 and ClpB4; At4g14670, At5g15450 and At2g25140). To define further the roles of these chaperones in plants we investigated their intracellular localization, evolutionary relationships, patterns of expression and the phenotypes of corresponding T-DNA insertion mutants. We first found that ClpB2 was misannotated; there is no functional ClpB/Hsp100 gene at this locus. By fusing the putative transit peptides of ClpB3 and ClpB4 with GFP, we showed that these proteins are targeted to the chloroplast and mitochondrion, respectively, and we therefore designated them as ClpB-p and ClpB-m. Phylogenetic analysis supports two major lineages of ClpB proteins in plants, an 'eukaryotic', cytosol/nuclear-localized group containing AtHsp101, and an organelle-localized lineage, containing both ClpB-p and ClpB-m. Although AtHsp101, ClpB-p and ClpB-m transcripts all accumulate dramatically at high temperatures, the T-DNA insertion mutants of ClpB-p and ClpB-m show no evidence of seedling heat stress phenotypes similar to those observed in AtHsp101 mutants. Strikingly, ClpB-p knockouts were seedling lethals, failing to accumulate chlorophyll or properly develop chloroplasts. Thus, in plants, the function of ClpB/Hsp100 proteins is not restricted to heat stress, but a specific member of the family provides housekeeping functions that are essential to chloroplast development.

The involvement of chloroplast HSP100/ClpB in the acquired thermotolerance in tomato

The chloroplast HSP100/ClpB is a newly documented member of the ClpB family, but little was known about its role in imparting thermotolerance to cells. A cDNA coding for a HSP100/ClpB homolog has been cloned from Lycopersicon esculentum and termed as Lehsp100/ClpB (the cDNA sequence of Lehsp100/ClpB has been submitted to the GenBank database under accession number: AB219939). The protein encoded by the cDNA was most similar to the putative chloroplast HSP100/ClpBs in higher plants and the ClpB from Cyanobacterium Synechococcus sp. A 97 kDa protein, which matched the predicted size of mature LeHSP100/ClpB, was immunologically detected in chloroplast isolated from heat-treated tomato plants. In addition, the fusion protein, combining the transit sequence of LeHSP100/ClpB and GFP, was found to be located in chloroplast based on the observations of fluorescent microscope images. These results indicated the chloroplast-localization of LeHSP100/ClpB. Both the transcript and the protein of Lehsp100/ClpB were not detected under normal growth conditions, but they were induced by increasingly higher temperatures. An antisense Lehsp100/ClpB cDNA fragment was introduced into the tomato by Agrobacterium-mediated transformation. Antisense lines exhibited an extreme repression of heat-induced expression of Lehsp100/ClpB. The levels of chloroplast HSP60 and small HSP in antisense lines were identical to those of the control plants. After plants preconditioned at 38 degrees C for 2 h were exposed to a lethal heat shock at 46 degrees C for 2 h, the antisense lines were greatly impaired and withered in 21 days of the recovery phase, whereas the untransformed control plants and the vector-transformed plants survived. Furthermore, chlorophyll fluorescence measurements showed that PS II in antisense lines were more susceptible to the thermal irreversible inactivation than the untransformed and vector-transformed control plants. This work provides the first example that induction of chloroplast LeHSP100/ClpB contributes to the acquisition of thermotolerance in higher plants.

An Arabidopsis chloroplast-targeted Hsp101 homologue, APG6, has an essential role in chloroplast development as well as heat-stress response

Analysis of albino or pale-green (apg) mutants is important for identifying nuclear genes responsible for chloroplast development and pigment synthesis. We have identified 38 apg mutants by screening 11 000 Arabidopsis Ds-tagged lines. One mutant, apg6, contains a Ds insertion in a gene encoding APG6 (ClpB3), a homologue of the heat-shock protein Hsp101 (ClpB1). We isolated somatic revertants and identified two Ds-tagged and one T-DNA-tagged mutant alleles of apg6. All three alleles gave the same pale-green phenotype. These results suggest that APG6 is important for chloroplast development. The APG6 protein contains a transit peptide and is localized in chloroplasts. The plastids of apg6 pale-green cells were smaller than those of the wild type, and contained undeveloped thylakoid membranes. APG6 mRNA accumulated in response to heat shock in various organs, but not in response to other abiotic stresses. Under normal conditions, APG6 is constitutively expressed in the root tips, the organ boundary region, the reproductive tissues of mature plants where plastids exist as proplastids, and slightly in the stems and leaves. In addition, constitutive overexpression of APG6 in transgenic plants inhibited chloroplast development and resulted in a mild pale-green phenotype. The amounts of chloroplast proteins related to photosynthesis were markedly decreased in apg6 mutants. These results suggest that APG6 functions as a molecular chaperone involved in plastid differentiation mediating internal thylakoid membrane formation and conferring thermotolerance to chloroplasts during heat stress. The APG6 protein is not only involved in heat-stress response in chloroplasts, but is also essential for chloroplast development.

Tobamovirus infection is independent of HSP101 mRNA induction and protein expression

Heat shock protein 101 (HSP101) has been implicated in tobamovirus infections by virtue of its ability to enhance translation of mRNAs possessing the 5'Omega-leader of Tobacco mosaic virus (TMV). Enhanced translation is mediated by HSP101 binding to a CAA-repeat motif in TMV Omega leader. CAA repeat sequences are present in the 5' leaders of other tobamoviruses including Oilseed rape mosaic virus (ORMV), which infects Arabidopsis thaliana. HSP101 is one of eight HSP100 gene family members encoded by the A. thaliana genome, and of these, HSP101 and HSP98.7 are predicted to encode proteins localized to the cytoplasm where they could potentially interact with TMV RNA. Analysis of the expression of the HSP100s showed that only HSP101 mRNA transcripts were induced significantly by ORMV in A. thaliana. The induction of HSP101 mRNA was also correlated with an increase in its protein levels and was independent of defense-related signaling pathways involving salicylic acid, jasmonic acid, or ethylene. A. thaliana mutants lacking HSP101, HSP98.7, or both supported wild-type levels of ORMV replication and movement. Similar results were obtained for TMV infection in Nicotiana benthamiana plants silenced for HSP101, demonstrating that HSP101 is not necessary for efficient tobamovirus infection.

Arabidopsis Hsa32, a novel heat shock protein, is essential for acquired thermotolerance during long recovery after acclimation

Plants and animals share similar mechanisms in the heat shock (HS) response, such as synthesis of the conserved HS proteins (Hsps). However, because plants are confined to a growing environment, in general they require unique features to cope with heat stress. Here, we report on the analysis of the function of a novel Hsp, heat-stress-associated 32-kD protein (Hsa32), which is highly conserved in land plants but absent in most other organisms. The gene responds to HS at the transcriptional level in moss (Physcomitrella patens), Arabidopsis (Arabidopsis thaliana), and rice (Oryza sativa). Like other Hsps, Hsa32 protein accumulates greatly in Arabidopsis seedlings after HS treatment. Disruption of Hsa32 by T-DNA insertion does not affect growth and development under normal conditions. However, the acquired thermotolerance in the knockout line was compromised following a long recovery period (>24 h) after acclimation HS treatment, when a severe HS challenge killed the mutant but not the wild-type plants, but no significant difference was observed if they were challenged within a short recovery period. Quantitative hypocotyl elongation assay also revealed that thermotolerance decayed faster in the absence of Hsa32 after a long recovery. Similar results were obtained in Arabidopsis transgenic plants with Hsa32 expression suppressed by RNA interference. Microarray analysis of the knockout mutant indicates that only the expression of Hsa32 was significantly altered in HS response. Taken together, our results suggest that Hsa32 is required not for induction but rather maintenance of acquired thermotolerance, a feature that could be important to plants.

Arabidopsis Hsa32, a novel heat shock protein, is essential for acquired thermotolerance during long recovery after acclimation

Plants and animals share similar mechanisms in the heat shock (HS) response, such as synthesis of the conserved HS proteins (Hsps). However, because plants are confined to a growing environment, in general they require unique features to cope with heat stress. Here, we report on the analysis of the function of a novel Hsp, heat-stress-associated 32-kD protein (Hsa32), which is highly conserved in land plants but absent in most other organisms. The gene responds to HS at the transcriptional level in moss (Physcomitrella patens), Arabidopsis (Arabidopsis thaliana), and rice (Oryza sativa). Like other Hsps, Hsa32 protein accumulates greatly in Arabidopsis seedlings after HS treatment. Disruption of Hsa32 by T-DNA insertion does not affect growth and development under normal conditions. However, the acquired thermotolerance in the knockout line was compromised following a long recovery period (>24 h) after acclimation HS treatment, when a severe HS challenge killed the mutant but not the wild-type plants, but no significant difference was observed if they were challenged within a short recovery period. Quantitative hypocotyl elongation assay also revealed that thermotolerance decayed faster in the absence of Hsa32 after a long recovery. Similar results were obtained in Arabidopsis transgenic plants with Hsa32 expression suppressed by RNA interference. Microarray analysis of the knockout mutant indicates that only the expression of Hsa32 was significantly altered in HS response. Taken together, our results suggest that Hsa32 is required not for induction but rather maintenance of acquired thermotolerance, a feature that could be important to plants.

The heat stress transcription factor HsfA2 serves as a regulatory amplifier of a subset of genes in the heat stress response in Arabidopsis

Within the Arabidopsis family of 21 heat stress transcription factors (Hsfs) HsfA2 is the strongest expressed member under heat stress (hs) conditions. Irrespective of the tissue, HsfA2 accumulates under heat stress similarly to other heat stress proteins (Hsps). A SALK T-DNA insertion line with a complete HsfA2-knockout was analyzed with respect to the changes in the transcriptome under heat stress conditions. Ascorbate peroxidase 2 (APX2) was identified as the most affected transcript in addition to several sHsps, individual members of the Hsp70 and Hsp100 family, as well as many transcripts of genes with yet unknown functions. For functional validation, the transcription activation potential of HsfA2 on GUS reporter constructs containing 1 kb upstream promoter sequences of selected target genes were analyzed using transient reporter assays in mesophyll protoplasts. By deletion analysis the promoter region of the strongest affected target gene APX2 was functionally mapped in detail to verify potential HsfA2 binding sites. By electrophoretic mobility shift assays we identified TATA-Box proximal clusters of heat stress elements (HSE) in the promoters of selected target genes as potential HsfA2 binding sites. The results presented here demonstrate that the expression of HsfA2 in Arabidopsis is strictly heat stress-dependent and this transcription factor represents a regulator of a subset of stress response genes in Arabidopsis.

The Chlamydomonas genome reveals its secrets: chaperone genes and the potential roles of their gene products in the chloroplast

The first draft of the Chlamydomonas nuclear genome was searched for genes potentially encoding members of the five major chaperone families, Hsp100/Clp, Hsp90, Hsp70, Hsp60, the small heat shock proteins, and the Hsp70 and Cpn60 co-chaperones GrpE and Cpn10/20, respectively. This search yielded 34 potential (co-)chaperone genes, among them those 8 that have been reported earlier inChlamydomonas. These 34 genes encode all the (co-)chaperones that have been expected for the different compartments and organelles from genome searches in Arabidopsis, where 74 genes have been described to encode basically the same set of (co-)chaperones. Genome data from Arabidopsis and Chlamydomonas on the five major chaperone families are compared and discussed, with particular emphasis on chloroplast chaperones.

Molecular characterization of rice hsp101: complementation of yeast hsp104 mutation by disaggregation of protein granules and differential expression in indica and japonica rice types

HSP100 protein is an important component of the heat-shock response in diverse organisms. Using specific primers based on cDNA sequence, rice hsp101 gene was PCR-amplified and sequenced. Southern analysis revealed that there appears to be a single gene per haploid genome coding for HSP101 protein in rice. Northern analysis showed that expression of hsp101 transcript is strictly heat-inducible and induction is transient in nature. In the temperature regime tested, 45 degrees C treatment to intact rice seedlings for 2 h showed maximal levels of hsp101 mRNA. Rice full-length hsp101 cDNA complemented yeast mutant disrupted for its own hsp104 gene by insertional mutagenesis, with efficacy that was comparable with Arabidopsis hsp101 cDNA. Electron micrographic evidence suggested that rice hsp101 cDNA in yeast is active in re-solubilizing the stress-induced protein granules in the post-stress recovery period. Rice hsp101 cDNA expression in hsp104 deficient yeast also caused recovery in tolerance against arsenite. Western analyses showed that this protein is expressed more rapidly during the stress period and retained for longer duration in the post-stress recovery period in japonica rice as compared to indica rice types. This is the first report wherein plant HSP100 protein expression is correlated to disappearance of protein granules in the yeast cells and distinct rice type-dependent protein expression patterns are reported.