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

A seed preferential heat shock transcription factor from wheat provides abiotic stress tolerance and yield enhancement in transgenic Arabidopsis under heat stress environment

Reduction in crop yield and quality due to various abiotic stresses is a worldwide phenomenon. In the present investigation, a heat shock factor (HSF) gene expressing preferentially in developing seed tissues of wheat grown under high temperatures was cloned. This newly identified heat shock factor possesses the characteristic domains of class A type plant HSFs and shows high similarity to rice OsHsfA2d, hence named as TaHsfA2d. The transcription factor activity of TaHsfA2d was confirmed through transactivation assay in yeast. Transgenic Arabidopsis plants overexpressing TaHsfA2d not only possess higher tolerance towards high temperature but also showed considerable tolerance to salinity and drought stresses, they also showed higher yield and biomass accumulation under constant heat stress conditions. Analysis of putative target genes of AtHSFA2 through quantitative RT-PCR showed higher and constitutive expression of several abiotic stress responsive genes in transgenic Arabidopsis plants over-expressing TaHsfA2d. Under stress conditions, TaHsfA2d can also functionally complement the T-DNA insertion mutants of AtHsfA2, although partially. These observations suggest that TaHsfA2d may be useful in molecular breeding of crop plants, especially wheat, to improve yield under abiotic stress conditions.

Heat shock factor OsHsfB2b negatively regulates drought and salt tolerance in rice

KEY MESSAGE

Expression of OsHsfB2b was strongly induced by heat, salt, ABA and PEG treatments. Drought and salt tolerances were significantly decreased by OsHsfB2b overexpression, but were enhanced by RNA interference.

ABSTRACT

Plants have more than 20 heat shock factors (Hsfs) that were designated class A, B, and C. Many members of Class A Hsfs were characterized as activators of transcription, but the functional roles of class B and C Hsfs have not been fully recognized. OsHsfB2b is a member of class B Hsfs in rice (Oryza sativa). Expression of OsHsfB2b was strongly induced by heat, salt, abscisic acid (ABA) and polyethylene glycol (PEG) treatments but was almost not affected by cold stress. Drought and salt tolerances were significantly decreased in OsHsfB2b-overexpressing transgenic rice, but were enhanced in the OsHsfB2b-RNAi transgenic rice. Under drought stress, the OsHsfB2b-overexpressing transgenic rice exhibited increased relative electrical conductivity (REC) and content of malondialdehyde (MDA) and decreased proline content compared with the wild type, while the lower REC and MDA content and increased proline content were found in the OsHsfB2b-RNAi transgenic rice. These results suggest that OsHsfB2b functions as a negative regulator in response to drought and salt stresses in rice, with its existing B3 repression domain (BRD) that might be necessary for the repressive activity. The present study revealed the potential value of OsHsfB2b in genetic improvement of rice.

Mutations in HISTONE ACETYLTRANSFERASE1 affect sugar response and gene expression in Arabidopsis

Nutrient response networks are likely to have been among the first response networks to evolve, as the ability to sense and respond to the levels of available nutrients is critical for all organisms. Although several forward genetic screens have been successful in identifying components of plant sugar-response networks, many components remain to be identified. Toward this end, a reverse genetic screen was conducted in Arabidopsis thaliana to identify additional components of sugar-response networks. This screen was based on the rationale that some of the genes involved in sugar-response networks are likely to be themselves sugar regulated at the steady-state mRNA level and to encode proteins with activities commonly associated with response networks. This rationale was validated by the identification of hac1 mutants that are defective in sugar response. HAC1 encodes a histone acetyltransferase. Histone acetyltransferases increase transcription of specific genes by acetylating histones associated with those genes. Mutations in HAC1 also cause reduced fertility, a moderate degree of resistance to paclobutrazol and altered transcript levels of specific genes. Previous research has shown that hac1 mutants exhibit delayed flowering. The sugar-response and fertility defects of hac1 mutants may be partially explained by decreased expression of AtPV42a and AtPV42b, which are putative components of plant SnRK1 complexes. SnRK1 complexes have been shown to function as central regulators of plant nutrient and energy status. Involvement of a histone acetyltransferase in sugar response provides a possible mechanism whereby nutritional status could exert long-term effects on plant development and metabolism.

A KH domain-containing putative RNA-binding protein is critical for heat stress-responsive gene regulation and thermotolerance in Arabidopsis

Heat stress is a severe environmental factor that significantly reduces plant growth and delays development. Heat stress factors (HSFs) are a class of transcription factors that are synthesized rapidly in response to elevations in temperature and are responsible for the transcription of many heat stress-responsive genes including those encoding heat shock proteins (HSPs). There are 21 HSFs in Arabidopsis, and recent studies have established that the HSFA1 family members are master regulators for the remaining HSFs. However, very little is known about upstream molecular factors that control the expression of HSFA1 genes and other HSF genes under heat stress. Through a forward genetic analysis, we identified RCF3, a K homology (KH) domain-containing nuclear-localized putative RNA-binding protein. RCF3 is a negative regulator of most HSFs, including HSFA1a, HSFA1b, and HSFA1d. In contrast, RCF3 positively controls the expression of HSFA1e, HSFA3, HSFA9, HSFB3, and DREB2C. Consistently with the overall increased accumulation of heat-responsive genes, the rcf3 mutant plants are more tolerant than the wild-type to heat stress. Together, our results suggest that a KH domain-containing putative RNA-binding protein RCF3 is an important upstream regulator for heat stress-responsive gene expression and thermotolerance in Arabidopsis.

Arabidopsis heat shock factor HsfA1a directly senses heat stress, pH changes, and hydrogen peroxide via the engagement of redox state

Arabidopsis heat shock factor HsfA1a is present in a latent, monomeric state under normal conditions; its activation involves heat stress-induced trimerization, binding to heat shock element in target promoters, and the acquisition of transcriptional competence. HsfA1a is an important regulator for heat stress-induced gene expression and thermotolerance. However, it is not clear whether HsfA1a is directly activated by stress and the mechanisms of the stress signaling are poorly understood. We analyzed HsfA1a activation by trimerization and DNA-binding assays in vitro and in vivo in response to heat stress, low/high pH, and hydrogen peroxide treatments. Our results show that purified recombinant HsfA1a was activated by these stress treatments in vitro. The same treatments also induced the binding to HSP18.2 and HSP70 promoters as examined by chromatin immunoprecipitation, and the HsfA1a DNA binding paralleled the mRNA expression of its target genes induced by different stresses. Stress-induced DNA-binding could be reversed, both in vitro and in vivo, by subsequent incubation with reducing agents (DTT, NADPH). These data suggest that HsfA1a can directly sense stress and become activated, and this process is dependent on the redox state. An N-terminal deletion of the amino acid residues from 48 to 74 negatively affected pH- and hydrogen peroxide-, but not heat-stress sensing.

Source-sink dynamics and proteomic reprogramming under elevated night temperature and their impact on rice yield and grain quality

High night temperatures (HNTs) can reduce significantly the global rice (Oryza sativa) yield and quality. A systematic analysis of HNT response at the physiological and molecular levels was performed under field conditions. Contrasting rice accessions, N22 (highly tolerant) and Gharib (susceptible), were evaluated at 22°C (control) and 28°C (HNT). Nitrogen (N) and nonstructural carbohydrate (NSC) translocation from different plant tissues into grains at key developmental stages, and their contribution to yield, grain-filling dynamics and quality aspects, were evaluated. Proteomic profiling of flag leaf and spikelets at 100% flowering and 12 d after flowering was conducted, and their reprogramming patterns were explored. Grain yield reduction in susceptible Gharib was traced back to the significant reduction in N and NSC translocation after flowering, resulting in reduced maximum and mean grain-filling rate, grain weight and grain quality. A combined increase in heat shock proteins (HSPs), Ca signaling proteins and efficient protein modification and repair mechanisms (particularly at the early grain-filling stage) enhanced N22 tolerance for HNT. The increased rate of grain filling and efficient proteomic protection, fueled by better assimilate translocation, overcome HNT tolerance in rice. Temporal and spatial proteome programming alters dynamically between key developmental stages and guides future transgenic and molecular analysis targeted towards crop improvement.

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

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

Transient expression assays in tobacco protoplasts

The sequence information generated through genome and transcriptome analysis from plant tissues has reached unprecedented sizes. Sequence homology-based annotations may provide hints for the possible function and roles of particular plant genes, but the functional annotation remains nonexistent or incomplete for many of them. To discover gene functions, transient expression assays are a valuable tool because they can be done more rapidly and at a higher scale than generating stably transformed tissues. Here, we describe a transient expression assay in protoplasts derived from suspension cells of tobacco (Nicotiana tabacum) for the study of the transactivation capacities of transcription factors. To enhance throughput and reproducibility, this method can be automated, allowing medium-throughput screening of interactions between large compendia of potential transcription factors and gene promoters.

Heat shock transcriptional factors in Malus domestica: identification, classification and expression analysis

Background

Heat shock transcriptional factors (Hsfs) play a crucial role in plant responses to biotic and abiotic stress conditions and in plant growth and development. Apple (Malus domestica Borkh) is an economically important fruit tree whose genome has been fully sequenced. So far, no detailed characterization of the Hsf gene family is available for this crop plant.

Results

A genome-wide analysis was carried out in Malus domestica to identify heat shock transcriptional factor (Hsf) genes, named MdHsfs. Twenty five MdHsfs were identified and classified in three main groups (class A, B and C) according to the structural characteristics and to the phylogenetic comparison with Arabidopsis thaliana and Populus trichocarpa. Chromosomal duplications were analyzed and segmental duplications were shown to have occurred more frequently in the expansion of Hsf genes in the apple genome. Furthermore, MdHsfs transcripts were detected in several apple organs, and expression changes were observed by quantitative real-time PCR (qRT-PCR) analysis in developing flowers and fruits as well as in leaves, harvested from trees grown in the field and exposed to the naturally increased temperatures.

Conclusions

The apple genome comprises 25 full length Hsf genes. The data obtained from this investigation contribute to a better understanding of the complexity of the Hsf gene family in apple, and provide the basis for further studies to dissect Hsf function during development as well as in response to environmental stimuli.

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.

Histone acetyltransferases in rice (Oryza sativa L.): phylogenetic analysis, subcellular localization and expression

BACKGROUND

Histone acetyltransferases (HATs) play an important role in eukaryotic transcription. Eight HATs identified in rice (OsHATs) can be organized into four families, namely the CBP (OsHAC701, OsHAC703, and OsHAC704), TAFII250 (OsHAF701), GNAT (OsHAG702, OsHAG703, and OsHAG704), and MYST (OsHAM701) families. The biological functions of HATs in rice remain unknown, so a comprehensive protein sequence analysis of the HAT families was conducted to investigate their potential functions. In addition, the subcellular localization and expression patterns of the eight OsHATs were analyzed.

RESULTS

On the basis of a phylogenetic and domain analysis, monocotyledonous CBP family proteins can be subdivided into two groups, namely Group I and Group II. Similarly, dicotyledonous CBP family proteins can be divided into two groups, namely Group A and Group B. High similarities of protein sequences, conserved domains and three-dimensional models were identified among OsHATs and their homologs in Arabidopsis thaliana and maize. Subcellular localization predictions indicated that all OsHATs might localize in both the nucleus and cytosol. Transient expression in Arabidopsis protoplasts confirmed the nuclear and cytosolic localization of OsHAC701, OsHAG702, and OsHAG704. Real-time quantitative polymerase chain reaction analysis demonstrated that the eight OsHATs were expressed in all tissues examined with significant differences in transcript abundance, and their expression was modulated by abscisic acid and salicylic acid as well as abiotic factors such as salt, cold, and heat stresses.

CONCLUSIONS

Both monocotyledonous and dicotyledonous CBP family proteins can be divided into two distinct groups, which suggest the possibility of functional diversification. The high similarities of protein sequences, conserved domains and three-dimensional models among OsHATs and their homologs in Arabidopsis and maize suggested that OsHATs have multiple functions. OsHAC701, OsHAG702, and OsHAG704 were localized in both the nucleus and cytosol in transient expression analyses with Arabidopsis protoplasts. OsHATs were expressed constitutively in rice, and their expression was regulated by exogenous hormones and abiotic stresses, which suggested that OsHATs may play important roles in plant defense responses.

Molecular communications between plant heat shock responses and disease resistance

As sessile, plants are continuously exposed to potential dangers including various abiotic stresses and pathogen attack. Although most studies focus on plant responses under an ideal condition to a specific stimulus, plants in nature must cope with a variety of stimuli at the same time. This indicates that it is critical for plants to fine-control distinct signaling pathways temporally and spatially for simultaneous and effective responses to various stresses. Global warming is currently a big issue threatening the future of humans. Reponses to high temperature affect many physiological processes in plants including growth and disease resistance, resulting in decrease of crop yield. Although plant heat stress and defense responses share important mediators such as calcium ions and heat shock proteins, it is thought that high temperature generally suppresses plant immunity. We therefore specifically discuss on interactions between plant heat and defense responses in this review hopefully for an integrated understanding of these responses in plants.

Arabidopsis HsfB1 and HsfB2b act as repressors of the expression of heat-inducible Hsfs but positively regulate the acquired thermotolerance

Many eukaryotes have from one to three heat shock factors (Hsfs), but plants have more than 20 Hsfs, designated class A, B, and C. Class A Hsfs are activators of transcription, but details of the roles of individual Hsfs have not been fully characterized. We show here that Arabidopsis (Arabidopsis thaliana) HsfB1 and HsfB2b, members of class B, are transcriptional repressors and negatively regulate the expression of heat-inducible Hsfs (HsfA2, HsfA7a, HsfB1, and HsfB2b) and several heat shock protein genes. In hsfb1 hsfb2b double mutant plants, the expression of a large number of heat-inducible genes was enhanced in the non-heat condition (23°C) and the plants exhibited slightly higher heat tolerance at 42°C than the wild type, similar to Pro35S:HsfA2 plants. In addition, under extended heat stress conditions, expression of the heat-inducible Hsf genes remained consistently higher in hsfb1 hsfb2b than in the wild type. These data indicate that HsfB1 and HsfB2b suppress the general heat shock response under non-heat-stress conditions and in the attenuating period. On the other hand, HsfB1 and HsfB2b appear to be necessary for the expression of heat stress-inducible heat shock protein genes under heat stress conditions, which is necessary for acquired thermotolerance. We show that the heat stress response is finely regulated by activation and repression activities of Hsfs in Arabidopsis.

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.

Heat shock factors in rice (Oryza sativa L.): genome-wide expression analysis during reproductive development and abiotic stress

Plants respond to heat stress by enhancing the expression of genes encoding heat shock protein (HSPs) genes through activation of heat shock factors (HSFs) which interact with heat shock elements present in the promoter of HSP genes. Plant HSFs have been divided into three conserved classes viz A, B and C. In the present study, a detailed analysis has been done of all rice HSFs, along with their spliced variants. Their chromosomal localization reveals that six HSFs are segmentally duplicated and four pairs of these segmentally duplicated HSF encoding genes show pseudo-functionalization. Expression profiling through microarray and quantitative real-time PCR showed that eight OsHsfs express at a higher level during seed development, while six HSFs are up-regulated in all the abiotic stresses studied. The expression of OsHsfA2a gene in particular was greatly stimulated by heat stress in both root and shoot tissues and also during panicle and seed development. OsHsfA3 was found more responsive to cold and drought stress, while OsHsfA7 and OsHsfA9 showed developing seed-specific expression. This study also revealed that spliced variants generally accumulated at a higher level in all the tissues examined. Different hormones/elicitors like ABA, brassinosteroids and salicylic acid also alter OsHsf gene expression. Calcium in combination with heat stress elevated further the level of HSF transcripts. Expression analysis by both microarray and real-time RT-PCR revealed a unique stable constitutive expression of OsHsfA1 across all the tissues and stresses. A detailed in silico analysis involving identification of unidentified domains has been done by MEME-motif tool in their full-length proteins as well as in DNA-binding domains. Analysis of 1 kb putative promoter region revealed presence of tissue-specific, abiotic stress and hormone-related cis-acting elements, correlating with expression under stress conditions.

The role of class A1 heat shock factors (HSFA1s) in response to heat and other stresses in Arabidopsis

In Arabidopsis, there are four homologs of class A1 heat shock factor (HSFA1) genes, which likely encode the master regulators of heat shock response (HSR). However, previous studies with double knockout (KO) mutants were unable to confirm this point probably due to functional redundancy. Here, we generated a quadruple KO (QK) and four triple KO mutants to dissect their functions. Our data show that members of the HSFA1 group not only play a pivotal role in HSR but also are involved in growth and development. Alterations in morphology and retardation in growth were observed in the quadruple but not in triple KO mutants. The basal and acquired thermotolerance capacity was dramatically decreased in the QK mutant but varied in triple KO mutants at different developmental stages. The transcriptomics profiles suggested that more than 65% of the heat stress (HS)-up-regulated genes were HSFA1 dependent. HSFA1s were also involved in the expression of several HS genes induced by H(2) O(2) , salt and mannitol, which is consistent with the increased sensitive phenotype of the QK mutant to the stress factors. In conclusion, the Arabidopsis HSFA1s function as the master regulators of HSR and participate as important components in other abiotic stress responses as well.

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.

Cloning and characterization of HsfA2 from Lily (Lilium longiflorum)

Heat shock transcription factors (Hsfs) are the terminal components of the signal transduction chain mediating the activation of genes responsive to both heat stress and a large number of chemical stressors. This paper aims to clone Hsf from lily and characterize its function by analyses of mRNA expression, transactivation activity and thermotolerance of transgenic Arabidopsis. In this study, the gene encoding HsfA2 with 1,053 bp open reading frame (ORF) was cloned by rapid amplification of cDNA ends (RACE) technique from Lilium longiflorum 'White heaven'. Multiple alignment and phylogenetic analyses showed that the deduced protein was a novel member of the Hsf class A2. Expression analyses by RT-PCR indicated that LlHsfA2 expression was induced by heat shock and H(2)O(2) treatment, but not by NaCl. It was also found that the expression of LlHsfA2 correlated with thermotolerance in Lilium longiflorum 'White heaven' and Oriental hybrid 'Acapulco' under heat stress. Furthermore, yeast one-hybrid assay showed that LlHsfA2 had transactivation activity. In addition, overexpression of LlHsfA2 activated the downstream genes including Hsp101, Hsp70, Hsp25.3 and Apx2 and enhanced the thermotolerance of transgenic Arabidopsis plants. Taken together, our data suggest that LlHsfA2 is a novel and functional HsfA2, involved in heat signaling pathway in lily and useful for improvement of thermotolerance in transgenic plants.

Promoter specificity and interactions between early and late Arabidopsis heat shock factors

The class A heat shock factors HsfA1a and HsfA1b are highly conserved, interacting regulators, responsible for the immediate-early transcription of a subset of heat shock genes in Arabidopsis. In order to determine functional cooperation between them, we used a reporter assay based on transient over-expression in Arabidopsis protoplasts. Reporter plasmids containing promoters of Hsf target genes fused with the GFP coding region were co-transformed with Hsf effector plasmids. The GFP reporter gene activity was quantified using flow cytometry. Three of the tested target gene promoters (Hsp25.3, Hsp18.1-CI, Hsp26.5) resulted in a strong reporter gene activity, with HsfA1a or HsfA1b alone, and significantly enhanced GFP fluorescence when both effectors were co-transformed. A second set of heat shock promoters (HsfA2, Hsp17.6CII, Hsp17.6C-CI) was activated to much lower levels. These data suggest that HsfA1a/1b cooperate synergistically at a number of target gene promoters. These targets are also regulated via the late HsfA2, which is the most strongly heat-induced class A-Hsf in Arabidopsis. HsfA2 has also the capacity to interact with HsfA1a and HsfA1b as determined by bimolecular fluorescence complementation (BiFC) in Arabidopsis protoplasts and yeast-two-hybrid assay. However, there was no synergistic effect on Hsp18.1-CI promoter-GFP reporter gene expression when HsfA2 was co-expressed with either HsfA1a or HsfA1b. These data provide evidence that interaction between early and late HSF is possible, but only interaction between the early Hsfs results in a synergistic enhancement of expression of certain target genes. The interaction of HsfA1a/A1b with the major-late HsfA2 may possibly support recruitment of HsfA2 and replacement of HsfA1a/A1b at the same target gene promoters.

Epigenetic control of plant immunity

In eukaryotic genomes, gene expression and DNA recombination are affected by structural chromatin traits. Chromatin structure is shaped by the activity of enzymes that either introduce covalent modifications in DNA and histone proteins or use energy from ATP to disrupt histone-DNA interactions. The genomic 'marks' that are generated by covalent modifications of histones and DNA, or by the deposition of histone variants, are susceptible to being altered in response to stress. Recent evidence has suggested that proteins generating these epigenetic marks play crucial roles in the defence against pathogens. Histone deacetylases are involved in the activation of jasmonic acid- and ethylene-sensitive defence mechanisms. ATP-dependent chromatin remodellers mediate the constitutive repression of the salicylic acid-dependent pathway, whereas histone methylation at the WRKY70 gene promoter affects the activation of this pathway. Interestingly, bacterial-infected tissues show a net reduction in DNA methylation, which may affect the disease resistance genes responsible for the surveillance against pathogens. As some epigenetic marks can be erased or maintained and transmitted to offspring, epigenetic mechanisms may provide plasticity for the dynamic control of emerging pathogens without the generation of genomic lesions.

Developmental and heat stress-regulated expression of HsfA2 and small heat shock proteins in tomato anthers

The high sensitivity of male reproductive cells to high temperatures may be due to an inadequate heat stress response. The results of a comprehensive expression analysis of HsfA2 and Hsp17-CII, two important members of the heat stress system, in the developing anthers of a heat-tolerant tomato genotype are reported here. A transcriptional analysis at different developmental anther/pollen stages was performed using semi-quantitative and real-time PCR. The messengers were localized using in situ RNA hybridization, and protein accumulation was monitored using immunoblot analysis. Based on the analysis of the gene and protein expression profiles, HsfA2 and Hsp17-CII are finely regulated during anther development and are further induced under both short and prolonged heat stress conditions. These data suggest that HsfA2 may be directly involved in the activation of protection mechanisms in the tomato anther during heat stress and, thereby, may contribute to tomato fruit set under adverse temperatures.

Identification and expression analysis of OsHsfs in rice

Heat stress transcription factors (Hsfs) are the central regulators of defense response to heat stress. We identified a total of 25 rice Hsf genes by genome-wide analysis of rice (Oryza sativa L.) genome, including the subspecies of O. japonica and O. indica. Proteins encoded by OsHsfs were divided into three classes according to their structures. Digital Northern analysis showed that OsHsfs were expressed constitutively. The expressions of these OsHsfs in response to heat stress and oxidative stress differed among the members of the gene family. Promoter analysis identified a number of stress-related cis-elements in the promoter regions of these OsHsfs. No significant correlation, however, was found between the heat-shock responses of genes and their cis-elements. Overall, our results provide a foundation for future research of OsHsfs function.

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.

The cytosolic protein response as a subcomponent of the wider heat shock response in Arabidopsis

In common with a range of environmental and biological stresses, heat shock results in the accumulation of misfolded proteins and a collection of downstream consequences for cellular homeostasis and growth. Within this complex array of responses, the sensing of and responses to misfolded proteins in specific subcellular compartments involves specific chaperones, transcriptional regulators, and expression profiles. Using biological (ectopic protein expression and virus infection) and chemical triggers for misfolded protein accumulation, we have profiled the transcriptional features of the response to misfolded protein accumulation in the cytosol (i.e., the cytoplasmic protein response [CPR]) and identified the effects as a subcomponent of the wider effects induced by heat shock. The CPR in Arabidopsis thaliana is associated with the heat shock promoter element and the involvement of specific heat shock factors (HSFs), notably HSFA2, which appears to be regulated by alternative splicing and non-sense-mediated decay. Characterization of Arabidopsis HSFA2 knockout and overexpression lines showed that HSFA2 is one of the regulatory components of the CPR.

Heat shock factors HsfB1 and HsfB2b are involved in the regulation of Pdf1.2 expression and pathogen resistance in Arabidopsis

In order to assess the functional roles of heat stress-induced class B-heat shock factors in Arabidopsis, we investigated T-DNA knockout mutants of AtHsfB1 and AtHsfB2b. Micorarray analysis of double knockout hsfB1/hsfB2b plants revealed as strong an up-regulation of the basal mRNA-levels of the defensin genes Pdf1.2a/b in mutant plants. The Pdf expression was further enhanced by jasmonic acid treatment or infection with the necrotrophic fungus Alternaria brassicicola. The single mutant hsfB2b and the double mutant hsfB1/B2b were significantly improved in disease resistance after A. brassicicola infection. There was no indication for a direct interaction of Hsf with the promoter of Pdf1.2, which is devoid of perfect HSE consensus Hsf-binding sequences. However, changes in the formation of late HsfA2-dependent HSE binding were detected in hsfB1/B2b plants. This suggests that HsfB1/B2b may interact with class A-Hsf in regulating the shut-off of the heat shock response. The identification of Pdf genes as targets of Hsf-dependent negative regulation is the first evidence for an interconnection of Hsf in the regulation of biotic and abiotic responses.

Cytosolic heat shock protein 90 regulates heat shock transcription factor in Arabidopsis thaliana

Plant survival requires the ability to acclimate to heat, which is involves the expression of heat-inducible genes. We found cytosolic heat shock protein (HSP) 90 serves as a negative regulator of heat shock transcription factor (HSF), which is responsible for the induction of heat-inducible genes in plant. Transient inhibition of HSP90 induces heat-inducible genes and heat acclimation in Arabidopsis thaliana seedlings. Most of upregulated genes by heat shock and HSP90 inhibitor treatments carry heat shock response element (HSE) in their promoter, which suggests that HSF participates in the response to HSP90 inhibition. A. thaliana HSP90.2 interacts with AtHsfA1d, which is one of the constitutively expressed HSFs in A. thaliana. Heat shock depleted cytosolic HSP90 activity, as shown by the activity of exogenously expressed glucocorticoid receptor (GR), which is a substrate of cytosolic HSP90. Thus, it appears that in the absence of heat shock, cytosolic HSP90 negatively regulates HsfA1. Upon heat shock, cytosolic HSP90 is transiently inactivated, and this may lead to the activation of HsfA1.

Transcriptional regulation of plant senescence: from functional genomics to systems biology

Leaf senescence is an active process that involves the increased expression of many hundreds of genes. Many putative transcription factors show enhanced transcription during leaf senescence in Arabidopsis and functional analysis of these should help to indicate their role in controlling gene expression during leaf senescence. In this paper, we describe the analysis of knockout insertion mutants in two different senescence-enhanced genes, one encodes a heat shock transcription factor and the other a zinc finger protein. Plants mutated in these genes show accelerated leaf senescence and reduced tolerance to drought stress, indicating that expression of these genes during senescence has a protective role to maintain viability during this essential developmental process. Analysis of gene expression changes in both mutants compared to the wild-type plants indicates an increased rate of senescence but does not show clearly the pathway that is dependent on these genes for expression. The complexities of signalling networks in plant stress and the plasticity of plant responses mean that the direct consequences of mutation are very difficult to define. The usefulness of this type of approach to address the burning question of how senescence is regulated is discussed, and an alternative approach aimed at a more global analysis of gene regulation using systems biology methods is described.

Distinct heat-shock element arrangements that mediate the heat shock, but not the late-embryogenesis induction of small heat-shock proteins, correlate with promoter activation in root-knot nematode feeding cells

Genes coding small heat-shock proteins (sHSPs) show distinct behaviours with respect to environmental and developmental signals. Their transcriptional regulation depends on particular combinations of heat stress cis-elements (heat-shock elements; HSEs) but many aspects regarding their regulation remain unclear. Cyst and root-knot nematodes induce, in the roots of infected plants, the differentiation of special feeding cells with high metabolic activity (syncytia and giant cells, respectively), a process accompanied by extensive gene expression changes. The Hahsp17.7G4 (G4) promoter was active in giant cells and its HSE arrangements were crucial for this activation. In the present work, we provide further basis to associate giant cell expression with the heat-shock response of this gene class, by analysing additional promoters. The Hahsp17.6G1 (G1) promoter, not induced by heat shock, was silent in giant cells, while Hahsp18.6G2 (G2), which responds to heat shock, was specifically induced in giant cells. In addition, a mutated Hahsp17.7G4 promoter version (G4MutP) with a strong heat-shock induction was also induced in giant cells. The responses of the different promoters correlated with distinct HSE configurations, which might have implications on differential trans-activation. Furthermore, the shortest giant cell and heat-shock-inducible sHSP promoter version analysed in tobacco (-83pb Hahsp17.7G4) fully maintained its expression profile in Arabidopsis. Cyst nematodes did not induce the Hahsp17.7G4 promoter, revealing additional specificity in the nematode response. These findings, together with the fact that the class I sHSP products of endogenous genes accumulated specifically in tobacco giant cells, support the idea that these nematode-induced giant cells represent a transcriptional state very similar to that produced by heat shock regarding this class of genes. The high metabolic rate of giant cells may result in unfolded proteins requiring class I sHSPs as chaperones, which might, somehow, mimic heat-shock and/or other stress responses.

Histone modifications and dynamic regulation of genome accessibility in plants

In all eukaryotes chromatin physically restricts the accessibility of the genome to regulatory proteins such as transcription factors. Plant model systems have been instrumental in demonstrating that this restriction is dynamic and changes during development and in response to exogenous cues. Among the multiple epigenetic mechanisms that alter chromatin to regulate gene expression, histone modifications play a major role. Recent studies in Arabidopsis have provided the first genome-wide histone modification maps, revealed important biological roles for histone modifications, and advanced our understanding of stimulus-dependent changes in histone modifications.

Cytosolic HSP90 regulates the heat shock response that is responsible for heat acclimation in Arabidopsis thaliana

Plant survival requires the ability to acclimate to heat. When plants are subjected to heat shock, the expression of various genes is induced, and the plants become tolerant of higher temperatures. We found that transient treatment with geldanamycin and radicicol, two heat shock protein 90 (HSP90) inhibitors, induced heat-inducible genes and heat acclimation in Arabidopsis thaliana seedlings. Heat shock reduced the activity of exogenously expressed glucocorticoid receptor (GR). Since GR activity depends on HSP90, this suggests that heat shock reduces cytosolic HSP90 activity in vivo. Microarray analysis revealed that many of the genes that are up-regulated by both heat shock and HSP90 inhibitors are involved in protein folding and degradation, suggesting that the activation of a protein maintenance system is a crucial part of this response. Most of these genes have heat shock response element-like motifs in their promoters, which suggests that heat shock transcription factor (HSF) is involved in the response to HSP90 inhibition. Several HSF genes are expressed constitutively in A. thaliana, including AtHsfA1d. Recombinant AtHsfA1d protein recognizes the heat shock response element motif and interacts with A. thaliana cytosolic HSP90, HSP90.2. Overexpression of a dominant negative form of HSP90.2 induced the heat-inducible gene. Thus, it appears that in the absence of heat shock, cytosolic HSP90 negatively regulates heat-inducible genes by actively suppressing HSF function. Upon heat shock, cytosolic HSP90 is transiently inactivated, which may lead to HSF activation.

The diversity of plant heat stress transcription factors

Compared with other eukaryotes with one to three heat stress transcription factors (Hsf), the plant Hsf family shows a striking multiplicity, with more than 20 members. Despite many conserved features, members of the Hsf family show a strong diversification of expression pattern and function within the family. Research on Arabidopsis Hsfs opened a new era with genome-wide transcriptome profiling in combination with the availability of knockout lines. The output from these analyses provides increasing evidence that individual Hsfs have unique functions as part of different signal transduction pathways operating in response to environmental stress and during development.

Heat shock transcription factor 1 opens chromatin structure of interleukin-6 promoter to facilitate binding of an activator or a repressor

Heat shock transcription factor 1 (HSF1) not only regulates expression of heat shock genes in response to elevated temperature, but is also involved in developmental processes by regulating genes such as cytokine genes. However, we did not know how HSF1 regulates non-heat shock genes. Here, we show that constitutive HSF1 binding to the interleukin (IL)-6 promoter is necessary for its maximal induction by lipopolysaccharide (LPS) stimulation in mouse embryo fibroblasts and peritoneal macrophages. Lack of HSF1 inhibited LPS-induced in vivo binding of an activator NF-kappaB and a repressor ATF3 to IL-6 promoter. Neither NF-kappaB nor ATF3 binds to the IL-6 promoter in unstimulated HSF1-null cells even if they were overexpressed. Treatment with histone deacetylase inhibitor or a DNA methylation inhibitor restored LPS-induced IL-6 expression in HSF1-null cells, and histone modification enzymes were recruited on the IL-6 promoter in the presence of HSF1. Consistently, chromatin structure of the IL-6 promoter in the presence of HSF1 was more open than that in its absence. These results indicate that HSF1 partially opens the chromatin structure of the IL-6 promoter for an activator or a repressor to bind to it, and provides a novel mechanism of gene regulation by HSF1.

Identification of novel conserved peptide uORF homology groups in Arabidopsis and rice reveals ancient eukaryotic origin of select groups and preferential association with transcription factor-encoding genes

BACKGROUND

Upstream open reading frames (uORFs) can mediate translational control over the largest, or major ORF (mORF) in response to starvation, polyamine concentrations, and sucrose concentrations. One plant uORF with conserved peptide sequences has been shown to exert this control in an amino acid sequence-dependent manner but generally it is not clear what kinds of genes are regulated, or how extensively this mechanism is invoked in a given genome.

RESULTS

By comparing full-length cDNA sequences from Arabidopsis and rice we identified 26 distinct homology groups of conserved peptide uORFs, only three of which have been reported previously. Pairwise Ka/Ks analysis showed that purifying selection had acted on nearly all conserved peptide uORFs and their associated mORFs. Functions of predicted mORF proteins could be inferred for 16 homology groups and many of these proteins appear to have a regulatory function, including 6 transcription factors, 5 signal transduction factors, 3 developmental signal molecules, a homolog of translation initiation factor eIF5, and a RING finger protein. Transcription factors are clearly overrepresented in this data set when compared to the frequency calculated for the entire genome (p = 1.2 x 10(-7)). Duplicate gene pairs arising from a whole genome duplication (ohnologs) with a conserved uORF are much more likely to have been retained in Arabidopsis (Arabidopsis thaliana) than are ohnologs of other genes (39% vs 14% of ancestral genes, p = 5 x 10(-3)). Two uORF groups were found in animals, indicating an ancient origin of these putative regulatory elements.

CONCLUSION

Conservation of uORF amino acid sequence, association with homologous mORFs over long evolutionary time periods, preferential retention after whole genome duplications, and preferential association with mORFs coding for transcription factors suggest that the conserved peptide uORFs identified in this study are strong candidates for translational controllers of regulatory genes.

Complexity of the heat stress response in plants

Plants have evolved a variety of responses to elevated temperatures that minimize damage and ensure protection of cellular homeostasis. New information about the structure and function of heat stress proteins and molecular chaperones has become available. At the same time, transcriptome analysis of Arabidopsis has revealed the involvement of factors other than classical heat stress responsive genes in thermotolerance. Recent reports suggest that both plant hormones and reactive oxygen species also contribute to heat stress signaling. Additionally, an increasing number of mutants that have altered thermotolerance have extended our understanding of the complexity of the heat stress response in plants.

Role of plant CBP/p300-like genes in the regulation of flowering time

CREB-binding protein (CBP) and its homolog p300 possess histone acetyltransferase activity and function as key transcriptional co-activators in the regulation of gene expression that controls differentiation and development in animals. However, the role of CBP/p300-like genes in plants has not yet been elucidated. Here, we show that Arabidopsis CBP/p300-like genes promote flowering by affecting the expression of a major floral repressor FLOWERING LOCUS C (FLC). Although animal CBP and p300 generally function as co-activators, Arabidopsis CBP/p300-like proteins are required for the negative regulation of FLC. This CBP/p300-mediated FLC repression may involve reversible protein acetylation independent of histone modification within FLC chromatin.

A heat-inducible transcription factor, HsfA2, is required for extension of acquired thermotolerance in Arabidopsis

The expression of heat shock proteins (Hsps) induced by nonlethal heat treatment confers acquired thermotolerance (AT) to organisms against subsequent challenges of otherwise lethal temperature. After the stress signal is removed, AT gradually decays, with decreased Hsps during recovery. AT of sufficient duration is critical for sessile organisms such as plants to survive repeated heat stress in their environment, but little is known regarding its regulation. To identify potential regulatory components, we took a reverse genetics approach by screening for Arabidopsis (Arabidopsis thaliana) T-DNA insertion mutants that show decreased thermotolerance after a long recovery (2 d) under nonstress conditions following an acclimation heat treatment. Among the tested mutants corresponding to 48 heat-induced genes, only the heat shock transcription factor HsfA2 knockout mutant showed an obvious phenotype. Following pretreatment at 37 degrees C, the mutant line was more sensitive to severe heat stress than the wild type after long but not short recovery periods, and this could be complemented by the introduction of a wild-type copy of the HsfA2 gene. Quantitative hypocotyl elongation assay also revealed that AT decayed faster in the absence of HsfA2. Significant reduction in the transcript levels of several highly heat-inducible genes was observed in HsfA2 knockout plants after 4 h recovery or 2 h prolonged heat stress. Immunoblot analysis showed that Hsa32 and class I small Hsp were less abundant in the mutant than in the wild type after long recovery. Our results suggest that HsfA2 as a heat-inducible transactivator sustains the expression of Hsp genes and extends the duration of AT in Arabidopsis.

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.

Could heat shock transcription factors function as hydrogen peroxide sensors in plants?

BACKGROUND

Heat shock transcription factors (Hsfs) are modular transcription factors encoded by a large gene family in plants. They bind to the consensus sequence 'nGAAnnTCCn' found in the promoters of many defence genes, and are thought to function as a highly redundant and flexible gene network that controls the response of plants to different environmental stress conditions, including biotic and abiotic stresses. Hsf proteins encoded by different genes exhibit a high degree of complexity in their interactions. They can potentially bind and activate their own promoters, as well as the promoters of other members of their gene family, and they can form homo- or heterotrimers resulting in altered nuclear localization, as well as enhanced or suppressed transcription.

SCOPE

In this review, we summarize recent studies on Hsf function in Arabidopsis and tomato and present evidence obtained from microarray expression studies in Arabidopsis that the Hsf gene network is highly flexible and specialized, with specific members and/or member combinations controlling the response of plants to particular stress conditions. In addition, we describe recent studies that support the hypothesis that certain Hsfs function as molecular sensors that directly sense reactive oxygen species (ROS) and control the expression of oxidative stress response genes during oxidative stress.

Proteome profiling of Populus euphratica Oliv. upon heat stress

BACKGROUND AND AIMS

Populus euphratica is a light-demanding species ecologically characterized as a pioneer. It grows in shelter belts along riversides, being part of the natural desert forest ecosystems in China and Middle Eastern countries. It is able to survive extreme temperatures, drought and salt stress, marking itself out as an important plant species to study the mechanisms responsible for survival of woody plants under heat stress.

METHODS

Heat effects were evaluated through electrolyte leakage on leaf discs, and LT(50) was determined to occur above 50 degrees C. Protein accumulation profiles of leaves from young plants submitted to 42/37 degrees C for 3 d in a phytotron were determined through 2D-PAGE, and a total of 45 % of up- and downregulated proteins were detected. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF)/TOF analysis, combined with searches in different databases, enabled the identification of 82 % of the selected spots.

KEY RESULTS

Short-term upregulated proteins are related to membrane destabilization and cytoskeleton restructuring, sulfur assimilation, thiamine and hydrophobic amino acid biosynthesis, and protein stability. Long-term upregulated proteins are involved in redox homeostasis and photosynthesis. Late downregulated proteins are involved mainly in carbon metabolism.

CONCLUSIONS

Moderate heat response involves proteins related to lipid biogenesis, cytoskeleton structure, sulfate assimilation, thiamine and hydrophobic amino acid biosynthesis, and nuclear transport. Photostasis is achieved through carbon metabolism adjustment, a decrease of photosystem II (PSII) abundance and an increase of PSI contribution to photosynthetic linear electron flow. Thioredoxin h may have a special role in this process in P. euphratica upon moderate heat exposure.

The maize heat shock factor-binding protein paralogs EMP2 and HSBP2 interact non-redundantly with specific heat shock factors

The heat shock response (HSR) is a conserved mechanism by which transcripts of heat shock protein (hsp) genes accumulate following mobilization of heat shock transcription factors (HSFs) in response to thermal stress. Studies in animals identified the heat shock factor-binding protein1 (HSBP1) that interacts with heat shock transcription factor1 (HSF1) during heat shock attenuation; overexpression analyses revealed that the coiled-coil protein HSBP1 functions as a negative regulator of the HSR. Zea mays contains two HSBP paralogs, EMP2 and HSBP2, which exhibit differential accumulation during the HSR and plant development. Embryo-lethal recessive emp2 mutations revealed that EMP2 is required for the down-regulation of hsp transcription during embryogenesis, whereas accumulation of HSBP2 is induced in seedlings following heat shock. Notwithstanding, no interaction has yet been demonstrated between a plant HSBP and a plant HSF. In this report 22 maize HSF isoforms are identified comprising three structural classes: HSF-A, HSF-B and HSF-C. Phylogenetic analysis of Arabidopsis, maize and rice HSFs reveals that at least nine ancestral HSF isoforms were present prior to the separation of monocot and eudicots, followed by differential amplification of HSF members in these lineages. Yeast two-hybrid analyses show that EMP2 and HSBP2 interact non-redundantly with specific HSF-A isoforms. Site-specific mutagenesis of HSBP2 reveals that interactions between hydrophobic residues within the coiled coil are required for HSF::HSBP2 binding; domain swapping demonstrate that the isoform specificity of HSF::HSBP interaction is conferred by residues outside of the coiled coil. These data suggest that the non-redundant functions of the maize HSBPs may be explained, at least in part, by the specificity of HSBP::HSF interactions during plant development.

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.

Functional interaction between two transcription factors involved in the developmental regulation of a small heat stress protein gene promoter

Hahsp17.6G1 is the promoter of a small heat stress protein (sHSP) from sunflower (Helianthus annuus) that is activated during zygotic embryogenesis, but which does not respond to heat stress. We report here the cloning of a transcription factor (TF), sunflower drought-responsive element binding factor 2 (HaDREB2), by one-hybrid interaction with functional cis-elements in Hahsp17.6G1. We have analyzed the functional interaction between HaDREB2 and a second transcription factor, sunflower heat stress factor A9 (HaHSFA9), which was previously assigned to the regulation of Hahsp17.6G1. HaDREB2 and HaHSFA9 synergistically trans-activate the Hahsp17.6G1 promoter in bombarded sunflower embryos. This synergistic interaction is heat stress factor (HSF) specific and requires the binding of both factors to the promoter. The C-terminal region of HaHSFA9 is sufficient for the HSF specificity. Our results represent an example of a functional interaction between members of the Apetala 2 (HaDREB2) and HSF (HaHSFA9) families of transcription factors. We suggest new roles in zygotic embryogenesis for specific members of the AP2 transcription factor family.

Identification of novel heat shock factor-dependent genes and biochemical pathways in Arabidopsis thaliana

In order to assess specific functional roles of plant heat shock transcription factors (HSF) we conducted a transcriptome analysis of Arabidopsis thaliana hsfA1a/hsfA1b double knock out mutants and wild-type plants. We used Affymetrix ATH1 microarrays (representing more than 24 000 genes) and conducted hybridizations for heat-treated or non-heat-treated leaf material of the respective lines. Heat stress had a severe impact on the transcriptome of mutant and wild-type plants. Approximately 11% of all monitored genes of the wild type showed a significant effect upon heat stress treatment. The difference in heat stress-induced gene expression between mutant and wild type revealed a number of HsfA1a/1b-regulated genes. Besides several heat shock protein and other stress-related genes, we found HSFA-1a/1b-regulated genes for other functions including protein biosynthesis and processing, signalling, metabolism and transport. By screening the profiling data for genes in biochemical pathways in which known HSF targets were involved, we discovered that at each step in the pathway leading to osmolytes, the expression of genes is regulated by heat stress and in several cases by HSF. Our results document that in the immediate early phase of the heat shock response HSF-dependent gene expression is not limited to known stress genes, which are involved in protection from proteotoxic effects. HsfA1a and HsfA1b-regulated gene expression also affects other pathways and mechanisms dealing with a broader range of physiological adaptations to stress.

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.