Mutants of Arabidopsis thaliana defective in the acquisition of tolerance to high temperature stress

The drnf1 Gene from the Drought-Adapted Cyanobacterium Nostoc flagelliforme Improved Salt Tolerance in Transgenic Synechocystis and Arabidopsis Plant

Environmental abiotic stresses are limiting factors for less tolerant organisms, including soil plants. Abiotic stress tolerance-associated genes from prokaryotic organisms are supposed to have a bright prospect for transgenic application. The drought-adapted cyanobacterium Nostoc flagelliforme is arising as a valuable prokaryotic biotic resource for gene excavation. In this study, we evaluated the salt-tolerant function and application potential of a candidate gene drnf1 from N. flagelliforme, which contains a P-loop NTPase (nucleoside-triphosphatase) domain, through heterologous expression in two model organisms Synechocystis sp. PCC 6803 and Arabidopsis thaliana. It was found that DRNF1 could confer significant salt tolerance in both transgenic organisms. In salt-stressed transgenic Synechocystis, DRNF1 could enhance the respiration rate; slow-down the accumulation of exopolysaccharides; up-regulate the expression of salt tolerance-related genes at a higher level, such as those related to glucosylglycerol synthesis, Na⁺/H⁺ antiport, and sugar metabolism; and maintain a better K⁺/Na⁺ homeostasis, as compared to the wild-type strain. These results imply that DRNF1 could facilitate salt tolerance by affecting the respiration metabolism and indirectly regulating the expression of important salt-tolerant genes. Arabidopsis was employed to evaluate the salt tolerance-conferring potential of DRNF1 in plants. The results show that it could enhance the seed germination and shoot growth of transgenic plants under saline conditions. In general, a novel prokaryotic salt-tolerant gene from N. flagelliforme was identified and characterized in this study, enriching the candidate gene pool for genetic engineering in plants.

Mapping of domains of heat stress transcription factor OsHsfA6a responsible for its transactivation activity

Elevated temperatures affect the growth and reproduction of crop plants and thus have become concern worldwide. Hsp101/ClpB protein is a major molecular chaperone, performing dis-aggregation of protein aggregates formed during heat stress. In rice, OsHsfA6a binds to the promoter of OsHsp101/ClpB-C and regulates its expression. In this study, analysis of C-terminal domains of ClassA OsHsfs revealed the presence of aromatic, hydrophobic, acidic (AHA) and nuclear export signal (NES) motifs in all the members. Using deletion constructs, we show that the activation potential of OsHsfA6a is confined in the C-terminal activation domain comprising of AHA and NES sequences. The results obtained in yeast were complemented with transient expression of reporter in protoplast (TERP) based assay. Detailed analysis of OsHsfA6a splice variants shows the presence of one full version and a DBD truncated smaller version whose existence needs experimental evidences. Phylogeny analysis revealed that OsHsfA6a has diverged from A6a/A6b forms of Arabidopsis and tomato and has no expressologs. OsHsfA6a in-silico network was enriched in MAP kinases along with Hsp70 and Hsp90 proteins. Thus, it appears that regulation of OsClpB-C by HsfA6a is unique in rice and activation potential of OsHsfA6a resides in the single AHA motif located in the C-terminal domain.

The heat responsive wheat TaRAD23 rescues developmental and thermotolerant defects of the rad23b mutant in Arabidopsis thaliana

High temperature severely damage the growth and development of crops with climate change. To effectively screen heat responsive proteins in wheat (Triticum aestivum L.), the isobaric tandem mass tag (TMT)-labeled quantitative proteomic analysis and quantitative real-time PCR (qRT-PCR) were performed. Here, we found that a wheat RADIATION SENSITIVE 23 protein, TaRAD23, was up-regulated at both protein and RNA levels by exposing to heat stress. Sequence homology analysis indicated that the TaRAD23 is a conserved protein, which is closely related to the Arabidopsis thaliana proteins AtRAD23B and AtRAD23A. Genetic knockout of AtRAD23B, but not AtRAD23A, shows multiple developmental defects, as well as sensitivity to heat stress. Meanwhile, we observed that constitutive overexpression of TaRAD23 in rad23b fully rescued developmental and thermotolerant defects of the mutant. Furthermore, qRT-PCR analysis of heat responsive genes in rad23b and its complementary lines suggested that suppression of the heat shock transcription factor AtHSFA2 and heat responsive genes (HSP70, HSP90, HSP17.6 and HSA32) may be the cause of the weaker thermotolerance in rad23b. Taken together, the data suggest that the heat responsive TaRAD23 is a functionally highly conserved protein that plays an important role in development, as well as the regulation in heat stress response network.

A cytosolic class II small heat shock protein, PfHSP17.2, confers resistance to heat, cold, and salt stresses in transgenic Arabidopsis

We cloned and characterized the full-length coding sequence of a small heat shock (sHSP) gene, PfHSP17.2, from Primula forrestii leaves following heat stress treatment. Homology and phylogenetic analysis suggested that PfHSP17.2 is a cytosolic class II sHSP, which was further supported by the cytosolic localization of transient expression of PfHSP17.2 fused with green fluorescent protein reporter. Expression analysis showed that PfHSP17.2 was highly inducible by heat stress in almost all the vegetative and generative tissues and was expressed under salt, cold, and oxidative stress conditions as well. Moreover, the expression of PfHSP17.2 in P. forrestii was detected in certain developmental growth stages. Transgenic Arabidopsis thaliana constitutively expressing PfHSP17.2 displayed increased thermotolerance and higher resistance to salt and cold compared with wild type plants. It is suggested that PfHSP17.2 plays a key role in heat and other abiotic stresses.

Linkage mapping of yeast cross protection connects gene expression variation to a higher-order organismal trait

Gene expression variation is extensive in nature, and is hypothesized to play a major role in shaping phenotypic diversity. However, connecting differences in gene expression across individuals to higher-order organismal traits is not trivial. In many cases, gene expression variation may be evolutionarily neutral, and in other cases expression variation may only affect phenotype under specific conditions. To understand connections between gene expression variation and stress defense phenotypes, we have been leveraging extensive natural variation in the gene expression response to acute ethanol in laboratory and wild Saccharomyces cerevisiae strains. Previous work found that the genetic architecture underlying these expression differences included dozens of “hotspot” loci that affected many transcripts in trans. In the present study, we provide new evidence that one of these expression QTL hotspot loci affects natural variation in one particular stress defense phenotype—ethanol-induced cross protection against severe doses of H₂O₂. A major causative polymorphism is in the heme-activated transcription factor Hap1p, which we show directly impacts cross protection, but not the basal H₂O₂ resistance of unstressed cells. This provides further support that distinct cellular mechanisms underlie basal and acquired stress resistance. We also show that Hap1p-dependent cross protection relies on novel regulation of cytosolic catalase T (Ctt1p) during ethanol stress in a wild oak strain. Because ethanol accumulation precedes aerobic respiration and accompanying reactive oxygen species formation, wild strains with the ability to anticipate impending oxidative stress would likely be at an advantage. This study highlights how strategically chosen traits that better correlate with gene expression changes can improve our power to identify novel connections between gene expression variation and higher-order organismal phenotypes.

A major goal in genetics is to understand how individuals with different genetic makeups respond to their environment. Understanding these “gene-environment interactions” is important for the development of personalized medicine. For example, gene-environment interactions can explain why some people are more sensitive to certain drugs or are more likely to get certain cancers. While the underlying causes of gene-environment interactions are unclear, one possibility is that differences in gene expression across individuals are responsible. In this study, we examined that possibility using baker’s yeast as a model. We were interested in a phenomenon called acquired stress resistance, where cells exposed to a mild dose of one stress can become resistant to an otherwise lethal dose of severe stress. This response is observed in diverse organisms ranging from bacteria to humans, though the specific mechanisms governing acquisition of higher stress resistance are poorly understood. To understand the differences between yeast strains with and without the ability to acquire further stress resistance, we employed genetic mapping. We found that part of the variation in acquired stress resistance was due to sequence differences in a key regulatory protein, thus providing new insight into how different individuals respond to acute environmental change.

Enhanced multiple stress tolerance in Arabidopsis by overexpression of the polar moss peptidyl prolyl isomerase FKBP12 gene

PaFKBP12 overexpression in Arabidopsis resulted in stress tolerance to heat, ABA, drought, and salt stress, in addition to growth promotion under normal conditions. Polytrichastrum alpinum (alpine haircap moss) is one of polar organisms that can withstand the severe conditions of the Antarctic. In this study, we report the isolation of a peptidyl prolyl isomerase FKBP12 gene (PaFKBP12) from P. alpinum collected in the Antarctic and its functional implications in development and stress responses in plants. In P. alpinum, PaFKBP12 expression was induced by heat and ABA. Overexpression of PaFKBP12 in Arabidopsis increased the plant size, which appeared to result from increased rates of cell cycle. Under heat stress conditions, PaFKBP12-overexpressing lines (PaFKBP12-OE) showed better growth and survival than the wild type. PaFKBP12-OE also showed higher root elongation rates, better shoot growth and enhanced survival at higher concentrations of ABA in comparison to the wild type. In addition, PaFKBP12-OE were more tolerant to drought and salt stress than the wild type. All these phenotypes were accompanied with higher induction of the stress responsive genes in PaFKBP12-OE than in the wild type. Taken together, our findings revealed important functions of PaFKBP12 in plant development and abiotic stress responses.

Plant molecular responses to the elevated ambient temperatures expected under global climate change

Environmental temperatures affect plant distribution, growth, and development. The Intergovernmental Panel on Climate Change (IPCC) predicts that global temperatures will rise by at least 1.5°C by the end of this century. Global temperature changes have already had a discernable impact on agriculture, phenology, and ecosystems. At the molecular level, extensive literature exists on the mechanism controlling plant responses to high temperature stress. However, few studies have focused on the molecular mechanisms behind plant responses to mild increases in ambient temperature. Previous research has found that moderately higher ambient temperatures can induce hypocotyl elongation and early flowering. Recent evidence demonstrates roles for the phytohormones auxin and ethylene in adaptive growth of plant roots to slightly higher ambient temperatures.

MicroRNA160 Modulates Plant Development and Heat Shock Protein Gene Expression to Mediate Heat Tolerance in Arabidopsis

Global warming is causing a negative impact on plant growth and adversely impacts on crop yield. MicroRNAs (miRNAs) are critical in regulating the expression of genes involved in plant development as well as defense responses. The effects of miRNAs on heat-stressed Arabidopsis warrants further investigation. Heat stress increased the expression of miR160 and its precursors but considerably reduced that of its targets, ARF10, ARF16, and ARF17. To study the roles of miR160 during heat stress, transgenic Arabidopsis plants overexpressing miR160 precursor a (160OE) and artificial miR160 (MIM160), which mimics an inhibitor of miR160, were created. T-DNA insertion mutants of miR160 targets were also used to examine their tolerances to heat stress. Results presented that overexpressing miR160 improved seed germination and seedling survival under heat stress. The lengths of hypocotyl elongation and rachis were also longer in 160OE than the wild-type (WT) plants under heat stress. Interestingly, MIM160 plants showed worse adaption to heat. In addition, arf10, arf16, and arf17 mutants presented similar phenotypes to 160OE under heat stress to advance abilities of thermotolerance. Moreover, transcriptome and qRT-PCR analyses revealed that HSP17.6A, HSP17.6II, HSP21, and HSP70B expression levels were regulated by heat in 160OE, MIM160, arf10, arf16, and arf17 plants. Hence, miR160 altered the expression of the heat shock proteins and plant development to allow plants to survive heat stress.

Candidate Genes and Molecular Markers Correlated to Physiological Traits for Heat Tolerance in Fine Fescue Cultivars

Heat stress is one of the major abiotic factors limiting the growth of cool-season grass species during summer season. The objectives of this study were to assess genetic variations in the transcript levels of selected genes in fine fescue cultivars differing in heat tolerance, and to identify single nucleotide polymorphism (SNP) markers associated with candidate genes related to heat tolerance. Plants of 26 cultivars of five fine fescue species (Festuca spp.) were subjected to heat stress (38/33 °C, day/night temperature) in controlled environmental growth chambers. Physiological analysis including leaf chlorophyll content, photochemical efficiency, and electrolyte leakage demonstrated significant genetic variations in heat tolerance among fine fescue cultivars. The transcript levels of selected genes involved in photosynthesis (RuBisCO activase, Photosystem II CP47 reaction center protein), carbohydrate metabolism (Sucrose synthase), energy production (ATP synthase), growth regulation (Actin), oxidative response (Catalase), and stress protection (Heat shock protein 90) were positively correlated with the physiological traits for heat tolerance. SNP markers for those candidate genes exhibited heterozygosity, which could also separate heat-sensitive and heat-tolerant cultivars into clusters. The development of SNP markers for candidate genes in heat tolerance may allow marker-assisted breeding for the development of new heat-tolerant cultivars in fine fescue and other cool-season grass species.

Mutations in eIF5B Confer Thermosensitive and Pleiotropic Phenotypes via Translation Defects in Arabidopsis thaliana

The conserved eukaryotic translation initiation factor 5B, eIF5B, is a GTPase that acts late in translation initiation. We found that an Arabidopsis thaliana mutant sensitive to hot temperatures 3 (hot3-1), which behaves as the wild type in the absence of stress but is unable to acclimate to high temperature, carries a missense mutation in the eIF5B1 gene (At1g76810), producing a temperature sensitive protein. A more severe, T-DNA insertion allele (hot3-2) causes pleiotropic developmental phenotypes. Surprisingly, Arabidopsis has three other eIF5B genes that do not substitute for eIF5B1; two of these appear to be in the process of pseudogenization. Polysome profiling and RNA-seq analysis of hot3-1 plants show delayed recovery of polysomes after heat stress and reduced translational efficiency (TE) of a subset of stress protective proteins, demonstrating the critical role of translational control early in heat acclimation. Plants carrying the severe hot3-2 allele show decreased TE of auxin-regulated, ribosome-related, and electron transport genes, even under optimal growth conditions. The hot3-2 data suggest that disrupting specific eIF5B interactions on the ribosome can, directly or indirectly, differentially affect translation. Thus, modulating eIF5B interactions could be another mechanism of gene-specific translational control.

Assessing Plant Tolerance to Acute Heat Stress

It is well-established that plants are able to acclimate to temperatures above or below the optimal temperature for their growth. Here, we provide protocols for assays that can be used quantitatively or qualitatively to assess the relative ability of plants to acquire tolerance to high temperature stress. The hypocotyl elongation assay described was developed to screen for mutants defective in the acquisition of tolerance to extreme temperature stress, and other assays were developed to further characterize mutant and transgenic plants for heat tolerance of other processes or at other growth stages. Although the protocols provide details for application to Arabidopsis thaliana, the same basic methods can be adopted to assay heat tolerance in other plant species.

Plant responses to environmental stresses-from gene to biotechnology

Increasing global population, urbanization and industrialization are increasing the rate of conversion of arable land into wasteland. Supplying food to an ever-increasing population is one of the biggest challenges that agriculturalists and plant scientists are currently confronting. Environmental stresses make this situation even graver. Despite the induction of several tolerance mechanisms, sensitive plants often fail to survive under environmental extremes. New technological approaches are imperative. Conventional breeding methods have a limited potential to improve plant genomes against environmental stress. Recently, genetic engineering has contributed enormously to the development of genetically modified varieties of different crops such as cotton, maize, rice, canola and soybean. The identification of stress-responsive genes and their subsequent introgression or overexpression within sensitive crop species are now being widely carried out by plant scientists. Engineering of important tolerance pathways, like antioxidant enzymes, osmolyte accumulation, membrane-localized transporters for efficient compartmentation of deleterious ions and accumulation of essential elements and resistance against pests or pathogens is also an area that has been intensively researched. In this review, the role of biotechnology and its successes, prospects and challenges in developing stress-tolerant crop cultivars are discussed.

The growing world of small heat shock proteins: from structure to functions

Small heat shock proteins (sHSPs) are present in all kingdoms of life and play fundamental roles in cell biology. sHSPs are key components of the cellular protein quality control system, acting as the first line of defense against conditions that affect protein homeostasis and proteome stability, from bacteria to plants to humans. sHSPs have the ability to bind to a large subset of substrates and to maintain them in a state competent for refolding or clearance with the assistance of the HSP70 machinery. sHSPs participate in a number of biological processes, from the cell cycle, to cell differentiation, from adaptation to stressful conditions, to apoptosis, and, even, to the transformation of a cell into a malignant state. As a consequence, sHSP malfunction has been implicated in abnormal placental development and preterm deliveries, in the prognosis of several types of cancer, and in the development of neurological diseases. Moreover, mutations in the genes encoding several mammalian sHSPs result in neurological, muscular, or cardiac age-related diseases in humans. Loss of protein homeostasis due to protein aggregation is typical of many age-related neurodegenerative and neuromuscular diseases. In light of the role of sHSPs in the clearance of un/misfolded aggregation-prone substrates, pharmacological modulation of sHSP expression or function and rescue of defective sHSPs represent possible routes to alleviate or cure protein conformation diseases. Here, we report the latest news and views on sHSPs discussed by many of the world's experts in the sHSP field during a dedicated workshop organized in Italy (Bertinoro, CEUB, October 12-15, 2016).

PECTIN METHYLESTERASE34 Contributes to Heat Tolerance through Its Role in Promoting Stomatal Movement

Pectin, a major component of the primary cell wall, is synthesized in the Golgi apparatus and exported to the cell wall in a highly methylesterified form, then is partially demethylesterified by pectin methylesterases (PMEs; EC 3.1.1.11). PME activity on the status of pectin methylesterification profoundly affects the properties of pectin and, thereby, is critical for plant development and the plant defense response, although the roles of PMEs under heat stress (HS) are poorly understood. Functional genome annotation predicts that at least 66 potential PME genes are contained in Arabidopsis (Arabidopsis thaliana). Thermotolerance assays of PME gene T-DNA insertion lines revealed two null mutant alleles of PME34 (At3g49220) that both consistently showed reduced thermotolerance. Nevertheless, their impairment was independently associated with the expression of HS-responsive genes. It was also observed that PME34 transcription was induced by abscisic acid and highly expressed in guard cells. We showed that the PME34 mutation has a defect in the control of stomatal movement and greatly altered PME and polygalacturonase (EC 3.2.1.15) activity, resulting in a heat-sensitive phenotype. PME34 has a role in the regulation of transpiration through the control of the stomatal aperture due to its cell wall-modifying enzyme activity during the HS response. Hence, PME34 is required for regulating guard cell wall flexibility to mediate the heat response in Arabidopsis.

Honey can repairing damage of liver tissue due to protein energy malnutrition through induction of endogenous stem cells

AIM

This study was to evaluate effect of honey in repairing damage of liver tissue due to energy protein malnutrition and in mobilization of endogenous stem cells.

MATERIALS AND METHODS

Male mice model of degenerative liver was obtained through food fasting but still have drinking water for 5 days. It caused energy protein malnutrition and damage of liver tissue. The administration of 50% (v/v) honey was performed for 10 consecutive days, while the positive control group was fasted and not given honey and the negative control not fasted and without honey. Observations of regeneration the liver tissue based on histologically examination, observation of Hsp70 expression, and homing signal based on vascular endothelial growth factor-1 (VEGF-1) expression using immunohistochemistry technique. Observation on expression of CD34 and CD45 as the marker of auto mobilization of hematopoietic stem cells using flow cytometry technique.

RESULTS

There is regeneration of the liver tissue due to protein energy malnutrition, decrease of Hsp70 expression, increase of VEGF-1 expression, and high expression of CD34 and CD45.

CONCLUSION

Honey can improve the liver tissue based on: (1) Mobilization of endogenous stem cells (CD34 and CD45); (2) Hsp70 and VEGF-1 expressions as regeneration marker of improvement, and (3) regeneration histologically of liver tissue.

Overexpression of the Rice SUMO E3 Ligase Gene OsSIZ1 in Cotton Enhances Drought and Heat Tolerance, and Substantially Improves Fiber Yields in the Field under Reduced Irrigation and Rainfed Conditions

The Arabidopsis SUMO E3 ligase gene AtSIZ1 plays important roles in plant response to abiotic stresses as loss of function in AtSIZ1 leads to increased sensitivity to drought, heat and salt stresses. Overexpression of the AtSIZ1 rice homolog, OsSIZ1, leads to increased heat and drought tolerance in bentgrass, suggesting that the function of the E3 ligase SIZ1 is highly conserved in plants and it plays a critical role in abiotic stress responses. To test the possibility that the SUMO E3 ligase could be used to engineer drought- and heat-tolerant crops, the rice gene OsSIZ1 was overexpressed in cotton. We report here that overexpression of OsSIZ1 in cotton results in higher net photosynthesis and better growth than wild-type cotton under drought and thermal stresses in growth chamber and greenhouse conditions. Additionally, this tolerance to abiotic stresses was correlated with higher fiber yield in both controlled-environment and field trials carried out under reduced irrigation and rainfed conditions. These results suggest that OsSIZ1 is a viable candidate gene to improve crop yields under water-limited and rainfed agricultural production systems.

Identification of core subunits of photosystem II as action sites of HSP21, which is activated by the GUN5-mediated retrograde pathway in Arabidopsis

Photosystem II (PSII) is the most thermolabile photosynthetic complex. Physiological evidence suggests that the small chloroplast heat-shock protein 21 (HSP21) is involved in plant thermotolerance, but the molecular mechanism of its action remains largely unknown. Here, we have provided genetic and biochemical evidence that HSP21 is activated by the GUN5-dependent retrograde signaling pathway, and stabilizes PSII by directly binding to its core subunits such as D1 and D2 proteins under heat stress. We further demonstrate that the constitutive expression of HSP21 sufficiently rescues the thermosensitive stability of PSII and survival defects of the gun5 mutant with dramatically improving granal stacking under heat stress, indicating that HSP21 is a key chaperone protein in maintaining the integrity of the thylakoid membrane system under heat stress. In line with our interpretation based on several lines of in vitro and in vivo protein-interaction evidence that HSP21 interacts with core subunits of PSII, the kinetics of HSP21 binding to the D1 and D2 proteins was determined by performing an analysis of microscale thermophoresis. Considering the major role of HSP21 in protecting the core subunits of PSII from thermal damage, its heat-responsive activation via the heat-shock transcription factor HsfA2 is critical for the survival of plants under heat stress. Our findings reveal an auto-adaptation loop pathway that plant cells optimize particular needs of chloroplasts in stabilizing photosynthetic complexes by relaying the GUN5-dependent plastid signal(s) to activate the heat-responsive expression of HSP21 in the nucleus during adaptation to heat stress in plants.

The Arabidopsis heat-intolerant 5 (hit5)/enhanced response to aba 1 (era1) mutant reveals the crucial role of protein farnesylation in plant responses to heat stress

Protein farnesylation is a post-translational modification known to regulate abscisic acid (ABA)-mediated drought tolerance in plants. However, it is unclear whether and to what extent protein farnesylation affects plant tolerance to high-temperature conditions. The Arabidopsis heat-intolerant 5 (hit5) mutant was isolated because it was thermosensitive to prolonged heat incubation at 37°C for 4 d but thermotolerant to sudden heat shock at 44°C for 40 min. Map-based cloning revealed that HIT5 encodes the β-subunit of the protein farnesyltransferase. hit5 was crossed with the aba-insensitive 3 (abi3) mutant, the aba-deficient 3 (aba3) mutant, and the heat shock protein 101 (hsp101) mutant, to characterize the HIT5-mediated heat stress response. hit5/abi3 and hit5/aba3 double mutants had the same temperature-dependent phenotypes as hit5. Additionally, exogenous supplementation of neither ABA nor the ABA synthesis inhibitor fluridone altered the temperature-dependent phenotypes of hit5. The hit5/hsp101 double mutant was still sensitive to prolonged heat incubation, yet its ability to tolerate sudden heat shock was lost. The results suggest that protein farnesylation either positively or negatively affects the ability of plants to survive heat stress, depending on the intensity and duration of high-temperature exposure, in an ABA-independent manner. HSP101 is involved in the hit5-derived heat shock tolerance phenotype.

Antarctic Moss Multiprotein Bridging Factor 1c Overexpression in Arabidopsis Resulted in Enhanced Tolerance to Salt Stress

Polytrichastrum alpinum is one of the moss species that survives extreme conditions in the Antarctic. In order to explore the functional benefits of moss genetic resources, P. alpinum multiprotein-bridging factor 1c gene (PaMBF1c) was isolated and characterized. The deduced amino acid sequence of PaMBF1c comprises of a multiprotein-bridging factor (MBF1) domain and a helix-turn-helix (HTH) domain. PaMBF1c expression was induced by different abiotic stresses in P. alpinum, implying its roles in stress responses. We overexpressed PaMBF1c in Arabidopsis and analyzed the resulting phenotypes in comparison with wild type and/or Arabidopsis MBF1c (AtMBF1c) overexpressors. Overexpression of PaMBF1c in Arabidopsis resulted in enhanced tolerance to salt and osmotic stress, as well as to cold and heat stress. More specifically, enhanced salt tolerance was observed in PaMBF1c overexpressors in comparison to wild type but not clearly observable in AtMBF1c overexpressing lines. Thus, these results implicate the evolution of PaMBF1c under salt-enriched Antarctic soil. RNA-Seq profiling of NaCl-treated plants revealed that 10 salt-stress inducible genes were already up-regulated in PaMBF1c overexpressing plants even before NaCl treatment. Gene ontology enrichment analysis with salt up-regulated genes in each line uncovered that the terms lipid metabolic process, ion transport, and cellular amino acid biosynthetic process were significantly enriched in PaMBF1c overexpressors. Additionally, gene enrichment analysis with salt down-regulated genes in each line revealed that the enriched categories in wild type were not significantly overrepresented in PaMBF1c overexpressing lines. The up-regulation of several genes only in PaMBF1c overexpressing lines suggest that enhanced salt tolerance in PaMBF1c-OE might involve reactive oxygen species detoxification, maintenance of ATP homeostasis, and facilitation of Ca²⁺ signaling. Interestingly, many salt down-regulated ribosome- and translation-related genes were not down-regulated in PaMBF1c overexpressing lines under salt stress. These differentially regulated genes by PaMBF1c overexpression could contribute to the enhanced tolerance in PaMBF1c overexpressing lines under salt stress.

In silico study on Arabidopsis BAG gene expression in response to environmental stresses

BAG (Bcl-2 athanogene) family proteins are conserved in a wide range of eukaryotes, and they have been proposed to play a crucial role in plant programmed cell death (PCD). During the past decade, with the help of advanced bioinformatics tools, seven homologs of BAG genes have been identified in the Arabidopsis genome; these genes are involved in pathogen attack and abiotic stress conditions. In this study, gene expression of Arabidopsis BAG family members under environmental stresses was analyzed using the Botany Array Resource (BAR) expression browser tool and the in silico data were partially confirmed by qRT-PCR analysis for the selected stress- and hormone-treated conditions related to environmental stresses. Particularly, the induction of AtBAG6 gene in response to heat shock was confirmed by using GUS reporter lines. The loss of the AtBAG6 gene resulted into impairment in basal thermotolerance of plant and showed enhanced cell death in response to heat stress. To elucidate the regulatory mechanisms of BAG genes, we analyzed ∼1-kbp promoter regions for the presence of stress-responsive elements. Our transcription profiling finally revealed that the Arabidopsis BAG genes differentially respond to environmental stresses under the control of specifically organized upstream regulatory elements.

Tissue Culture as a Source of Replicates in Nonmodel Plants: Variation in Cold Response in Arabidopsis lyrata ssp. petraea

While genotype-environment interaction is increasingly receiving attention by ecologists and evolutionary biologists, such studies need genetically homogeneous replicates-a challenging hurdle in outcrossing plants. This could be potentially overcome by using tissue culture techniques. However, plants regenerated from tissue culture may show aberrant phenotypes and "somaclonal" variation. Here, we examined somaclonal variation due to tissue culturing using the response to cold treatment of photosynthetic efficiency (chlorophyll fluorescence measurements for Fv/Fm, Fv'/Fm', and ΦPSII, representing maximum efficiency of photosynthesis for dark- and light-adapted leaves, and the actual electron transport operating efficiency, respectively, which are reliable indicators of photoinhibition and damage to the photosynthetic electron transport system). We compared this to variation among half-sibling seedlings from three different families of Arabidopsis lyrata ssp. petraea Somaclonal variation was limited, and we could detect within-family variation in change in chlorophyll fluorescence due to cold shock successfully with the help of tissue-culture derived replicates. Icelandic and Norwegian families exhibited higher chlorophyll fluorescence, suggesting higher performance after cold shock, than a Swedish family. Although the main effect of tissue culture on Fv/Fm, Fv'/Fm', and ΦPSII was small, there were significant interactions between tissue culture and family, suggesting that the effect of tissue culture is genotype-specific. Tissue-cultured plantlets were less affected by cold treatment than seedlings, but to a different extent in each family. These interactive effects, however, were comparable to, or much smaller than the single effect of family. These results suggest that tissue culture is a useful method for obtaining genetically homogenous replicates for studying genotype-environment interaction related to adaptively-relevant phenotypes, such as cold response, in nonmodel outcrossing plants.

Reactive electrophilic oxylipins trigger a heat stress-like response through HSFA1 transcription factors

Abiotic and biotic stresses are often characterized by an induction of reactive electrophile species (RES) such as the jasmonate 12-oxo-phytodienoic acid (OPDA) or the structurally related phytoprostanes. Previously, RES oxylipins have been shown massively to induce heat-shock-response (HSR) genes including HSP101 chaperones. Moreover, jasmonates have been reported to play a role in basal thermotolerance. We show that representative HSR marker genes are strongly induced by RES oxylipins through the four master regulator transcription factors HSFA1a, b, d, and e essential for short-term adaptation to heat stress in Arabidopsis. When compared with Arabidopsis seedlings treated at the optimal acclimation temperature of 37 °C, the exogenous application of RES oxylipins at 20 °C induced a much weaker induction of HSP101 at both the gene and protein expression levels which, however, was not sufficient to confer short-term acquired thermotolerance. Moreover, jasmonate-deficient mutant lines displayed a wild-type-like HSR and were not compromised in acquiring thermotolerance. Hence, the OPDA- and RES oxylipin-induced HSR is not sufficient to protect seedlings from severe heat stress but may help plants to cope better with stresses associated with protein unfolding by inducing a battery of chaperones in the absence of heat.

Class I and II Small Heat Shock Proteins Together with HSP101 Protect Protein Translation Factors during Heat Stress

The ubiquitous small heat shock proteins (sHSPs) are well documented to act in vitro as molecular chaperones to prevent the irreversible aggregation of heat-sensitive proteins. However, the in vivo activities of sHSPs remain unclear. To investigate the two most abundant classes of plant cytosolic sHSPs (class I [CI] and class II [CII]), RNA interference (RNAi) and overexpression lines were created in Arabidopsis (Arabidopsis thaliana) and shown to have reduced and enhanced tolerance, respectively, to extreme heat stress. Affinity purification of CI and CII sHSPs from heat-stressed seedlings recovered eukaryotic translation elongation factor (eEF) 1B (α-, β-, and γ-subunits) and eukaryotic translation initiation factor 4A (three isoforms), although the association with CI sHSPs was stronger and additional proteins involved in translation were recovered with CI sHSPs. eEF1B subunits became partially insoluble during heat stress and, in the CI and CII RNAi lines, showed reduced recovery to the soluble cell fraction after heat stress, which was also dependent on HSP101. Furthermore, after heat stress, CI sHSPs showed increased retention in the insoluble fraction in the CII RNAi line and vice versa. Immunolocalization revealed that both CI and CII sHSPs were present in cytosolic foci, some of which colocalized with HSP101 and with eEF1Bγ and eEF1Bβ. Thus, CI and CII sHSPs have both unique and overlapping functions and act either directly or indirectly to protect specific translation factors in cytosolic stress granules.

The Heat Stress Factor HSFA6b Connects ABA Signaling and ABA-Mediated Heat Responses

Heat stress response (HSR) is a conserved mechanism developed to increase the expression of heat shock proteins (HSPs) via a heat shock factor (HSF)-dependent mechanism. Signaling by the stress phytohormone abscisic acid (ABA) is involved in acquired thermotolerance as well. Analysis of Arabidopsis (Arabidopsis thaliana) microarray databases revealed that the expression of HSFA6b, a class A HSF, extensively increased with salinity, osmotic, and cold stresses, but not heat. Here, we show that HSFA6b plays a pivotal role in the response to ABA and in thermotolerance. Salt-inducible HSFA6b expression was down-regulated in ABA-insensitive and -deficient mutants; however, exogenous ABA application restored expression in ABA-deficient, but not -insensitive plants. Thus, ABA signaling is required for proper HSFA6b expression. A transcriptional activation assay of protoplasts revealed that ABA treatment and coexpression of an ABA signaling master effector, ABA-RESPONSIVE ELEMENT-BINDING PROTEIN1, could activate the HSFA6b promoter. In addition, HSFA6b directly bound to the promoter of DEHYDRATION-RESPONSIVE ELEMENT-BINDING PROTEIN2A and enhanced its expression. Analysis of ABA responses in seed germination, cotyledon greening, and root growth as well as salt and drought tolerance in HSFA6b-null, overexpression, and dominant negative mutants revealed that HSFA6b is a positive regulator participating in ABA-mediated salt and drought resistance. Thermoprotection tests showed that HSFA6b was required for thermotolerance acquisition. Our study reveals a network in which HSFA6b operates as a downstream regulator of the ABA-mediated stress response and is required for heat stress resistance. This new ABA-signaling pathway is integrated into the complex HSR network in planta.

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.

The plastid metalloprotease FtsH6 and small heat shock protein HSP21 jointly regulate thermomemory in Arabidopsis

Acquired tolerance to heat stress is an increased resistance to elevated temperature following a prior exposure to heat. The maintenance of acquired thermotolerance in the absence of intervening stress is called 'thermomemory' but the mechanistic basis for this memory is not well defined. Here we show that Arabidopsis HSP21, a plastidial small heat shock protein that rapidly accumulates after heat stress and remains abundant during the thermomemory phase, is a crucial component of thermomemory. Sustained memory requires that HSP21 levels remain high. Through pharmacological interrogation and transcriptome profiling, we show that the plastid-localized metalloprotease FtsH6 regulates HSP21 abundance. Lack of a functional FtsH6 protein promotes HSP21 accumulation during the later stages of thermomemory and increases thermomemory capacity. Our results thus reveal the presence of a plastidial FtsH6-HSP21 control module for thermomemory in plants.

The roles of mitochondrial transcription termination factors (MTERFs) in plants

Stress such as salinity, cold, heat or drought affect plant growth and development, and frequently result in diminished productivity. Unlike animals, plants are sedentary organisms that must withstand and cope with environmental stresses. During evolution, plants have developed strategies to successfully adapt to or tolerate such stresses, which might have led to the expansion and functional diversification of gene families. Some new genes may have acquired functions that could differ from those of their animal homologues, e.g. in response to abiotic stress. The mitochondrial transcription termination factor (MTERF) family could be a good example of this. Originally identified and characterized in metazoans, MTERFs regulate transcription, translation and DNA replication in vertebrate mitochondria. Plant genomes harbor a considerably larger number of MTERFs than animals. Nonetheless, only eight plant MTERFs have been characterized, which encode chloroplast or mitochondrial proteins. Mutations in MTERFs alter the expression of organelle genes and impair chloroplast or mitochondria development. This information is transmitted to the nucleus, probably through retrograde signaling, because mterf plants often exhibit changes in nuclear gene expression. This study summarizes the recent findings, mainly from the analysis of mterf mutants, which support an emerging role for plant MTERFs in response to abiotic stress.

Acclimation to high temperature during pollen development

KEY MESSAGE

Pollen heat acclimation. As a consequence of global warming, plants have to face more severe and more frequently occurring periods of high temperature stress. While this affects the whole plant, development of the male gametophyte, the pollen, seems to be the most sensitive process. Given the great importance of functioning pollen for the plant life cycle and for agricultural production, it is necessary to understand this sensitivity. While changes in temperature affect different components of all cells and require a cellular response and acclimation, high temperature effects and responses in developing pollen are distinct from vegetative tissues at several points. This could be related to specific physiological characteristics of developing pollen and supporting tissues which make them vulnerable to high temperature, or its derived effects such as ROS accumulation and carbohydrate starvation. But also expression of heat stress-responsive genes shows unique patterns in developing pollen when compared to vegetative tissues that might explain the failure to withstand high temperatures. As an alternative to viewing pollen failure under high temperature as a result of inherent sensitivity of a specific developmental process, we end by discussing whether it might actually be an adaptation.

Habitat-Associated Life History and Stress-Tolerance Variation in Arabidopsis arenosa

Weediness in ephemeral plants is commonly characterized by rapid cycling, prolific all-in flowering, and loss of perenniality. Many species made transitions to weediness of this sort, which can be advantageous in high-disturbance or human-associated habitats. The molecular basis of this shift, however, remains mostly mysterious. Here, we use transcriptome sequencing, genome resequencing scans for selection, and stress tolerance assays to study a weedy population of the otherwise nonweedy Arabidopsis arenosa, an obligately outbreeding relative of Arabidopsis thaliana Although weedy A. arenosa is widespread, a single genetic lineage colonized railways throughout central and northern Europe. We show that railway plants, in contrast to plants from sheltered outcrops in hill/mountain regions, are rapid cycling, have lost the vernalization requirement, show prolific flowering, and do not return to vegetative growth. Comparing transcriptomes of railway and mountain plants across time courses with and without vernalization, we found that railway plants have sharply abrogated vernalization responsiveness and high constitutive expression of heat- and cold-responsive genes. Railway plants also have strong constitutive heat shock and freezing tolerance compared with mountain plants, where tolerance must be induced. We found 20 genes with good evidence of selection in the railway population. One of these, LATE ELONGATED HYPOCOTYL, is known in A. thaliana to regulate many stress-response genes that we found to be differentially regulated among the distinct habitats. Our data suggest that, beyond life history regulation, other traits like basal stress tolerance also are associated with the evolution of weediness in A. arenosa.

Characterization of 5'UTR of rice ClpB-C/Hsp100 gene: evidence of its involvement in post-transcriptional regulation

Rice (Oryza sativa) ClpB-C (OsClpB-C) protein is expressed upon heat stress in vegetative tissues and constitutively in seeds. We produced stably transformed Arabidopsis plants carrying β-glucuronidase (Gus) reporter gene downstream to 1-kb OsClpB-C promoter (1kbPro plants). In the 1kbPro plants, expression of Gus transcript and protein followed the expression pattern of OsClpB-C gene in rice plants, i.e., heat induced in vegetative tissues and constitutive in seeds. Next, we produced transgenic Arabidopsis plants containing Gus downstream to 862-bp fragment of OsClpB-C promoter [lacking 138 nucleotides from 3' end of the 5'untranslated region (5'UTR); ∆UTR plants). In ∆UTR plants, Gus transcript was expressed in heat-inducible manner, but strikingly, Gus protein levels were negligible after heat treatment. However, Gus protein was expressed in ∆UTR seedlings at levels comparable to 1kbPro seedlings when recovery treatment of 22 °C/2 h was given post heat stress (38 °C/15 min). This suggests that 5'UTR of OsClpB-C gene is involved in its post-transcriptional regulation and is an obligate requirement for protein expression during persistent heat stress. Furthermore, the Gus transcript levels were higher in the polysomal RNA fraction in heat-stressed seedlings of 1kbPro plants as compared to ∆UTR plants, indicating that 5'UTR aids in assembly of ribosomes onto the Gus transcript during heat stress. Unlike the case of seedlings, Gus protein was formed constitutively in ∆UTR seeds at levels comparable to 1kbPro seeds. Hence, the function of 5'UTR of OsClpB-C is dispensable for expression in seeds.

Histone acetyltransferase GCN5 is essential for heat stress-responsive gene activation and thermotolerance in Arabidopsis

Exposure to temperatures exceeding the normal optimum levels, or heat stress (HS), constitutes an environmental disruption for plants, resulting in severe growth and development retardation. Here we show that loss of function of the Arabidopsis histone acetyltransferase GCN5 results in serious defects in terms of thermotolerance, and considerably impairs the transcriptional activation of HS-responsive genes. Notably, expression of several key regulators such as the HS transcription factors HSFA2 and HSFA3, Multiprotein Bridging Factor 1c (MBF1c) and UV-HYPERSENSITIVE 6 (UVH6) is down-regulated in the gcn5 mutant under HS compared with the wild-type. Chromatin immunoprecipitation (ChIP) assays indicated that GCN5 protein is enriched at the promoter regions of HSFA3 and UVH6 genes, but not in HSFA2 and MBF1c, and that GCN5 facilitates H3K9 and H3K14 acetylation, which are associated with HSFA3 and UVH6 activation under HS. Moreover, constitutive expression of UVH6 in the gcn5 mutant partially restores heat tolerance. Taken together, our data indicate that GCN5 plays a key role in the preservation of thermotolerance via versatile regulation in Arabidopsis. In addition, expression of the wheat TaGCN5 gene re-establishes heat tolerance in Arabidopsis gcn5 mutant plants, suggesting that GCN5-mediated thermotolerance may be conserved between Arabidopsis and wheat.

An antimicrobial peptide essential for bacterial survival in the nitrogen-fixing symbiosis

In the nitrogen-fixing symbiosis between legume hosts and rhizobia, the bacteria are engulfed by a plant cell membrane to become intracellular organelles. In the model legume Medicago truncatula, internalization and differentiation of Sinorhizobium (also known as Ensifer) meliloti is a prerequisite for nitrogen fixation. The host mechanisms that ensure the long-term survival of differentiating intracellular bacteria (bacteroids) in this unusual association are unclear. The M. truncatula defective nitrogen fixation4 (dnf4) mutant is unable to form a productive symbiosis, even though late symbiotic marker genes are expressed in mutant nodules. We discovered that in the dnf4 mutant, bacteroids can apparently differentiate, but they fail to persist within host cells in the process. We found that the DNF4 gene encodes NCR211, a member of the family of nodule-specific cysteine-rich (NCR) peptides. The phenotype of dnf4 suggests that NCR211 acts to promote the intracellular survival of differentiating bacteroids. The greatest expression of DNF4 was observed in the nodule interzone II-III, where bacteroids undergo differentiation. A translational fusion of DNF4 with GFP localizes to the peribacteroid space, and synthetic NCR211 prevents free-living S. meliloti from forming colonies, in contrast to mock controls, suggesting that DNF4 may interact with bacteroids directly or indirectly for its function. Our findings indicate that a successful symbiosis requires host effectors that not only induce bacterial differentiation, but also that maintain intracellular bacteroids during the host-symbiont interaction. The discovery of NCR211 peptides that maintain bacterial survival inside host cells has important implications for improving legume crops.

A Potential Role for Mitochondrial Produced Reactive Oxygen Species in Salicylic Acid-Mediated Plant Acquired Thermotolerance

To characterize the function of salicylic acid (SA) in acquired thermotolerance, the effects of heat shock (HS) on wild-type and sid2 (for SA induction deficient 2) was investigated. After HS treatment, the survival ratio of sid2 mutant was lower than that of wild-type. However, pretreatment with hydrogen peroxide (H2O2) rescued the sid2 heat sensitivity. HsfA2 is a key component of acquired thermotolerance in Arabidopsis. The expression of HsfA2 induced by SA was highest among those of heat-inducible Hsfs (HsfA2, HsfA7a, HsfA3, HsfB1, and HsfB2) in response to HS. Furthermore, the application of AsA, an H2O2 scavenger, significantly reduced the expression level of HsfA2 induced by SA. Although SA enhanced the survival of sid2 mutant, no significant effect on the hsfA2 mutant was observed, suggesting that HsfA2 is responsible for SA-induced acquired thermotolerance as a downstream factor. Further, real-time PCR analysis revealed that after HS treatment, SA also up-regulated mRNA transcription of HS protein (Hsp) genes through AtHsfA2. Time course experiments showed an increase in the fluorescence intensity of DCF in the mitochondria occurred earlier than in other regions of the protoplasts in response to SA. The cytochrome reductase activity analysis in isolated mitochondria demonstrated that SA-induced mitochondrial ROS possibly originated from complex III in the respiration chain. Collectively, our data suggest that SA functions and acts upstream of AtHsfA2 in acquired thermotolerance, which requires a pathway with H2O2 production involved and is dependent on increased expression of Hsp genes.

Metazoan Hsp70-based protein disaggregases: emergence and mechanisms

Proteotoxic stresses and aging cause breakdown of cellular protein homeostasis, allowing misfolded proteins to form aggregates, which dedicated molecular machines have evolved to solubilize. In bacteria, fungi, protozoa and plants protein disaggregation involves an Hsp70•J-protein chaperone system, which loads and activates a powerful AAA+ ATPase (Hsp100) disaggregase onto protein aggregate substrates. Metazoans lack cytosolic and nuclear Hsp100 disaggregases but still eliminate protein aggregates. This longstanding puzzle of protein quality control is now resolved. Robust protein disaggregation activity recently shown for the metazoan Hsp70-based disaggregases relies instead on a crucial cooperation between two J-protein classes and interaction with the Hsp110 co-chaperone. An expanding multiplicity of Hsp70 and J-protein family members in metazoan cells facilitates different configurations of this Hsp70-based disaggregase allowing unprecedented versatility and specificity in protein disaggregation. Here we review the architecture, operation, and adaptability of the emerging metazoan disaggregation system and discuss how this evolved.

Metazoan Hsp70-based protein disaggregases: emergence and mechanisms

Proteotoxic stresses and aging cause breakdown of cellular protein homeostasis, allowing misfolded proteins to form aggregates, which dedicated molecular machines have evolved to solubilize. In bacteria, fungi, protozoa and plants protein disaggregation involves an Hsp70•J-protein chaperone system, which loads and activates a powerful AAA+ ATPase (Hsp100) disaggregase onto protein aggregate substrates. Metazoans lack cytosolic and nuclear Hsp100 disaggregases but still eliminate protein aggregates. This longstanding puzzle of protein quality control is now resolved. Robust protein disaggregation activity recently shown for the metazoan Hsp70-based disaggregases relies instead on a crucial cooperation between two J-protein classes and interaction with the Hsp110 co-chaperone. An expanding multiplicity of Hsp70 and J-protein family members in metazoan cells facilitates different configurations of this Hsp70-based disaggregase allowing unprecedented versatility and specificity in protein disaggregation. Here we review the architecture, operation, and adaptability of the emerging metazoan disaggregation system and discuss how this evolved.

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.

The Wheat NAC Transcription Factor TaNAC2L Is Regulated at the Transcriptional and Post-Translational Levels and Promotes Heat Stress Tolerance in Transgenic Arabidopsis

Heat stress poses a serious threat to global crop production. In efforts that aim to mitigate the adverse effects of heat stress on crops, a variety of genetic tools are being used to develop plants with improved thermotolerance. The characterization of important regulators of heat stress tolerance provides essential information for this aim. In this study, we examine the wheat (Triticum aestivum) NAC transcription factor gene TaNAC2L. High temperature induced TaNAC2L expression in wheat and overexpression of TaNAC2L in Arabidopsis thaliana enhanced acquired heat tolerance without causing obvious alterations in phenotype compared with wild type under normal conditions. TaNAC2L overexpression also activated the expression of heat-related genes in the transgenic Arabidopsis plants, suggesting that TaNAC2L may improve heat tolerance by regulating the expression of stress-responsive genes. Notably, TaNAC2L is also regulated at the post-translational level and might be degraded via a proteasome-mediated pathway. Thus, this wheat transcription factor may have potential uses in enhancing thermotolerance in crops.

Accumulation of extra-chloroplastic triacylglycerols in Arabidopsis seedlings during heat acclimation

Heat acclimation enables plants to tolerate and survive short-term heat stress on hot days. In Arabidopsis thaliana, a genetically programmed heat shock response can be rapidly triggered in the temperature range of 32-38°C through activation of heat shock transcription factors (HSF). The heat shock response leads to heat acclimation and confers short-term protection against temperatures above 40°C. However, little is known about metabolic adjustments during heat acclimation.Untargeted metabolite analyses of A. thaliana seedlings revealed that levels of polyunsaturated triacylglycerols (TG) rapidly and dramatically increase during heat acclimation. TG accumulation was found to be temperature dependent in a temperature range of 32-50°C (optimum at 42°C) and reversible after a return from 37°C to normal growth temperatures. Heat-induced TGs accumulated in extra-chloroplastic compartments and increased in both roots and shoots to a similar extent. Analysis of mutants deficient in all four HSFA1 master regulator genes or the HSFA2 gene revealed that TG accumulation was not dependent on HSFs. Moreover, the TG response was not limited to heat stress because drought and salt stress also triggered an accumulation of TGs, but not short-term osmotic, cold, and high light stress. Lipid analysis revealed that heat-induced accumulation of TGs was not due to massive de novo fatty acid synthesis. It is hypothesized that TGs serve as transient stores for fatty acids that may be required for membrane remodelling during heat acclimation.

Cooperation of Hsp70 and Hsp100 chaperone machines in protein disaggregation

Unicellular and sessile organisms are particularly exposed to environmental stress such as heat shock causing accumulation and aggregation of misfolded protein species. To counteract protein aggregation, bacteria, fungi, and plants encode a bi-chaperone system composed of ATP-dependent Hsp70 and hexameric Hsp100 (ClpB/Hsp104) chaperones, which rescue aggregated proteins and provide thermotolerance to cells. The partners act in a hierarchic manner with Hsp70 chaperones coating first the surface of protein aggregates and next recruiting Hsp100 through direct physical interaction. Hsp100 proteins bind to the ATPase domain of Hsp70 via their unique M-domain. This extra domain functions as a molecular toggle allosterically controlling ATPase and threading activities of Hsp100. Interactions between neighboring M-domains and the ATPase ring keep Hsp100 in a repressed state exhibiting low ATP turnover. Breakage of intermolecular M-domain interactions and dissociation of M-domains from the ATPase ring relieves repression and allows for Hsp70 interaction. Hsp70 binding in turn stabilizes Hsp100 in the activated state and primes Hsp100 ATPase domains for high activity upon substrate interaction. Hsp70 thereby couples Hsp100 substrate binding and motor activation. Hsp100 activation presumably relies on increased subunit cooperation leading to high ATP turnover and threading power. This Hsp70-mediated activity control of Hsp100 is crucial for cell viability as permanently activated Hsp100 variants are toxic. Hsp100 activation requires simultaneous binding of multiple Hsp70 partners, restricting high Hsp100 activity to the surface of protein aggregates and ensuring Hsp100 substrate specificity.

Enhancement of reproductive heat tolerance in plants

Comparison of average crop yields with reported record yields has shown that major crops exhibit annual average yields three- to seven-fold lower than record yields because of unfavorable environments. The current study investigated the enhancement of pollen heat tolerance through expressing an Arabidopsis thaliana heat shock protein 101 (AtHSP101) that is not normally expressed in pollen but reported to play a crucial role in vegetative thermotolerance. The AtHSP101 construct under the control of the constitutive ocs/mas 'superpromoter' was transformed into cotton Coker 312 and tobacco SRI lines via Agrobacterium mediated transformation. Thermotolerance of pollen was evaluated by in vitro pollen germination studies. Comparing with those of wild type and transgenic null lines, pollen from AtHSP101 transgenic tobacco and cotton lines exhibited significantly higher germination rate and much greater pollen tube elongation under elevated temperatures or after a heat exposure. In addition, significant increases in boll set and seed numbers were also observed in transgenic cotton lines exposed to elevated day and night temperatures in both greenhouse and field studies. The results of this study suggest that enhancing heat tolerance of reproductive tissues in plant holds promise in the development of crops with improved yield production and yield sustainability in unfavorable environments.

Heat stress in grapevine: the pros and cons of acclimation

Heat stress is a major limiting factor of grapevine production and quality. Acclimation and recovery are essential to ensure plant survival, and the recovery mechanisms can be independent of the heat response mechanisms. An experimental set up with and without acclimation to heat followed by recovery [stepwise acclimation and recovery (SAR) and stepwise recovery (SR), respectively] was applied to two grapevine varieties, Touriga Nacional (TN), and Trincadeira (TR), with different tolerance to abiotic stress. Major differences were found between leaves of SAR and SR, especially after recovery; in SAR, almost all parameters returned to basal levels while in SR they remained altered. Acclimation led to a swifter and short-term antioxidative response, affecting the plant to a lesser extent than SR. Significant differences were found among varieties: upon stress, TN significantly increased ascorbate and glutathione reduction levels, boosting the cell's redox-buffering capacity, while TR needed to synthesize both metabolites, its response being insufficient to keep the redox state at working levels. TR was affected by stress for a longer period and the up-regulation pattern of antioxidative stress genes was more obvious. In TN, heat shock proteins were significantly induced, but the canonical heat-stress gene signature was not evident probably because no shutdown of the housekeeping metabolism was needed.

Non-equilibrium conformational dynamics in the function of molecular chaperones

Why do chaperones need ATP hydrolysis to help proteins reach their native, functional states? In this review, we highlight the most recent experimental and theoretical evidences suggesting that ATP hydrolysis allows molecular chaperones to escape the bounds imposed by equilibrium thermodynamics. We argue here that energy consumption must be fully taken into account to understand the mechanism of these intrinsically non-equilibrium machines and we propose a novel perspective in the way the relation between function and ATP hydrolysis is viewed.

Antimycin A sensitive pathway independent from PGR5 cyclic electron transfer triggers non-photochemical reduction of PQ pool and state transitions in Arabidopsis thaliana

We investigated the mechanism involved in triggering state transitions at 40°C in Arabidopsis thaliana. Leaves (1-6 week old) exposed to 40°C exhibited state II transition indicating its role as one of the earliest stress responsive mechanism apart from regulation of light energy distribution between photosystem (PS)II and PSI. Post illumination transients (rise in Fo') revealed that non-photochemical reduction of PQ pool at 40°C in dark is responsible for activation of STN7 kinase, consequently light harvesting complex (LHC)II phosphorylation leading to state II condition. Later, in pgr5 mutant, non-photochemical reduction of PQ pool was observed indicating the involvement of alternative electron transfer routes. In chlororespiratory mutant crr2-2, state II transition occurred signifying that the reduction of PQ pool is independent from NDH mediated cyclic electron transfer. Further, antimycin A inhibitor studies in wt and mutants revealed its inhibitory action on non-photochemical reduction of PQ pool affecting both LHCII phosphorylation and migration to PSI which leads to state I. Thus, our study showed that antimycin A sensitive pathway independent from PGR5 dependent cyclic electron transfer, is responsible for inducing non-photochemical reduction of PQ pool and state transitions at 40°C.

Reactive short-chain leaf volatiles act as powerful inducers of abiotic stress-related gene expression

Abiotic stresses cause serious damage to plants; therefore, plants undergo a complicated stress response through signal transduction originating from environmental stimuli. Here we show that a subset of short-chain leaf volatiles with an α, β-unsaturated carbonyl bond in their structure (reactive short-chain leaf volatiles, RSLVs) like (E)-2-hexenal and (E)-2-butenal can act as signal chemicals that strongly induce the gene expression of abiotic-related transcription factors, such as heat stress-related transcription factors (HSFA2, MBF1c) and other abiotic stress-related transcription factors (DREB2A, ZATs). RSLV-induced expression of HSFA2 and MBF1c was eliminated in HSFA1s-, known as heat stress response master regulators, knockout mutant, whereas those of DREB2A and ZATs were not, suggesting that the RSLV signaling pathway is composed of HSFA1-dependent and -independent pathways. RSLV treatment induced production of chaperon proteins, and the RSLV-treated Arabidopsis thus demonstrated enhanced abiotic stress tolerance. Because oxidative stress treatment enhanced RSLV production, we concluded that commonly found RSLVs produced by environmental stresses are powerful inducer of abiotic stress-related gene expression as oxidative stress signals.

Translatome profiling in dormant and nondormant sunflower (Helianthus annuus) seeds highlights post-transcriptional regulation of germination

Seed dormancy, which blocks germination in apparently favourable conditions, is a key regulatory control point of plant population establishment. As germination requires de novo translation, its regulation by dormancy is likely to be related to the association of individual transcripts to polysomes. Here, the polysome-associated mRNAs, that is, the translatome, were fractionated and characterized with microarrays in dormant and nondormant sunflower (Helianthus annuus) embryos during their imbibition at 10°C, a temperature preventing germination of dormant embryos. Profiling of mRNAs in polysomal complexes revealed that the translatome differs between germinating and nongerminating embryos. Association of transcripts with polysomes reached a maximum after 15 h of imbibition; at this time-point 194 polysome-associated transcripts were specifically found in nondormant embryos and 47 in dormant embryos only. The proteins corresponding to the polysomal mRNAs in nondormant embryos appeared to be very pertinent for germination and were involved mainly in transport, regulation of transcription or cell wall modifications. This work demonstrates that seed germination results from a timely regulated and selective recruitment of mRNAs to polysomes, thus opening novel fields of investigation for the understanding of this developmental process.

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.

A wheat lipid transfer protein 3 could enhance the basal thermotolerance and oxidative stress resistance of Arabidopsis

Wheat (Triticum aestivum L.) is one of the major grain crops, and heat stress adversely affects wheat production in many regions of the world. Previously, we found a heat-responsive gene named Lipid Transfer Protein 3 (TaLTP3) in wheat. TaLTP3 was deduced to be regulated by cold, ABA, MeJA, Auxin and oxidative stress according to cis-acting motifs in its promoter sequences. In this study, we show that TaLTP3 is responsive to prolonged water deficit, salt or ABA treatment in wheat seedlings. Also, TaLTP3 accumulation was observed after the plant suffered from heat stress both at the seedling and the grain-filling stages. TaLTP3 protein was localized in the cell membrane and cytoplasm of tobacco epidermal cells. Overexpression of TaLTP3 in yeast imparted tolerance to heat stress compared to cells expressing the vector alone. Most importantly, transgenic Arabidopsis plants engineered to overexpress TaLTP3 showed higher thermotolerance than control plants at the seedling stage. Further investigation indicated that transgenic lines decreased H₂O₂ accumulation and membrane injury under heat stress. Taken together, our results demonstrate that TaLTP3 confers heat stress tolerance possibly through reactive oxygen species (ROS) scavenging.