Chloroplast protein synthesis elongation factor, EF-Tu, reduces thermal aggregation of rubisco activase

Valsa mali effector Vm_04797 interacts with adaptor protein MdAP-2β to manipulate host autophagy

Apple Valsa canker, caused by the ascomycete fungus Valsa mali, employs virulence effectors to disturb host immunity and poses a substantial threat to the apple industry. However, our understanding of how V. mali effectors regulate host defense responses remains limited. Here, we identified the V. mali effector Vm_04797, which was upregulated during the early infection stage. Vm_04797, a secreted protein, suppressed Inverted formin 1 (INF1)-triggered cell death in Nicotiana benthamiana and performed virulence functions inside plant cells. Vm_04797 deletion mutants showed substantially reduced virulence toward apple. The adaptor protein MdAP-2β positively regulated apple Valsa canker resistance and was targeted and degraded by Vm_04797 via the ubiquitination pathway. The in vitro analysis suggested that Vm_04797 possesses E3 ubiquitin ligase activity. Further analysis revealed that MdAP-2β is involved in autophagy by interacting with Malus domestica autophagy protein 16 MdATG16 and promoting its accumulation. By degrading MdAP-2β, Vm_04797 inhibited autophagic flux, thereby disrupting the defense response mediated by autophagy. Our findings provide insights into the molecular mechanisms employed by the effectors of E3 ubiquitin ligase activity in ascomycete fungi to regulate host immunity.

Elongation factor MdEF-Tu coordinates with heat shock protein MdHsp70 to enhance apple thermotolerance

High-temperature stress results in protein misfolding/unfolding and subsequently promotes the accumulation of cytotoxic protein aggregates that can compromise cell survival. Heat shock proteins (HSPs) function as molecular chaperones that coordinate the refolding and degradation of aggregated proteins to mitigate the detrimental effects of high temperatures. However, the relationship between HSPs and protein aggregates in apples under high temperatures remains unclear. Here, we show that an apple (Malus domestica) chloroplast-localized, heat-sensitive elongation factor Tu (MdEF-Tu), positively regulates apple thermotolerance when it is overexpressed. Transgenic apple plants exhibited higher photosynthetic capacity and better integrity of chloroplasts during heat stress. Under high temperatures, MdEF-Tu formed insoluble aggregates accompanied by ubiquitination modifications. Furthermore, we identified a chaperone heat shock protein (MdHsp70), as an interacting protein of MdEF-Tu. Moreover, we observed obviously elevated MdHsp70 levels in 35S: MdEF-Tu apple plants that prevented the accumulation of ubiquitinated MdEF-Tu aggregates, which positively contributes to the thermotolerance of the transgenic plants. Overall, our results provide new insights into the molecular chaperone function of MdHsp70, which mediates the homeostasis of thermosensitive proteins under high temperatures.

Genetic variation for terminal heat stress tolerance in winter wheat

In many regions worldwide wheat (Triticum aestivum L.) plants experience terminal high temperature stress during the grain filling stage, which is a leading cause for single seed weight decrease and consequently for grain yield reduction. An approach to mitigate high temperature damage is to develop tolerant cultivars using the conventional breeding approach which involves identifying tolerant lines and then incorporating the tolerant traits in commercial varieties. In this study, we evaluated the terminal heat stress tolerance of 304 diverse elite winter wheat lines from wheat breeding programs in the US, Australia, and Serbia in controlled environmental conditions. Chlorophyll content and yield traits were measured and calculated as the percentage of non-stress control. The results showed that there was significant genetic variation for chlorophyll retention and seed weight under heat stress conditions. The positive correlation between the percent of chlorophyll content and the percent of single seed weight was significant. Two possible mechanisms of heat tolerance during grain filling were proposed. One represented by wheat line OK05723W might be mainly through the current photosynthesis since the high percentage of single seed weight was accompanied with high percentages of chlorophyll content and high shoot dry weight, and the other represented by wheat Line TX04M410164 might be mainly through the relocation of reserves since the high percentage of single seed weight was accompanied with low percentages of chlorophyll content and low shoot dry weight under heat stress. The tolerant genotypes identified in this study should be useful for breeding programs after further validation.

Candidate Genes Associated with Abiotic Stress Response in Plants as Tools to Engineer Tolerance to Drought, Salinity and Extreme Temperatures in Wheat: An Overview

Wheat represents one of the most important staple food crops worldwide and its genetic improvement is fundamental to meeting the global demand of the growing population. However, the environmental stresses, worsened by climate change, and the increasing deterioration of arable land make it very difficult to fulfil this demand. In light of this, the tolerance of wheat to abiotic stresses has become a key objective of genetic improvement, as an effective strategy to ensure high yields without increasing the cultivated land. Genetic erosion related to modern agriculture, whereby elite, high-yielding wheat varieties are the product of high selection pressure, has reduced the overall genetic diversity, including the allelic diversity of genes that could be advantageous for adaptation to adverse environmental conditions. This makes traditional breeding a less effective or slower approach to generating new stress-tolerant wheat varieties. Either mining for the diversity of not-adapted large germplasm pools, or generating new diversity, are the mainstream approaches to be pursued. The advent of genetic engineering has opened the possibility to create new plant variability and its application has provided a strong complement to traditional breeding. Genetic engineering strategies such as transgenesis and genome editing have then provided the opportunity to improve environmental tolerance traits of agronomic importance in cultivated species. As for wheat, several laboratories worldwide have successfully produced transgenic wheat lines with enhanced tolerance to abiotic stresses, and, more recently, significant improvements in the CRISPR/Cas9 tools available for targeted variations within the wheat genome have been achieved. In light of this, the present review aims to provide successful examples of genetic engineering applications for the improvement of wheat adaptation to drought, salinity and extreme temperatures, which represent the most frequent and most severe events causing the greatest losses in wheat production worldwide.

Nucleus and chloroplast: A necessary understanding to overcome heat stress in Pinus radiata

The recovery and maintenance of plant homeostasis under stressful environments are complex processes involving organelle crosstalk for a coordinated cellular response. Here, we revealed through nuclear and chloroplast subcellular proteomics, biochemical cell profiles and targeted transcriptomics how chloroplasts and nuclei developed their responses under increased temperatures in a long-lived species (Pinus radiata). Parallel to photosynthetic impairment and reactive oxygen species production in the chloroplast, a DNA damage response was triggered in the nucleus followed by an altered chromatin conformation. In addition, in the nuclei, we found several proteins, such as HEMERA or WHIRLY, which change their locations from the chloroplasts to the nuclei carrying the stress message. Additionally, our data showed a deep rearrangement of RNA metabolism in both organelles, revealing microRNAs and AGO1 as potential regulators of the acclimation mechanisms. Altogether, our study highlights the synchronisation among the different stages required for thermotolerance acquisition in P. radiata, pointing out the role of chromatin conformation and posttranscriptional gene regulation in overcoming heat stress and assuring plant survival for the following years.

A β-ketoacyl carrier protein reductase confers heat tolerance via the regulation of fatty acid biosynthesis and stress signaling in rice

Heat stress is a major environmental threat affecting crop growth and productivity. However, the molecular mechanisms associated with plant responses to heat stress are poorly understood. Here, we identified a heat stress-sensitive mutant, hts1, in rice. HTS1 encodes a thylakoid membrane-localized β-ketoacyl carrier protein reductase (KAR) involved in de novo fatty acid biosynthesis. Phylogenetic and bioinformatic analysis showed that HTS1 probably originated from streptophyte algae and is evolutionarily conserved in land plants. Thermostable HTS1 is predominantly expressed in green tissues and strongly induced by heat stress, but is less responsive to salinity, cold and drought treatments. An amino acid substitution at A254T in HTS1 causes a significant decrease in KAR enzymatic activity and, consequently, impairs fatty acid synthesis and lipid metabolism in the hts1 mutant, especially under heat stress. Compared to the wild-type, the hts1 mutant exhibited heat-induced higher H₂ O₂ accumulation, a larger Ca²⁺ influx to mesophyll cells, and more damage to membranes and chloroplasts. Also, disrupted heat stress signaling in the hts1 mutant depresses the transcriptional activation of HsfA2s and the downstream target genes. We suggest that HTS1 is critical for underpinning membrane stability, chloroplast integrity and stress signaling for heat tolerance in rice.

Sphingobacterium sp. BHU-AV3 Induces Salt Tolerance in Tomato by Enhancing Antioxidant Activities and Energy Metabolism

Salt tolerant bacteria can be helpful in improving a plant's tolerance to salinity. Although plant-bacteria interactions in response to salt stress have been characterized, the precise molecular mechanisms by which bacterial inoculation alleviates salt stress in plants are still poorly explored. In the present study, we aimed to determine the role of a salt-tolerant plant growth-promoting rhizobacteria (PGPR) Sphingobacterium BHU-AV3 for improving salt tolerance in tomato through investigating the physiological responses of tomato roots and leaves under salinity stress. Tomato plants inoculated with BHU-AV3 and challenged with 200 mM NaCl exhibited less senescence, positively correlated with the maintenance of ion balance, lowered reactive oxygen species (ROS), and increased proline content compared to the non-inoculated plants. BHU-AV3-inoculated plant leaves were less affected by oxidative stress, as evident from a reduction in superoxide contents, cell death, and lipid peroxidation. The reduction in ROS level was associated with the increased antioxidant enzyme activities along with multiple-isoform expression [peroxidase (POD), polyphenol oxidase (PPO), and superoxide dismutase (SOD)] in plant roots. Additionally, BHU-AV3 inoculation induced the expression of proteins involved in (i) energy production [ATP synthase], (ii) carbohydrate metabolism (enolase), (iii) thiamine biosynthesis protein, (iv) translation protein (elongation factor 1 alpha), and the antioxidant defense system (catalase) in tomato roots. These findings have provided insight into the molecular mechanisms of bacteria-mediated alleviation of salt stress in plants. From the study, we can conclude that BHU-AV3 inoculation effectively induces antioxidant systems and energy metabolism in tomato roots, which leads to whole plant protection during salt stress through induced systemic tolerance.

Expression of protein synthesis elongation factors in winter wheat and oat in response to heat stress

The aim of our work was to examine the expression and accumulation of EF-Tu and eEF1A in grain filing stage of five genotypes of winter wheat and one oat genotype in conditions of heat stress. In addition, the correlation between accumulation of elongation factors eEF1A and EF-Tu, and yield components of cereals in the field was investigated. Flag leaf protein samples were analyzed by immunoblotting. Flag leaves were collected under conditions of moderate (23 °C; MT) and high air temperature (38 °C; HT) in a field experiment. After the harvest, grain yield was determined. The yield components, the weight of dry seed and grains number per spike, were assessed in the stage of full physiological maturity of investigated cultivars. Obtained results revealed a difference in the level of EF-Tu accumulation both under conditions of moderate air temperatures and conditions of heat stress among investigated cultivars. Cultivar Zvezdana was the only one that showed increase in EF-Tu accumulation under HT (25%) compared to MT. Immunoblot analysis indicated that the highest increase of eEF1A accumulation (43%) in relation to moderate temperature was detected in cultivar Talas. A significant, positive, linear correlation was found between the expression of eEF1A and small grains productivity under heat-stress conditions.

Metabolic Reprogramming in Chloroplasts under Heat Stress in Plants

Increases in ambient temperatures have been a severe threat to crop production in many countries around the world under climate change. Chloroplasts serve as metabolic centers and play a key role in physiological adaptive processes to heat stress. In addition to expressing heat shock proteins that protect proteins from heat-induced damage, metabolic reprogramming occurs during adaptive physiological processes in chloroplasts. Heat stress leads to inhibition of plant photosynthetic activity by damaging key components functioning in a variety of metabolic processes, with concomitant reductions in biomass production and crop yield. In this review article, we will focus on events through extensive and transient metabolic reprogramming in response to heat stress, which included chlorophyll breakdown, generation of reactive oxygen species (ROS), antioxidant defense, protein turnover, and metabolic alterations with carbon assimilation. Such diverse metabolic reprogramming in chloroplasts is required for systemic acquired acclimation to heat stress in plants.

Comparative proteomics illustrates the complexity of drought resistance mechanisms in two wheat (Triticum aestivum L.) cultivars under dehydration and rehydration

BACKGROUND

Drought stress is one of the most adverse environmental constraints to plant growth and productivity. Comparative proteomics of drought-tolerant and sensitive wheat genotypes is a strategy to understand the complexity of molecular mechanism of wheat in response to drought. This study attempted to extend findings regarding the potential proteomic dynamics in wheat under drought stress and to enrich the research content of drought tolerance mechanism.

RESULTS

A comparative proteomics approach was applied to analyze proteome change of Xihan No. 2 (a drought-tolerant cultivar) and Longchun 23 (a drought-sensitive cultivar) subjected to a range of dehydration treatments (18 h, 24 h and 48 h) and rehydration treatment (R24 h) using 2-DE, respectively. Quantitative image analysis showed a total of 172 protein spots in Xihan No. 2 and 215 spots from Longchun 23 with their abundance significantly altered (p < 0.05) more than 2.5-fold. Out of these spots, a total of 84 and 64 differentially abundant proteins were identified by MALDI-TOF/TOF MS in Xihan No. 2 and Longchun 23, respectively. Most of these identified proteins were involved in metabolism, photosynthesis, defence and protein translation/processing/degradation in both two cultivars. In addition, the proteins involved in redox homeostasis, energy, transcription, cellular structure, signalling and transport were also identified. Furthermore, the comparative analysis of drought-responsive proteome allowed for the general elucidation of the major mechanisms associated with differential responses to drought of both two cultivars. These cellular processes work more cooperatively to re-establish homeostasis in Xihan No. 2 than Longchun 23. The resistance mechanisms of Xihan No. 2 mainly included changes in the metabolism of carbohydrates and amino acids as well as in the activation of more antioxidation and defense systems and in the levels of proteins involved in ATP synthesis and protein degradation/refolding.

CONCLUSIONS

This study revealed that the levels of a number of proteins involved in various cellular processes were affected by drought stress in two wheat cultivars with different drought tolerance. The results showed that there exist specific responses to drought in Xihan No. 2 and Longchun 23. The proposed hypothetical model would explain the interaction of these identified proteins that are associated with drought-responses in two cultivars, and help in developing strategies to improve drought tolerance in wheat.

Leaf Proteome Analysis Reveals Prospective Drought and Heat Stress Response Mechanisms in Soybean

Drought and heat are among the major abiotic stresses that affect soybean crops worldwide. During the current investigation, the effect of drought, heat, and drought plus heat stresses was compared in the leaves of two soybean varieties, Surge and Davison, combining 2D-DIGE proteomic data with physiology and biochemical analyses. We demonstrated how 25 differentially expressed photosynthesis-related proteins affect RuBisCO regulation, electron transport, Calvin cycle, and carbon fixation during drought and heat stress. We also observed higher abundance of heat stress-induced EF-Tu protein in Surge. It is possible that EF-Tu might have activated heat tolerance mechanisms in the soybean. Higher level expressions of heat shock-related protein seem to be regulating the heat tolerance mechanisms. This study identifies the differential expression of various abiotic stress-responsive proteins that regulate various molecular processes and signaling cascades. One inevitable outcome from the biochemical and proteomics assays of this study is that increase of ROS levels during drought stress does not show significant changes at the phenotypic level in Davison and this seems to be due to a higher amount of carbonic anhydrase accumulation in the cell which aids the cell to become more resistant to cytotoxic concentrations of H2O2.

Proteome Analysis Reveals Extensive Light Stress-Response Reprogramming in the Seagrass Zostera muelleri (Alismatales, Zosteraceae) Metabolism

Seagrasses are marine ecosystem engineers that are currently declining in abundance at an alarming rate due to both natural and anthropogenic disturbances in ecological niches. Despite reports on the morphological and physiological adaptations of seagrasses to extreme environments, little is known of the molecular mechanisms underlying photo-acclimation, and/or tolerance in these marine plants. This study applies the two-dimensional isoelectric focusing (2D-IEF) proteomics approach to identify photo-acclimation/tolerance proteins in the marine seagrass Zostera muelleri. For this, Z. muelleri was exposed for 10 days in laboratory mesocosms to saturating (control, 200 μmol photons m^-2 s^-1), super-saturating (SSL, 600 μmol photons m^-2 s^-1), and limited light (LL, 20 μmol photons m^-2 s^-1) irradiance conditions. Using LC-MS/MS analysis, 93 and 40 protein spots were differentially regulated under SSL and LL conditions, respectively, when compared to the control. In contrast to the LL condition, Z. muelleri robustly tolerated super-saturation light than control conditions, evidenced by their higher relative maximum electron transport rate and minimum saturating irradiance values. Proteomic analyses revealed up-regulation and/or appearances of proteins belonging to the Calvin-Benson and Krebs cycle, glycolysis, the glycine cleavage system of photorespiration, and the antioxidant system. These proteins, together with those from the inter-connected glutamate-proline-GABA pathway, shaped Z. muelleri photosynthesis and growth under SSL conditions. In contrast, the LL condition negatively impacted the metabolic activities of Z. muelleri by down-regulating key metabolic enzymes for photosynthesis and the metabolism of carbohydrates and amino acids, which is consistent with the observation with lower photosynthetic performance under LL condition. This study provides novel insights into the underlying molecular photo-acclimation mechanisms in Z. muelleri, in addition to identifying protein-based biomarkers that could be used as early indicators to detect acute/chronic light stress in seagrasses to monitor seagrass health.

Proteomic analysis of Arabidopsis thaliana (L.) Heynh responses to a generalist sucking pest (Myzus persicae Sulzer)

Herbivorous insects can cause severe cellular changes to plant foliage following infestations, depending on feeding behaviour. Here, a proteomic study was conducted to investigate the influence of green peach aphid (Myzus persicae Sulzer) as a polyphagous pest on the defence response of Arabidopsis thaliana (L.) Heynh after aphid colony establishment on the host plant (3 days). Analysis of about 574 protein spots on 2-DE gels revealed 31 differentially expressed protein spots. Twenty out of these 31 differential proteins were selected for analysis by mass spectrometry. In 12 of the 20 analysed spots, we identified seven and nine proteins using MALDI-TOF-MS and LC-ESI-MS/MS, respectively. Of the analysed spots, 25% contain two proteins. Different metabolic pathways were modulated in Arabidopsis leaves according to aphid feeding: most corresponded to carbohydrate, amino acid and energy metabolism, photosynthesis, defence response and translation. This paper has established a survey of early alterations induced in the proteome of Arabidopsis by M. persicae aphids. It provides valuable insights into the complex responses of plants to biological stress, particularly for herbivorous insects with sucking feeding behaviour.

Roles of Protein Synthesis Elongation Factor EF-Tu in Heat Tolerance in Plants

EF-Tu proteins of plastids, mitochondria, and the cytosolic counterpart EF-1α in plants, as well as EF-Tu proteins of bacteria, are highly conserved and multifunctional. The functions of EF-Tu include transporting the aminoacyl-tRNA complex to the A site of the ribosome during protein biosynthesis; chaperone activity in protecting other proteins from aggregation caused by environmental stresses, facilitating renaturation of proteins when conditions return to normal; displaying a protein disulfide isomerase activity; participating in the degradation of N-terminally blocked proteins by the proteasome; eliciting innate immunity and triggering resistance to pathogenic bacteria in plants; participating in transcription when an E. coli host is infected with phages. EF-Tu genes are upregulated by abiotic stresses in plants, and EF-Tu plays important role in stress responses. Expression of a plant EF-Tu gene confers heat tolerance in E. coli, maize knock-out EF-Tu null mutants are heat susceptible, and over-expression of an EF-Tu gene improves heat tolerance in crop plants. This review paper summarizes the current knowledge of EF-Tu proteins in stress responses in plants and progress on application of EF-Tu for developing crop varieties tolerant to abiotic stresses, such as high temperatures.

Expression of elongation factor (EF)-Tu is correlated with prognosis of gastric adenocarcinomas

Altered expressions of mitochondria elongation factor Tu (EF-Tu) have been observed in certain types of cancers, including gastric cancer cell lines, but the impact of the alterations in gastric adenocarcinoma remains unclear. The purpose of this study was to investigate the expression of EF-Tu in gastric adenocarcinoma and to assess its clinical significance. A total of 104 paired resected gastric adenocarcinoma and corresponding normal specimens were collected in this study. EF-Tu expression was assessed by immunohistochemical staining. The correlation of EF-Tu expression and patients’ clinicopathological parameters was statically evaluated and the prognostic significance of EF-Tu expression was assessed by univariate and multivariate analyses. Forty-nine out of 104 (47.1%) gastric adenocarcinoma specimens showed high expression of EF-Tu, while the remaining 55 specimens showed weak or negative expression of EF-Tu. In contrast, EF-Tu high expression was detected in 62.5% (65 of 104) normal tissues. Down-regulation of EF-Tu was associated with serosal invasion (P = 0.042) and node involvement (P = 0.005), and down-regulation of EF-Tu was correlated with poor overall survival (P = 0.020). In curative resection (R0) patients, there were also significant differences (P = 0.043). In the multivariate analysis, the EF-Tu expression remained a significant independent prognostic factor (P = 0.038). Our results indicate that EF-Tu is expressed in both gastric adenocarcinoma and corresponding normal tissues. Down-regulation of EF-Tu expression is associated with advanced disease stage and EF-Tu expression maybe served as an independent prognostic factor.

Analysis of transgenic wheat (Triticum aestivum L.) harboring a maize (Zea mays L.) gene for plastid EF-Tu: segregation pattern, expression and effects of the transgene

We previously reported that transgenic wheat (Triticum aestivum L.) carrying a maize (Zea mays L.) gene (Zmeftu1) for chloroplast protein synthesis elongation factor, EF-Tu, displays reduced thermal aggregation of leaf proteins, reduced injury to photosynthetic membranes (thylakoids), and enhanced rate of CO(2) fixation following exposure to heat stress (18 h at 45 degrees C) [Fu et al. in Plant Mol Biol 68:277-288, 2008]. In the current study, we investigated the segregation pattern and expression of the transgene Zmeftu1 and determined the grain yield of transgenic plants after exposure to a brief heat stress (18 h at 45 degrees C). We also assessed thermal aggregation of soluble leaf proteins in transgenic plants, testing the hypothesis that increased levels of EF-Tu will lead to a non-specific protection of leaf proteins against thermal aggregation. The transgenic wheat displayed a single-gene pattern of segregation of Zmeftu1. Zmeftu1 was expressed, and the transgenic plants synthesized and accumulated three anti-EF-Tu cross-reacting polypeptides of similar molecular mass but different pI, suggesting the possibility of posttranslational modification of this protein. The transgenic plants also showed better grain yield after exposure to heat stress compared with their non-transgenic counterparts. Soluble leaf proteins of various molecular masses displayed lower thermal aggregation in transgenic than in non-transgenic wheat. The results suggest that overexpression of chloroplast EF-Tu can be beneficial to wheat tolerance to heat stress. Moreover, the results also support the hypothesis that EF-Tu contributes to heat tolerance by acting as a molecular chaperone and protecting heat-labile proteins from thermal aggregation in a non-specific manner.

Comparative proteomic analysis of the short-term responses of rice roots and leaves to cadmium

Cadmium (Cd) is a non-essential heavy metal that is recognized as a major environmental pollutant. While Cd responses and toxicities in some plant species have been well established, there are few reports about the effects of short-term exposure to Cd on rice, a model monocotyledonous plant, at the proteome level. To investigate the effect of Cd in rice, we monitored the influence of Cd exposure on root and leaf proteomes. After Cd treatment, root and leaf tissues were separately collected and leaf proteins were fractionated with polyethylene glycol. Differentially regulated proteins were selected after image analysis and identified using MALDI-TOF MS. A total of 36 proteins were up- or down-regulated following Cd treatment. As expected, total glutathione levels were significantly decreased in Cd-treated roots, and approximately half of the up-regulated proteins in roots were involved in responses to oxidative stress. These results suggested that prompt antioxidative responses might be necessary for the reduction of Cd-induced oxidative stress in roots but not in leaves. In addition, RNA gel blot analysis showed that the proteins identified in the proteomic analysis were also differentially regulated at the transcriptional level. Collectively, our study provides insights into the integrated molecular mechanisms of early responses to Cd in rice.

Identification of potential DREB2C targets in Arabidopsis thaliana plants overexpressing DREB2C using proteomic analysis

The dehydration responsive element binding protein 2C (DREB2C) is a dehydration responsive element/C-repeat (DRE/CRT)-motif binding transcription factor that induced by mild heat stress. Previous experiments established that overexpression of DREB2C cDNA driven by the cauliflower mosaic virus 35S promoter (35S:DREB2C) resulted in increased heat tolerance in Arabidopsis. We first analyzed the proteomic profiles in wild-type and 35S:DREB2C plants at a normal temperature (22 degrees C), but could not detect any differences between the proteomes of wild-type and 35S:DREB2C plants. The transcript level of DREB2C in 35S:DREB2C plants after treatment with mild heat stress was increased more than two times compared with expression in 35S:DREB2C plants under unstressed condition. A proteomic approach was used to decipher the molecular mechanisms underlying thermotolerance in 35S:DREB2C Arabidopsis plants. Eleven protein spots were identified as being differentially regulated in 35S:DREB2C plants. Moreover, in silico motif analysis showed that peptidyl-prolyl isomerase ROC4, glutathione transferase 8, pyridoxal biosynthesis protein PDX1, and elongation factor Tu contained one or more DRE/CRT motifs. To our knowledge, this study is the first to identify possible targets of DREB2C transcription factors at the protein level. The proteomic results were in agreement with transcriptional data.

Heat tolerance and expression of protein synthesis elongation factors, EF-Tu and EF-1α, in spring wheat

Protein elongation factors, EF-Tu and EF-1α, have been implicated in cell response to heat stress. We investigated the expression (accumulation) of EF-Tu and EF-1α in mature plants of spring wheat cultivars Kukri and Excalibur, and tested the hypothesis that cultivars with contrasting tolerance to heat stress differ in the accumulation of these elongation factors under prolonged exposure to high temperature (16 days at 36/30°C). In addition, we investigated the expression of EF-Tu and EF-1α in young plants experiencing a 24-h heat shock (43°C). Excalibur showed better tolerance to heat stress than Kukri. Heat stress induced accumulation of EF-Tu and EF-1α in mature plants of both cultivars, but to a greater extent in Excalibur. Young plants did not show appreciable accumulation of EF-Tu in response to heat shock. However, these plants showed increased accumulation of EF-1α and the accumulation appeared greater in Excalibur than in Kukri. The results support the hypothesis that EF-Tu plays a role in heat tolerance in spring wheat. The results also suggest that EF-1α may be of importance to wheat response to heat stress.

Rubisco activase and wheat productivity under heat-stress conditions

Rubisco activase (RCA) constrains the photosynthetic potential of plants at high temperatures (heat stress). Endogenous levels of RCA could serve as an important determinant of plant productivity under heat-stress conditions. Thus, in this study, the possible relationship between expression levels of RCA and plant yield in 11 European cultivars of winter wheat following prolonged exposure to heat stress was investigated. In addition, the effect of a short-term heat stress on RCA expression in four genotypes of wheat, five genotypes of maize, and one genotype of Arabidopsis thaliana was examined. Immunoblots prepared from leaf protein extracts from control plants showed three RCA cross-reacting bands in wheat and two RCA cross-reacting bands in maize and Arabidopsis. The molecular mass of the observed bands was in the range between 40 kDa and 46 kDa. Heat stress affected RCA expression in a few genotypes of wheat and maize but not in Arabidopsis. In wheat, heat stress slightly modulated the relative amounts of RCA in some cultivars. In maize, heat stress did not seem to affect the existing RCA isoforms (40 kDa and 43 kDa) but induced the accumulation of a new putative RCA of 45-46 kDa. The new putative 45-46 kDa RCA was not seen in a genotype of maize (ZPL 389) that has been shown to display an exceptional sensitivity to heat stress. A significant, positive, linear correlation was found between the expression of wheat 45-46 kDa RCA and plant productivity under heat-stress conditions. Results support the hypothesis that endogenous levels of RCA could play an important role in plant productivity under supraoptimal temperature conditions.

Heterologous expression of a plastid EF-Tu reduces protein thermal aggregation and enhances CO2 fixation in wheat (Triticum aestivum) following heat stress

Heat stress is a major constraint to wheat production and negatively impacts grain quality, causing tremendous economic losses, and may become a more troublesome factor due to global warming. At the cellular level, heat stress causes denaturation and aggregation of proteins and injury to membranes leading to alterations in metabolic fluxes. Protein aggregation is irreversible, and protection of proteins from thermal aggregation is a strategy a cell uses to tolerate heat stress. Here we report on the development of transgenic wheat (Triticum aestivum) events, expressing a maize gene coding for plastidal protein synthesis elongation factor (EF-Tu), which, compared to non-transgenic plants, display reduced thermal aggregation of leaf proteins, reduced heat injury to photosynthetic membranes (thylakoids), and enhanced rate of CO(2) fixation after exposure to heat stress. The results support the concept that EF-Tu ameliorates negative effects of heat stress by acting as a molecular chaperone. This is the first demonstration of the introduction of a plastidal EF-Tu in plants that leads to protection against heat injury and enhanced photosynthesis after heat stress. This is also the first demonstration that a gene other than HSP gene can be used for improvement of heat tolerance and that the improvement is possible in a species that has a complex genome, hexaploid wheat. The results strongly suggest that heat tolerance of wheat, and possibly other crop plants, can be improved by modulating expression of plastidal EF-Tu and/or by selection of genotypes with increased endogenous levels of this protein.