cor Gene expression in barley mutants affected in chloroplast development and photosynthetic electron transport

Crosstalk between Light- and Temperature-Mediated Processes under Cold and Heat Stress Conditions in Plants

Extreme temperatures are among the most important stressors limiting plant growth and development. Results indicate that light substantially influences the acclimation processes to both low and high temperatures, and it may affect the level of stress injury. The interaction between light and temperature in the regulation of stress acclimation mechanisms is complex, and both light intensity and spectral composition play an important role. Higher light intensities may lead to overexcitation of the photosynthetic electron transport chain; while different wavelengths may act through different photoreceptors. These may induce various stress signalling processes, leading to regulation of stomatal movement, antioxidant and osmoregulation capacities, hormonal actions, and other stress-related pathways. In recent years, we have significantly expanded our knowledge in both light and temperature sensing and signalling. The present review provides a synthesis of results for understanding how light influences the acclimation of plants to extreme low or high temperatures, including the sensing mechanisms and molecular crosstalk processes.

Freezing Tolerance of Lolium multiflorum/Festuca arundinacea Introgression Forms is Associated with the High Activity of Antioxidant System and Adjustment of Photosynthetic Activity under Cold Acclimation

Though winter-hardiness is a complex trait, freezing tolerance was proved to be its main component. Species from temperate regions acquire tolerance to freezing in a process of cold acclimation, which is associated with the exposure of plants to low but non-freezing temperatures. However, mechanisms of cold acclimation in Lolium-Festuca grasses, important for forage production in Europe, have not been fully recognized. Thus, two L. multiflorum/F. arundinacea introgression forms with distinct freezing tolerance were used herein as models in the comprehensive research to dissect these mechanisms in that group of plants. The work was focused on: (i) analysis of cellular membranes' integrity; (ii) analysis of plant photosynthetic capacity (chlorophyll fluorescence; gas exchange; gene expression, protein accumulation, and activity of selected enzymes of the Calvin cycle); (iii) analysis of plant antioxidant capacity (reactive oxygen species generation; gene expression, protein accumulation, and activity of selected enzymes); and (iv) analysis of Cor14b accumulation, under cold acclimation. The more freezing tolerant introgression form revealed a higher integrity of membranes, an ability to cold acclimate its photosynthetic apparatus and higher water use efficiency after three weeks of cold acclimation, as well as a higher capacity of the antioxidant system and a lower content of reactive oxygen species in low temperature.

Temperature and Light-Quality-Dependent Regulation of Freezing Tolerance in Barley

It is established that, besides the cold, incident light also has a crucial role in the cold acclimation process. To elucidate the interaction between these two external hardening factors, barley plantlets were grown under different light conditions with low, normal, and high light intensities at 5 and 15 °C. The expression of the HvCBF14 gene and two well-characterized members of the C-repeat binding factor (CBF)-regulon HvCOR14b and HvDHN5 were studied. In general, the expression level of the studied genes was several fold higher at 5 °C than that at 15 °C independently of the applied light intensity or the spectra. The complementary far-red (FR) illumination induced the expression of HvCBF14 and also its target gene HvCOR14b at both temperatures. However, this supplementation did not affect significantly the expression of HvDHN5. To test the physiological effects of these changes in environmental conditions, freezing tests were also performed. In all the cases, we found that the reduced R:FR ratio increased the frost tolerance of barley at every incident light intensity. These results show that the combined effects of cold, light intensity, and the modification of the R:FR light ratio can greatly influence the gene expression pattern of the plants, which can result in increased plant frost tolerance.

Identification of genes from the ICE-CBF-COR pathway under cold stress in Aegilops-Triticum composite group and the evolution analysis with those from Triticeae

Adverse environmental conditions limit various aspects of plant growth, productivity, and ecological distribution. To get more insights into the signaling pathways under low temperature, we identified 10 C-repeat binding factors (CBFs), 9 inducer of CBF expression (ICEs) and 10 cold-responsive (CORs) genes from Aegilops-Triticum composite group under cold stress. Conserved amino acids analysis revealed that all CBF, ICE, COR contained specific and typical functional domains. Phylogenetic analysis of CBF proteins from Triticeae showed that these CBF homologs were divided into 11 groups. CBFs from Triticum were found in every group, which shows that these CBFs generated prior to the divergence of the subfamilies of Triticeae. The evolutionary relationship among the ICE and COR proteins in Poaceae were divided into four groups with high multispecies specificity, respectively. Moreover, expression analysis revealed that mRNA accumulation was altered by cold treatment and the genes of three types involved in the ICE-CBF-COR signaling pathway were induced by cold stress. Together, the results make CBF, ICE, COR genes family in Triticeae more abundant, and provide a starting point for future studies on transcriptional regulatory network for improvement of chilling tolerance in crop.

Lipid signalling mediated by PLD/PA modulates proline and H2O2 levels in barley seedlings exposed to short- and long-term chilling stress

Phospholipase D (PLD) hydrolyses phospholipids to yield phosphatidic acid (PA) and a head group, and is involved in responses to a variety of environmental stresses, including chilling and freezing stress. Barley responses to chilling stress (induced by incubating seedlings at 4 °C) are dynamic and the duration of stress, either short (0-180 min) or long-term (24-36 h) had a significant impact on the response. We investigated the roles of PLD/PA in responses of barley (Hordeum vulgare) seedlings to short and long-term chilling stress, based on regulation of proline and reactive oxygen species (ROS) levels. Short-term chilling stress caused rapid and transient increases in PLD activity, proline level, and ROS levels in young leaves. PLD has the ability to catalyse the transphosphatidylation reaction leading to formation of phosphatidylalcohol (preferentially, to PA). Pre-treatment of seedlings with 1-butanol significantly increased proline synthesis but decreased ROS (H₂O₂) formation. These observations suggest that PLD is a negative regulator of proline synthesis, whereas PA/PLD promote ROS signals. Exogenous PA pre-treatment reduced the proline synthesis but enhanced H₂O₂ formation. Effects of long-term chilling stress on barley seedlings differed from those of short-term chilling stress. E.g., PLD activity was significantly reduced in young leaves and roots, whereas proline synthesis and ROS signals were increased in roots. Exogenous ROS application enhanced proline level while exogenous proline application reduced ROS level and modulated some effects of long-term chilling stress. Our findings suggest that PLD contributes to signalling pathways in responses to short-term chilling stress in barley seedling, through regulation of the balance between proline and ROS levels. In contrast, reduced PLD activity in the response to long-term chilling stress did not affect proline level. Increased ROS levels may reflect an antioxidant system that is affected by chilling stress and positively compensated by changes in proline level. Implications of our findings are discussed in regard to adaptation strategies of barley seedlings to low temperatures.

Freezing tolerance revisited-effects of variable temperatures on gene regulation in temperate grasses and legumes

Climate change creates new patterns of seasonal climate variation with higher temperatures, longer growth seasons and more variable winter climates. This is challenging the winter survival of perennial herbaceous plants. In this review, we focus on the effects of variable temperatures during autumn/winter/spring, and its interactions with light, on the development and maintenance of freezing tolerance. Cold temperatures induce changes at several organizational levels in the plant (cold acclimation), leading to the development of freezing tolerance, which can be reduced/lost during warm spells (deacclimation) in winters, and attained again during cold spells (reacclimation). We summarize how temperature interacts with components of the light regime (photoperiod, PSII excitation pressure, irradiance, and light quality) in determining changes in the transcriptome, proteome and metabolome.

The homeodomain transcription factor TaHDZipI-2 from wheat regulates frost tolerance, flowering time and spike development in transgenic barley

Homeodomain leucine zipper class I (HD-Zip I) transcription factors (TFs) play key roles in the regulation of plant growth and development under stresses. Functions of the TaHDZipI-2 gene isolated from the endosperm of developing wheat grain were revealed. Molecular characterization of TaHDZipI-2 protein included studies of its dimerisation, protein-DNA interactions and gene activation properties using pull-down assays, in-yeast methods and transient expression assays in wheat cells. The analysis of TaHDZipI-2 gene functions was performed using transgenic barley plants. It included comparison of developmental phenotypes, yield components, grain quality, frost tolerance and the levels of expression of potential target genes in transgenic and control plants. Transgenic TaHDZipI-2 lines showed characteristic phenotypic features that included reduced growth rates, reduced biomass, early flowering, light-coloured leaves and narrowly elongated spikes. Transgenic lines produced 25-40% more seeds per spike than control plants, but with 50-60% smaller grain size. In vivo lipid imaging exposed changes in the distribution of lipids between the embryo and endosperm in transgenic seeds. Transgenic lines were significantly more tolerant to frost than control plants. Our data suggest the role of TaHDZipI-2 in controlling several key processes underlying frost tolerance, transition to flowering and spike development.

Light-quality and temperature-dependent CBF14 gene expression modulates freezing tolerance in cereals

C-repeat binding factor 14 (CBF14) is a plant transcription factor that regulates a set of cold-induced genes, contributing to enhanced frost tolerance during cold acclimation. Many CBF genes are induced by cool temperatures and regulated by day length and light quality, which affect the amount of accumulated freezing tolerance. Here we show that a low red to far-red ratio in white light enhances CBF14 expression and increases frost tolerance at 15°C in winter Triticum aesitivum and Hordeum vulgare genotypes, but not in T. monococcum (einkorn), which has a relatively low freezing tolerance. Low red to far-red ratio enhances the expression of PHYA in all three species, but induces PHYB expression only in einkorn. Based on our results, a model is proposed to illustrate the supposed positive effect of phytochrome A and the negative influence of phytochrome B on the enhancement of freezing tolerance in cereals in response to spectral changes of incident light.

KEY WORDS

CBF-regulon, barley, cereals, cold acclimation, freezing tolerance, light regulation, low red/far-red ratio, phytochrome, wheat.

Vernalization Requirement and the Chromosomal VRN1-Region can Affect Freezing Tolerance and Expression of Cold-Regulated Genes in Festuca pratensis

Plants adapted to cold winters go through annual cycles of gain followed by loss of freezing tolerance (cold acclimation and deacclimation). Warm spells during winter and early spring can cause deacclimation, and if temperatures drop, freezing damage may occur. Many plants are vernalized during winter, a process making them competent to flower in the following summer. In winter cereals, a coincidence in the timing of vernalization saturation, deacclimation, downregulation of cold-induced genes, and reduced ability to reacclimate, occurs under long photoperiods and is under control of the main regulator of vernalization requirement in cereals, VRN1, and/or closely linked gene(s). Thus, the probability of freezing damage after a warm spell may depend on both vernalization saturation and photoperiod. We investigated the role of vernalization and the VRN1-region on freezing tolerance of meadow fescue (Festuca pratensis Huds.), a perennial grass species. Two F2 populations, divergently selected for high and low vernalization requirement, were studied. Each genotype was characterized for the copy number of one of the four parental haplotypes of the VRN1-region. Clonal plants were cold acclimated for 2 weeks or vernalized/cold acclimated for a total of 9 weeks, after which the F2 populations reached different levels of vernalization saturation. Vernalized and cold acclimated plants were deacclimated for 1 week and then reacclimated for 2 weeks. All treatments were given at 8 h photoperiod. Flowering response, freezing tolerance and expression of the cold-induced genes VRN1, MADS3, CBF6, COR14B, CR7 (BLT14), LOS2, and IRI1 was measured. We found that some genotypes can lose some freezing tolerance after vernalization and a deacclimation-reacclimation cycle. The relationship between vernalization and freezing tolerance was complex. We found effects of the VRN1-region on freezing tolerance in plants cold acclimated for 2 weeks, timing of heading after 9 weeks of vernalization, expression of COR14B, CBF6, and LOS2 in vernalized and/or deacclimated treatments, and restoration of freezing tolerance during reacclimation. While expression of VRN1, COR14B, CBF6, LOS2, and IRI1 was correlated, CR7 was associated with vernalization requirement by other mechanisms, and appeared to play a role in freezing tolerance in reacclimated plants.

Cloning, expression analysis, and functional characterization of two secretory phospholipases A2 in durum wheat (Triticum durum Desf.)

We previously isolated four cDNAs in durum wheat, TdsPLA2I, TdsPLA2II, TdsPLA2III and TdsPLA2IV, that encode proteins with homology to plant secretory phospholipases A2 (sPLA2s) (Verlotta et al., Int. J. Mol. Sci., 14, 2013, 5146-5169). In this study, we have further characterized TdsPLA2II and TdsPLA2III sequences that, on the basis of our previous findings, might encode sPLA2 isoforms with different features. Functional analysis revealed that, similarly to other known sPLA2s, TdsPLA2II and TdsPLA2III have an optimum at pH 9.0, require Ca(2+), are heat stable, and are inhibited by the disulfide-bond-reducing agent dithiothreitol. However, differences emerged between these TdsPLA2 isoforms. Transcript analysis revealed that the TdsPLA2III gene is highly up-regulated under different environmental stresses; conversely, the TdsPLA2II gene is expressed at constant levels under almost all of the stress conditions examined. Moreover, TdsPLA2II is saturated at micromolar substrate and Ca(2+) concentrations, whereas TdsPLA2III requires millimolar concentrations to reach maximal activity. This suggests that TdsPLA2II normally functions under optimal conditions in vivo, whereas TdsPLA2III is only partially activated, depending on the specific phospholipid and Ca(2+) levels. Altogether these data lead to the hypothesis that in vivo TdsPLA2II and TdsPLA2III are differently regulated at both molecular and biochemical level and that TdsPLA2III plays a major role in durum wheat response to adverse environmental conditions.

The Recovery of Plastid Function Is Required for Optimal Response to Low Temperatures in Arabidopsis

Cold acclimation is an essential response in higher plants to survive freezing temperatures. Here, we report that two independent mutant alleles of the H-subunit of Mg-chelatase, CHLH, gun5-1 and cch in Arabidopsis are sensitive to low temperatures. Plants were grown in photoperiodic conditions and exposed to low temperatures for short- and long-term periods. Tetrapyrrole biosynthesis was initially significantly inhibited in response to low temperature but recovered in wild type (Col-0), although the tetrapyrrole levels were lower in cold compared to control conditions. The gun5-1 and cch alleles showed an inability to recover chlorophyll biosynthesis in addition to a significant decrease in freezing tolerance. We found that the impaired plastid function in the CHLH mutant plants resulted in compromised de novo protein synthesis at low temperatures. The expression of the transcription factors CBF1-3 was super-induced in gun5-1 and cch mutant alleles but expression levels of their target genes, COR15a, COR47 and COR78 were similar or even lower compared to Col-0. In addition, the protein levels of COR15a were lower in gun5-1 and cch and a general defect in protein synthesis could be seen in the gun5-1 mutant following a 35S labelling experiment performed at low temperature. Taken together, our results demonstrate the importance of a functional chloroplast for the cold acclimation process and further suggest that impaired plastid function could result in inhibition of protein synthesis at low temperature.

Low temperature tolerance in plants: Changes at the protein level

Low temperature (LT) is one of several important environmental stresses influencing plant performance and distribution. Adaptation to LT is a highly dynamic stress-response phenomenon and involves complex cross-talk between different regulatory levels. Although plants differ in their sensitivity to LT, in temperate species low nonfreezing temperatures cause noticeable alterations in various biochemical and physiological processes that can potentially improve freezing tolerance. This adaptation is associated with changes in the expression pattern of genes and their protein products. Proteins are the major players in most cellular events and are directly involved in plant LT responses, thereby proteome analysis could help uncover additional novel proteins associated with LT tolerance. Proteomics is recommended as an appropriate strategy for complementing transcriptome level changes and characterizing translational and post-translational regulations. In this review, we considered alterations in the expression and accumulation of proteins in response to LT stress in the three major cereal crops produced worldwide (wheat, barley, and rice). LT stress down-regulates many photosynthesis-related proteins. On the contrary, pathways/protein sets that are up-regulated by LT include carbohydrate metabolism (ATP formation), ROS scavenging, redox adjustment, cell wall remodelling, cytoskeletal rearrangements, cryoprotection, defence/detoxification. These modifications are common adaptation reactions also observed in the plant model Arabidopsis, thus representing key potential biomarkers and critical intervention points for improving LT tolerance of crop plants in cold regions with short summers. We believe that an assessment of the proteome within a broad time frame and during the different phenological stages may disclose the molecular mechanisms related to the developmental regulation of LT tolerance and facilitate the progress of genetically engineered stress-resistant plant varieties.

The cold response of CBF genes in barley is regulated by distinct signaling mechanisms

Cold acclimation ability is crucial in the winter survival of cereals. In this process CBF transcription factors play key role, therefore understanding the regulation of these genes might provide useful knowledge for molecular breeding. In the present study the signal transduction pathways leading to the cold induction of different CBF genes were investigated in barley cv. Nure using pharmacological approach. Our results showed that the cold induced expression of CBF9 and CBF14 transcription factors is regulated by phospholipase C, phospholipase D pathways and calcium. On the contrary, these pathways have negative effect on the cold induction of CBF12 that is regulated by a different, as yet unidentified pathway. The diversity in the regulation of these transcription factors corresponds to their sequence based phylogenetic relationships suggesting that their evolutionary separation happened on structural, functional and regulational levels as well. On the CBF effector gene level, the signaling regulation is more complex, resultant effect of multiple pathways.

Photosynthesis-dependent physiological and genetic crosstalk between cold acclimation and cold-induced resistance to fungal pathogens in triticale (Triticosecale Wittm.)

The breeding for resistance against fungal pathogens in winter triticale (Triticosecale Wittm.) continues to be hindered by a complexity of the resistance mechanisms, strong interaction with environmental conditions, and dependence on the plant genotype. We showed, that temperature below 4 °C induced the plant genotype-dependent resistance against the fungal pathogen Microdochium nivale. The mechanism involved, at least, the adjustment of the reactions in the PSII proximity and photoprotection, followed by an improvement of the growth and development. The genotypes capable to develop the cold-induced resistance, showed a higher maximum quantum yield of PSII and a more efficient integration of the primary photochemistry of light reactions with the dark reactions. Moreover, induction of the photoprotective mechanism, involving at least the peroxidases scavenging hydrogen peroxide, was observed for such genotypes. Adjustment of the photosynthesis and stress acclimation has enabled fast plant growth and avoidance of the developmental stages sensitive to fungal infection. The same mechanisms allowed the quick regrow of plants during the post-disease period. In contrast, genotypes that were unable to develop resistance despite cold hardening had less flexible balancing of the photoprotection and photoinhibition processes. Traits related to: photosynthesis-dependent cold-acclimation and cold-induced resistance; biomass accumulation and growth; as well as protection system involving peroxidases; were integrated also at a genetic level. Analysing 95 lines of the mapping population SaKa3006×Modus we determined region on chromosomes 5B and 7R shared within all tested traits. Moreover, similar expression pattern of a set of the genes related to PSII was determined with the metaanalysis of the multiple microarray experiments. Comparable results for peroxidases, involving APXs and GPXs and followed by PRXs, indicated a similar function during cold acclimation and defense responses. These data provide a new insight into the cross talk between cold acclimation and cold-induced resistance in triticale, indicating a key role of photosynthesis-related processes.

Proteins with high turnover rate in barley leaves estimated by proteome analysis combined with in planta isotope labeling

Protein turnover is a key component in cellular homeostasis; however, there is little quantitative information on degradation kinetics for individual plant proteins. We have used (15)N labeling of barley (Hordeum vulgare) plants and gas chromatography-mass spectrometry analysis of free amino acids and liquid chromatography-mass spectrometry analysis of proteins to track the enrichment of (15)N into the amino acid pools in barley leaves and then into tryptic peptides derived from newly synthesized proteins. Using information on the rate of growth of barley leaves combined with the rate of degradation of (14)N-labeled proteins, we calculate the turnover rates of 508 different proteins in barley and show that they vary by more than 100-fold. There was approximately a 9-h lag from label application until (15)N incorporation could be reliably quantified in extracted peptides. Using this information and assuming constant translation rates for proteins during the time course, we were able to quantify degradation rates for several proteins that exhibit half-lives on the order of hours. Our workflow, involving a stringent series of mass spectrometry filtering steps, demonstrates that (15)N labeling can be used for large-scale liquid chromatography-mass spectrometry studies of protein turnover in plants. We identify a series of abundant proteins in photosynthesis, photorespiration, and specific subunits of chlorophyll biosynthesis that turn over significantly more rapidly than the average protein involved in these processes. We also highlight a series of proteins that turn over as rapidly as the well-known D1 subunit of photosystem II. While these proteins need further verification for rapid degradation in vivo, they cluster in chlorophyll and thiamine biosynthesis.

Hv-CBF2A overexpression in barley accelerates COR gene transcript accumulation and acquisition of freezing tolerance during cold acclimation

C-Repeat Binding Factors (CBFs) are DNA-binding transcriptional activators of gene pathways imparting freezing tolerance. Poaceae contain three CBF subfamilies, two of which, HvCBF3/CBFIII and HvCBF4/CBFIV, are unique to this taxon. To gain mechanistic insight into HvCBF4/CBFIV CBFs we overexpressed Hv-CBF2A in spring barley (Hordeum vulgare) cultivar 'Golden Promise'. The Hv-CBF2A overexpressing lines exhibited stunted growth, poor yield, and greater freezing tolerance compared to non-transformed 'Golden Promise'. Differences in freezing tolerance were apparent only upon cold acclimation. During cold acclimation freezing tolerance of the Hv-CBF2A overexpressing lines increased more rapidly than that of 'Golden Promise' and paralleled the freezing tolerance of the winter hardy barley 'Dicktoo'. Transcript levels of candidate CBF target genes, COR14B and DHN5 were increased in the overexpressor lines at warm temperatures, and at cold temperatures they accumulated to much higher levels in the Hv-CBF2A overexpressors than in 'Golden Promise'. Hv-CBF2A overexpression also increased transcript levels of other CBF genes at FROST RESISTANCE-H2-H2 (FR-H2) possessing CRT/DRE sites in their upstream regions, the most notable of which was CBF12. CBF12 transcript levels exhibited a relatively constant incremental increase above levels in 'Golden Promise' both at warm and cold. These data indicate that Hv-CBF2A activates target genes at warm temperatures and that transcript accumulation for some of these targets is greatly enhanced by cold temperatures.

Transgenic barley: a prospective tool for biotechnology and agriculture

Barley (Hordeum vulgare L.) is one of the founder crops of agriculture, and today it is the fourth most important cereal grain worldwide. Barley is used as malt in brewing and distilling industry, as an additive for animal feed, and as a component of various food and bread for human consumption. Progress in stable genetic transformation of barley ensures a potential for improvement of its agronomic performance or use of barley in various biotechnological and industrial applications. Recently, barley grain has been successfully used in molecular farming as a promising bioreactor adapted for production of human therapeutic proteins or animal vaccines. In addition to development of reliable transformation technologies, an extensive amount of various barley genetic resources and tools such as sequence data, microarrays, genetic maps, and databases has been generated. Current status on barley transformation technologies including gene transfer techniques, targets, and progeny stabilization, recent trials for improvement of agricultural traits and performance of barley, especially in relation to increased biotic and abiotic stress tolerance, and potential use of barley grain as a protein production platform have been reviewed in this study. Overall, barley represents a promising tool for both agricultural and biotechnological transgenic approaches, and is considered an ancient but rediscovered crop as a model industrial platform for molecular farming.

Low temperatures induce rapid changes in chromatin state and transcript levels of the cereal VERNALIZATION1 gene

Transcriptional activation of the VERNALIZATION1 gene mediates the acceleration of flowering by prolonged cold (vernalization) in temperate cereals. This study examined the earliest stages of the transcriptional response of VRN1 to low temperatures. Time-course analyses, using a sensitive quantitative PCR assay, showed that in sprouting barley seedlings VRN1 transcripts begin to accumulate within 24 hours of the onset of cold. The kinetics of the initial transcriptional response of VRN1 to cold was similar to the cold-induced genes DEHYDRIN5 (DHN5) and COLD REGULATED 14B (COR14B), but occurred at lower levels compared to cold acclimation genes or the response to longer cold treatments. Temperatures between 15 and -2 °C induced expression of VRN1 within 24 hours, with a maximal response observed between 2 and -2 °C. Transcriptional induction was also observed in undifferentiated callus cells. There were significant increases in histone acetylation levels at the VRN1 locus in response to 24-hour cold treatment. Sodium butyrate, a histone deacetylation inhibitor, triggered an increase in histone acetylation at VRN1 chromatin and elevated VRN1 transcript levels. The transcriptional response of VRN1 to short-term cold treatment was examined in near-isogenic lines that have different VRN1 genotypes, showing that an allele of the barley VRN1 gene with an insertion in the first intron and high basal expression levels has a reduced transcriptional response to short term cold treatment. This study suggests that low-temperature induction of VRN1 is a cellular response to cold triggered by the same mechanisms that mediate low-temperature induction of cold acclimation genes.

Harden the chloroplast to protect the plant

The chloroplast is the central switch of the plant's response to cold and light stress. The ability of many plant species to develop a cold tolerant phenotype is dependent on the presence of light and photosynthetic activity during low-temperature growth. Light exposure at low temperature stimulates an over-reduction of the plastoquinone pool as well as the accumulation of reactive oxygen species, and both metabolic conditions generate a retrograde signal controlling nuclear gene expression. At the same time the chloroplast is the target of many cold acclimation processes which are the results of the chloroplast-nucleus cross-talk. Often, the extent of cold acclimation of the chloroplast is tightly correlated with the overall plant tolerance to chilling and freezing temperatures, a finding suggesting that the chloroplast cold acclimation could be the rate limiting factor in the adaptation to low temperature.

Identification of a protein network interacting with TdRF1, a wheat RING ubiquitin ligase with a protective role against cellular dehydration

Plants exploit ubiquitination to modulate the proteome with the final aim to ensure environmental adaptation and developmental plasticity. Ubiquitination targets are specifically driven to degradation through the action of E3 ubiquitin ligases. Genetic analyses have indicated wide functions of ubiquitination in plant life; nevertheless, despite the large number of predicted E3s, only a few of them have been characterized so far, and only a few ubiquitination targets are known. In this work, we characterized durum wheat (Triticum durum) RING Finger1 (TdRF1) as a durum wheat nuclear ubiquitin ligase. Moreover, its barley (Hordeum vulgare) homolog was shown to protect cells from dehydration stress. A protein network interacting with TdRF1 has been defined. The transcription factor WHEAT BEL1-TYPE HOMEODOMAIN1 (WBLH1) was degraded in a TdRF1-dependent manner through the 26S proteasome in vivo, the mitogen-activated protein kinase TdWNK5 [for Triticum durum WITH NO LYSINE (K)5] was able to phosphorylate TdRF1 in vitro, and the RING-finger protein WHEAT VIVIPAROUS-INTERACTING PROTEIN2 (WVIP2) was shown to have a strong E3 ligase activity. The genes coding for the TdRF1 interactors were all responsive to cold and/or dehydration stress, and a negative regulative function in dehydration tolerance was observed for the barley homolog of WVIP2. A role in the control of plant development was previously known, or predictable based on homology, for wheat BEL1-type homeodomain1(WBLH1). Thus, TdRF1 E3 ligase might act regulating the response to abiotic stress and remodeling plant development in response to environmental constraints.

Reactive oxygen species and transcript analysis upon excess light treatment in wild-type Arabidopsis thaliana vs a photosensitive mutant lacking zeaxanthin and lutein

BACKGROUND

Reactive oxygen species (ROS) are unavoidable by-products of oxygenic photosynthesis, causing progressive oxidative damage and ultimately cell death. Despite their destructive activity they are also signalling molecules, priming the acclimatory response to stress stimuli.

RESULTS

To investigate this role further, we exposed wild type Arabidopsis thaliana plants and the double mutant npq1lut2 to excess light. The mutant does not produce the xanthophylls lutein and zeaxanthin, whose key roles include ROS scavenging and prevention of ROS synthesis. Biochemical analysis revealed that singlet oxygen (1O2) accumulated to higher levels in the mutant while other ROS were unaffected, allowing to define the transcriptomic signature of the acclimatory response mediated by 1O2 which is enhanced by the lack of these xanthophylls species. The group of genes differentially regulated in npq1lut2 is enriched in sequences encoding chloroplast proteins involved in cell protection against the damaging effect of ROS. Among the early fine-tuned components, are proteins involved in tetrapyrrole biosynthesis, chlorophyll catabolism, protein import, folding and turnover, synthesis and membrane insertion of photosynthetic subunits. Up to now, the flu mutant was the only biological system adopted to define the regulation of gene expression by 1O2. In this work, we propose the use of mutants accumulating 1O2 by mechanisms different from those activated in flu to better identify ROS signalling.

CONCLUSIONS

We propose that the lack of zeaxanthin and lutein leads to 1O2 accumulation and this represents a signalling pathway in the early stages of stress acclimation, beside the response to ADP/ATP ratio and to the redox state of both plastoquinone pool. Chloroplasts respond to 1O2 accumulation by undergoing a significant change in composition and function towards a fast acclimatory response. The physiological implications of this signalling specificity are discussed.

Retrograde signaling pathway from plastid to nucleus

Plastids are a diverse group of organelles found in plants and some parasites. Because genes encoding plastid proteins are divided between the nuclear and plastid genomes, coordinated expression of genes in two separate genomes is indispensable for plastid function. To coordinate nuclear gene expression with the functional or metabolic state of plastids, plant cells have acquired a retrograde signaling pathway from plastid to nucleus, also known as the plastid signaling pathway. To date, several metabolic processes within plastids have been shown to affect the expression of nuclear genes. Recent progress in this field has also revealed that the plastid signaling pathway interacts and shares common components with other intracellular signaling pathways. This review summarizes our current knowledge on retrograde signaling from plastid to nucleus in plant cells and its role in plant growth and development.

Molecular mechanisms underlying frost tolerance in perennial grasses adapted to cold climates

We review recent progress in understanding cold and freezing stress responses in forage grass species, notably Lolium and Festuca species. The chromosomal positions of important frost tolerance and winter survival QTLs on Festuca and Lolium chromosomes 4 and 5 are most likely orthologs of QTLs on Triticeae chromosome 5 which correspond to a cluster of CBF-genes and the major vernalization gene. Gene expression and protein accumulation analyses after cold acclimation shed light on general responses to cold stress. These responses involve modulation of transcription levels of genes encoding proteins involved in cell signalling, cellular transport and proteins associated with the cell membrane. Also, abundance levels of proteins directly involved in photosynthesis were found to be different between genotypes of differing frost tolerance levels, stressing the importance of the link between the function of the photosynthetic apparatus under cold stress and frost tolerance levels. The significance of the ability to undergo photosynthetic acclimation and avoid photoinhibition is also evident from numerous studies in forage grasses. Other interesting candidate mechanisms for freezing tolerance in forage grasses are molecular responses to cold stress which have evolved after the divergence of temperate grasses. This includes metabolic machinery for synthesis of fructans and novel ice-binding proteins.

Transgene expression systems in the Triticeae cereals

The control of transgene expression is vital both for the elucidation of gene function and for the engineering of transgenic crops. Given the dominance of the Triticeae cereals in the agricultural economy of the temperate world, the development of well-performing transgene expression systems of known functionality is of primary importance. Transgenes can be expressed either transiently or stably. Transient expression systems based on direct or virus-mediated gene transfer are particularly useful in situations where the need is to rapidly screen large numbers of genes. However, an unequivocal understanding of gene function generally requires that a transgene functions throughout the plant's life and is transmitted through the sexual cycle, since this alone allows its effect to be decoupled from the plant's response to the generally stressful gene transfer event. Temporal, spatial and quantitative control of a transgene's expression depends on its regulatory environment, which includes both its promoter and certain associated untranslated region sequences. While many transgenic approaches aim to manipulate plant phenotype via ectopic gene expression, a transgene sequence can be also configured to down-regulate the expression of its endogenous counterpart, a strategy which exploits the natural gene silencing machinery of plants. In this review, current technical opportunities for controlling transgene expression in the Triticeae species are described. Apart from protocols for transient and stable gene transfer, the choice of promoters and other untranslated regulatory elements, we also consider signal peptides, as they too govern the abundance and particularly the sub-cellular localization of transgene products.

Regulation of freezing tolerance and flowering in temperate cereals: the VRN-1 connection

In winter wheat (Triticum spp.) and barley (Hordeum vulgare) varieties, long exposures to nonfreezing cold temperatures accelerate flowering time (vernalization) and improve freezing tolerance (cold acclimation). However, when plants initiate their reproductive development, freezing tolerance decreases, suggesting a connection between the two processes. To better understand this connection, we used two diploid wheat (Triticum monococcum) mutants, maintained vegetative phase (mvp), that carry deletions encompassing VRN-1, the major vernalization gene in temperate cereals. Homozygous mvp/mvp plants never flower, whereas plants carrying at least one functional VRN-1 copy (Mvp/-) exhibit normal flowering and high transcript levels of VRN-1 under long days. The Mvp/- plants showed reduced freezing tolerance and reduced transcript levels of several cold-induced C-REPEAT BINDING FACTOR transcription factors and COLD REGULATED genes (COR) relative to the mvp/mvp plants. Diploid wheat accessions with mutations in the VRN-1 promoter, resulting in high transcript levels under both long and short days, showed a significant down-regulation of COR14b under long days but not under short days. Taken together, these studies suggest that VRN-1 is required for the initiation of the regulatory cascade that down-regulates the cold acclimation pathway but that additional genes regulated by long days are required for the down-regulation of the COR genes. In addition, our results show that allelic variation in VRN-1 is sufficient to determine differences in freezing tolerance, suggesting that quantitative trait loci for freezing tolerance previously mapped on this chromosome region are likely a pleiotropic effect of VRN-1 rather than the effect of a separate closely linked locus (FROST RESISTANCE-1), as proposed in early freezing tolerance studies.

Bilateral communication between plastid and the nucleus: plastid protein import and plastid-to-nucleus retrograde signaling

Plastids are a diverse group of organelles found in plants and some parasites. Chloroplasts are the archetypical plastids and are present in photosynthetic plant cells. Because most plastid proteins are encoded by the nuclear genome, plastid biogenesis relies on importing these proteins into the plastid. On the other hand, changes in functional or metabolic states of plastids have been known to affect the expression of nuclear genes encoding plastid proteins, and are collectively called "plastid signals." This regulation is also important for maintaining plastid function. This review focuses on the roles of these anterograde and retrograde pathways in plastid biogenesis and environmental adaptation.

Comparative expression of Cbf genes in the Triticeae under different acclimation induction temperatures

In plants, the C-repeat binding factors (Cbfs) are believed to regulate low-temperature (LT) tolerance. However, most functional studies of Cbfs have focused on characterizing expression after an LT shock and have not quantified differences associated with variable temperature induction or the rate of response to LT treatment. In the Triticeae, rye (Secale cereale L.) is one of the most LT-tolerant species, and is an excellent model to study and compare Cbf LT induction and expression profiles. Here, we report the isolation of rye Cbf genes (ScCbfs) and compare their expression levels in spring- and winter-habit rye cultivars and their orthologs in two winter-habit wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) cultivars. Eleven ScCbfs were isolated spanning all four major phylogenetic groups. Nine of the ScCbfs mapped to 5RL and one to chromosome 2R. Cbf expression levels were variable, with stronger expression in winter- versus spring-habit rye cultivars but no clear relationship with cultivar differences in LT, down-stream cold-regulated gene expression and Cbf expression were detected. Some Cbfs were expressed only at warmer acclimation temperatures in all three species and their expression was repressed at the end of an 8-h dark period at warmer temperatures, which may reflect a temperature-dependent, light-regulated diurnal response. Our work indicates that Cbf expression is regulated by complex genotype by time by induction-temperature interactions, emphasizing that sample timing, induction-temperature and light-related factors must receive greater consideration in future studies involving functional characterization of LT-induced genes in cereals.

Identification of leaf proteins differentially accumulated during cold acclimation between Festuca pratensis plants with distinct levels of frost tolerance

Festuca pratensis (meadow fescue) as the most frost-tolerant species within the Lolium-Festuca complex was used as a model for research aimed at identifying the cellular components involved in the cold acclimation (CA) of forage grasses. The work presented here also comprises the first comprehensive proteomic research on CA in a group of monocotyledonous species which are able to withstand winter conditions. Individual F. pratensis plants with contrasting levels of frost tolerance, high frost tolerant (HFT) and low frost tolerant (LFT) plants, were selected for comparative proteomic research. The work focused on the analysis of leaf protein accumulation before and after 2, 8, and 26 h, and 3, 5, 7, 14, and 21 d of CA, using high-throughput two-dimensional electrophoresis, and on the identification of proteins which were accumulated differentially between the selected plants by the application of mass spectrometry. The analyses of approximately 800 protein profiles revealed a total of 41 (5.1%) proteins that showed a minimum of a 1.5-fold difference in abundance, at a minimum of one time point of CA for HFT and LFT genotypes. It was shown that significant differences in profiles of protein accumulation between the analysed plants appeared relatively early during cold acclimation, most often after 26 h (on the 2nd day) of CA and one-half of the differentially accumulated proteins were all parts of the photosynthetic apparatus. Several proteins identified here have been reported to be differentially accumulated during cold conditions for the first time in this paper. The functions of the selected proteins in plant cells and their probable influence on the level of frost tolerance in F. pratensis, are discussed.

The effects of cold acclimation on photosynthetic apparatus and the expression of COR14b in four genotypes of barley (Hordeum vulgare) contrasting in their tolerance to freezing and high-light treatment in cold conditions

BACKGROUND AND AIMS

Cold acclimation modifies the balance of the energy absorbed and metabolized in the dark processes of photosynthesis, which may affect the expression of cold-regulated (COR) genes. At the same time, a gradual acclimation to the relatively high light conditions is observed, thereby minimizing the potential for photo-oxidative damage. As a result, the resistance to photoinhibition in the cold has often been identified as a trait closely related to freezing tolerance. Using four barley genotypes that differentially express both traits, the effect of cold acclimation on freezing tolerance and high-light tolerance was studied together with the expression of COR14b, one of the best-characterized barley COR genes.

METHODS

Plants were cold acclimated for 2 weeks at 2 degrees C. Freezing tolerance was studied by means of electrolyte leakage. Changes in photosynthetic apparatus and high-light tolerance were monitored by means of chlorophyll fluorescence. Accumulation of COR14b and some proteins important in photosynthetic acclimation to cold were studied with western analysis. COR14b transcript accumulation during cold acclimation was assessed with real-time PCR.

KEY RESULTS

Cold acclimation increased both freezing tolerance and high-light tolerance, especially when plants were treated with high light after non-lethal freezing. In all plants, cold acclimation triggered the increase in photosynthetic capacity during high-light treatment. In two plants that were characterized by higher high-light tolerance but lower freezing tolerance, higher accumulation of COR14b transcript and protein was observed after 7 d and 14 d of cold acclimation, while a higher transient induction of COR14b expression was observed in freezing-tolerant plants during the first day of cold acclimation. High-light tolerant plants were also characterized with a higher level of PsbS accumulation and more efficient dissipation of excess light energy.

CONCLUSIONS

Accumulation of COR14b in barley seems to be important for resistance to combined freezing and high-light tolerance, but not for freezing tolerance per se.

The change of chlorophyll fluorescence parameters in winter barley during recovery after freezing shock and as affected by cold acclimation and irradiance

The change of chlorophyll fluorescence parameters in froze leaves of 3 leaf-age seedlings were examined using two winter barley cultivars (Chumai 1 and Mo 103) differing in cold tolerance to investigate physiological response to low temperature as affected by cold acclimation (under 3/1 degrees C, day/night for 5 days before freezing treatment) and irradiation size (high irradiance: 380+/-25 micromol m(-2)s(-1) and low irradiance: 60+/-25 micromol m(-2)s(-1)) during recovery. The results showed that non-lethal freezing shock (exposed to -8 degrees C for 18 h) did not obviously affect maximum quantum efficiency in photosystem II (PSII), but dramatically increased non-photochemical quenching and reduced effective quantum yield in PSII. Cold acclimation significantly improved stability of photosynthetic function of leaves after freezing stress through buffering excessive energy and alleviating photoinhibition during recovery, indicating it increased recovery ability of barley plants from freezing injury. High irradiance was quite harmful to the stability of PSII in barley plants during recovery from freezing injury. The electron transport rate of PSII varied with cold-acclimation, irradiance and genotype. Cold acclimation caused significant increase in electron transport rate of PSII for relatively tolerant cultivar Mo 103, but not for relatively sensitive cultivar Chumai 1. It can be concluded that some chlorophyll fluorescence parameters during recovery from freezing shock may be used as the indicators in identification and evaluation of cold tolerance in barley.

Identification of Maize Kernel Endosperm Proteins Associated with Resistance to Aflatoxin Contamination by Aspergillus flavus

ABSTRACT Aflatoxins are carcinogens produced mainly by Aspergillus flavus during infection of susceptible crops such as maize (Zea mays). Previously, embryo proteins from maize genotypes resistant or susceptible to A. flavus infection were compared using proteomics, and resistance-associated proteins were identified. Here, we report the comparison of maize endosperm proteins from five resistant and five susceptible genotypes, and the identification of additional resistance-associated proteins using the same approach. Ten protein spots were upregulated twofold or higher in resistant lines compared with susceptible ones. Peptide sequencing of these proteins identified them as a globulin-2 protein, late embryogenesis abundant proteins (LEA3 and LEA14), a stress-related peroxiredoxin antioxidant (PER1), heat-shock proteins (HSP17.2), a cold-regulated protein (COR), and an antifungal trypsin-inhibitor protein (TI). The gene encoding one such upregulated protein, PER1, was cloned and overexpressed in Escherichia coli. The overexpressed PER1 protein demonstrated peroxidase activity in vitro. In addition, per1 expression was significantly higher in the resistant genotype Mp420 than in the susceptible genotype B73 during the late stage of kernel development, and was significantly induced upon A. flavus infection, suggesting that it may play an important role in enhancing kernel stress tolerance and aflatoxin resistance. The significance of other identified proteins to host resistance and stress tolerance also is discussed.

Gene expression patterns in near isogenic lines for wheat rust resistance gene lr34/yr18

ABSTRACT The Lr34/Yr18 resistance gene provides durable, adult-plant, slow rusting resistance to leaf rust, yellow rust, and several other diseases of wheat. Flag leaves may exhibit spontaneous leaf tip necrosis and tips are more resistant than leaf bases. Despite the importance of this gene, the mechanism of resistance is unknown. Patterns of expression for 55,052 transcripts were examined by microarray analysis in mock-inoculated flag leaves of two pairs of wheat near isogenic lines for Lr34/Yr18 (Jupateco 73S/Jupateco 73R and Thatcher/Thatcher-Lr34). The Thatcher isolines were also examined for patterns of expression after inoculation with leaf rust. Mock-inoculated leaf tips of resistant plants showed up-regulation of 57 transcripts generally associated with ABA inducibility, osmotic stress, cold stress, and/or seed maturation. Several transcripts may be useful as expression markers for Lr34/Yr18. Five transcripts were also up-regulated in resistant leaf bases. The possible role of these transcripts in resistance is discussed. In mock-inoculated plants, pathogenesis-related (PR) proteins were not up-regulated in resistant flag leaves compared with that in susceptible flag leaves. In inoculated plants, the same set of PR proteins was up-regulated in both resistant and susceptible flag leaves. However, expression was often higher in resistant plants, suggesting a possible role for Lr34/Yr18 in priming of defense responses.

Expression levels of barley Cbf genes at the Frost resistance-H2 locus are dependent upon alleles at Fr-H1 and Fr-H2

Genetic analyses have identified two loci in wheat and barley that mediate the capacity to overwinter in temperate climates. One locus co-segregates with VRN-1, which affects the vernalization requirement. This locus is known as Frost resistance-1 (Fr-1). The second locus, Fr-2, is coincident with a cluster of more than 12 Cbf genes. Cbf homologs in Arabidopsis thaliana play a key regulatory role in cold acclimatization and the acquisition of freezing tolerance. Here we report that the Hordeum vulgare (barley) locus VRN-H1/Fr-H1 affects expression of multiple barley Cbf genes at Fr-H2. RNA blot analyses, conducted on a 'Nure'x'Tremois' barley mapping population segregating for VRN-H1/Fr-H1 and Fr-H2, revealed that transcript levels of all cold-induced Cbf genes at Fr-H2 were significantly higher in recombinants harboring the vrn-H1 winter allele than in recombinants harboring the Vrn-H1 spring allele. Steady-state Cbf2 and Cbf4 levels were also significantly higher in recombinants harboring the Nure allele at Fr-H2. Additional experiments indicated that, in vrn-H1 genotypes requiring vernalization, Cbf expression levels were dampened after plants were vernalized, and dampened Cbf expression was accompanied by robust expression of Vrn-1. Cbf levels were also significantly higher in plants grown under short days than under long days. Experiments in wheat and rye indicated that similar regulatory mechanisms occurred in these plants. These results suggest that VRN-H1/Fr-H1 acts in part to repress or attenuate expression of the Cbf at Fr-H2; and that the greater level of low temperature tolerance attributable to the Nure Fr-H2 allele may be due to the greater accumulation of Cbf2 and Cbf4 transcripts during normal growth and development.

Interplay between cold-responsive gene regulation, metabolism and RNA processing during plant cold acclimation

Temperate plants are capable of developing freezing tolerance when they are exposed to low nonfreezing temperatures. Acquired freezing tolerance involves extensive reprogramming of gene expression and metabolism. Recent full-genome transcript profiling studies, in combination with mutational and transgenic plant analyses, have provided a snapshot of the complex transcriptional network that operates under cold stress. Ubiquitination-mediated proteosomal protein degradation has a crucial role in regulating one of the upstream transcription factors, INDUCER OF CBF EXPRESSION 1 (ICE1), and thus in controlling the cold-responsive transcriptome. The changes in expression of hundreds of genes in response to cold temperatures are followed by increases in the levels of hundreds of metabolites, some of which are known to have protective effects against the damaging effects of cold stress. Genetic analysis has revealed important roles for cellular metabolic signals, and for RNA splicing, export and secondary structure unwinding, in regulating cold-responsive gene expression and chilling and freezing tolerance.

Arabidopsis Cor15am is a chloroplast stromal protein that has cryoprotective activity and forms oligomers

Many plants acquire increased freezing tolerance when they are exposed to nonfreezing temperatures of a certain duration. This process is known as cold acclimation and allows plants to protect themselves from freezing injury. A wide variety of polypeptides are induced during cold acclimation, among which is one encoded by COR15A in Arabidopsis (Arabidopsis thaliana). Previous studies showed that the COR15A gene encodes a small, plastid-targeted polypeptide that is processed to a mature form called Cor15am. In this study, we examined the biochemical properties and activities of Cor15am in more detail. We provide evidence that Cor15am localizes almost exclusively to the chloroplast stroma. In addition, the cold-regulated accumulation of Cor15am is affected by chloroplast functionality. Both gel-filtration chromatography and protein cross-linking reveal that Cor15am forms oligomers in the stroma of chloroplasts. Although Cor15am accumulates in response to low temperature, cold acclimation is not a prerequisite for oligomerization of Cor15am. Structural analysis suggests that Cor15am is composed of both ordered and random structures, and can stay soluble with small structural change after boiling and freeze-thaw treatments. Recombinant Cor15am exhibits in vitro cryoprotection of a freeze-labile enzyme, l-lactate dehydrogenase. Furthermore, Cor15am is capable of associating with l-lactate dehydrogenase in vitro and with potential stromal substrates in vivo. On the basis of these results, we propose that Arabidopsis Cor15am is a cryoprotective protein that forms oligomers in the chloroplast stroma, and that direct association of Cor15am with its substrates is part of its cryoprotective mechanism.

Photosynthesis research in Italy: a review

This historical review was compiled and edited by Giorgio Forti, whereas the other authors of the different sections are listed alphabetically after his name, below the title of the paper; they are also listed in the individual sections. This review deals with the research on photosynthesis performed in several Italian laboratories during the last 50 years; it includes research done, in collaboration, at several international laboratories, particularly USA, UK, Switzerland, Hungary, Germany, France, Finland, Denmark, and Austria. Wherever pertinent, references are provided, especially to other historical papers in Govindjee et al. [Govindjee, Beatty JT, Gest H, Allen JF (eds) (2005) Discoveries in Photosynthesis. Springer, Dordrecht]. This paper covers the physical and chemical events starting with the absorption of a quantum of light by a pigment molecule to the conversion of the radiation energy into the stable chemical forms of the reducing power and of ATP. It describes the work done on the structure, function and regulation of the photosynthetic apparatus in higher plants, unicellular algae and in photosynthetic bacteria. Phenomena such as photoinhibition and the protection from it are also included. Research in biophysics of photosynthesis in Padova (Italy) is discussed by G.M. Giacometti and G. Giacometti (2006).

Gene duplication, exon gain and neofunctionalization of OEP16-related genes in land plants

OEP16, a channel protein of the outer membrane of chloroplasts, has been implicated in amino acid transport and in the substrate-dependent import of protochlorophyllide oxidoreductase A. Two major clades of OEP16-related sequences were identified in land plants (OEP16-L and OEP16-S), which arose by a gene duplication event predating the divergence of seed plants and bryophytes. Remarkably, in angiosperms, OEP16-S genes evolved by gaining an additional exon that extends an interhelical loop domain in the pore-forming region of the protein. We analysed the sequence, structure and expression of the corresponding Arabidopsis genes (atOEP16-S and atOEP16-L) and demonstrated that following duplication, both genes diverged in terms of expression patterns and coding sequence. AtOEP16-S, which contains multiple G-box ABA-responsive elements (ABREs) in the promoter region, is regulated by ABI3 and ABI5 and is strongly expressed during the maturation phase in seeds and pollen grains, both desiccation-tolerant tissues. In contrast, atOEP-L, which lacks promoter ABREs, is expressed predominantly in leaves, is induced strongly by low-temperature stress and shows weak induction in response to osmotic stress, salicylic acid and exogenous ABA. Our results indicate that gene duplication, exon gain and regulatory sequence evolution each played a role in the divergence of OEP16 homologues in plants.

Transcriptome analysis of cold acclimation in barley albina and xantha mutants

Previously, we have shown that barley (Hordeum vulgare) plants carrying a mutation preventing chloroplast development are completely frost susceptible as well as impaired in the expression of several cold-regulated genes. Here we investigated the transcriptome of barley albina and xantha mutants and the corresponding wild type to assess the effect of the chloroplast on expression of cold-regulated genes. First, by comparing control wild type against cold-hardened wild-type plants 2,735 probe sets with statistically significant changes (P = 0.05; > or = 2-fold change) were identified. Expression of these wild-type cold-regulated genes was then analyzed in control and cold-hardened mutants. Only about 11% of the genes cold regulated in wild type were regulated to a similar extent in all genotypes (chloroplast-independent cold-regulated genes); this class includes many genes known to be under C-repeat binding factor control. C-repeat binding factor genes were also equally induced in mutants and wild-type plants. About 67% of wild-type cold-regulated genes were not regulated by cold in any mutant (chloroplast-dependent cold-regulated genes). We found that the lack of cold regulation in the mutants is due to the presence of signaling pathway(s) normally cold activated in wild type but constitutively active in the mutants, as well as to the disruption of low-temperature signaling pathway(s) due to the absence of active chloroplasts. We also found that photooxidative stress signaling pathway is constitutively active in the mutants. These results demonstrate the major role of the chloroplast in the control of the molecular adaptation to cold.

Mapping of barley homologs to genes that regulate low temperature tolerance in Arabidopsis

We investigated the allelic nature and map locations of Hordeum vulgare (barley) homologs to three classes of Arabidopsis low temperature (LT) regulatory genes-CBFs, ICE1, and ZAT12-to determine if there were any candidates for winterhardiness-related quantitative trait loci (QTL). We phenotyped the Dicktoo x Morex (DxM) mapping population under controlled freezing conditions and in addition to the previously reported 5H-L Fr-H1 QTL, observed three additional LT tolerance QTLs on 1H-L, 4H-S, and 4H-L. We identified and assigned either linkage map or chromosome locations to 1 ICE1 homolog, 2 ZAT12 homologs, and 17 of 20 CBF homologs. Twelve of the CBF genes were located on 5H-L and the 11 with assigned linkage map positions formed 2 tandem clusters on 5H-L. A subset of these CBF genes was confirmed to be physically linked, validating the map position clustering. The tandem CBF clusters are not candidates for the DxM LT tolerance Fr-H1 QTL, as they are approximately 30 cM distal to the QTL peak. No LT tolerance QTL was detected in conjunction with the CBF gene clusters in Dicktoo x Morex. However, comparative mapping using common markers and BIN positions established the CBF clusters are coincident with reported Triticeae LT tolerance and COR gene accumulation QTLs and suggest one or more of the CBF genes may be candidates for Fr-H2 in some germplasm combinations. These results suggest members of the CBF gene family may function as components of winter-hardiness in the Triticeae and underscore both the importance of extending results from model systems to economically important crop species and in viewing QTL mapping results in the context of multiple germplasm combinations.

Structural, functional, and phylogenetic characterization of a large CBF gene family in barley

CBFs are key regulators in the Arabidopsis cold signaling pathway. We used Hordeum vulgare (barley), an important crop and a diploid Triticeae model, to characterize the CBF family from a low temperature tolerant cereal. We report that barley contains a large CBF family consisting of at least 20 genes (HvCBFs) comprising three multigene phylogenetic groupings designated the HvCBF1-, HvCBF3-, and HvCBF4-subgroups. For the HvCBF1- and HvCBF3-subgroups, there are comparable levels of phylogenetic diversity among rice, a cold-sensitive cereal, and the cold-hardy Triticeae. For the HvCBF4-subgroup, while similar diversity levels are observed in the Triticeae, only a single ancestral rice member was identified. The barley CBFs share many functional characteristics with dicot CBFs, including a general primary domain structure, transcript accumulation in response to cold, specific binding to the CRT motif, and the capacity to induce cor gene expression when ectopically expressed in Arabidopsis. Individual HvCBF genes differed in response to abiotic stress types and in the response time frame, suggesting different sets of HvCBF genes are employed relative to particular stresses. HvCBFs specifically bound monocot and dicot cor gene CRT elements in vitro under both warm and cold conditions; however, binding of HvCBF4-subgroup members was cold dependent. The temperature-independent HvCBFs activated cor gene expression at warm temperatures in transgenic Arabidopsis, while the cold-dependent HvCBF4-subgroup members of three Triticeae species did not. These results suggest that in the Triticeae - as in Arabidopsis - members of the CBF gene family function as fundamental components of the winter hardiness regulon.

The effect of overexpression of two Brassica CBF/DREB1-like transcription factors on photosynthetic capacity and freezing tolerance in Brassica napus

The effects of overexpression of two Brassica CBF/DREB1-like transcription factors (BNCBF5 and 17) in Brassica napus cv. Westar were studied. In addition to developing constitutive freezing tolerance and constitutively accumulating COR gene mRNAs, BNCBF5- and 17-overexpressing plants also accumulate moderate transcript levels of genes involved in photosynthesis and chloroplast development as identified by microarray and Northern analyses. These include GLK1- and GLK2-like transcription factors involved in chloroplast photosynthetic development, chloroplast stroma cyclophilin ROC4 (AtCYP20-3), beta-amylase and triose-P/Pi translocator. In parallel with these changes, increases in photosynthetic efficiency and capacity, pigment pool sizes, increased capacities of the Calvin cycle enzymes, and enzymes of starch and sucrose biosynthesis, as well as glycolysis and oxaloacetate/malate exchange are seen, suggesting that BNCBF overexpression has partially mimicked cold-induced photosynthetic acclimation constitutively. Taken together, these results suggest that BNCBF/DREB1 overexpression in Brassica not only resulted in increased constitutive freezing tolerance but also partially regulated chloroplast development to increase photochemical efficiency and photosynthetic capacity.

Low temperature promotes intron retention in two e-cor genes of durum wheat

Following the screening of a suppression subtractive library developed from durum wheat plants exposed to low temperature for 6 h, two early cold-regulated (e-cor) genes have been isolated. These genes, coding putatively for a ribokinase (7H8) and a C3H2C3 RING-finger protein (6G2), were characterized by the stress-induced retention of a subset of introns in the mature mRNA. This feature was dependent on cold for 7H8 and on cold and dehydration for 6G2. When other genes, such as the stress-related gene WCOR410c, coding for a dehydrin (one intron), or a gene coding for a putative ATP binding cassette transporter (16 introns) were analyzed, no cold-dependent intron retention was observed. Cold-induced intron retention was not observed in mutants defective in the chloroplast development; nevertheless treatment with cycloheximide in the absence of cold was able to promote intron retention for the 7H8 e-cor gene. These results suggest that the cold-induced intron retention reflects the response of the spliceosoma to specific environmental signals transduced to the splicing protein factors through a chloroplast-dependent pathway. Notably, when the 7H8 Arabidopsis orthologous gene was analyzed, no stress induction in terms of mRNA abundance and no cold-dependent intron retention was detected. Otherwise, 6G2 Arabidopsis homologous sequences sharing the same genomic structure of the durum wheat 6G2 showed a similar intron retention event although not strictly dependent on stress.

Hv-WRKY38: a new transcription factor involved in cold- and drought-response in barley

WRKY proteins constitute a large family of plant specific transcription factors implicated in many different processes. Here we describe Hv-WRKY38, a barley gene coding for a WRKY protein, whose expression is involved in cold and drought stress response. Hv-WRKY38 was early and transiently expressed during exposure to low non-freezing temperature, in ABA-independent manner. Furthermore, it showed a continuous induction during dehydration and freezing treatments. A WRKY38:YFP fusion protein was found to localise into the nucleus upon introduction into epidermal onion cells. Bacterially expressed Hv-WRKY38 was able to bind in vitro to the W-box element (T)TGAC(C/T) also recognisable by other WRKY proteins. Hv-WRKY38 genomic DNA was sequenced and mapped onto the centromeric region of the barley chromosome 6H. Arabidopsis and rice sequences homologous to Hv-WRKY38 were also identified. Our results indicate that Hv-WRKY38 transcription factor may play a regulatory role in abiotic stress response.

Two loci on chromosome 5H determine low-temperature tolerance in a 'Nure' (winter) x 'Tremois' (spring) barley map

Barley ( Hordeum vulgare subsp. vulgare) is an economically important diploid model for the Triticeae; and a better understanding of low-temperature tolerance mechanisms could significantly improve the yield of fall-sown cereals. We developed a new resource for genetic analysis of winter hardiness-related traits, the 'Nure' x 'Tremois' linkage map, based on a doubled-haploid population that is segregating for low-temperature tolerance and vernalization requirement. Three measures of low-temperature tolerance and one measure of vernalization requirement were used and, for all traits, QTLs were mapped on chromosome 5H. The vernalization response QTL coincides with previous reports at the Vrn-1/Fr1 region of the Triticeae. We also found coincident QTLs at this position for all measures of low-temperature tolerance. Using Composite Interval Mapping, a second proximal set, of coincident QTLs for low-temperature tolerance, and the accumulation of two different COR proteins (COR14b and TMC-Ap3) was identified. The HvCBF4 locus, or another member of the CBF loci clustered in this region, is the candidate gene underlying this QTL. There is a CRT/DRE recognition site in the promoter of cor14b with which a CBF protein could interact. These results support the hypothesis that highly conserved regulatory factors, such as members of the CBF gene family, may regulate the stress responses of a wide range of plant species.