Differential and coordinated expression of Cbf and Cor/Lea genes during long-term cold acclimation in two wheat cultivars showing distinct levels of freezing tolerance

Identification of DREB Family Genes in Banana and Their Function under Drought and Cold Stress

Bananas are one of the most important cash crops in the tropics and subtropics. Drought and low-temperature stress affect the growth of banana. The DREB (dehydration responsive element binding protein) gene family, as one of the major transcription factor families, plays crucial roles in defense against abiotic stress. Currently, systematic analyses of the banana DREB (MaDREB) gene family have not yet been reported. In this study, 103 members of the MaDREB gene family were identified in the banana genome. In addition, transcriptomic analysis results revealed that MaDREBs responded to drought and cold stress. The expression of MaDREB14/22/51 was induced by drought and cold stress; these geneswere selected for further analysis. The qRT-PCR validation results confirmed the transcriptome results. Additionally, transgenic Arabidopsis plants overexpressing MaDREB14/22/51 exhibited enhanced resistance to drought and cold stress by reducing MDA content and increasing PRO and soluble sugar content. This study enhances our understanding of the function of the MaDREB gene family, provides new insights into their regulatory role under abiotic stress, and lays a good foundation for improving drought and cold stress-tolerant banana verities.

Transcriptome analysis during vernalization in wheat (Triticum aestivum L.)

BACKGROUND

Vernalization, as a vital process in the life cycle of winter cereal, has important effects on floral organ formation and flowering time. Many morphological changes together with molecular changes occur during the vernalization period. Here, we used transcriptome sequencing to analyze the transcriptomic changes in wheat leaves before, during and after vernalization using the winter wheat cultivar 'Shiluan02-1'.

RESULTS

A total of 16,370 differentially expressed genes were obtained across different vernalization periods. Gene Ontology enrichment analysis revealed that photoperiodism, photoprotection, photosynthesis, lipid transport and biosynthetic process, and chlorophyll metabolic process were closely related to vernalization. In addition, AP2/ERF, C2H2, bHLH, WRKY, MYB, MYB-related, and NAC transcription factors were significantly enriched during vernalization, and the transcription factor expression patterns suggested the intricate regulation of transcription factor modules in plant vernalization pathways. Analysis of gene expression patterns of the MADS-box transcription factor genes showed different expression patterns during vernalization phases, among which VERNALIZATION1 (VRN1) genes were found to gradually increase during vernalization periods from V0 to V35, while decline in the V42 phase, then increase after vernalization. The Tavrt-2 gene cooperated with Tavrn1 to regulate flowering induced by vernalization, and its expression level was rapidly increased by vernalization but declined in the V42 phase and then increased after vernalization. Some genes from the ICE-CBF-COR pathway were also identified, and additional analysis indicated that some key genes related to phytohormone biosynthesis and signal transduction were enriched during the vernalization period, such as gibberellic acid, ethylene, abscisic acid and jasmonic acid biosynthesis and signaling pathway genes.

CONCLUSIONS

Our study provides valuable molecular information for future studies on wheat vernalization regulation and also serves as an excellent reference for future wheat breeding.

How Does Diurnal and Nocturnal Warming Affect the Freezing Resistance of Antarctic Vascular Plants?

The Antarctic Peninsula has rapidly warmed up in past decades, and global warming has exhibited an asymmetric trend; therefore, it is interesting to understand whether nocturnal or diurnal warming is the most relevant for plant cold deacclimation. This study aimed to evaluate the effect of diurnal and nocturnal warming on Antarctic vascular plant's freezing resistance under laboratory conditions. This was studied by measuring the lethal temperature for 50% of tissue (LT₅₀), ice nucleation temperature (INT), and freezing point (FP) on Deschampsia antarctica and Colobanthus quitensis plants. Additionally, soluble carbohydrates content and dehydrin levels were analyzed during nocturnal and diurnal temperatures increase. Nocturnal warming led to a 7 °C increase in the LT₅₀ of D. antarctica and reduced dehydrin-like peptide expression. Meanwhile, C. quitensis warmed plants reduce their LT₅₀ to about 3.6 °C. Both species reduce their sucrose content by more than 28% in warming treatments. Therefore, nocturnal warming leads to cold deacclimation in both plant species, while C. quitensis plants are also cold-deacclimated upon warm days. This suggests that even when the remaining freezing resistance of both species allows them to tolerate summer freezing events, C. quitensis can reach its boundaries of freezing vulnerability in the near future if warming in the Antarctic Peninsula progress.

Genome-wide identification and expression analysis of DREB genes in alfalfa (Medicago sativa) in response to cold stress

Dehydration-responsive element-binding proteins (DREBs) belong to members of the AP2/ERF transcription factor superfamily, which has been reported to involve various abiotic-stress responses and tolerance in plants. However, research on the DREB-family is still limited in alfalfa (Medicago sativa L.), a forage legume cultivated worldwide. The recent genome-sequence release of the alfalfa cultivar "XinJiangDaYe" allowed us to identify 172 DREBs by a multi-step homolog search. The phylogenetic analysis indicated that such MsDREBs could be classified into 5 groups, namely A-1 (56 members), A-2 (39), A-3 (3), A-4 (61) and 13 (A-5 (13), thus adding substantial new members to the DREB-family in alfalfa. Furthermore, a comprehensive survey in silico of conserved motif, gene structure, molecular weight, and isoelectric point (pI) as well as gene expression was conducted. The resulting data showed that, for cold-stress response, 33 differentially expressed MsDREBs were identified with a threshold of Log2-fold > 1, and most of which were transcriptionally upregulated within 48 h during a cold treatment(s). Moreover, the expression profiling of MsDREBs from two ecotypes of alfalfa subspecies i.e. M. sativa ssp. falcata (F56, from a colder region of Central Asia) and M. sativa ssp. sativa (B47, from Near East) revealed that most of the cold-stress responsive MsDREBs exhibited a significantly lower expression in F56, leading to a proposal of the existence of a distinct mechanism(s) for cold tolerance regulated by DREB-related action, which would have been evolved in alfalfa with a genotypic specificity. Additionally, by examining the transcriptome of a freezing-tolerance species (M. sativa cv. Zhaodong), eight DREBs were found to be implicated in a long-term freezing-stress adaptation with a great potential. Taken together, the current genome-wide identification in alfalfa points to the importance of some MsDREBs in the cold-stress response, providing some promising molecular targets to be functionally characterized for the improvement of cold tolerance in crops including alfalfa.

Sugarcane ScDREB2B-1 Confers Drought Stress Tolerance in Transgenic Nicotiana benthamiana by Regulating the ABA Signal, ROS Level and Stress-Related Gene Expression

The dehydration-responsive element-binding protein (DREB) is a subgroup member of the AP2/ERF family and actively participates in the response of plants to abiotic stress. Although DREB genes have been studied in a variety of plant species, there are few reports of DREB genes in sugarcane (Saccharum spp.). In this study, a novel full-length cDNA sequence of the ScDREB2B-1 gene was cloned from the Saccharum hybrid ROC22, whose encoding protein contained only one AP2-conserved domain and was clustered into the DREB (A-2) subgroup. The diverse promoter elements in the ScDREB2B-1 gene and the accumulated transcripts of its homologous gene (SsAP2/ERF-107) in S. spontaneum under drought stress suggest that the ScDREB2B-1 gene may play a role in drought response. In addition, reverse transcription quantitative PCR analysis showed that the expression level of the ScDREB2B-1 gene was upregulated in the root and leaf of ROC22 under polyethylene glycol, sodium chloride and abscisic acid (ABA) treatments. The yeast two-hybrid experiment demonstrated that ScDREB2B-1 had transcriptional self-activation activity. Compared with wild-type plants, the overexpression of the ScDREB2B-1 gene improved the drought tolerance of the transgenic Nicotiana benthamiana by activating the ABA pathway to enhance the expression of the ABA-responsive gene (NbNCED) and ABA content, regulate the intracellular reactive oxygen species (ROS) level (enhance the transcripts of ROS synthase-related gene NbRbohB and the activities of catalase, peroxidase and superoxide dismutase) and increase the relative water content, proline content and expression level of osmotic stress-related genes (NbERD and NbLEA). Collectively, our data indicate that ScDREB2B-1 is a stress-inducible and ABA-responsive transcription factor gene that responds to drought stress by regulating ABA signaling, ROS levels and stress-related gene expression. This study contributes to a better understanding of the biological function of ScDREB2B-1, which could serve as a foundation for future resistance breeding in sugarcane.

Genome-wide analysis of the DREB family genes and functional identification of the involvement of BrDREB2B in abiotic stress in wucai (Brassica campestris L.)

Dehydration responsive element binding protein (DREB) is a significant transcription factor class known to be implicated in abiotic stresses. In this study, we systematically conducted a genome-wide identification and expression analysis of the DREB gene family, including gene structures, evolutionary relationships, chromosome distribution, conserved domains, and expression patterns. A total of 65 DREB family gene members were identified in Chinese cabbage (Brassica rapa L.) and were classified into five subgroups based on phylogenetic analysis. Through analysis of the conserved domains of BrDREB family genes, only one exon existed in the gene structure. Through the analysis of cis-acting elements, these genes were mainly involved in hormone regulation and adversity stress. In order to identify the function of BrDREB2B, overexpressed transgenic Arabidopsis was constructed. After different stress treatments, the germination rate, root growth, survival rate, and various plant physiological indicators were measured. The results showed that transgenic Arabidopsis thaliana plants overexpressing BrDREB2B exhibited enhanced tolerance to salt, heat and drought stresses. Taken together, our results are the first to report the BrDREB2B gene response to drought and heat stresses in Chinese cabbage and provide a basis for further studies to determine the function of BrDREBs in response to abiotic stresses.

Transcription factors as key molecular target to strengthen the drought stress tolerance in plants

Amid apprehension of global climate change, crop plants are inevitably confronted with a myriad of abiotic stress factors during their growth that inflicts a serious threat to their development and overall productivity. These abiotic stresses comprise extreme temperature, pH, high saline soil, and drought stress. Among different abiotic stresses, drought is considered the most calamitous stressor with its serious impact on the crops' yield stability. The development of climate-resilient crops that withstands reduced water availability is a major focus of the scientific fraternity to ensure the food security of the sharply increasing population. Numerous studies aim to recognize the key regulators of molecular and biochemical processes associated with drought stress tolerance response. A few potential candidates are now considered as promising targets for crop improvement. Transcription factors act as a key regulatory switch controlling the gene expression of diverse biological processes and, eventually, the metabolic processes. Understanding the role and regulation of the transcription factors will facilitate the crop improvement strategies intending to develop and deliver agronomically-superior crops. Therefore, in this review, we have emphasized the molecular avenues of the transcription factors that can be exploited to engineer drought tolerance potential in crop plants. We have discussed the molecular role of several transcription factors, such as basic leucine zipper (bZIP), dehydration responsive element binding (DREB), DNA binding with one finger (DOF), heat shock factor (HSF), MYB, NAC, TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP), and WRKY. We have also highlighted candidate transcription factors that can be used for the development of drought-tolerant crops.

Membrane-Enriched Proteomics Link Ribosome Accumulation and Proteome Reprogramming With Cold Acclimation in Barley Root Meristems

Due to their sessile nature, plants rely on root systems to mediate many biotic and abiotic cues. To overcome these challenges, the root proteome is shaped to specific responses. Proteome-wide reprogramming events are magnified in meristems due to their active protein production. Using meristems as a test system, here, we study the major rewiring that plants undergo during cold acclimation. We performed tandem mass tag-based bottom-up quantitative proteomics of two consecutive segments of barley seminal root apexes subjected to suboptimal temperatures. After comparing changes in total and ribosomal protein (RP) fraction-enriched contents with shifts in individual protein abundances, we report ribosome accumulation accompanied by an intricate translational reprogramming in the distal apex zone. Reprogramming ranges from increases in ribosome biogenesis to protein folding factors and suggests roles for cold-specific RP paralogs. Ribosome biogenesis is the largest cellular investment; thus, the vast accumulation of ribosomes and specific translation-related proteins during cold acclimation could imply a divergent ribosomal population that would lead to a proteome shift across the root. Consequently, beyond the translational reprogramming, we report a proteome rewiring. First, triggered protein accumulation includes spliceosome activity in the root tip and a ubiquitous upregulation of glutathione production and S-glutathionylation (S-GSH) assemblage machineries in both root zones. Second, triggered protein depletion includes intrinsically enriched proteins in the tip-adjacent zone, which comprise the plant immune system. In summary, ribosome and translation-related protein accumulation happens concomitantly to a proteome reprogramming in barley root meristems during cold acclimation. The cold-accumulated proteome is functionally implicated in feedbacking transcript to protein translation at both ends and could guide cold acclimation.

COR/LEA Proteins as Indicators of Frost Tolerance in Triticeae: A Comparison of Controlled versus Field Conditions

Low temperatures in the autumn induce enhanced expression/relative accumulation of several cold-inducible transcripts/proteins with protective functions from Late-embryogenesis-abundant (LEA) superfamily including dehydrins. Several studies dealing with plants grown under controlled conditions revealed a correlation (significant quantitative relationship) between dehydrin transcript/protein relative accumulation and plant frost tolerance. However, to apply these results in breeding, field experiments are necessary. The aim of the review is to provide a summary of the studies dealing with the relationships between plant acquired frost tolerance and COR/LEA transcripts/proteins relative accumulation in cereals grown in controlled and field conditions. The impacts of cold acclimation and vernalisation processes on the ability of winter-type Triticeae to accumulate COR/LEA proteins are discussed. The factors determining dehydrin relative accumulation under controlled cold acclimation treatments versus field trials during winter seasons are discussed. In conclusion, it can be stated that dehydrins could be used as suitable indicators of winter survival in field-grown winter cereals but only in plant prior to the fulfilment of vernalisation requirement.

A DREB1 gene from zoysiagrass enhances Arabidopsis tolerance to temperature stresses without growth inhibition

The DREB (dehydration-responsive element binding) protein family comprises transcription factors that can increase the survivability of a plant under abiotic stresses by regulating expression of multiple genes and altering downstream metabolism at the cost of growth retardation and developmental delay. In this study, a gene for the DREB1-type transcription factor, designated ZjDREB1.4, was isolated from zoysiagrass (Zoysia japonica Steud.), a popular warm-season turfgrass. This gene contains a conserved AP2/ERF DNA-binding domain flanking the signature sequence of DREB1 and belongs to a DREB1 branch in the grass family that expands in the warm-season species. The expression of ZjDREB1.4 was significantly induced by chilling stress (4-15 °C), moderately induced by salt stress, and only slightly induced by drought stress. The product of ZjDREB1.4 was targeted to the nucleus and showed strong transactivation activity but weak binding to the DRE with ACCGAC as the core sequence. The ZjDREB1.4 protein bound to GCCGAC more preferentially than to ACCGAC. Overexpression of ZjDREB1.4 in Arabidopsis induced the expression of multiple genes including a part of the CBF-regulon, and moderately increased the levels of proline and soluble sugars under normal growth conditions. The transgenic Arabidopsis plants showed an increase in tolerance to high and freezing temperature stresses without obvious growth inhibition and with only a few days delay in bolting. ZjDREB1.4 is potentially useful for producing transgenic plants that are tolerant to high temperature and/or cold stresses with few negative effects.

CBF/DREB transcription factor genes play role in cadmium tolerance and phytoaccumulation in Ricinus communis under molybdenum treatments

The C-repeat binding factor/dehydration responsive element binding proteins (CBF/DREB) constitute a large group of transcriptional factors. Their role in abiotic stresses such as drought, salinity and low temperature tolerance in plants have been well established, while little information about their role in metals stress tolerance is available. Transcriptomic analyses of four genes (DREB-1A, DREB-1B, DREB-1F and CBF) were carried out in industrially important plant Ricinus communis under cadmium (Cd) and molybdenum (Mo) treatments. Cadmium (in soil) and Mo (as foliar spray) were used separate as well as in combinations. All the genes (except DREB 1A) expressed under Cd stress, while Mo further enhanced their expression. The proline (55.68 ± 5.51 ppm) and phenolic (120.00 ± 14.40 ppm) contents were significant increase in combination treatments of Cd and Mo. Positive and significant correlations of DREB 1B, DREB 1F and CBF genes expressions with free proline (0.92, 0.93 and 0.88 respectively), phenolic (075, 0.77 and 0.62 respectively) contents and Cd accumulation were demonstrated. Nucleotide sequence of R. comunis DREB1F and CBF genes showed more than 80% homology with related genes of other flowering plants. Predicted amino acids sequence of R. communis DREB 1F and CBF protein fragment demonstrated more than 75% homology with related proteins from other flowering plants.

Identification of Rice Genes Associated With Enhanced Cold Tolerance by Comparative Transcriptome Analysis With Two Transgenic Rice Plants Overexpressing DaCBF4 or DaCBF7, Isolated From Antarctic Flowering Plant Deschampsia antarctica

Few plant species can survive in Antarctica, the harshest environment for living organisms. Deschampsia antarctica is the only natural grass species to have adapted to and colonized the maritime Antarctic. To investigate the molecular mechanism of the Antarctic adaptation of this plant, we identified and characterized D. antarctica C-repeat binding factor 4 (DaCBF4), which belongs to monocot CBF group IV. The transcript level of DaCBF4 in D. antarctica was markedly increased by cold and dehydration stress. To assess the roles of DaCBF4 in plants, we generated a DaCBF4-overexpressing transgenic rice plant (Ubi:DaCBF4) and analyzed its abiotic stress response phenotype. Ubi:DaCBF4 displayed enhanced tolerance to cold stress without growth retardation under any condition compared to wild-type plants. Because the cold-specific phenotype of Ubi:DaCBF4 was similar to that of Ubi:DaCBF7 (Byun et al., 2015), we screened for the genes responsible for the improved cold tolerance in rice by selecting differentially regulated genes in both transgenic rice lines. By comparative transcriptome analysis using RNA-seq, we identified 9 and 15 genes under normal and cold-stress conditions, respectively, as putative downstream targets of the two D. antarctica CBFs. Overall, our results suggest that Antarctic hairgrass DaCBF4 mediates the cold-stress response of transgenic rice plants by adjusting the expression levels of a set of stress-responsive genes in transgenic rice plants. Moreover, selected downstream target genes will be useful for genetic engineering to enhance the cold tolerance of cereal plants, including rice.

Elucidating the role of osmotic, ionic and major salt responsive transcript components towards salinity tolerance in contrasting chickpea (Cicer arietinum L.) genotypes

The growth of chickpea (Cicer arietinum L.) is extremely hampered by salt stress. Understanding of physio-biochemical and molecular attributes along with morphological traits contributing to the salinity tolerance is important for developing salt tolerant chickpea varieties. To explore these facts, two genotypes CSG8962 and HC5 with contrasting salt tolerance were evaluated in the salinity stress (Control and 120 mM NaCl) conditions. CSG8962 maintained lower Na/K ratio in root and shoot, trammeled Na translocation to the shoots from roots compared to HC5 which ascribed to better exclusion of salt from its roots and compartmentation in the shoot. In chickpea, salt stress specifically induced genes/sequences involved at several levels in the salt stress signaling pathway. Higher induction of trehalose 6 phosphate synthase and protein kinase genes pertaining to the osmotic and signaling modules, respectively, were evident in CSG8962 compared to HC5. Further transcripts of late embryogenesis abundant, non-specific lipid transfer protein, HI and 219 genes/sequences were also highly induced in CSG8962 compared to HC5 which emphasizes the better protection of cellular membranous network and membrane-bound macromolecules under salt stress. This further suppressed the stress enhanced electrolyte leakage, loss of turgidity, promoted the higher compatible solute accumulation and maintained better cellular ion homoeostasis in CSG8962 compared to HC5. Our study further adds to the importance of these genes in salt tolerance by comparing their behavior in contrasting chickpea genotypes.

Transcription factors involved in drought tolerance and their possible role in developing drought tolerant cultivars with emphasis on wheat (Triticum aestivum L.)

TFs involved in drought tolerance in plants may be utilized in future for developing drought tolerant cultivars of wheat and some other crops. Plants have developed a fairly complex stress response system to deal with drought and other abiotic stresses. These response systems often make use of transcription factors (TFs); a gene encoding a specific TF together with -its target genes constitute a regulon, and take part in signal transduction to activate/silence genes involved in response to drought. Since, five specific families of TFs (out of >80 known families of TFs) have gained widespread attention on account of their significant role in drought tolerance in plants, TFs and regulons belonging to these five multi-gene families (AP2/EREBP, bZIP, MYB/MYC, NAC and WRKY) have been described and their role in improving drought tolerance discussed in this brief review. These TFs often undergo reversible phosphorylation to perform their function, and are also involved in complex networks. Therefore, some details about reversible phosphorylation of TFs by different protein kinases/phosphatases and the co-regulatory networks, which involve either only TFs or TFs with miRNAs, have also been discussed. Literature on transgenics involving genes encoding TFs and that on QTLs and markers associated with TF genes involved in drought tolerance has also been reviewed. Throughout the review, there is a major emphasis on wheat as an important crop, although examples from the model cereal rice (sometimes maize also), and the model plant Arabidopsis have also been used. This knowledge base may eventually allow the use of TF genes for development of drought tolerant cultivars, particularly in wheat.

Expansion and stress responses of AP2/EREBP superfamily in Brachypodium distachyon

APETALA2/ethylene-responsive element binding protein (AP2/EREBP) transcription factors constitute one of the largest and most conserved gene families in plant, and play essential roles in growth, development and stress response. Except a few members, the AP2/EREBP family has not been characterized in Brachypodium distachyon, a model plant of Poaceae. We performed a genome-wide study of this family in B. distachyon by phylogenetic analyses, transactivation assays and transcript profiling. A total of 149 AP2/EREBP genes were identified and divided into four subfamilies, i.e., ERF (ethylene responsive factor), DREB (dehydration responsive element binding gene), RAV (related to ABI3/VP) and AP2. Tandem duplication was a major force in expanding B. distachyon AP2/EREBP (BdAP2/EREBP) family. Despite a significant expansion, genomic organizations of BdAP2/EREBPs were monotonous as the majority of them, except those of AP2 subfamily, had no intron. An analysis of transcription activities of several closely related and duplicated BdDREB genes showed their functional divergence and redundancy in evolution. The expression of BdAP2/EREBPs in different tissues and the expression of DREB/ERF subfamilies in B. distachyon, wheat and rice under abiotic stresses were investigated by next-generation sequencing and microarray profiling. Our results are valuable for further function analysis of stress tolerant AP2/EREBP genes in B. distachyon.

Line differences in Cor/Lea and fructan biosynthesis-related gene transcript accumulation are related to distinct freezing tolerance levels in synthetic wheat hexaploids

In common wheat, cultivar differences in freezing tolerance are considered to be mainly due to allelic differences at two major loci controlling freezing tolerance. One of the two loci, Fr-2, is coincident with a cluster of genes encoding C-repeat binding factors (CBFs), which induce downstream Cor/Lea genes during cold acclimation. Here, we conducted microarray analysis to study comprehensive changes in gene expression profile under long-term low-temperature (LT) treatment and to identify other LT-responsive genes related to cold acclimation in leaves of seedlings and crown tissues of a synthetic hexaploid wheat line. The microarray analysis revealed marked up-regulation of a number of Cor/Lea genes and fructan biosynthesis-related genes under the long-term LT treatment. For validation of the microarray data, we selected four synthetic wheat lines that contain the A and B genomes from the tetraploid wheat cultivar Langdon and the diverse D genomes originating from different Aegilops tauschii accessions with distinct levels of freezing tolerance after cold acclimation. Quantitative RT-PCR showed increased transcript levels of the Cor/Lea, CBF, and fructan biosynthesis-related genes in more freezing-tolerant lines than in sensitive lines. After a 14-day LT treatment, a significant difference in fructan accumulation was observed among the four lines. Therefore, the fructan biosynthetic pathway is associated with cold acclimation in development of wheat freezing tolerance and is another pathway related to diversity in freezing tolerance, in addition to the CBF-mediated Cor/Lea expression pathway.

Cell Wall Metabolism in Response to Abiotic Stress

This review focuses on the responses of the plant cell wall to several abiotic stresses including drought, flooding, heat, cold, salt, heavy metals, light, and air pollutants. The effects of stress on cell wall metabolism are discussed at the physiological (morphogenic), transcriptomic, proteomic and biochemical levels. The analysis of a large set of data shows that the plant response is highly complex. The overall effects of most abiotic stress are often dependent on the plant species, the genotype, the age of the plant, the timing of the stress application, and the intensity of this stress. This shows the difficulty of identifying a common pattern of stress response in cell wall architecture that could enable adaptation and/or resistance to abiotic stress. However, in most cases, two main mechanisms can be highlighted: (i) an increased level in xyloglucan endotransglucosylase/hydrolase (XTH) and expansin proteins, associated with an increase in the degree of rhamnogalacturonan I branching that maintains cell wall plasticity and (ii) an increased cell wall thickening by reinforcement of the secondary wall with hemicellulose and lignin deposition. Taken together, these results show the need to undertake large-scale analyses, using multidisciplinary approaches, to unravel the consequences of stress on the cell wall. This will help identify the key components that could be targeted to improve biomass production under stress conditions.

Structural alteration of cell wall pectins accompanies pea development in response to cold

Pea (Pisum sativum) cell wall metabolism in response to chilling was investigated in a frost-sensitive genotype 'Terese' and a frost-tolerant genotype 'Champagne'. Cell walls isolated from stipules of cold acclimated and non-acclimated plants showed that cold temperatures induce changes in polymers containing xylose, arabinose, galactose and galacturonic acid residues. In the tolerant cultivar Champagne, acclimation is accompanied by increases in homogalacturonan, xylogalacturonan and highly branched Rhamnogalacturonan I with branched and unbranched (1→5)-α-arabinans and (1→4)-β-galactans. In contrast, the sensitive cultivar Terese accumulates substantial amounts of (1→4)-β-xylans and glucuronoxylan, but not the pectins. Greater JIM7 labeling was observed in Champagne compared to Terese, indicating that cold acclimation also induces an increase in the degree of methylesterification of pectins. Significant decrease in polygalacturonase activities in both genotypes were observed at the end of cold acclimation. These data indicate a role for esterified pectins in cold tolerance. The possible functions for pectins and their associated arabinans and galactans in cold acclimation are discussed.

Molecular characterization of cotton C-repeat/dehydration-responsive element binding factor genes that are involved in response to cold stress

Low temperature, drought and salinity are major abiotic stresses that influence survival, productivity and geographical distribution of many important crops across the globe. The C-repeat/dehydration-responsive element binding transcription factors (CBF/DREB) are important proteins involved in response to abiotic stresses in plants. In this study, twenty-one CBF genes were identified in cotton (Gossypium hirsutum) by bioinformatic approach. The twenty-one CBF genes (named as GhCBF1--GhCBF21) were characterized to encode proteins that share high similarity with those plant cold stress-related CBF proteins, which contain the classic AP2 domain of 58 amino acid residues. Phylogenetic analysis revealed that the isolated cotton CBF genes can be classified into 4 groups: GhCBF I, GhCBF II, GhCBF III and GhCBF IV. RT-PCR analysis indicated that GhCBF genes were up-regulated in cotton plants under cold stress. Furthermore, four GhCBF genes were up-regulated in cotton under salinity and drought treatments. Our data provided valuable information for further exploring the roles of the CBF genes in cotton development and in response to cold stress.

Cross-talk between abscisic acid-dependent and abscisic acid-independent pathways during abiotic stress

Salinity, drought and low temperature are the common forms of abiotic stress encountered by land plants. To cope with these adverse environmental factors, plants execute several physiological and metabolic responses. Both osmotic stress (elicited by water deficit or high salt) and cold stress increase the endogenous level of the phytohormone abscisic acid (ABA). ABA-dependent stomatal closure to reduce water loss is associated with small signaling molecules like nitric oxide, reactive oxygen species and cytosolic free calcium, and mediated by rapidly altering ion fluxes in guard cells. ABA also triggers the expression of osmotic stress-responsive (OR) genes, which usually contain single/multiple copies of cis-acting sequence called abscisic acid-responsive element (ABRE) in their upstream regions, mostly recognized by the basic leucine zipper-transcription factors (TFs), namely, ABA-responsive element-binding protein/ABA-binding factor. Another conserved sequence called the dehydration-responsive element (DRE)/C-repeat, responding to cold or osmotic stress, but not to ABA, occurs in some OR promoters, to which the DRE-binding protein/C-repeat-binding factor binds. In contrast, there are genes or TFs containing both DRE/CRT and ABRE, which can integrate input stimuli from salinity, drought, cold and ABA signaling pathways, thereby enabling cross-tolerance to multiple stresses. A strong candidate that mediates such cross-talk is calcium, which serves as a common second messenger for abiotic stress conditions and ABA. The present review highlights the involvement of both ABA-dependent and ABA-independent signaling components and their interaction or convergence in activating the stress genes. We restrict our discussion to salinity, drought and cold stress.

A major quantitative trait locus for cold-responsive gene expression is linked to frost-resistance gene Fr-A2 in common wheat

Low temperature induces expression of Cor (cold-responsive)/Lea (late embryogenesis-abundant) gene family members through C-repeat binding factor (CBF) transcription factors in common wheat. However, the relationship between the genetic loci controlling cold-responsive gene expression and freezing tolerance is unclear. In expression quantitative trait locus (eQTL) analysis, accumulated transcripts of Cor/Lea and CBF genes were quantified in recombinant inbred lines derived from a cross between two common wheat cultivars with different levels of freezing tolerance. Four eQTLs controlling five cold-responsive genes were found, and the major eQTL with the greatest effect was located on the long arm of chromosome 5A. At least the 1D and 5A eQTLs played important roles in development of freezing tolerance in common wheat. The chromosomal location of the 5A eQTL, controlling four cold-responsive genes, coincided with a region homoeologous to a frost-tolerance locus (Fr-A (m) 2) reported as a CBF cluster region in einkorn wheat. The 5A eQTL plays a significant role through Cor/Lea gene expression in cold acclimation of wheat. In addition, our results suggest that one or more CBF copies at the Fr-2 region positively regulate other copies, which might amplify the positive effects of the CBF cluster on downstream Cor/Lea gene activation.

Induction of DREB2A pathway with repression of E2F, jasmonic acid biosynthetic and photosynthesis pathways in cold acclimation-specific freeze-resistant wheat crown

Winter wheat lines can achieve cold acclimation (development of tolerance to freezing temperatures) and vernalization (delay in transition from vegetative to reproductive phase) in response to low non-freezing temperatures. To describe cold-acclimation-specific processes and pathways, we utilized cold acclimation transcriptomic data from two lines varying in freeze survival but not vernalization. These lines, designated freeze-resistant (FR) and freeze-susceptible (FS), were the source of crown tissue RNA. Well-annotated differentially expressed genes (p ≤ 0.005 and fold change ≥ 2 in response to 4 weeks cold acclimation) were used for gene ontology and pathway analysis. "Abiotic stimuli" was identified as the most enriched and unique for FR. Unique to FS was "cytoplasmic components." Pathway analysis revealed the "triacylglycerol degradation" pathway as significantly downregulated and common to both FR and FS. The most enriched of FR pathways was "neighbors of DREB2A," with the highest positive median fold change. The "13-LOX and 13-HPL" and the "E2F" pathways were enriched in FR only with a negative median fold change. The "jasmonic acid biosynthesis" pathway and four "photosynthetic-associated" pathways were enriched in both FR and FS but with a more negative median fold change in FR than in FS. A pathway unique to FS was "binding partners of LHCA1," which was enriched only in FS with a significant negative median fold change. We propose that the DREB2A, E2F, jasmonic acid biosynthesis, and photosynthetic pathways are critical for discrimination between cold-acclimated lines varying in freeze survival.

Metabolite profiling reveals novel multi-level cold responses in the diploid model Fragaria vesca (woodland strawberry)

Winter freezing damage is a crucial factor in overwintering crops such as the octoploid strawberry (Fragaria × ananassa Duch.) when grown in a perennial cultivation system. Our study aimed at assessing metabolic processes and regulatory mechanisms in the close-related diploid model woodland strawberry (Fragaria vescaL.) during a 10-days cold acclimation experiment. Based on gas chromatography/time-of-flight-mass spectrometry (GC/TOF-MS) metabolite profiling of three F. vesca genotypes, clear distinctions could be made between leaves and non-photosynthesizing roots, underscoring the evolvement of organ-dependent cold acclimation strategies. Carbohydrate and amino acid metabolism, photosynthetic acclimation, and antioxidant and detoxification systems (ascorbate pathway) were strongly affected. Metabolic changes in F. vesca included the strong modulation of central metabolism, and induction of osmotically-active sugars (fructose, glucose), amino acids (aspartic acid), and amines (putrescine). In contrast, a distinct impact on the amino acid proline, known to be cold-induced in other plant systems, was conspicuously absent. Levels of galactinol and raffinose, key metabolites of the cold-inducible raffinose pathway, were drastically enhanced in both leaves and roots throughout the cold acclimation period of 10 days. Furthermore, initial freezing tests and multifaceted GC/TOF-MS data processing (Venn diagrams, independent component analysis, hierarchical clustering) showed that changes in metabolite pools of cold-acclimated F. vesca were clearly influenced by genotype.

Identification and expression analysis of CBF/DREB1 and COR15 genes in mutants of Brassica oleracea var. botrytis with enhanced proline production and frost resistance

Frost resistant mutants of Brassica oleracea var. botrytis were investigated for the presence of CBF/DREB1 and COR15a gene products and induced frost resistance. Total RNA of clones was isolated after 3 h, 6 h, 24 h and 14 d acclimation at 4 °C and proteins and free proline were isolated after 14 d acclimation. cDNA was produced using RT-PCR and the first CBF gene in B. oleracea detected and did quantify. Through SDS-PAGE and Western blotting, the COR15a protein was detected for the first time in B. oleracea. The results confirmed the first report of the presence of BoCBF/DREB1 in B. oleracea and this only appeared under cold acclimation. The sequence analysis of predicted amino acids revealed a very high homology (90%) with CBF sequences of other Brassica species (BnCBF5/DREB1, BrDREB1 and BjDREB1B) and homology reduced to 67% when compared to plants other than Brassicas. BoCBF/DREB1 transcript levels increased up to 24 h acclimation and then declined. Some mutants showed BoCBF/DREB1 expression at 3 h while others only after 6 h and 24 h acclimation. The genotypes showed positive significant correlation between BoCBF/DREB1 expression and frost resistance (R(2) = 0.9343). The proline level under acclimation increased about 8 fold and demonstrated positive and significant correlation with BoCBF/DREB1 expression. Proline also showed positive and significant correlation with frost resistance under cold acclimation but very not under non-acclimation. All clones were positive for COR15a protein after 14 d cold acclimation and expression correlated with frost resistance. Under non-acclimation COR15a was constitutively expressed in 3 mutants.

Role of DREBs in regulation of abiotic stress responses in plants

Abiotic stresses such as drought, high salinity, and cold are common adverse environmental conditions that significantly influence plant growth and productivity worldwide. The phytohormone abscisic acid (ABA) plays an important role in physiological and developmental responses as well as in co-ordinating various stress signal transduction pathways in plants. DREBs (dehydration responsive element binding) are important plant transcription factors (TFs) that regulate the expression of many stress-inducible genes mostly in an ABA-independent manner and play a critical role in improving the abiotic stress tolerance of plants by interacting with a DRE/CRT cis-element present in the promoter region of various abiotic stress-responsive genes. This review summarizes recent studies highlighting the role of the DRE-binding family of TFs in the adaptive responses to different abiotic stresses and their structural and functional characters with emphasis on the expression and regulation of DREBs. The practical and application value of DREBs in crop improvement, such as stress tolerance engineering as well as marker-assisted selection (MAS), has also been discussed.

Improvement of stress tolerance of wheat and barley by modulation of expression of DREB/CBF factors

Transcription factors have been shown to control the activity of multiple stress response genes in a coordinated manner and therefore represent attractive targets for application in molecular plant breeding. We investigated the possibility of modulating the transcriptional regulation of drought and cold responses in the agriculturally important species, wheat and barley, with a view to increase drought and frost tolerance. Transgenic wheat and barley plants were generated showing constitutive (double 35S) and drought-inducible (maize Rab17) expression of the TaDREB2 and TaDREB3 transcription factors isolated from wheat grain. Transgenic populations with constitutive over-expression showed slower growth, delayed flowering and lower grain yields relative to the nontransgenic controls. However, both the TaDREB2 and TaDREB3 transgenic plants showed improved survival under severe drought conditions relative to nontransgenic controls. There were two components to the drought tolerance: real (activation of drought-stress-inducible genes) and 'seeming' (consumption of less water as a result of smaller size and/or slower growth of transgenics compared to controls). The undesired changes in plant development associated with the 'seeming' component of tolerance could be alleviated by using a drought-inducible promoter. In addition to drought tolerance, both TaDREB2 and TaDREB3 transgenic plants with constitutive over-expression of the transgene showed a significant improvement in frost tolerance. The increased expression of TaDREB2 and TaDREB3 lead to elevated expression in the transgenics of 10 other CBF/DREB genes and a large number of stress responsive LEA/COR/DHN genes known to be responsible for the protection of cell from damage and desiccation under stress.

Natural variation in the freezing tolerance of Arabidopsis thaliana: effects of RNAi-induced CBF depletion and QTL localisation vary among accessions

Plants from temperate regions are able to withstand freezing temperatures and to increase their freezing tolerance during exposure to low, but non-freezing, temperatures through a process known as cold acclimation. Key regulatory proteins in this process are the cold-induced CBF1, 2 and 3 transcription factors which control many cold regulated genes. Although much work has focused on this signal transduction pathway, the details of its regulation and of its quantitative contribution to cold acclimation are still unclear. Here, we have used the large natural variation present in the 48 accessions of the Versailles core collection of Arabidopsis thaliana to further elucidate the function of the CBF transcription factors. CBF gene expression studies showed that the freezing sensitive accessions had mostly low expression levels 2h after transfer of plants to 5°C, while the most tolerant accessions showed a wide range of CBF expression levels. To investigate the quantitative contribution of CBF expression to plant freezing tolerance and low temperature growth performance, RNAi lines targeting all three CBF genes were produced in eight different accessions. We observed striking differences between different accessions in the effects that reduced CBF expression had on freezing tolerance, while effects on growth were generally too small to draw firm conclusions. Analysis of CBF expression indicated a tight co-regulation between CBF1 and CBF3, while the relationship between the expression levels of CBF2 and CBF1 or CBF3 strongly depended on the genetic background of the RNAi lines. In agreement with the observed differences between the different accessions, QTL analyses with two different RIL populations indicated that QTL localisation varies strongly between populations. Collectively, these results show that both the regulation of the CBF genes and their relative contribution to freezing tolerance strongly depend on the accession studied. In addition, natural variation is suggested to be an interesting source of novel regulatory pathways and genes that may be useful in the future for improving plant freezing tolerance.

Expression of dehydrins in wheat and barley under different temperatures

The review summarizes recent knowledge on the expression of cold-inducible dehydrins with a special attention to Wcs120 and Dhn5 genes in wheat and barley plants under different temperatures. When plants are exposed to cold, dehydrins start accumulating both in freezing-tolerant and freezing-susceptible plants; however, their accumulation correlates with plant acquired frost tolerance (FT). During a long-term cold acclimation (CA), dehydrin accumulation is significantly affected by Vrn1/Fr1 locus and the expression of the major vernalization gene VRN1, respectively. A different dynamics of dehydrin transcripts and proteins during CA is also observed. Transcripts reach their maximum within the first week of CA while proteins gradually accumulate until vernalization. Vernalization is associated with a significant decrease in dehydrin accumulation while the decrease of acquired FT is delayed. Studies carried out on plants grown at moderately cold temperatures (9-20 °C) have shown that both dehydrin transcripts and proteins can be detected even at these temperatures and that plants with different FT levels can be distinguished according to dehydrin accumulation without any exposure to severe cold. In conclusion, the potential use of these results in the breeding programmes aimed at the enhancement of wheat and barley FT is discussed.

Plant responses to cold: Transcriptome analysis of wheat

Temperature and light are important environmental stimuli that have a profound influence on the growth and development of plants. Wheat varieties can be divided on the basis of whether they require an extended period of cold to flower (vernalization). Varieties that have a requirement for vernalization also tend to be winter hardy and are able to withstand quite extreme subzero temperatures. This capacity, however, is not constitutive and plants require a period of exposure to low, non-freezing temperatures to acquire freezing tolerance: this process is referred to as cold acclimation. Cold acclimation and the acquisition of freezing tolerance require the orchestration of many different, seemingly disparate physiological and biochemical changes. These changes are, at least in part, mediated through the differential expression of many genes. Some of these genes code for effector molecules that participate directly to alleviate stress. Others code for proteins involved in signal transduction or transcription factors that control the expression of further banks of genes. In this review, we provide an overview of some of the main features of cold acclimation with particular focus on transcriptome reprogramming. In doing so, we highlight some of the important differences between cold-hardy and cold-sensitive varieties. An understanding of these processes is of great potential importance because cold and freezing stress are major limiting factors for growing crop plants and periodically account for significant losses in plant productivity.

Identification of quantitative trait loci for ABA responsiveness at the seedling stage associated with ABA-regulated gene expression in common wheat

Responsiveness to abscisic acid (ABA) during vegetative growth plays an important role in regulating adaptive responses to various environmental conditions, including activation of a number of ABA-responsive genes. However, the relationship between gene expression and responsiveness to ABA at the seedling stage has not been well studied in wheat. In the present study, quantitative trait locus (QTL) analysis for ABA responsiveness at the seedling stage was performed using recombinant inbred lines derived from a cross between common wheat cultivars showing different ABA responsiveness. Five QTLs were found to be significant, located on chromosomes 1B, 2A, 3A, 6D and 7B. The QTL with the greatest effect was located on chromosome 6D and explained 11.12% of the variance in ABA responsiveness. The other QTLs each accounted for approximately 5-8% of the phenotypic variation. Expression analyses of three ABA-responsive Cor/Lea genes, Wdhn13, Wrab15 and Wrab17, showed that allelic differences in QTLs on chromosomes 2A, 6D and 7B influenced expression of these genes in seedlings treated with ABA. The 3A QTL appeared to be involved in the regulatory system of Wdhn13 and Wrab15, but not Wrab17. The effects of the 2A and 6D QTLs on gene expression were relatively large. The combination of alleles at the QTLs resulted in an additive or synergistic effect on Cor/Lea expression. These results indicate that the QTLs influencing ABA responsiveness are associated with ABA-regulated gene expression and suggest that the QTL on chromosome 6D with the largest effect acts as a key regulator of ABA responses including seedling growth arrest and gene expression during the vegetative stage.

Discovery and expression profile analysis of AP2/ERF family genes from Triticum aestivum

Throughout its development, common wheat, Triticum aestivum responds to different kinds of adverse abiotic and biotic stress by expressing specific genes that allow it to adapt to these stresses. In this process, genes in the AP2/ERF family encode transcriptional regulators involved in diverse developmental and physiological processes play critical roles. Here, we established an extensive picture of the AP2/ERF family genes in wheat. From 960, 174 ESTs of T. aestivum, 117 putative AP2/ERF family genes were identified by in silico analysis based on the presence of the conserved AP2/ERF domain amino acid sequence of Arabidopsis thaliana. Based on the model species A. thaliana, the AP2/ERF TFs from T. aestivum were classified into five subfamilies with the following number of members: DREB (57), ERF (47), AP2 (9), RAV (3) and Soloist (1). Using the available EST information as a source of expression data, the putative AP2/ERF family genes from T. aestivum were detected in nine kinds of tissues. Transcripts of the genes were shown to be most abundant in leaves, followed by roots and seeds, and the least abundant in stem. Most of the T. aestivum AP2/ERF family genes showed some tissue specificity.

Effects of molybdenum on expression of cold-responsive genes in abscisic acid (ABA)-dependent and ABA-independent pathways in winter wheat under low-temperature stress

BACKGROUND AND AIMS

Molybdenum (Mo) is an essential trace element for higher plants. It has been shown that application of Mo enhances the cold resistance of winter wheat. In order to improve our understanding of the molecular mechanisms of cold resistance arising from application of Mo in winter wheat, investigations were made regarding the transcription of cold-responsive (COR) genes in abscisic acid (ABA)-dependent and ABA-independent pathways in winter wheat regulated by Mo application under low-temperature stress.

METHODS

Two cultivars of winter wheat (Triticum aestivum), Mo-efficient cultivar '97003' and Mo-inefficient cultivar '97014', were grown in control (-Mo) and Mo fertilizer (+Mo) treatments for 40 d at 15/12 degrees C (day/night), and the temperature was then reduced to 5/2 degrees C (day/night) to create low-temperature stress. Aldehyde oxidase (AO) activities, ABA contents, the transcripts of basic leucine zipper (bZIP)-type transcription factor (TF) genes, ABA-dependent COR genes, CBF/DREB transcription factor genes and ABA-independent COR genes were investigated at 0, 3, 6 and 48 h post cold stress.

KEY RESULTS

Mo application significantly increased AO activity, ABA levels, and expression of bZIP-type TF genes (Wlip19 and Wabi5) and ABA-dependent COR genes (Wrab15, Wrab17, Wrab18 and Wrab19). Mo application increased expression levels of CBF/DREB transcription factor genes (TaCBF and Wcbf2-1) and ABA-independent COR genes (Wcs120, Wcs19, Wcor14 and Wcor15) after 3 and 6 h exposure to low temperature.

CONCLUSIONS

Mo might regulate the expression of ABA-dependent COR genes through the pathway: Mo --> AO --> ABA --> bZIP --> ABA-dependent COR genes in winter wheat. The response of the ABA-dependent pathway to Mo was prior to that of the ABA-independent pathway. Similarities and differences between the Mo-efficient and Mo-inefficient wheat cultivars in response to Mo under cold stress are discussed.

Variation in dehydration tolerance, ABA sensitivity and related gene expression patterns in D-genome progenitor and synthetic hexaploid wheat lines

The wild wheat Aegilops tauschii Coss. has extensive natural variation available for breeding of common wheat. Drought stress tolerance is closely related to abscisic acid (ABA) sensitivity. In this study, 17 synthetic hexaploid wheat lines, produced by crossing the tetraploid wheat cultivar Langdon with 17 accessions of Ae. tauschii, were used for comparative analysis of natural variation in drought tolerance and ABA sensitivity. Ae. tauschii showed wide natural variation, with weak association between the traits. Drought-sensitive accessions of Ae. tauschii exhibited significantly less ABA sensitivity. D-genome variations observed at the diploid genome level were not necessarily reflected in synthetic wheats. However, synthetic wheats derived from the parental Ae. tauschii accessions with high drought tolerance were significantly more tolerant to drought stress than those from drought-sensitive accessions. Moreover, synthetic wheats with high drought tolerance showed significantly higher ABA sensitivity than drought-sensitive synthetic lines. In the hexaploid genetic background, therefore, weak association of ABA sensitivity with drought tolerance was observed. To study differences in gene expression patterns between stress-tolerant and -sensitive lines, levels of two Cor/Lea and three transcription factor gene transcripts were compared. The more tolerant accession of Ae. tauschii tended to accumulate more abundant transcripts of the examined genes than the sensitive accession under stress conditions. The expression patterns in the synthetic wheats seemed to be additive for parental lines exposed to drought and ABA treatments. However, the transcript levels of transcription factor genes in the synthetic wheats did not necessarily correspond to the postulated levels based on expression in parental lines. Allopolyploidization altered the expression levels of the stress-responsive genes in synthetic wheats.

Post-acclimation transcriptome adjustment is a major factor in freezing tolerance of winter wheat

Cold-acclimated winter wheat plants were slowly frozen to -10 degrees C, and then the temperature was either maintained at -10 degrees C or was lowered further to -12 degrees C. Expression levels of a total of 423 genes were significantly altered in these treatments; genes upregulated outnumbered those downregulated by about a 9:1 ratio. Sixty-eight genes were upregulated at least fivefold in all freezing treatments; 17 of these 68 encoded transcription factors including C-repeat binding factor (Cbf), WRKY, or other Zn-finger proteins, indicating strong upregulation of genes involved in transcription regulation. Sixteen of the 68 highly upregulated genes encoded kinases, phosphatases, calcium trafficking-related proteins, or glycosyltransferases, indicating upregulation of genes involved in signal transduction. Six genes encoding chlorophyll a/b binding-like proteins were upregulated uniquely in response to the -12 degrees C treatment, suggesting a protective role of pigment-binding proteins in freezing stress response. Most genes responded similarly in the very freezing tolerant cultivar Norstar and in the moderately freezing tolerant Tiber, but some genes responded in opposite fashion in the two cultivars. These results showed that wheat crowns actively adapt as the temperature declines to potentially damaging levels, and genetic variation for this ability exists among cultivars.

Characterization of the TaAIDFa gene encoding a CRT/DRE-binding factor responsive to drought, high-salt, and cold stress in wheat

Dehydration responsive element-binding factors (DBFs) belong to the AP2/ERF superfamily and play vital regulatory roles in abiotic stress responses in plants. In this study, we isolated three novel homologs of the DBF gene family in wheat (Triticum aestivum L.) by screening a drought-induced cDNA library and designated them as TaAIDFs (T. aestivum abiotic stress-induced DBFs). Compared to TaAIDFb and TaAIDFc, TaAIDFa lacks a short Ser/Thr-rich region, a putative phosphorylation site, following the AP2/ERF domain. The TaAIDFa gene, located on chromosome 3BS, is interrupted by a single intron at the 17th Arg (R) in the N-terminal domain. The N-terminal region of the TaAIDFa protein modulates nuclear localization. The TaAIDFa protein is capable of binding to CRT/DRE elements in vitro and in vivo, and of trans-activating reporter gene expression in yeast cells. The TaAIDFa promoter, with various stress-related cis-acting elements, drives expression of the GUS reporter gene in wheat calli under stress conditions. This was further confirmed by responses of TaAIDFa transcripts to drought, salinity, low-temperature, and exogenous ABA. Furthermore, overexpression of TaAIDFa activated CRT/DRE-containing genes under normal growth conditions, and improved drought and osmotic stress tolerances in transgenic Arabidopsis plants. These results suggested that TaAIDFa encodes a CRT/DRE element-binding factor that might be involved in multiple abiotic stress signal transduction pathways.

Positive role of a wheat HvABI5 ortholog in abiotic stress response of seedlings

ABA-responsive element binding protein (AREB) and ABA-responsive element binding factor (ABF), members of the basic region/leucine zipper (bZIP)-type protein family, act as major transcription factors in ABA-responsive gene expression under abiotic stress conditions in Arabidopsis. Barley HvABI5 and rice transcription factor responsible for ABA regulation 1 (TRAB1) are homologues of AREB/ABF and are expressed in drought- and ABA-treated seedlings. However, no direct evidence has shown an association of an AREB/ABF-type transcription factor with stress tolerance in cereals. To understand the molecular basis of abiotic stress tolerance through a cereal AREB/ABF-type transcription factor, a wheat HvABI5 ortholog, Wabi5, was isolated and characterized. Wabi5 expression was activated by low temperature, drought and exogenous ABA treatment, and its expression pattern differed between two wheat accessions with distinct levels of stress tolerance and ABA sensitivity. Wabi5-expressing transgenic tobacco plants showed a significant increase in tolerance to abiotic stresses such as freezing, osmotic and salt stresses and a hypersensitivity to exogenous ABA in the seedling stage compared with wild-type plants. Expression of a GUS reporter gene under the control of promoters of three wheat cold-responsive/late embryogenesis abundant (Cor/Lea) genes, Wdhn13, Wrab18 and Wrab19, was enhanced by ectopic Wabi5 expression in wheat callus and tobacco plants. These results clearly indicated that WABI5 functions as a transcriptional regulator of the Cor/Lea genes in multiple abiotic stress responses in common wheat.

Identification of candidate CBF genes for the frost tolerance locus Fr-Am2 in Triticum monococcum

A cluster of eleven CBF genes was recently mapped to the Frost resistance-2 (Fr-Am2) locus on chromosome 5 of diploid wheat (Triticum monococcum) using a cross between frost tolerant accession G3116 and frost sensitive DV92. The Fr-Am2 locus was mapped at the peak of two overlapping quantitative trait loci (QTL), one for frost survival and the other for differential expression of the cold regulated gene COR14b. Seven lines with recombination events within the CBF cluster were used to identify CBF candidate genes for these QTL. The lines carrying the critical recombination events were tested for whole plant frost survival and for differential transcript levels of cold induced COR14b and DHN5 genes. The strongest effect for these traits was associated to the linked TmCBF12, TmCBF14 and TmCBF15 genes, with the G3116 allele conferring improved frost tolerance and higher levels of COR14b and DHN5 transcript at mild cold temperatures (12-15 degrees C) than the DV92 allele. Comparison of CBF protein sequences revealed that the DV92 TmCBF12 protein contains a deletion of five amino acids in the AP2 DNA binding domain. Electrophoretic Mobility Shift Assays (EMSA) confirmed that the protein encoded by this allele cannot bind to the CRT/DRE (C-repeat/ dehydration-responsive element) motif present in the promoters of several cold induced genes. A smaller effect on frost tolerance was mapped to the distal group of CBF genes including TmCBF16. Transcript levels of TmCBF16, as well as those of TmCBF12 and TmCBF15 were up-regulated at mild cold temperatures in G3116 but not in DV92. Higher threshold induction temperatures can result in earlier initiation of the cold acclimation process and better resistance to subsequent freezing temperatures. The non-functional TmCBF12 allele in DV92 can also contribute to its lower frost tolerance.

Mitochondrial alternative pathway is associated with development of freezing tolerance in common wheat

Cold acclimation is an adaptive process for acquiring cold/freezing tolerance in wheat. To clarify the cultivar difference of freezing tolerance, we compared mitochondrial respiration activity and the expression profile of alternative oxidase (AOX) genes under low-temperature conditions using two common wheat cultivars differing in freezing tolerance. During cold acclimation, the respiration capacity of the alternative pathway significantly increased in a freezing-tolerant cultivar compared with a freezing-sensitive cultivar. More abundant accumulation of the AOX and uncoupling protein gene transcripts was also observed under the low-temperature conditions in the tolerant cultivar than in the sensitive cultivar. These results suggest that the mitochondrial alternative pathway might be partly associated with the cold acclimation and freezing tolerance in wheat.

Development of abiotic stress tolerance via bZIP-type transcription factor LIP19 in common wheat

Cereal lip19 genes encoding bZIP-type transcription factors are assumed to play a regulatory role in gene expression during the cold acclimation process. However, no direct evidence shows an association of LIP19-type bZIPs with stress tolerance or activation of stress-responsive Cor/Lea genes. To understand the molecular basis of development of abiotic stress tolerance through the LIP19 transcription factor, a wheat lip19 homologue, Wlip19, was isolated and characterized. Wlip19 expression was activated by low temperature in seedlings and was higher in a freezing-tolerant cultivar than in a freezing-sensitive one. Wlip19 also responded to drought and exogenous ABA treatment. Wlip19-expressing transgenic tobacco showed a significant increase in abiotic stress tolerance, especially freezing tolerance. Expression of a GUS reporter gene under the control of promoter sequences of four wheat Cor/Lea genes, Wdhn13, Wrab17, Wrab18, and Wrab19, was enhanced by Wlip19 expression in wheat callus and tobacco plants. These results indicate that WLIP19 acts as a transcriptional regulator of Cor/Lea genes in the development of abiotic stress tolerance. Moreover, direct protein-protein interaction between WLIP19 and a wheat OBF1 homologue TaOBF1, another bZIP-type transcription factor, was observed, suggesting that this interaction is conserved in cereals.

Increased freezing tolerance through up-regulation of downstream genes via the wheat CBF gene in transgenic tobacco

The wheat (Triticum aestivum L.) CBF gene family is assumed to play important roles in development of low-temperature and freezing tolerance through activation of the downstream Cor/Lea genes. However, no direct evidence shows association of the wheat CBF genes with stress tolerance or any interaction between wheat CBF transcription factors and Cor/Lea gene activation. Here, we introduced Wcbf2, one of the wheat CBF genes, into the tobacco (Nicotiana tabacum L.) genome. Expression of Wcbf2 significantly increased the level of freezing tolerance in the transgenic tobacco plants without phenotypic retardation, and altered the expression patterns of tobacco genes, including cold-responsive genes. A transgenic tobacco plant expressing Wcbf2 was crossed to other transgenic plants expressing a GUS reporter gene under control of the wheat Cor/Lea gene promoter. Analysis of the F(1) plants showed that the WCBF2 protein positively regulated at least the expression of Wdhn13 and Wrab17. These results strongly indicate that WCBF2 functions as a transcription factor in the development of freezing tolerance in common wheat.

Increased freezing tolerance in an ABA-hypersensitive mutant of common wheat

To study role of abscisic acid (ABA) in cold acclimation and cold/freezing tolerance in wheat, we analyzed an ABA-hypersensitive mutant of Triticum aestivum, named ABA27. ABA-hypersensitivity in ABA27 was confirmed by bioassays involving germination and seedling growth and expression analysis of ABA-responsive genes in comparison with the parental cultivar 'Chihoku-komugi' (Chihoku). ABA27 showed significantly increased freezing tolerance in seedlings without cold acclimation. ABA-treated seedlings of ABA27 accumulated more transcripts of ABA-responsive genes Cor/Lea and their putative transcription factor (TF) genes than Chihoku under both normal and low-temperature (LT) conditions. Non-ABA-regulated Cor/Lea transcripts showed higher accumulation in ABA27 also under normal temperature. These results suggest that the elevated ABA sensitivity in ABA27 contributes to the improved freezing tolerance through increased expression of the ABA-regulated LT signal pathway. Based on these and previous results obtained in an ABA-less-sensitive mutant, it is suggested that both positive and negative regulation of ABA response is involved in the basic mechanism of freezing tolerance in wheat.

Quantitative expression analysis of selected COR genes reveals their differential expression in leaf and crown tissues of wheat (Triticum aestivum L.) during an extended low temperature acclimation regimen

A number of COR genes (COld-Regulated genes) have been implicated in the acquisition of low temperature (LT) tolerance in wheat (Triticum aestivum L.). This study compared the relative expression patterns of selected COR genes in leaf and crown tissues of wheat near-isogenic lines to increase understanding of the molecular mechanisms underlying LT acclimation. Reciprocal near-isogenic lines were generated such that the dominant Vrn-A1 and recessive vrn-A1 loci were interchanged in a spring cv. Manitou and a winter cv. Norstar. Phenological development, acquisition of LT tolerance, and WCS120 polypeptide accumulation in these genotypes proceeded at rates similar to those previously reported for 6 degrees C acclimation from 0 to 98 d. However, a differential accumulation of WCS120 polypeptide and expression of the COR genes Wcs120, Wcor410, and Wcor14 was observed in the leaf and crown tissues. COR gene transcript levels peaked at 2 d of the acclimation period in both tissues and differences among genotypes were most evident at this time. COR gene expression was highest for the LT-tolerant and lowest for the tender genotypes. However, expression rates were divergent enough in genotypes with intermediate hardiness that comparisons among tissues and/or times during acclimation often resulted in variable interpretations of the relative expression of the COR genes in the determination of LT tolerance. These observations emphasize the need to pay close attention to experimental conditions, sampling times, and genotype and tissue selection in experiments designed to identify the critical genetic components that interact to determine LT acclimation.

The CBF gene family in hexaploid wheat and its relationship to the phylogenetic complexity of cereal CBFs

Most temperate plants tolerate both chilling and freezing temperatures whereas many species from tropical regions suffer chilling injury when exposed to temperatures slightly above freezing. Cold acclimation induces the expression of cold-regulated genes needed to protect plants against freezing stress. This induction is mediated, in part, by the CBF transcription factor family. To understand the evolution and function of this family in cereals, we identified and characterized 15 different CBF genes from hexaploid wheat. Our analyses reveal that wheat species, T. aestivum and T. monococcum, may contain up to 25 different CBF genes, and that Poaceae CBFs can be classified into 10 groups that share a common phylogenetic origin and similar structural characteristics. Six of these groups (IIIc, IIId, IVa, IVb, IVc and IVd) are found only in the Pooideae suggesting they represent the CBF response machinery that evolved recently during colonization of temperate habitats. Expression studies reveal that five of the Pooideae-specific groups display higher constitutive and low temperature inducible expression in the winter cultivar, and a diurnal regulation pattern during growth at warm temperature. The higher constitutive and inducible expression within these CBF groups is an inherited trait that may play a predominant role in the superior low temperature tolerance capacity of winter cultivars and possibly be a basis of genetic variability in freezing tolerance within the Pooideae subfamily.