Eskimo1 mutants of Arabidopsis are constitutively freezing-tolerant

Plants in a cold climate

Plants are able to survive prolonged exposure to sub-zero temperatures; this ability is enhanced by pre-exposure to low, but above-zero temperatures. This process, known as cold acclimation, is briefly reviewed from the perception of cold, through transduction of the low-temperature signal to functional analysis of cold-induced gene products. The stresses that freezing of apoplastic water imposes on plant cells is considered and what is understood about the mechanisms that plants use to combat those stresses discussed, with particular emphasis on the role of the extracellular matrix.

Molecular genetic analysis of cold-regulated gene transcription

Chilling and freezing temperatures adversely affect the productivity and quality of crops. Hence improving the cold hardiness of crop plants is an important goal in agriculture, which demands a clear understanding of cold stress signal perception and transduction. Pharmacological and biochemical evidence shows that membrane rigidification followed by cytoskeleton rearrangement, Ca(2+) influx and Ca(2+)-dependent phosphorylation are involved in cold stress signal transduction. Cold-responsive genes are regulated through C-repeat/dehydration-responsive elements (CRT/DRE) and abscisic acid (ABA)-responsive element cis elements by transacting factors C-repeat binding factors/dehydration-responsive element binding proteins (CBFs/DREBs) and basic leucine zippers (bZIPs) (SGBF1), respectively. We have carried out a forward genetic analysis using chemically mutagenized Arabidopsis plants expressing cold-responsive RD29A promoter-driven luciferase to dissect cold signal transduction. We have isolated the fiery1 (fry1) mutant and cloned the FRY1 gene, which encodes an inositol polyphosphate 1-phosphatase. The fry1 plants showed enhanced induction of stress genes in response to cold, ABA, salt and dehydration due to higher accumulation of the second messenger, inositol (1,4,5)- triphosphate (IP(3)). Thus our study provides genetic evidence suggesting that cold signal is transduced through changes in IP(3) levels. We have also identified the hos1 mutation, which showed super induction of cold-responsive genes and their transcriptional activators. Molecular cloning and characterization revealed that HOS1 encodes a ring finger protein, which has been implicated as an E3 ubiquitin conjugating enzyme. HOS1 is present in the cytoplasm at normal growth temperatures but accumulates in the nucleus upon cold stress. HOS1 appears to regulate temperature sensing by the cell as cold-responsive gene expression occurs in the hos1 mutant at relatively warm temperatures. Thus HOS1 is a negative regulator, which may be functionally linked to cellular thermosensors to modulate cold-responsive gene transcription.

LOS2, a genetic locus required for cold-responsive gene transcription encodes a bi-functional enolase

The Arabidopsis mutation, los2, impairs cold-responsive gene transcription, acquired freezing tolerance and plant resistance to chilling under certain conditions. LOS2 was isolated through positional cloning and shown to encode an enolase in the glycolytic pathway. In animal cells, enolase has also been known to function as a transcription factor that represses the expression of c-myc by binding to the c-myc gene promoter. LOS2 fused to green fluorescent protein is targeted to the nucleus as well as to the cytoplasm. LOS2/enolase protein can bind to the cis-element of the human c-myc gene promoter and to the gene promoter of STZ/ZAT10, a zinc finger transcriptional repressor from Arabidopsis. STZ/ZAT10 expression is induced rapidly and transiently by cold in the wild type, and this induction is stronger and more sustained in the los2 mutant. Furthermore, the expression of a RD29A-LUC reporter gene is repressed significantly by STZ/ZAT10 in transient expression assays in Arabidopsis leaves. Our results demonstrate that cold-responsive gene transcription in plants is controlled by a bi-functional enolase.

A plant for all seasons: alterations in photosynthetic carbon metabolism during cold acclimation in Arabidopsis

Low temperatures lead to the inhibition of sucrose synthesis and photosynthesis. The biochemical and physiological adaptations of plants to low temperatures include the post-translational activation and increased expression of enzymes of the sucrose synthesis pathway, the changed expression of Calvin cycle enzymes, and changes in the leaf protein content. Recent progress has been made in understanding both the signals that trigger these processes and how the regulation of photosynthetic carbon metabolism interacts with other processes during cold acclimation.

Chromosome regions and stress-related sequences involved in resistance to abiotic stress in Triticeae

Drought, low temperature and salinity are the most important abiotic stress factors limiting crop productivity. A genomic map of major loci and QTLs affecting stress tolerance in Triticeae identified the crucial role of the group 5 chromosomes, where the highest concentration of QTLs and major loci controlling plant's adaptation to the environment (heading date, frost and salt tolerance) has been found. In addition, a conserved region with a major role in drought tolerance has been localized to the group 7 chromosomes. Extensive molecular biological studies have led to the cloning of many stress-related genes and responsive elements. The expression of some stress-related genes was shown to be linked to stress-tolerant QTLs, suggesting that these genes may represent the molecular basis of stress tolerance. The development of suitable genetic tools will allow the role of stress-related sequences and their relationship with stress-tolerant loci to be established in the near future.

Molecular and genetic aspects of plant responses to osmotic stress

Drought, high salinity and freezing impose osmotic stress on plants. Plants respond to the stress in part by modulating gene expression, which eventually leads to the restoration of cellular homeostasis, detoxification of toxins and recovery of growth. The signal transduction pathways mediating these adaptations can be dissected by combining forward and reverse genetic approaches with molecular, biochemical and physiological studies. Arabidopsis is a useful genetic model system for this purpose and its relatives including the halophyte Thellungiella halophila, can serve as valuable complementary genetic model systems.

Altered levels of proline dehydrogenase cause hypersensitivity to proline and its analogs in Arabidopsis

Pro dehydrogenase (PDH) catalyzes the first and rate-limiting step in the Pro catabolic pathway. In Arabidopsis, this enzyme is encoded by At-PDH. To investigate the role of Pro catabolism in plants, we generated transgenic Arabidopsis plants with altered levels of PDH by sense (PDH-S plants) and antisense (PDH-AS plants) strategies. Free Pro levels were reduced by up to 50% in PDH-S plants under stress and recovery conditions and enhanced by a maximum of 25% in PDH-AS plants, despite large modifications of the At-PDH transcript and At-PDH protein levels. A similar trend in free Pro levels was observed in the PDH-S and PDH-AS seeds without visible effects on germination or growth. Under stress conditions, PDH transgenic plants showed no signs of change in osmotolerance. However, addition of exogenous Pro increased survival rates of salt-stressed PDH-S plants by 30%. Isotope-labeling studies showed that the conversion of [14C]Pro to Glu was reduced in PDH-AS plants and increased in PDH-S plants, especially under stress conditions. Furthermore, PDH-AS plants were hypersensitive to exogenous Pro, whereas PDH-S plants were sensitive to Pro analogs. These findings demonstrate that altered At-PDH levels lead to weakly modified free Pro accumulation with a limited impact on plant development and growth, suggesting a tight control of Pro homeostasis and/or gene redundancy.

Cold signalling associated with vernalization in Arabidopsis thaliana does not involve CBF1 or abscisic acid

In genotypes of Arabidopsis that exhibit a winter-annual flowering habit, floral induction in response to extended cold exposure (vernalization) is mediated by repression of the flowering-inhibitor gene FLC. We are interested in identifying components of the cold signal transduction pathway leading to FLC repression. We examined the potential involvement of two factors that are known to play roles in plant cold responses: (1) CBF1, a cold-responsive transcription factor that is involved in activating the cold acclimation response, and (2) the phytohormone abscisic acid (ABA), which has traditionally been associated with plant cold responses. We introduced a transgene driving constitutive expression of CBF1 into a winter-annual genotype of Arabidopsis. In transgenic lines expressing CBF1 mRNA to high levels, FLC mRNA expression was not repressed, and flowering was not accelerated relative to control plants. We also introduced mutations that compromise ABA biosynthesis or sensitivity into a winter-annual genotype and found that the vernalization response was not affected. Finally, we found that presumed increases in ABA levels, as a result of direct application of the hormone or severe water stress, were insufficient to substitute for cold to induce flowering. Taken together, these findings indicate that vernalization involves a pathway that is distinct from cold-response mechanisms involving CBF1, cold-regulated genes under CBF1 control, and ABA.

Altered levels of proline dehydrogenase cause hypersensitivity to proline and its analogs in Arabidopsis

Pro dehydrogenase (PDH) catalyzes the first and rate-limiting step in the Pro catabolic pathway. In Arabidopsis, this enzyme is encoded by At-PDH. To investigate the role of Pro catabolism in plants, we generated transgenic Arabidopsis plants with altered levels of PDH by sense (PDH-S plants) and antisense (PDH-AS plants) strategies. Free Pro levels were reduced by up to 50% in PDH-S plants under stress and recovery conditions and enhanced by a maximum of 25% in PDH-AS plants, despite large modifications of the At-PDH transcript and At-PDH protein levels. A similar trend in free Pro levels was observed in the PDH-S and PDH-AS seeds without visible effects on germination or growth. Under stress conditions, PDH transgenic plants showed no signs of change in osmotolerance. However, addition of exogenous Pro increased survival rates of salt-stressed PDH-S plants by 30%. Isotope-labeling studies showed that the conversion of [14C]Pro to Glu was reduced in PDH-AS plants and increased in PDH-S plants, especially under stress conditions. Furthermore, PDH-AS plants were hypersensitive to exogenous Pro, whereas PDH-S plants were sensitive to Pro analogs. These findings demonstrate that altered At-PDH levels lead to weakly modified free Pro accumulation with a limited impact on plant development and growth, suggesting a tight control of Pro homeostasis and/or gene redundancy.

Cell signaling under salt, water and cold stresses

Forward genetics and biochemical approaches to studying plant responses to salt, water and cold stresses began to bear fruit recently. Analysis of salt overly sensitive (sos) Arabidopsis mutants revealed a novel calcium-regulated protein kinase pathway for response to the ionic aspect of salt stress. In-gel kinase assays identified several SOS-independent protein kinases that are either activated specifically by osmotic stress or by multiple abiotic and biotic stresses. Molecular analysis revealed a transcriptional cascade in cold-regulated gene expression.

The Arabidopsis LOS5/ABA3 locus encodes a molybdenum cofactor sulfurase and modulates cold stress- and osmotic stress-responsive gene expression

To understand low temperature and osmotic stress signaling in plants, we isolated and characterized two allelic Arabidopsis mutants, los5-1 and los5-2, which are impaired in gene induction by cold and osmotic stresses. Expression of RD29A-LUC (the firefly luciferase reporter gene under the control of the stress-responsive RD29A promoter) in response to cold and salt/drought is reduced in the los5 mutants, but the response to abscisic acid (ABA) remains unaltered. RNA gel blot analysis indicates that the los5 mutation reduces the induction of several stress-responsive genes by cold and severely diminishes or even completely blocks the induction of RD29A, COR15, COR47, RD22, and P5CS by osmotic stresses. los5 mutant plants are compromised in their tolerance to freezing, salt, or drought stress. los5 plants are ABA deficient, as indicated by increased transpirational water loss and reduced accumulation of ABA under drought stress in the mutant. A comparison with another ABA-deficient mutant, aba1, reveals that the impaired low-temperature gene regulation is specific to the los5 mutation. Genetic tests suggest that los5 is allelic to aba3. Map-based cloning reveals that LOS5/ABA3 encodes a molybdenum cofactor (MoCo) sulfurase. MoCo sulfurase catalyzes the generation of the sulfurylated form of MoCo, a cofactor required by aldehyde oxidase that functions in the last step of ABA biosynthesis in plants. The LOS5/ABA3 gene is expressed ubiquitously in different plant parts, and the expression level increases in response to drought, salt, or ABA treatment. Our results show that LOS5/ABA3 is a key regulator of ABA biosynthesis, stress-responsive gene expression, and stress tolerance.

The Arabidopsis LOS5/ABA3 locus encodes a molybdenum cofactor sulfurase and modulates cold stress- and osmotic stress-responsive gene expression

To understand low temperature and osmotic stress signaling in plants, we isolated and characterized two allelic Arabidopsis mutants, los5-1 and los5-2, which are impaired in gene induction by cold and osmotic stresses. Expression of RD29A-LUC (the firefly luciferase reporter gene under the control of the stress-responsive RD29A promoter) in response to cold and salt/drought is reduced in the los5 mutants, but the response to abscisic acid (ABA) remains unaltered. RNA gel blot analysis indicates that the los5 mutation reduces the induction of several stress-responsive genes by cold and severely diminishes or even completely blocks the induction of RD29A, COR15, COR47, RD22, and P5CS by osmotic stresses. los5 mutant plants are compromised in their tolerance to freezing, salt, or drought stress. los5 plants are ABA deficient, as indicated by increased transpirational water loss and reduced accumulation of ABA under drought stress in the mutant. A comparison with another ABA-deficient mutant, aba1, reveals that the impaired low-temperature gene regulation is specific to the los5 mutation. Genetic tests suggest that los5 is allelic to aba3. Map-based cloning reveals that LOS5/ABA3 encodes a molybdenum cofactor (MoCo) sulfurase. MoCo sulfurase catalyzes the generation of the sulfurylated form of MoCo, a cofactor required by aldehyde oxidase that functions in the last step of ABA biosynthesis in plants. The LOS5/ABA3 gene is expressed ubiquitously in different plant parts, and the expression level increases in response to drought, salt, or ABA treatment. Our results show that LOS5/ABA3 is a key regulator of ABA biosynthesis, stress-responsive gene expression, and stress tolerance.

The Arabidopsis LOS5/ABA3 locus encodes a molybdenum cofactor sulfurase and modulates cold stress- and osmotic stress-responsive gene expression

To understand low temperature and osmotic stress signaling in plants, we isolated and characterized two allelic Arabidopsis mutants, los5-1 and los5-2, which are impaired in gene induction by cold and osmotic stresses. Expression of RD29A-LUC (the firefly luciferase reporter gene under the control of the stress-responsive RD29A promoter) in response to cold and salt/drought is reduced in the los5 mutants, but the response to abscisic acid (ABA) remains unaltered. RNA gel blot analysis indicates that the los5 mutation reduces the induction of several stress-responsive genes by cold and severely diminishes or even completely blocks the induction of RD29A, COR15, COR47, RD22, and P5CS by osmotic stresses. los5 mutant plants are compromised in their tolerance to freezing, salt, or drought stress. los5 plants are ABA deficient, as indicated by increased transpirational water loss and reduced accumulation of ABA under drought stress in the mutant. A comparison with another ABA-deficient mutant, aba1, reveals that the impaired low-temperature gene regulation is specific to the los5 mutation. Genetic tests suggest that los5 is allelic to aba3. Map-based cloning reveals that LOS5/ABA3 encodes a molybdenum cofactor (MoCo) sulfurase. MoCo sulfurase catalyzes the generation of the sulfurylated form of MoCo, a cofactor required by aldehyde oxidase that functions in the last step of ABA biosynthesis in plants. The LOS5/ABA3 gene is expressed ubiquitously in different plant parts, and the expression level increases in response to drought, salt, or ABA treatment. Our results show that LOS5/ABA3 is a key regulator of ABA biosynthesis, stress-responsive gene expression, and stress tolerance.

Temperature sensing and cold acclimation

The fundamental question in cold acclimation is how do plants perceive the low but nonfreezing temperatures that activate cold acclimation responses. New findings in the past year suggest that changes in membrane fluidity, cytoskeleton rearrangement, and calcium influxes are among the earliest events taking place in plants upon exposure to low nonfreezing temperatures. In the cyanobacterium Synechocystis PCC6803, temperature change is detected by at least two separate sensors. One of these measures membrane fluidity using a classical two-component system involving histidine kinases and a response regulator in a His-to-Asp phosphorelay. Although these Synechocystis results may not be directly relevant to cold acclimation, they can guide our thinking as we search for biological thermometers in higher plants.

Abiotic stress signal transduction in plants: Molecular and genetic perspectives

Low temperature, drought and salinity are major adverse environmental factors that limit plant productivity. Understanding the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses is essential for engineering stress-tolerant crop plants. Molecular and biochemical studies suggest that abiotic stress signaling in plants involves receptor-coupled phosphorelay, phosphoinositol-induced Ca2+ changes, mitogen-activated protein kinase cascades and transcriptional activation of stress-responsive genes. In addition, protein posttranslational modifications and adapter or scaffold-mediated protein-protein interactions are also important in abiotic stress signal transduction. Most of these signaling modules, however, have not been genetically established to function in plant abiotic stress signal transduction. To overcome the scarcity of abiotic stress-specific phenotypes for conventional genetic screens, molecular genetic analysis using stress-responsive promoter-driven reporter is suggested as an alternative approach to genetically dissect abiotic stress signaling networks in plants.

The Arabidopsis HOS1 gene negatively regulates cold signal transduction and encodes a RING finger protein that displays cold-regulated nucleo--cytoplasmic partitioning

Low temperature is one of the most important environmental stimuli that control gene transcription programs and development in plants. In Arabidopsis thaliana, the HOS1 locus is a key negative regulator of low temperature-responsive gene transcription. The recessive hos1 mutation causes enhanced induction of the CBF transcription factors by low temperature as well as of their downstream cold-responsive genes. The hos1 mutant plants flower early, and this correlates with a low level of Flowering Locus C gene expression. The HOS1 gene was isolated through positional cloning. HOS1 encodes a novel protein with a RING finger motif near the amino terminus. HOS1 is ubiquitously expressed in all plant tissues. HOS1--GFP translational fusion studies reveal that HOS1 protein resides in the cytoplasm at normal growth temperatures. However, in response to low temperature treatments, HOS1 accumulates in the nucleus. Ectopic expression of HOS1 in wild-type plants causes cosuppression of HOS1 expression and mimics the hos1 mutant phenotypes.

A novel cold-inducible zinc finger protein from soybean, SCOF-1, enhances cold tolerance in transgenic plants

Cold stress on plants induces changes in the transcription of cold response genes. A cDNA clone encoding C2H2-type zinc finger protein, SCOF-1, was isolated from soybean. The transcription of SCOF-1 is specifically induced by low temperature and abscisic acid (ABA) but not by dehydration or high salinity. Constitutive overexpression of SCOF-1 induced cold-regulated (COR) gene expression and enhanced cold tolerance of non-acclimated transgenic Arabidopsis and tobacco plants. SCOF-1 localized to the nucleus but did not bind directly to either C-repeat/dehydration (CRT/DRE) or ABA responsive element (ABRE), cis-acting DNA regulatory elements present in COR gene promoters. However, SCOF-1 greatly enhanced the DNA binding activity of SGBF-1, a soybean G-box binding bZIP transcription factor, to ABRE in vitro. SCOF-1 also interacted with SGBF-1 in a yeast two-hybrid system. The SGBF-1 transactivated the beta-glucuronidase reporter gene driven by the ABRE element in Arabidopsis leaf protoplasts. Furthermore, the SCOF-1 enhanced ABRE-dependent gene expression mediated by SGBF-1. These results suggest that SCOF-1 may function as a positive regulator of COR gene expression mediated by ABRE via protein-protein interaction, which in turn enhances cold tolerance of plants.

Overexpression of the Arabidopsis CBF3 transcriptional activator mimics multiple biochemical changes associated with cold acclimation

We further investigated the role of the Arabidopsis CBF regulatory genes in cold acclimation, the process whereby certain plants increase in freezing tolerance upon exposure to low temperature. The CBF genes, which are rapidly induced in response to low temperature, encode transcriptional activators that control the expression of genes containing the C-repeat/dehydration responsive element DNA regulatory element in their promoters. Constitutive expression of either CBF1 or CBF3 (also known as DREB1b and DREB1a, respectively) in transgenic Arabidopsis plants has been shown to induce the expression of target COR (cold-regulated) genes and to enhance freezing tolerance in nonacclimated plants. Here we demonstrate that overexpression of CBF3 in Arabidopsis also increases the freezing tolerance of cold-acclimated plants. Moreover, we show that it results in multiple biochemical changes associated with cold acclimation: CBF3-expressing plants had elevated levels of proline (Pro) and total soluble sugars, including sucrose, raffinose, glucose, and fructose. Plants overexpressing CBF3 also had elevated P5CS transcript levels suggesting that the increase in Pro levels resulted, at least in part, from increased expression of the key Pro biosynthetic enzyme Delta(1)-pyrroline-5-carboxylate synthase. These results lead us to propose that CBF3 integrates the activation of multiple components of the cold acclimation response.

The role of inorganic phosphate in the development of freezing tolerance and the acclimatization of photosynthesis to low temperature is revealed by the pho mutants of Arabidopsis thaliana

Low temperature inhibits sucrose synthesis, leading to a phosphate-limitation of photosynthesis. We have used the Arabidopsis pho1-2 and pho2-1 mutants with decreased and increased shoot phosphate, respectively, to investigate whether low phosphate triggers cold acclimatization of photosynthetic carbon metabolism. Wild-type Arabidopsis, pho1-2 and pho2-1 were grown at 23 degrees C and transferred to 5 degrees C to investigate acclimatization in pre-existing leaves and in new leaves developing at 5 degrees C. The development of frost tolerance and the accumulation of proline and sugars was unaltered or improved in pho1-2, and impaired in pho2-1. Sucrose phosphate synthase and cytoplasmic fructose-1,6-bisphosphatase activity and protein increase after transfer to 5 degrees C. This increase was accentuated in pho1-2 and attenuated in pho2-1. RBCS and LHCB2 transcript levels decrease in pre-formed wild-type leaves after transfer to 5 degrees C and recover in new leaves that develop at 5 degrees C. The initial decrease was attenuated in pho1-2, and accentuated in pho2-1, where the recovery in new leaves was also suppressed. Rubisco activity increased in wild-type leaves that developed at 5 degrees C. This increase was accentuated in pho1-2 and absent in pho2-1. NADP-glyceraldehyde-3-phosphate dehydrogenase, plastidic fructose-1,6-bisphosphatase and aldolase activity increase relative to phosphoglycerate kinase, transketolase and phosphoribulokinase in wild-type leaves at 5 degrees C. This shift was accentuated in pho1-2 and reversed in pho2-1. Transcript levels for COR genes increase transiently 1 day after transfer to 5 degrees C but were very low in leaves that developed at 5 degrees C in wild-type Arabidopsis, pho1-2 and pho2-1. We conclude that low phosphate plays an important role in triggering cold acclimatization of leaves, leading in particular to an increase of Rubisco expression, changes in other Calvin cycle enzymes to minimize sequestration of phosphate in metabolites, and increased expression of sucrose biosynthesis enzymes.

Molecular responses to dehydration and low temperature: differences and cross-talk between two stress signaling pathways

Recently, a major transcription system that controls abscisic-acid-independent gene expression in response to dehydration and low temperature has been identified. The system includes the DRE/CRT (dehydration-responsive element/C-repeat) cis-acting element and its DNA-binding protein, DREB/CBF (DRE-binding protein/C-repeat binding factor), which has an AP2 domain. DREB/CBF contains two subclasses, DREB1/CBF and DREB2, which are induced by cold and dehydration, respectively, and control the expression of various genes involved in stress tolerance. Recent studies are providing evidence of differences between dehydration-signaling and cold-stress-signaling cascades, and of cross-talk between them.

Genomic approaches to plant stress tolerance

Past efforts to improve plant tolerance to drought, high salinity and low-temperature through breeding and genetic engineering have had limited success owing to the genetic complexity of stress responses. Progress is now anticipated through comparative genomics studies of an evolutionarily diverse set of model organisms, and through the use of techniques such as high-throughput analysis of expressed sequence tags, large-scale parallel analysis of gene expression, targeted or random mutagenesis, and gain-of-function or mutant complementation. The discovery of novel genes, determination of their expression patterns in response to abiotic stress, and an improved understanding of their roles in stress adaptation (obtained by the use of functional genomics) will provide the basis of effective engineering strategies leading to greater stress tolerance.

Genetic analysis of salt-tolerant mutants in Arabidopsis thaliana

Stress caused by the increased salinity of irrigated fields impairs plant growth and is one of the major constraints that limits crop productivity in many important agricultural areas. As a contribution to solving such agronomic problems, we have carried out a large-scale screening for Arabidopsis thaliana mutants induced on different genetic backgrounds by EMS treatment, fast neutron bombardment, or T-DNA insertions. From the 675,500 seeds we screened, 17 mutant lines were isolated, all but one of which yielded 25-70% germination levels on 250 mm NaCl medium, a condition in which their ancestor ecotypes are unable to germinate. Monogenic recessive inheritance of NaCl-tolerant germination was displayed with incomplete penetrance by all the selected mutants, which fell into five complementation groups. These were named SALOBRENO (SAN) and mapped relative to polymorphic microsatellites, the map positions of three of them suggesting that they are novel genes. Strains carrying mutations in the SAN1-SAN4 genes display similar responses to both ionic effects and osmotic pressure, their germination being NaCl and mannitol tolerant but KCl and Na(2)SO(4) sensitive. In addition, NaCl-, KCl-, and mannitol-tolerant as well as abscisic-acid-insensitive germination was displayed by sañ5, whose genetic and molecular characterization indicates that it carries an extremely hypomorphic or null allele of the ABI4 gene, its deduced protein product lacking the APETALA2 DNA binding domain.

Antisense suppression of proline degradation improves tolerance to freezing and salinity in Arabidopsis thaliana

Synthesis, degradation, and transport of proline (Pro) are thought to cooperatively control its endogenous levels in higher plants in response to environmental conditions. To evaluate the function of Pro degradation in the regulation of the levels of Pro and to elucidate roles of Pro in stress tolerance, we generated antisense transgenic Arabidopsis plants with an AtProDH cDNA encoding proline dehydrogenase (ProDH), which catalyzes Pro degradation. Several transgenic lines accumulated Pro at higher levels than wild-type plants, providing evidence for a key role of ProDH in Pro degradation in Arabidopsis. These antisense transgenics were more tolerant to freezing and high salinity than wild-type plants, showing a positive correlation between Pro accumulation and stress tolerance in plants.

Interactions and intersections of plant signaling pathways

Plant signal transduction is a rapidly expanding field of research, and during the last decade a wealth of insight into how plants perceive and transmit signals as part of normal development and in response to environmental cues has been and is continuing to be unraveled. Although ?signaling cascades are often viewed as linear chains of events it is now becoming increasingly apparent, through the use of cell biological, molecular and genetic approaches, that plant signal transduction involves extensive cross-talk between different pathways. The numerous interactions and intersections which take place are potentially important to modulate and balance the various inputs from different signaling cascades so that plants can integrate all this information to execute the proper developmental responses.

Characterization of the transcriptional activator CBF1 from Arabidopsis thaliana. Evidence for cold denaturation in regions outside of the DNA binding domain

A transcriptional activator, CBF1, from Arabidopsis thaliana, which has the AP2 domain for DNA binding and regulates the cold acclimation response, was overexpressed in Escherichia coli, purified, and characterized. Analyses of the interaction between CBF1 and the C-repeat/dehydration-responsive element by fluorescence measurement showed that CBF1 binds to C-repeat/dehydration-responsive element as a monomer irrespective of the temperature. CD spectra of the intact and truncated CBF1 proteins (1-213, 41-213, 41-157, and 41-146) were measured to examine the temperature-dependent changes of the secondary structure of CBF1. The results suggested that the CBF1 protein has regions exhibiting reversible cold denaturation in the range between 30 and -5 degrees C and also has a region exhibiting thermal denaturation between 40 and 60 degrees C. This cold denaturation occurred in both the N-terminal and acidic regions. The thermal denaturation occurred in the region encompassing the AP2 domain. The difference between the retention time of CBF1 at 4 degrees C and that at 25 degrees C in gel filtration, and the decrease of the sedimentation coefficient, s20,w, caused by the temperature change from 25 to 3 degrees C, strongly suggested that the cold denaturation was accompanied by the extension of the molecule. The possible cold denaturation observed here might be a physiologically important structural response of CBF1 to cold stress.

PLANT COLD ACCLIMATION: Freezing Tolerance Genes and Regulatory Mechanisms

Many plants increase in freezing tolerance upon exposure to low nonfreezing temperatures, a phenomenon known as cold acclimation. In this review, recent advances in determining the nature and function of genes with roles in freezing tolerance and the mechanisms involved in low temperature gene regulation and signal transduction are described. One of the important conclusions to emerge from these studies is that cold acclimation includes the expression of certain cold-induced genes that function to stabilize membranes against freeze-induced injury. In addition, a family of Arabidopsis transcription factors, the CBF/DREB1 proteins, have been identified that control the expression of a regulon of cold-induced genes that increase plant freezing tolerance. These results along with many of the others summarized here further our understanding of the basic mechanisms that plants have evolved to survive freezing temperatures. In addition, the findings have potential practical applications as freezing temperatures are a major factor limiting the geographical locations suitable for growing crop and horticultural plants and periodically account for significant losses in plant productivity.

Cold-induced freezing tolerance in Arabidopsis

Changes in the physiology of plant leaves are correlated with enhanced freezing tolerance and include accumulation of compatible solutes, changes in membrane composition and behavior, and altered gene expression. Some of these changes are required for enhanced freezing tolerance, whereas others are merely consequences of low temperature. In this study we demonstrated that a combination of cold and light is required for enhanced freezing tolerance in Arabidopsis leaves, and this combination is associated with the accumulation of soluble sugars and proline. Sugar accumulation was evident within 2 h after a shift to low temperature, which preceded measured changes in freezing tolerance. In contrast, significant freezing tolerance was attained before the accumulation of proline or major changes in the percentage of dry weight were detected. Many mRNAs also rapidly accumulated in response to low temperature. All of the cold-induced mRNAs that we examined accumulated at low temperature even in the absence of light, when there was no enhancement of freezing tolerance. Thus, the accumulation of these mRNAs is insufficient for cold-induced freezing tolerance.

The sfr6 mutation in Arabidopsis suppresses low-temperature induction of genes dependent on the CRT/DRE sequence motif

The sfr mutations, which result in sensitivity to freezing after cold acclimation, define genes that are required for freezing tolerance. We tested plants homozygous for mutations sfr2 to sfr7 for cold-induced gene expression and found that sfr 6 plants were deficient in cold-inducible expression of the genes KIN1, COR15a, and LTI78, which all contain the C repeat/dehydration-responsive element (CRT/DRE) motif in their promoters. Similarly, sfr 6 plants failed to induce KIN1 normally in response to either osmotic stress or the application of abscisic acid. In contrast, cold-inducible expression of genes CBF1, CBF2, CBF3, and ATP5CS1, which lack the CRT/DRE motif, was not affected. The freezing-sensitive phenotype that defines sfr 6 also was found to be tightly linked to the gene expression phenotype. To determine whether the failure of cold induction of CRT/DRE-containing genes in sfr 6 was due to altered low-temperature calcium signaling, cold-induced cytosolic-free calcium ([Ca2+]cyt) elevations were investigated in the sfr 6 mutant, but these were found to be indistinguishable from those of the wild type. We discuss the possibilities that CRT/DRE binding proteins (such as CBF1) require activation to play a role in transcription and that the SFR6 protein is a vital component of their activation.

The sfr6 mutation in Arabidopsis suppresses low-temperature induction of genes dependent on the CRT/DRE sequence motif

The sfr mutations, which result in sensitivity to freezing after cold acclimation, define genes that are required for freezing tolerance. We tested plants homozygous for mutations sfr2 to sfr7 for cold-induced gene expression and found that sfr 6 plants were deficient in cold-inducible expression of the genes KIN1, COR15a, and LTI78, which all contain the C repeat/dehydration-responsive element (CRT/DRE) motif in their promoters. Similarly, sfr 6 plants failed to induce KIN1 normally in response to either osmotic stress or the application of abscisic acid. In contrast, cold-inducible expression of genes CBF1, CBF2, CBF3, and ATP5CS1, which lack the CRT/DRE motif, was not affected. The freezing-sensitive phenotype that defines sfr 6 also was found to be tightly linked to the gene expression phenotype. To determine whether the failure of cold induction of CRT/DRE-containing genes in sfr 6 was due to altered low-temperature calcium signaling, cold-induced cytosolic-free calcium ([Ca2+]cyt) elevations were investigated in the sfr 6 mutant, but these were found to be indistinguishable from those of the wild type. We discuss the possibilities that CRT/DRE binding proteins (such as CBF1) require activation to play a role in transcription and that the SFR6 protein is a vital component of their activation.

Map positions of SFR genes in relation to other freezing-related genes of Arabidopsis thaliana

We determined the map positions of seven SFR genes and compared them to the positions of 51 genes suspected of involvement in freezing tolerance in Arabidopsis thaliana. The SFR genes were recognized by the freezing sensitivity of mutants; the others (including 14 whose map positions we have determined) were genes whose expression is induced by low temperature, genes involved in abscisic acid (ABA) biosynthesis and perception, and genes involved in tolerance of oxidative stress. The comparison of map positions indicated a limited set of potential identities, some of which were eliminated by further mapping or by an allelism test.

Arabidopsis thaliana: a model plant for genome analysis

Arabidopsis thaliana is a small plant in the mustard family that has become the model system of choice for research in plant biology. Significant advances in understanding plant growth and development have been made by focusing on the molecular genetics of this simple angiosperm. The 120-megabase genome of Arabidopsis is organized into five chromosomes and contains an estimated 20,000 genes. More than 30 megabases of annotated genomic sequence has already been deposited in GenBank by a consortium of laboratories in Europe, Japan, and the United States. The entire genome is scheduled to be sequenced by the end of the year 2000. Reaching this milestone should enhance the value of Arabidopsis as a model for plant biology and the analysis of complex organisms in general.

Cold stress: manipulating freezing tolerance in plants

A single transcription factor can trigger induction of the freezing-tolerant state in Arabidopsis. Is such a factor all that is lacking in non-hardy plants?