Cold Acclimation Treatment Research Articles

Differential effects of cold acclimation and abscisic acid on free amino acid composition in wheat

The effect of cold acclimation and abscisic acid (ABA) treatment on the free amino acid composition was compared in Chinese Spring chromosome 5A substitution lines with different levels of freezing tolerance. The total amino acid content gradually increased during the 3-week cold acclimation period, while the effect of ABA became visible only after 7 d. The ratio of members of the glutamate family increased during cold acclimation and the ratio of amino acids belonging to the aspartate family decreased. Opposite changes were observed after treatment with ABA. Consistently with these results, ABA only induced a major increase in the Asn content, while the Asp, Glu, Gln and Pro levels were greatly induced by cold. A corresponding alteration at the gene expression level was only found for Pro and Glu. With the exception of Pro, cold- or ABA-induced changes in the amino acid levelsor Pro, did not correlate with the freezing tolerance of the three genotypes examined and were not affected by chromosome 5A. Since cold acclimation induced the accumulation of most of the amino acids, while ABA had a significant effect only on Asn, the cold-induced changes in free amino acid levels were probably not mediated by ABA.

Expression of sHSP genes as affected by heat shock and cold acclimation in relation to chilling tolerance in plum fruit

Three full-length cytosol small heat shock protein (sHSP) genes, including two class I sHSP (CI sHSP) and one class II sHSP (CII sHSP) cDNAs, termed Ps-CI sHSP1, Ps-CI sHSP2 and Ps-CII sHSP1 respectively, were isolated and characterized from plum fruit at harvest. Their expression in relation to heat shock and cold acclimation-induced chilling tolerance were investigated. Heat shock treatment by dipping the fruit in water at 55 °C hot for 2 min and cold acclimation by conditioning the fruit at 8 °C for 5 d prior to storage at 2 °C could effectively reduce malondialdehyde (MDA) content and alleviate chilling injury. Furthermore, accumulation of Ps-CII sHSP1 mRNA transcripts in the fruit during the subsequent storage at 2 °C was remarkably enhanced by heat shock and cold acclimation treatments. These data suggest that heat shock and cold acclimation treatments induced the expression of Ps-CII sHSP1, which may be involved in chilling tolerance of the fruit caused by these treatments.

MEDIUM AND LONG-TERM STORAGE OF RUBUS GERMPLASM

The United States Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository at Corvallis, Oregon preserves genetic resources for Rubus. The in vitro collection includes about 200 accessions. In vitro cold storage of these accessions is at 4°C with 12 h of low light. Storage facilities for germplasm collections vary, but one week of cold acclimation followed by 4°C storage in the dark or with a photoperiod is acceptable for most Rubus germplasm when quarterly evaluation inventories are used to determine timing of repropagation. A reduced-nitrogen medium extends room temperature storage to nine months and is a useful alternative for cold sensitive and tropical genotypes which typically only survive for a short time in cold storage. Meristems of 34 cold-acclimated genotypes of Rubus (blackberry and raspberry) were successfully cryopreserved by slow cooling through optimization of cryoprotectants, cooling rates and cold acclimation. Alternating low temperatures as a cold acclimation (CA) treatment improved recovery of shoot tips cryopreserved by slow freezing. The length of the CA required varied from 1 to 10 weeks and was genotype dependent. Cryopreserved Rubus shoot tips produced shoots directly from either apical meristems or axillary buds, but not from callus. Shooting increased and callus formation decreased when IBA was eliminated from the recovery medium. Shoot tips of 25 genotypes in 9 Rubus species were successfully cryopreserved using encapsulation-dehydration with recovery of 60–100%. Four genotypes of 3 species were tested using PVS2 vitrification with 71% average regrowth. A protocol for cryopreservation of Rubus germplasm should include a CA period of 2–10 weeks and recovery on auxin-free medium. These studies confirm that all three cryopreservation protocols may be used for cryopreservation of a wide range of Rubus genetic resources.

Clinal variation in freezing tolerance among natural accessions of Arabidopsis thaliana

Low temperature represents a form of abiotic stress that varies predictably with latitude and altitude and to which organisms have evolved multiple physiological responses. Plants provide an especially useful experimental system for investigating the ecological and evolutionary dynamics of tolerance to low temperature because of their sessile lifestyle and inability to escape ambient atmospheric conditions. Here, intraspecific variation in freezing tolerance was investigated in Arabidopsis thaliana by conducting freezing tolerance assays on 71 accessions collected from across the native range of the species. Assays were performed at multiple minimum temperatures and on both cold-acclimated and non-cold-acclimated individuals. Considerable variation in freezing tolerance was observed among accessions both with and without a prior cold-acclimation treatment, suggesting that differences among accessions in cold-acclimation capacity as well as differences in intrinsic physiology contribute to variation in this phenotype. A highly significant positive relationship was observed between freezing tolerance and latitude of origin of accessions, consistent with a major role for natural selection in shaping variation in this phenotype. Clinal variation in freezing tolerance in A. thaliana coupled with considerable knowledge of the underlying genetics and physiology of this phenotype should allow evolutionary genetic analysis at multiple levels.

Tissue expression map of a large number of expressed sequence tags and its application to in silico screening of stress response genes in common wheat

In order to assess global changes in gene expression patterns in stress-induced tissues, we conducted large-scale analysis of expressed sequence tags (ESTs) in common wheat. Twenty-one cDNA libraries derived from stress-induced tissues, such as callus, as well as liquid cultures and abiotic stress conditions (temperature treatment, desiccation, photoperiod, moisture and ABA) were constructed. Several thousand colonies were randomly selected from each of these 21 cDNA libraries and sequenced from both the 5' and 3' ends. By computing abundantly expressed ESTs, correlated expression patterns of genes across the tissues were monitored. Furthermore, the relationships between gene expression profiles among the stress-induced tissues were inferred from the gene expression patterns. Multi-dimensional analysis of EST data is analogous to microarray experiments. As an example, genes specifically induced and/or suppressed by cold acclimation and heat-shock treatments were selected in silico. Four hundred and ninety genes showing fivefold induction or 218 genes for suppression in comparison to the control expression level were selected. These selected genes were annotated with the BLAST search. Furthermore, gene ontology was conducted for these genes with the InterPro search. Because genes regulated in response to temperature treatment were successfully selected, this method can be applied to other stress-treated tissues. Then, the method was applied to screen genes in response to abiotic stresses such as drought and ABA treatments. In silico selection of screened genes from virtual display should provide a powerful tool for functional plant genomics.

Morphological and physiological changes in roots of various wheat genotypes as related to cytoskeleton disruption

A depolymerizing effect of anti-microtubule drug oryzalin on the roots of three winter wheat (Triticum aestivum L.) cultivars contrasting in their frost-resistance was studied. The influence of plant cold acclimation (3°C, 7 days) and ABA treatment (30 μM) on oryzalin action was evaluated. Plant growing in the presence of 10 μM oryzalin under optimum temperature of 23°C resulted in the root-length decrease by 19–24% and root-apex swelling. All cells, especially in the root cortex, changed their radial dimensions. The cells acquired a rounded or irregular shape and increased in size. This indicates the loss of correct cell growth polarity. Most pronounced changes in the root apex diameter and most severe linear growth suppression were observed in the cultivar of moderate frost-resistance. The roots of this cultivar contained the highest amounts of actin and tubulins, as was evident from the immunoblot analysis. The effect of oryzalin on root growth and apex swelling was correlated with the content of actin in the roots of different wheat cultivars. Cold acclimation and exogenous ABA reduced (or prevented) oryzalin action on roots in a cultivar-specific manner. The conclusion was made that the bulk of the cytoskeletal net determined the efficiency of the cytoskeletal control of plant growth and morphogenesis. During autumn and winter periods, this is important for a better adaptation to temperature fluctuations of moderately frost-resistant plants, which are characterized by a high ecological plasticity.

Plasma membrane lipid alterations induced by cold acclimation and abscisic acid treatment of winter wheat seedlings differing in frost resistance

Cold acclimation of plants affects many aspects of metabolism. Changes in plasma membrane lipids have always been considered to be important for development of frost resistance and survival at subzero temperatures. We studied different cultivars of winter wheat (Triticum aestivum L.) that differed in frost resistance induced either by cold acclimation or treatment with the plant hormone abscisic acid (ABA). Plasma membranes were isolated from non-acclimated and cold- as well as from ABA-acclimated plants, and were subjected to detailed lipid analysis. Cold acclimation affected virtually all plasma membrane lipid components and their constituents, resulting in both increases and decreases, which varied between the three groups of plants investigated. Including the cold-induced variations observed in the few plant species studied in detail previously, cerebrosides were the only components reduced by cold acclimation in all plants. In wheat, more uniform and consistent patterns were obtained when considering colligative parameters such as total free sterols, phospholipids or glycolipids, either as the proportion of total lipids or based on plasma membrane protein. The parameter which changed most significantly in parallel to the increase of inducible frost resistance in the three groups of plants was the ratio of free sterols/glycolipids, which increased. ABA treatment resulted in qualitatively similar effects in only one cultivar, but in general these changes were less pronounced. Compared to changes in transcription rates of several cold-induced genes and in the concentration of various compatible solutes reported for other plants, the observed changes in plasma membrane lipids are minor ones. This may indicate that acclimation-induced changes can be accomplished by posttranscriptional regulation of enzymatic activities, which is in agreement with the failure to detect significant changes in transcription of the corresponding genes during cold induction.

Protein, leucine aminopeptidase, esterase, acid phosphatase and photosynthetic responses of oleander ( Nerium oleander L.) during cold acclimation and freezing treatments

Six-month-old oleander (Nerium oleander L.) pot plants, derived from vegetative propagation by cuttings, were tested for their ability to cold hardening. Damage of the non-acclimated (NA) plants was visible when treated by low freezing temperatures (below -2 degrees C). The responses of total proteins, leucine aminopeptidase (LAP), esterase (EST) and acid phosphatase (ACP) isoforms of NA and cold-acclimated (CA; 4 degrees C for 14 days) plants were compared using polyacrylamide gel electrophoresis. These molecular markers were also compared in NA and CA plants which received for 2h temperatures of 0, -2, -4, -6 and -8 degrees C. A new 38-kDa polypeptide appeared from day 7 to 14 during the acclimation treatment in the bark extracts and on day 14 in the leaf extracts. The above-mentioned polypeptide band (38 kDa) strongly appeared in all freezing treatments (0, -2, -4, -6 and -8 degrees C) in both bark and leaf extracts of the CA plants. Alterations in the number and the intensity of LAP and EST isoforms as well as in the intensity of ACP isoforms were observed in both bark and leaf of the CA oleander plants. A newly expressed EST isoform is proposed as biochemical marker for the cold acclimation treatment. CO2 assimilation rates (A) as well as transpiration rates (E) in NA plants were positive in 0 degrees C and negative in all temperatures below zero in the freezing treatments. In contrast, CO2 assimilation rates (A) and transpiration rates (E) were positive in CA plants in all temperatures of freezing treatment. A significant decrease (P<0.05) in chlorophyll (Chl) a, Chl a+b concentration and Chl a/b ratio were noticed in oleander plants during the acclimation treatment (from day 0 to 14), while Chl b concentration was unchanged at the respective time. On the other hand, no significant (P<0.05) differences were observed in the freezing treatments.

Molecular and structural analyses of a novel temperature stress-induced lipocalin from wheat and Arabidopsis

Two cDNAs corresponding to a novel lipocalin were identified from wheat and Arabidopsis. The two cDNAs designated Tatil for Triticum aestivum L. temperature-induced lipocalin and Attil for Arabidopsis thaliana temperature-induced lipocalin encode polypeptides of 190 and 186 amino acids respectively. Structure analyses indicated the presence of the three structurally conserved regions that characterize lipocalins. Sequence analyses revealed that this novel class of plant lipocalin shares homology with three evolutionarily related lipocalins: the mammalian apolipoprotein D (ApoD), the bacterial lipocalin and the insect Lazarillo. The comparison of the putative tertiary structures of both the human ApoD and the wheat TaTIL suggest that the two proteins differ in membrane attachment and ligand interaction. Northern analyses demonstrated that Tatil and Attil transcripts are upregulated during cold acclimation and heat-shock treatment. The putative functions of this novel class of plant lipocalins during temperature stresses are discussed.

The microtubule stability increases in abscisic acid-treated and cold-acclimated differentiating vascular root tissues of wheat

The effects of low temperature and exogenous abscisic acid (ABA) on the structure of microtubule (MT) cytoskeleton in root cells of three winter wheat cultivars with different frost resistance were investigated with immunocytochemical and related methods. The effects of the different treatments on the MT cytoskeleton were measured as tolerance to the MT depolymerising agent, oryzalin. The cold acclimation (3 °C, 7 d) and ABA treatment (30 μmol/L ABA in the growth medium) of the plants increased the MT stability in differentiating vascular tissue of roots but not in the root meristem or elongation zone. The comparison of the structure of MT cytoskeleton in differentiating vascular tissue from control, cold-acclimated, ABA-treated and both cold-acclimated and ABA-treated roots of different wheat cultivars indicated that the changes in the structure of MT cytoskeleton were modified by the genotype of the wheat cultivar. An oryzalin-sensitive MT cytoskeleton was typical to the cold-resistant cultivars, in which it appeared to respond efficiently to ABA treatment and to cold acclimation by cytoskeletal changes that may improve the cold resistance of the cultivar.

Glycine betaine involvement in freezing tolerance and water stress in Arabidopsis thaliana

Levels of endogenous glycine betaine in the leaves were measured in response to cold acclimation, water stress and exogenous ABA application in Arabidopsis thaliana. The endogenous glycine betaine level in the leaves increased sharply during cold acclimation treatment as plants gained freezing tolerance. When glycine betaine (10 mM) was applied exogenously to the plants as a foliar spray, the freezing tolerance increased from −3.1 to −4.5°C. In addition, when ABA (1 mM) was applied exogenously, the endogenous glycine betaine level and the freezing tolerance in the leaves increased. However, the increase in the leaf glycine betaine level induced by ABA was only about half of that by the cold acclimation treatment. Furthermore, when plants were subjected to water stress (leaf water potential of ∼−1.6 MPa), the endogenous leaf glycine betaine level increased by about 18-fold over that in the control plants. Water stress lead to significant increase in the freezing tolerance, which was slightly less than that induced by the cold acclimation treatment. The results suggest that glycine betaine is involved in the induction of freezing tolerance in response to cold acclimation, ABA, and water stress in Arabidopsis plants.

Glycine betaine accumulation and induction of cold tolerance in strawberry ( Fragaria X ananassa Duch.) plants

Endogenous glycine betaine levels in the leaves of strawberry ( Fragaria X ananassa Duch.) plants were determined during cold acclimation and in response to exogenous abscisic acid (ABA) application. Glycine betaine levels in the leaves increased nearly two-fold after 4 weeks of cold acclimation treatment during which the cold tolerance of leaves increased from −5.8 to −17°C. Exogenous application of ABA (100 μM) to plants triggered glycine betaine accumulation in unhardened plants. It also increased cold tolerance of leaves in both unhardened and cold-hardened plants. Similar to ABA, exogenous glycine betaine was effective in inducing cold tolerance in unhardened and cold-hardening plants. Exogenous application of glycine betaine (2 mM) to unhardened plants increased the cold tolerance of leaves almost two-fold within 72 h of application. In addition, it improved freezing survival and regrowth in whole plants. The results suggest that glycine betaine is involved in inducing cold tolerance both in response to exogenous ABA and during natural cold acclimation of strawberry plants.

Expression of Heat-stable and Putative Dehydrin Proteins during Nonacclimated and Cold-acclimated Conditions in Several Tuber-bearing Solanum Species

Recent studies suggest cold-regulated heat-stable proteins mitigate the potential damaging effects of low water activity associated with freezing. A proposed function of these proteins is stabilization of enzymes during exposure of plants to subzero temperatures. To test this hypothesis for tuber-bearing Solanum L. species we determined the quantitative expression of heat-stable proteins, the qualitative changes in dehydrin proteins, and the capacity of heat-stable proteins to cryoprotect a freeze-thaw labile enzyme lactate dehydrogenase (LDH). We used five tuber-bearing Solanum species (S. tuberosum L. `Red Pontiac', S. acaule Bitter, S. sanctae rosea Hawkes, S. commersonii Dunal, and S. cardiophyllum Bitter), which vary in nonacclimated relative freezing tolerance (NA RFT), acclimated relative freezing tolerance (AC RFT), and acclimation capacity (ACC). The protein fraction containing a mixture of heat-stable proteins demonstrated cryoprotective capacities greater or equal to other cryoprotective compounds (bovine serum albumin, polyethylene glycol, glycerol, and sucrose). Heat-stable proteins extracted from acclimated S. commersonii had superior cryoprotective capacity than those extracted from nonacclimated S. commersonii plants. Interestingly, in the presence of these proteins extracted from acclimated plants (in S. commersonii and S. sanctae rosea), LDH activity was elevated above that of unfrozen controls. No quantitative relationships were found between heat-stable protein concentration and NA RFT, AC RFT, or ACC among the five species. This was also true for dehydrin protein expression. Cold acclimation treatment resulted in increased dehydrin expression for acclimating and nonacclimating species. In three of the cold acclimating species (S. acaule, S. sanctae rosea, and S. commersonii), an increase in dehydrin expression may play a role in increased freezing tolerance during cold acclimation. In the cold sensitive, nonacclimating species (S. tuberosum and S. cardiophyllum), however, an increase in dehydrin level maybe related to the response of these species to changed (perhaps stressful) environment during cold treatment. By exploiting the genetic variation in NA RFT and ACC for five tuber-bearing species, we were able to gain new insight into the complexity of the relationship between heat-stable protein and cold response.

Ultrastructural and protein changes in cell suspension cultures of peach associated with low temperature-induced cold acclimation and abscisic acid treatment

Cell suspension cultures were initiated from callus derived from xylem tissues of peach [Prunus persica (L.) Batsch]. Cold acclimation was induced (LT50 of-13°C) in cell suspensions at 3°C in the dark for 10 days. Freezing tolerance returned to the level of nonacclimated cells (LT50 of −4.5°C) when cold-acclimated cells were transferred to 24°C (in dark) for 3 days. Addition of 75 μM abscisic acid (ABA) to the growth medium failed to induce cold acclimation after cells were cultured for 5 days at 24°C. Microvacuolation, cytoplasmic augmentation and disappearance of starch grains were observed in cells that were cold-acclimated by exposure to low temperature. Similar ultrastructural alterations were not observed in ABA-treated cells. Several qualitative and quantitative changes in proteins were noted during both cold acclimation and ABA treatment. Both the ultrastructural and protein changes observed during cold acclimation were reversed during deacclimation. The relationship of these changes to cold acclimation in peach cell-cultures is discussed.

Responses to ABA and Cold Acclimation Are Genotype Dependent for Cryopreserved Blackberry and Raspberry Meristems

The meristems of cold-acclimated cultivars and species of Rubus (blackberry and raspberry) can be successfully cryopreserved through optimization of cryoprotectants, cooling rates, and cold acclimation treatments. Genotypes that do not cold acclimate are difficult to cryopreserve and new methods are needed for this group of plants. The effects of abscisic acid (ABA) (50 μM) and 7-day cold acclimation (CA) treatments on the survival of cryopreserved apical meristems was determined for five Rubus genotypes (Blackberries: Rubus cissoides Cunn. and Rubus hybrid cvs. Hillemeyer and Silvan, and Raspberries: R. parvifolius L., and R. parviflorus Nutt.). ABA alone did not significantly improve the survival of any of the genotypes tested. The combination of ABA and CA significantty improved the survival of R. cissoides, but neither was effective alone. Cold acclimation with or without ABA significantly improved the survival of meristems of 'Hillemeyer,' 'Silvan,' and R. parvifolius L. Survival of R. parviflorus Nutt. meristems was not significantly affected by any of the treatments. In contrast to cytokinin effects in other culture systems, the cytokinin benzyladenine did not significantly affect survival of 'Hillemeyer' and showed no interaction with ABA or CA. Murashige and Skoog-based recovery medium produced higher survival rates than did Anderson's medium for the nine genotypes tested. Overall, genotype played a large role in the survival of cryopreserved Rubus meristems. In this system ABA does not substitute for CA, but in some genotypes may interact with CA to increase survival through a synergistic effect.

Histochemical arguments for muscular non-shivering thermogenesis in muscovy ducklings.

1. The histochemical characteristics of gastrocnemius muscle were investigated in 6-week-old cold-acclimated (5 weeks, 4 degrees C) and glucagon-treated (5 weeks, 25 degrees C, 103 nmol/kg I.P. twice daily) muscovy ducklings, two groups able to develop non-shivering thermogenesis in vivo. A comparison was made with thermoneutral controls (25 degrees C) of the same age. All animals were fed ad libitum. Fibre type, fibre area and capillary supply have been studied. Further, a quantitative histochemical method for mitochondrial Mg(2+)-ATPase activity was developed to characterize the mitochondrial coupling state in situ. 2. White gastrocnemius was composed of fast glycolytic (FG) and fast oxidative glycolytic (FOG) fibres, while red gastrocnemius contained FOG and slow oxidative (SO) fibres. In white gastrocnemius, the proportion of FG fibres was higher in glucagon-treated than in control or cold-acclimated ducklings. In red gastrocnemius, the proportion of SO fibres was higher in both cold-acclimated and glucagon-treated ducklings than in controls. The area of all fibres was generally lower in glucagon-treated than in other ducklings. 3. The capillary density was higher in both red and white components of the gastrocnemius muscle in cold-acclimated and glucagon-treated than in control ducklings, as a result of an increased number of capillaries around each fibre. 4. In all fibres, except the FG type in cold-acclimated ducklings, the staining intensity of the Mg(2+)-ATPase reaction was higher in cold-acclimated and glucagon-treated than in control ducklings whereas the staining intensity with maximal decoupling of oxidative phosphorylation by dinitrophenol was unchanged. This indicated a more loose-coupled state of mitochondria in situ in all fibres of cold-acclimated ducklings, and in FOG fibres of white gastrocnemius and SO fibres of red gastrocnemius in glucagon-treated ducklings. 5. These results indicated a higher oxidative metabolism of skeletal muscle in both cold-acclimated and glucagon-treated than in control ducklings, and for most of the parameters studied, a similarity between cold acclimation and glucagon treatment. Because of the higher loose-coupled state of muscle mitochondria in cold-acclimated and glucagon-treated than in control ducklings, the higher oxidative capacity of skeletal muscle in these ducklings could be used for heat production rather than ATP synthesis and account for muscular non-shivering thermogenesis.

Cryopreservation of in vitro-cultured multiple bud clusters of asparagus (Asparagus officinalis L. cv Hiroshimagreen (2n=30) by the techniques of vitrification

A culture line of asparagus forming green bulbous structures consisting of numerous multiple bud clusters designated "bud clusters" was induced from a meristem culture of asparagus (Asparagus officinalis L.cv. Hiroshimagreen, 2n=30). Small cubic segments (2 mm3) cut from bud clusters were cryopreserved using three different cryogenic protocols. Only vitrification produced very high levels of shoot formation after cooling to -196°C. Segments were treated with a vitrification solution (PVS2) at 25°C for 45 min or at 0°C for 120 min prior to a direct plunge into liquid nitrogen. After rapid warming, the segments were expelled into Murashige and Skoog medium containing 1.2 M sucrose for 10 min and then plated on agar shoot outgrowth medium. The average rate of shoot formation of vitrified segments producing normal shoots was near 90% without any preculture and/or cold-acclimation treatment. Revived segments resumed growth within 3 days and developed about three shoots per segment. In vitro-cultured bud clusters appear promising as material for cryopreserving asparagus germplasm.

Relationship between Freezing Tolerance of Root-Tip Cells and Cold Stability of Microtubules in Rye (Secale cereale L. cv Puma)

The response of cortical microtubules to low temperature and freezing was assessed for root tips of cold-acclimated and non-acclimated winter rye (Secale cereale L. cv Puma) seedlings using indirect immunofluorescence microscopy with antitubulin antibodies. Roots cooled to 0 or -3 degrees C were fixed for immunofluorescence microscopy at these temperatures or after an additional hour at 4 degrees C. Typical arrays of cortical microtubules were present in root-tip cells of seedlings exposed to the cold-acclimation treatment of 4 degrees C for 2 days. Microtubules in these cold-acclimated cells were more easily depolymerized by a 0 degrees C treatment than microtubules in root-tip cells of nonacclimated, 22 degrees C-grown seedlings. Microtubules were still present in some cells of both nonacclimated and cold-acclimated roots at 0 and -3 degrees C; however, the number of microtubules in these cells was lower than in controls. Microtubules remaining during the -3 degrees C freeze were shorter than microtubules in unfrozen control cells. Repolymerization of microtubules after both the 0 and -3 degrees C treatments occurred within 1 h. Root tips of nonacclimated seedlings had an LT-50 of -9 degrees C. Cold acclimation lowered this value to -14 degrees C. Treatment of 22 degrees C-grown seedlings for 24 h with the microtubule-stabilizing drug taxol caused a decrease in the freezing tolerance of root tips, indicated by a LT-50 of -3 degrees C. Treatment with D-secotaxol, an analog of taxol that was less effective in stabilizing microtubules, did not alter the freezing tolerance. We interpret these data to indicate that a degree of depolymerization of microtubules is necessary for realization of maximum freezing tolerance in root-tip cells of rye.

RNA Synthesis in Spring and Winter Wheat during Cold Acclimation

AbstractMarked increases were obtained in the levels of ribosomal and soluble RNA fractions in winter wheat submitted to cold acclimation temperatures. In comparison, the change in the rRNA and sRNA of spring wheat, grown under similar conditions, were less pronounced.With the cold acclimation treatment, sRNA accumulated by winter wheat shows an important change in its (G + C)/ (A +Ψ+ U) ratio, whereas no change in base composition is observed in the sRNA of spring wheat or the rRNA of both spring and winter wheat.It is suggested that in winter wheat, cold acclimation temperatures potentiate increased synthesis of specific sRNAs, this possibly being part of the increased capacity for protein synthesis associated with cold acclimation.