Body temperature regulation during acclimation to cold and hypoxia in rats

Extreme environmental conditions present challenges for thermoregulation in homoeothermic organisms such as mammals. Such challenges are exacerbated when two stressors are experienced simultaneously and each stimulus evokes opposing physiological responses. This is the case of cold, which induces an increase in thermogenesis, and hypoxia, which suppresses metabolism conserving oxygen and preventing hypoxaemia. As an initial approach to understanding the thermoregulatory responses to cold and hypoxia in a small mammal, we explored the effects of acclimation to these two stressors on the body temperature (T_b) and the daily and ultradian T_b variations of Sprague-Dawley rats. As T_b is influenced by sleep-wake cycles, these T_b variations reflect underlying adjustments in set-point and thermosensitivity. The T_b of rats decreased precipitously during initial hypoxic exposure which was more pronounced in cold (T_b=33.4±0.13) than in room temperature (T_b=35.74±0.17) conditions. This decline was followed by an increase in T_b stabilising at a new level ~0.5 °C and ~1.4 °C below normoxic values at room and cold temperatures, respectively. Daily T_b variations were blunted during hypoxia with a greater effect in the cold. Ultradian T_b variations exhibited daily rhythmicity that disappeared under hypoxia, independent of ambient temperature. The adjustments in T_b during hypoxia and/or cold are in agreement with the hypothesis that an initial decrease in the T_b set-point is followed by its partial re-establishment with chronic hypoxia. This rebound of the T_b set-point might reflect cellular adjustments that would allow animals to better deal with low oxygen conditions, diminishing the drive for a lower T_b set-point. Cold and hypoxia are characteristic of high altitude environments. Understanding how mammals cope with changes in oxygen and temperature will shed light into their ability to colonize new environments along altitudinal clines and increase our understanding of how T_b is regulated under stimuli that impose contrasting physiological constraints.     

A global survey of gene regulation during cold acclimation in Arabidopsis thaliana

Many temperate plant species such as Arabidopsis thaliana are able to increase their freezing tolerance when exposed to low, nonfreezing temperatures in a process called cold acclimation. This process is accompanied by complex changes in gene expression. Previous studies have investigated these changes but have mainly focused on individual or small groups of genes. We present a comprehensive statistical analysis of the genome-wide changes of gene expression in response to 14 d of cold acclimation in Arabidopsis, and provide a large-scale validation of these data by comparing datasets obtained for the Affymetrix ATH1 Genechip and MWG 50-mer oligonucleotide whole-genome microarrays. We combine these datasets with existing published and publicly available data investigating Arabidopsis gene expression in response to low temperature. All data are integrated into a database detailing the cold responsiveness of 22,043 genes as a function of time of exposure at low temperature. We concentrate our functional analysis on global changes marking relevant pathways or functional groups of genes. These analyses provide a statistical basis for many previously reported changes, identify so far unreported changes, and show which processes predominate during different times of cold acclimation. This approach offers the fullest characterization of global changes in gene expression in response to low temperature available to date.     

Role of glutathione in adaptation and signalling during chilling and cold acclimation in plants

To test the hypothesis that antioxidant systems flexibly adjust to short-term, diurnal fluctuations of ambient environmental conditions, ascorbate-related systems were studied over several day/night cycles in mature sun-acclimated leaves of field-grown beech trees ( Fagus sylvatica ). Light-dependent increases in the activities of ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate radical reductase (MDAR, EC 1.1.5.4) and glutathione reductase (GR, EC 1.6.4.2) were not observed. Lowest activities of APX and MDAR were found on hot, sunny days. A strong negative correlation occurred between APX activities and ambient temperatures suggesting that this enzyme was temperature- rather than light-regulated. In contrast to the enzymatic defences, ascorbate levels increased by about 30% under bright sunlight suggesting that protection from excess light is mediated via the adjustment of metabolites. Under these conditions the apparent electron transport rate exceeded the capacity for assimilation and the dehydroascorbate pool increased twofold. Since dehydroascorbate reductase activities were hardly detected, MDAR activities seemed to be the major enzyme to keep ascorbate in its reduced state. However, MDAR appeared to be insufficient to maintain the redox balance of the ascorbate pool under high light intensities in the field.     

植物冷驯化的分子机理研究进展

植物冷驯化是一个非常复杂的过程,包括植物将感受到的低温信号转变成生化信号,以激活冷诱导基因的启动子,刺激特定的mRNA的转录,并在特定的组织中合成冷驯化蛋白.冷驯化蛋白通过增强膜脂流动性和阻止胞间冰晶形成等方式,以保护细胞免受低温伤害.冷驯化基因的表达以转录后调控为主,也有转录调控.某些冷诱导基因也可受ABA或其他环境胁迫(如高温、干旱、高盐、脱水等)诱导表达.     

Photosynthesis,photoinhibition and low temperature acclimation in cold tolerant plants

Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The photosynthetic response of plants to low temperature is dependent upon time of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperature shiftsinhibits whereas low temperature growthstimulates electron transport capacity and carbon assimilation. However, the photosynthetic response to low temperature is clearly species and cultivar dependent. Winter annuals and algae which actively grow and develop at low temperature and moderate irradiance acquire a resistance to irradiance 5- to 6-fold higher than their growth irradiance. Resistance to short-term photoinhibition (hours) in winter cereals is a reflection of the increased capacity to keep Q_A oxidized under high light conditions and low temperature. This is due to an increased capacity for photosynthesis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance of cereals. It is proposed that the enhanced photosynthetic capacity reflects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage of carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Long-term exposure (months) of cereals to low temperature photoinhibition indicates that this reduction of photochemical efficiency of PS II represents a stable, long-term down regulation of PS II to match the energy requirements for CO₂ fixation. Thus, photoinhibition in vivo should be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which necessarily results in damage or injury to plants. Not all cold tolerant, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering evergreens become dormant during the cold hardening period and generally remain susceptible to photoinhibition. It is concluded that the photosynthetic response to low temperatures and susceptibility to photoinhibition are consequences of the overwintering strategy of the plant species.     

Changes in monoamines in rat hypothalamus during cold acclimation

Action of norepinephrine (NE), serotonin (5-HT) and dopamine (DA) in the hypothalamus have been reported to play key roles in several homeostatic functions, including thermoregulation. The purpose of this study was to clarify differences in concentrations of NE, 5-HT and DA in several hypothalamic regions after cold exposure of different durations. Rats were exposed to a cold environment (5 °C) for 3 hours (3H), 1 day (1D), 7 days (7D), 14 days (14D), or 28 days (28D). After cold exposure, each hypothalamic region was immediately extracted and homogenized. NE, 5-HT and DA in the extract were measured by high-performance liquid chromatography. We observed marked differences in the concentration of NE in each hypothalamic region after cold exposures. NE in the preoptic area was high only in the 3H group, while it was elevated in the 7D, 14D and 28D groups in the ventromedial hypothalamus. On the other hand, NE in the posterior hypothalamus was low in the 3H, 1D, 7D and 14D groups. Cold exposure did not affect concentrations of 5-HT and DA in these hypothalamic regions. Our results suggest the involvement of NE in each hypothalamic region in maintenance of body temperature, and that the neuronally active site in the hypothalamus seems to change during cold acclimation.     

Accumulation of glycinebetaine during cold acclimation and freezing tolerance in leaves of winter and spring barley plants

A study was performed to examine whether or not betaine (glycinebetaine), a compatible solute, is accumulated in response to cold stress and is involved in mechanisms that protect plants from freezing injury. For this purpose, we used near-isogenic lines of barley, with each line differing only in a single gene for the spring type of growth habit; the various lines were produced by back-crosses to a recurrent cultivar of the winter type. The winter type of growth habit requires a low temperature for triggering of flower development (vernalization), whereas the spring type does not. Betaine was accumulated to five times the basal level over the course of 3 weeks at low temperature (5 °C) in the winter-type cultivar and in a spring-sh line having the sh gene for the spring-type growth habit, but the level was only doubled in the spring-Sh3 line, which carried the Sh₃ gene for the spring-type growth habit. Among near-isogenic lines of the same cultivar, the levels of betaine accumulated in leaves at low temperature were well correlated with the percentages (on a dry weight basis) of green leaves that survived freezing injury (-5 °C). This observation indicates the possibility, separate from the recognized role of betaine in the response to salinity and/or drought, that betaine accumulates in response to cold stress and that the accumulation of betaine during cold acclimation is associated to some extent with freezing tolerance in leaves of barley plants.     

Changes in the level of oxidized and reduced pyridine nucleotides during cold acclimation of winter rape plants

Prolonged cold (2°C) treatment of winter rape plants ( Brassica napus L. var. oleifera L. cv. Górczański) markedly modified the pattern of leaf growth and brought about changes in the level of pyridine nucleotides already during the first few days of treatment. The NAD⁺, NADP⁺ and NADPH levels markedly increased but there was practically no effect on the NADH level. Changes in the respective nucleotide levels were reflected by changes in anabolic and catabolic reduction charges. The former increased by 70%, whereas the latter decreased by 44%. Alterations in pyridine nucleotide levels and the reduction charges are discussed in terms of possible mechanisms involved, as well as in terms of their role in plant adaptation to cold.     

Differential expression of wheat genes during cold acclimation

Overwintering crops such as winter wheat display significant increase in freezing tolerance during a period of cold acclimation (CA). To gain better understanding of molecular mechanisms of CA, it is important to unravel functions and regulations of CA-associated genes. Differential screening of a cDNA library constructed from cold acclimated crown tissue of winter wheat identified three novel CA-associated cDNA clones. Nucleotide sequence analysis showed that the clones encode a high mobility globular protein (HMGB1), a glycine-rich RNA-binding protein (TaGRP2), and a LEA D-11 dehydrin (DHN14). Accumulation of the three mRNAs during 14 days of CA was differentially regulated. In response to drought, and ABA, DHN14 mRNA rapidly accumulated while HMGB1 and TaGRP2 mRNA levels remained unchanged. The possible functions of each of these genes in cold acclimation are discussed.     

Differential expression of wheat genes during cold acclimation

Overwintering crops such as winter wheat display a significant increase in freezing tolerance during periods of cold acclimation (CA). To gain a better understanding of the molecular mechanisms of CA, it is important to unravel the functions and regulations of CA-associated genes. Differential screening of a cDNA library constructed from cold acclimated crown tissue of winter wheat identified three novel CA-associated cDNA clones. Nucleotide sequence analysis showed that the clones encode a high mobility globular protein (HMGB1), a glycine-rich RNA-binding protein (TaGRP2), and a LEAD-11 dehydrin (DHN14). Accumulation of the three mRNAs during 14 days of CA was differentially regulated. In response to drought, and ABA, DHN14 mRNA rapidly accumulated while HMGB1 and TaGRP2 mRNA levels remained unchanged. The possible functions of each of these genes in cold acclimation are discussed. The text was submitted by the authors in English.     

Impaired chloroplast positioning affects photosynthetic capacity and regulation of the central carbohydrate metabolism during cold acclimation

Photosynthesis Research - Photosynthesis and carbohydrate metabolism of higher plants need to be tightly regulated to prevent tissue damage during environmental changes. The intracellular position...