Seedling growth, mitochondrial characteristics, and alternative respiratory capacity of corn genotypes differing in cold tolerance

Four maize (Zea mays L.) inbreds representing genetic differences in seedling cold tolerance were used to determine the effect of growth temperatures on dry weight accumulation and mitochondrial properties, especially the alternative oxidase capacity. Seedlings were grown in darkness at 30°C (constant), 14°C (constant), and 15°C for 16 hours and 8°C for 8 hours. Inbreds B73 and B49 were characterized as cold tolerant while G50 and G84 were cold sensitive. Shoot growth rate of cold-sensitive inbreds in the lower temperatures was slower relative to the tolerant inbreds. Mesocotyl tissue was particularly sensitive to low temperatures during growth after germination. There were no significant differences in relative rates of mitochondrial respiration in the cold-tolerant compared to cold-sensitive inbreds measured at 25°C. Mitochondria from all seedlings grown at all temperatures had the ability to phosphorylate as indicated by the observation of respiratory control. This result indicated that differences in low temperature growth were probably not related to mitochondrial function at low temperatures. Alternative oxidase capacity was higher in mitochondria from seedlings of all inbreds grown at 14°C compared to 30°C. Capacities in seedlings of 14°C-grown B73 and G50 were higher than in B49 and G84. Capacities in seedlings grown for 16 hours at 15°C and 8 hours at 8°C were similar to those from 14°C-grown except in G50 which was lower and similar to those grown at 30°C. Mesocotyl tissue was the most responsive tissue to low growth temperature. Coleoptile plus leaf tissue responded similarly but contained lower capacities. Antibody probing of western blots of mitochondrial proteins confirmed that differences in alternative oxidase capacities were due to differences in levels of the alternative oxidase protein. Male sterile lines of B73 were also grown under the three different temperature regimes. These lines grew equally as well as the normal B73 at all temperatures and the response of alternative oxidase capacity and protein to low growth temperature was similar to normal B73.     

Respiration and Alternative Oxidase in Corn Seedling Tissues during Germination at Different Temperatures

Respiration rates of Zea mays L. seedling tissues grown at 30 and 14°C were measured at 25°C at different stages of seedling growth. Accumulation of heat units was used to define the developmental stages to compare respiration between the two temperatures. At both temperatures, respiration rates of most tissues were highest at the youngest stages, then declined with age. Respiration rates of mesocotyl tissue were the most responsive to temperature, being nearly twofold higher when grown at 14 compared to 30°C. Alternative pathway respiration increased concomitantly with respiration and was higher in mesocotyls grown in the cold. When seedlings were started at 30 then transferred to 14°C, the increase in alternative pathway respiration due to cold was not observed unless the seedlings were transferred before 2 days of growth. Seedlings transferred to 14°C after growth at 30°C for 2 days had the same alternative oxidase capacity as seedlings grown at 30°C. Seedlings grown at 14°C for 10 to 12 days, then transferred to 30°C, lost alternative pathway respiratory capacity over a period of 2 to 3 days. Western blots of mitochondrial proteins indicated that this loss of capacity was due to a loss of the alternative oxidase protein. Some in vitro characteristics of mitochondria were determined. The temperature optimum for measurement of alternative oxidase capacity was 15 to 20°C. At 41°C, very little alternative oxidase was measured, i.e., the mitochondrial oxygen uptake was almost completely sensitive to cyanide. This inactivation at 41°C was reversible. After incubation at 41°C, the alternative oxidase capacity measured at 25°C was the similar to when it was measured at that temperature directly. Isolated mitochondria lost alternative oxidase capacity at the same rate when incubated at 41°C as they did when incubated at 25°C. Increasing the supply of electrons to isolated mitochondria increased the degree of engagement of the alternative pathway, whereas lower temperature decreased the degree of engagement. Lower temperatures did not increase the degree of engagement of the pathway in intact tissues. We interpret these observations to indicate that the greater capacity of alternative oxidase in cold-grown seedlings is a consequence of development at these low temperatures which results in elevated respiration rates. Low temperature itself does not cause greater capacity or engagement of the alternative oxidase in mitochondria that have developed under warm temperatures. Our hypothesis would be that the low growth temperatures require the seedlings to have a higher respiration rate for some reason, e.g., to prevent the accumulation of a toxic metabolite, and that the alternative pathway functions in that respiration.     

Metabolic changes associated with vernalization of wheat. I. Carbohydrate and nitrogen patterns

A spring wheat (Triticum aestivum) and an obligate winter wheat (Triticum compactum) variety were each grown for 5 weeks in controlled environments at 2° and 25°. The threshold for flower induction in the winter wheat was 4 to 5 weeks at 2°, whereas the spring wheat had no low temperature requirement for flowering. Changes in the levels of carbohydrate and nitrogen fractions in the wheat leaves were determined during their growth in the cold and warm environments. There was an enhanced accumulation of the 5 carbohydrate fractions in both wheat varieties grown at 2° compared to 25°. Highly significant differences in the levels of sucrose, oligosaccharides, and starch were found between the spring and winter varieties grown at 2°. The winter wheat seedlings grown at 2° accumulated much more of these carbohydrates than the corresponding spring wheat. The carbohydrate patterns in both varieties grown at 25° were nearly identical except for the final 2 weeks of growth.

The level of nitrogenous substances in the tissues grown at 2° was much higher than in the corresponding tissues grown at 25°. The only significant difference between the spring and winter varieties was in the soluble protein fraction. This fraction rose nearly 3-fold in the winter variety grown at 2°, whereas it remained nearly constant in the similarly grown spring wheat. Most of the changing chemical patterns observed in relation to the vernalization treatment appear to be metabolic alterations associated with low temperature rather than alterations directly related with the vernalization response.

     

Effect of growth temperature and temperature shifts on spinach leaf morphology and photosynthesis

The growth kinetics of spinach plants (Spinacia oleracea L. cv Savoy) grown at 5°C or 16°C were determined to allow us to compare leaf tissues of the same developmental stage rather than chronological age. The second leaf pairs reached full expansion at a plant age of 32 and 92 days for the 16°C and 5°C plants, respectively. Growth at 5°C resulted in an increased leaf area, dry weight, dry weight per area, and leaf thickness. Despite these changes, pigment content and composition, room temperature in vivo fluorescence, and apparent quantum yield and light-saturated rates of CO₂ exchange or O₂ evolution were not affected by the growth temperature. Furthermore, 5°C expanded leaves were found to be more resistant to photoinhibition at 5°C than were 16°C expanded leaves. Thus, it is concluded that spinach grown at low temperature is not stressed. However, shifting spinach leaves from 5°C to 16°C or from 16°C to 5°C for 12 days after full leaf expansion had occurred resulted in a 20 to 25% reduction in apparent quantum yields and 50 to 60% reduction in light saturated rates of both CO₂ exchange and O₂ evolution. This was not accompanied by a change in the pigment content or composition or in the room temperature in vivo fluorescence. It appears that leaf aging during the temperature shift period can account for the reduction in photosynthesis. Comparison of cold-hardened and non-hardened winter rye (Secale cereale L. cv Muskateer) with spinach by in vivo fluorescence indicated that rye is more sensitive to both short term and longer duration temperature shifts than is spinach. Thus, susceptibility to an abrupt temperature shift appears to be species dependent.     

Low Temperature Development Induces a Specific Decrease in trans-Delta-Hexadecenoic Acid Content which Influences LHCII Organization

Lipid and fatty acid analyses were performed on whole leaf extracts and isolated thylakoids from winter rye (Secale cereale L. cv Puma) grown at 5°C cold-hardened rye (RH) and 20°C nonhardened rye (RNH). Although no significant change in total lipid content was observed, growth at low, cold-hardening temperature resulted in a specific 67% (thylakoids) to 74% (whole leaves) decrease in the trans-Δ³-hexadecenoic acid (trans-16:1) level associated with phosphatidyldiacylglycerol (PG). Electron spin resonance and differential scanning calorimetry (DSC) indicated no significant difference in the fluidity of RH and RNH thylakoids. Separation of chlorophyll-protein complexes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the ratio of oligomeric light harvesting complex:monomeric light harvesting complex (LHCII₁:LHCII₃) was 2-fold higher in RNH than RH thylakoids. The ratio of CP1a:CP1 was also 1.5-fold higher in RNH than RH thylakoids. Analyses of winter rye grown at 20, 15, 10, and 5°C indicated that both, the trans-16:1 acid levels in PG and the LHCII₁:LHCII₃ decreased concomitantly with a decrease in growth temperature. Above 40°C, differential scanning calorimetry of RNH thylakoids indicated the presence of five major endotherms (47, 60, 67, 73, and 86°C). Although the general features of the temperature transitions observed above 40°C in RH thylakoids were similar to those observed for RNH thylakoids, the transitions at 60 and 73°C were resolved as inflections only and RH thylakoids exhibited transitions at 45 and 84°C which were 2°C lower than those observed in RNH thylakoids. Since polypeptide and lipid compositions of RH and RNH thylakoids were very similar, we suggest that these differences reflect alterations in thylakoid membrane organization. Specifically, it is suggested that low developmental temperature modulates LHCII organization such that oligomeric LHCII predominates in RNH thylakoids whereas a monomeric or an intermediate form of LHCII predominates in RH thylakoids. Furthermore, we conclude that low developmental temperature modulates LHCII organization by specifically altering the fatty composition of thylakoid PG.     

Site of Synthesis of NADPH: Protochlorophyllide Oxidoreductase in Rye (Secale cereale)

The site of synthesis of the plastid membrane-located enzyme, protochlorophyllide reductase, has been determined. Plastid ribosome-deficient and normal rye (Secale cereale L., cv Rheidol) plants were grown in darkness at 33°C and 22°C, respectively. Extracts from these plants were analyzed for the levels of different ribosomal RNAs and cytochrome f and the activity of a number of enzymes with well-established sites of synthesis. The results confirmed that the higher temperature had induced a specific inhibition of protein synthesis in the plastids. The activity and level of protochlorophyllide reductase was unaffected by growth at the higher temperature, suggesting it to be a cytoplasmically synthesized enzyme.     

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°C were fixed for immunofluorescence microscopy at these temperatures or after an additional hour at 4°C. Typical arrays of cortical microtubules were present in root-tip cells of seedlings exposed to the cold-acclimation treatment of 4°C for 2 days. Microtubules in these cold-acclimated cells were more easily depolymerized by a 0°C treatment than microtubules in root-tip cells of nonacclimated, 22°C-grown seedlings. Microtubules were still present in some cells of both nonacclimated and cold-acclimated roots at 0 and −3°C; however, the number of microtubules in these cells was lower than in controls. Microtubules remaining during the −3°C freeze were shorter than microtubules in unfrozen control cells. Repolymerization of microtubules after both the 0 and −3°C treatments occurred within 1 h. Root tips of nonacclimated seedlings had an LT-50 of −9°C. Cold acclimation lowered this value to −14°C. Treatment of 22°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°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.     

Effect of preincubation temperature on in vitro light saturated photosystem I activity in thylakoids isolated from cold hardened and nonhardened rye

Thylakoids isolated from winter rye (Secale cereale L. cv Muskateer) grown at 5°C or 20°C were compared with respect to their capacity to exhibit an increase in light saturated rates of photosystem I (PSI) electron transport (ascorbate/dichlorophenolindophenol → methylviologen) after dark preincubation at temperatures between 0 and 60°C. Thylakoids isolated in the presence or absence of Na⁺/Mg²⁺ from 20°C grown rye exhibited transient, 40 to 60% increases in light saturated rates of PSI activity at all preincubation temperatures between 5 and 60°C. This increase in PSI activity appeared to occur independently of the electron donor employed. The capacity to exhibit this in vitro induced increase in PSI activity was examined during biogenesis of rye thylakoids under intermittent light conditions at 20°C. Only after exposure to 48 cycles (1 cycle = 118 minutes dark + 2 min light) of intermittent light did rye thylakoids exhibit an increase in light saturated rates of PSI activity even though PSI activity could be detected after 24 cycles. In contrast to thylakoids from 20°C grown rye, thylakoids isolated from 5°C grown rye in the presence of Na⁺/Mg²⁺ exhibited no increase in light saturated PSI activity after preincubation at any temperature between 0 and 60°C. This was not due to damage to PSI electron transport in thylakoids isolated from 5°C grown plants since light saturated PSI activity was 60% higher in 5°C thylakoids than 20°C thylakoids prior to in vitro dark preincubation. However, a two-fold increase in light saturated PSI activity of 5°C thylakoids could be observed after dark preincubation only when 5°C thylakoids were initially isolated in the absence of Na⁺/Mg²⁺. We suggest that 5°C rye thylakoids, isolated in the presence of these cations, exhibit light saturated PSI electron transport which may be closer to the maximum rate attainable in vitro than 20°C thylakoids and hence cannot be increased further by dark preincubation.     

Induction of Freezing Tolerance in Spinach during Cold Acclimation

Spinach (Spinacia oleracea L.) seedlings, grown in soil or on an agar medium in vitro, became cold acclimated when exposed to a constant 5°C. Plants subjected to cold acclimation, beginning 1 week postgermination, attained freezing tolerance levels similar to that achieved by seedlings that were cold acclimated beginning 3 weeks after sowing. Seedlings at 1 week of age had only cotyledonary leaves, while 3-week-old seedlings had developed true leaves. Plants grown in vitro were able to increase in freezing tolerance, but were slightly less hardy than soil-grown plants. These results suggest that spinach, a cool-season crop that begins growth in early spring when subzero temperatures are likely, can undergo cold acclimation at the earliest stages of development following germination. Axenic seedlings, grown in vitro, were used to develop a noninjurious radiolabeling technique. Leaf proteins were radiolabeled to specific activities of 10⁵ counts per minute per microgram at 25°C or 5 × 10⁴ counts per minute per microgram at 5°C over a 24 hour period. The ability to radiolabel leaf proteins of in vitro grown plants to high specific activities at low temperature, without injury or microbial contamination, will facilitate studies of cold acclimation.     

Thermal Tolerance of Zymomonas mobilis: Temperature-Induced Changes in Membrane Composition

The membrane composition of Zymomonas mobilis changed dramatically in response to growth temperature. With increasing temperature, the proportion of vaccenic acid declined with an increase in myristic acid, the proportion of phosphatidylcholine and cardiolipin increased with decreases in phosphatidylethanolamine and phosphatidylglycerol, and the phospholipid/protein ratio of the membrane declined. These changes in membrane composition were correlated with changes in thermal tolerance and with changes in membrane fluidity. Cells grown at 20°C were more sensitive to inactivation at 45°C than were cells grown at 30°C, as expected. However, cells grown at 41°C (near the maximal growth temperature for Z. mobilis) were hypersensitive to thermal inactivation, suggesting that cells may be damaged during growth at this temperature. When cells were held at 45°C, soluble proteins from cells grown at 41°C were rapidly lost into the surrounding buffer in contrast to cells grown at lower temperatures. The synthesis of phospholipid-deficient membranes during growth at 41°C was proposed as being responsible for this increased thermal sensitivity.     

Fluorescence Properties Indicate that Photosystem II Reaction Centers and Light-Harvesting Complex Are Modified by Low Temperature Growth in Winter Rye

Thylakoids isolated from winter rye (Secale cereale L. cv Puma) grown at 20°C (nonhardened rye, RNH) or 5°C (cold-hardened rye, RH) were characterized using chlorophyll (Chl) fluorescence. Low temperature fluorescence emission spectra of RH thylakoids contained emission bands at 680 and 695 nanometers not present in RNH thylakoids which were interpreted as changes in the association of light-harvesting Chl a/b proteins and photosystem II (PSII) reaction centers. RH thylakoids also exhibited a decrease in the emission ratio of 742/685 nanometers relative to RNH thylakoids.

Room temperature fluorescence induction revealed that a larger proportion of Chl in RH thylakoids was inactive in transferring energy to PSII reaction centers when compared with RNH thylakoids. Fluorescence induction kinetics at 20°C indicated that RNH and RH thylakoids contained the same proportions of fast (α) and slow (β) components of the biphasic induction curve. In RH thylakoids, however, the rate constant for α components increased and the rate constant for β components decreased relative to RNH thylakoids. Thus, energy was transferred more quickly within a PSII reaction center complex in RH thylakoids. In addition, PSII reaction centers in RH thylakoids were less connected, thus reducing energy transfers between reaction center complexes. We concluded that both PSII reaction centers and light-harvesting Chl a/b proteins had been modified during development of rye chloroplasts at 5°C.

     

Effect of Temperature on In Vivo Protein Synthetic Capacity in Escherichia coli

In this report, we examine the effect of temperature on protein synthesis. The rate of protein accumulation is determined by three factors: the number of working ribosomes, the rate at which ribosomes are working, and the rate of protein degradation. Measurements of RNA/protein ratios and the levels of individual ribosomal proteins and rRNA show that the cellular amount of ribosomal machinery in Escherichia coli is constant between 25 and 37°C. Within this range, in a given medium, temperature affects ribosomal function the same as it affects overall growth. Two independent methodologies show that the peptide chain elongation rate increases as a function of temperature identically to growth rate up to 37°C. Unlike the growth rate, however, the elongation rate continues to increase up to 44°C at the same rate as between 25 and 37°C. Our results show that the peptide elongation rate is not rate limiting for growth at high temperature. Taking into consideration the number of ribosomes per unit of cell mass, there is an apparent excess of protein synthetic capacity in these cells, indicating a dramatic increase in protein degradation at high temperature. Temperature shift experiments show that peptide chain elongation rate increases immediately, which supports a mechanism of heat shock response induction in which an increase in unfolded, newly translated protein induces this response. In addition, we find that at low temperature (15°C), cells contain a pool of nontranslating ribosomes which do not contribute to cell growth, supporting the idea that there is a defect in initiation at low temperature.     

Tubulin Isotypes in Rye Roots Are Altered during Cold Acclimation

The cold stability of cortical microtubules in root-tip cells of winter rye (Secale cereale L. cv Puma) is altered by growth temperature (GP Kerr, JV Carter [1990] Plant Physiol 93:77-82). One hypothesis for the basis of this alteration is that different tubulin isotypes are present at different growth temperatures, and that the cold stability of microtubules is affected by these isotypic differences. We have explored the first part of this hypothesis by comparing protein extracts from roots of seedlings grown for 2 days at 22°C (nonacclimated) or for an additional 2 or 4 days at 4°C (cold-acclimated). Immunoblots of two-dimensional polyacrylamide gels were probed with monoclonal antibodies to α- and β-tubulin. At least six α- and seven β-tubulins were present in the extracts from both the nonacclimated and cold-acclimated roots. Changes in electrophoretic mobility and isotype number of both α- and β-tubulin were observed after only 2 days at 4°C. Further changes in tubulin were observed after 4 days at 4°C. Changes in α-tubulin were more pronounced than those in β-tubulin.     

Effect of altering the root-zone temperature on growth, translocation, carbon exchange rate, and leaf starch accumulation in the tomato

Tomato seedlings (Lycopersicon esculentum Mill. cv Vendor) were grown hydroponically with their root systems maintained at a constant temperature for a 2-week period commencing with the appearance of the first true leaf. Based on fresh and dry weight and leaf area, the optimal root-zone temperature for seedling growth was 30°C. The carbon exchange rate of the leaves was also found to increase with rising root-zone temperature up to 30°C. However, a more complex relationship seems to exist between root-zone temperature and the accumulation of ¹⁴C-labeled assimilates in the roots; inasmuch as there is no enhancement in this accumulation at the most growth promoting root-zone temperatures (22-30°C).     

Low temperature development of winter rye leaves alters the detergent solubilization of thylakoid membranes

Thylakoids isolated from leaves of winter rye (Secale cereale L. cv Puma) grown at either 20 or 5°C were extracted with the nonionic detergents Triton X-100 and octyl glucoside. Less total chlorophyll was extracted from 5°C thylakoids by these detergents under all conditions, including pretreatment with cations. Thylakoids from either 20 or 5°C leaves were solubilized in 0.7% Triton X-100 and centrifuged on sucrose gradients to purify the light harvesting complex (LHCII). Greater yields of LHCII were obtained by cation precipitation of particles derived from 20°C thylakoids than from 5°C thylakoids. When 20 and 5°C thylakoids were phosphorylated and completely solubilized in sodium dodecyl sulfate, no differences were observed in the ³²Pi-labeling characteristics of the membrane polypeptides. However, when phosphorylated thylakoids were extracted with octyl glucoside, extraction of LHCII associated with the 5°C thylakoids was markedly reduced in comparison with the extraction of LHCII from 20°C membranes. Since 20 and 5°C thylakoids exhibited significant differences in the Chl content and Chl a/b ratios of membrane fractions produced after solubilization with either Triton X-100 or octyl glucoside, and since few differences between the proteins of the two membranes could be observed following complete denaturation in sodium dodecyl sulfate, we conclude that the integral structure of the thylakoid membrane is affected during rye leaf development at low temperature.     

The Effect of Growth and Measurement Temperature on the Activity of the Alternative Respiratory Pathway

A postulated role of the CN-resistant alternative respiratory pathway in plants is the maintenance of mitochondrial electron transport at low temperatures that would otherwise inhibit the main phosphorylating pathway and prevent the formation of toxic reactive oxygen species. This role is supported by the observation that alternative oxidase protein levels often increase when plants are subjected to growth at low temperatures. We used oxygen isotope fractionation to measure the distribution of electrons between the main and alternative pathways in mung bean (Vigna radiata) and soybean (Glycine max) following growth at low temperature. The amount of alternative oxidase protein in mung bean grown at 19°C increased over 2-fold in both hypocotyls and leaves compared with plants grown at 28°C but was unchanged in soybean cotyledons grown at 14°C compared with plants grown at 28°C. When the short-term response of tissue respiration was measured over the temperature range of 35°C to 9°C, decreases in the activities of both main and alternative pathway respiration were observed regardless of the growth temperature, and the relative partitioning of electrons to the alternative pathway generally decreased as the temperature was lowered. However, cold-grown mung bean plants that up-regulated the level of alternative oxidase protein maintained a greater electron partitioning to the alternative oxidase when measured at temperatures below 19°C supporting a role for the alternative pathway in response to low temperatures in mung bean. This response was not observed in soybean cotyledons, in which high levels of alternative pathway activity were seen at both high and low temperatures.     

Control of Lignin Peroxidase Production by Phanerochaete chrysosporium INA-12 by Temperature Shifting

There are two temperature optima connected with lignin peroxidase synthesis by Phanerochaete chrysosporium INA-12. One, at 37°C, is for the mycelium-growing phase; the other, at 30°C, is for the lignin peroxidase-producing phase. One of six extracellular proteins with ligninase activity increased when cultures were grown at 30°C for the entire fermentation period or when cultures were grown at 37°C for the first 2 days of incubation and then shifted to 30°C, compared with the activity of control cultures grown at 37°C for the entire fermentation period. The unsaturation of fatty acid (Δ/mole) of P. chrysosporium INA-12 mycelium decreased from 1.25 to 1.03 when the growth temperature was shifted from 20 to 40°C.     

Response of Lactobacillus helveticus PR4 to Heat Stress during Propagation in Cheese Whey with a Gradient of Decreasing Temperatures

The heat stress response was studied in Lactobacillus helveticus PR4 during propagation in cheese whey with a gradient of naturally decreasing temperature (55 to 20°C). Growth under a gradient of decreasing temperature was compared to growth at a constant temperature of 42°C. Proteinase, peptidase, and acidification activities of L. helveticus PR4 were found to be higher in cells harvested when 40°C was reached by a gradient of decreasing temperature than in cells grown at constant temperature of 42°C. When cells grown under a temperature gradient were harvested after an initial exposure of 35 min to 55°C followed by decreases in temperature to 40 (3 h), 30 (5 h 30 min), or 20°C (13 h 30 min) and were then compared with cells grown for the same time at a constant temperature of 42°C, a frequently transient induction of the levels of expression of 48 proteins was found by two-dimensional electrophoresis analysis. Expression of most of these proteins increased following cooling from 55 to 40°C (3 h). Sixteen of these proteins were subjected to N-terminal and matrix-assisted laser desorption ionization-time of flight mass spectrometry analyses. They were identified as stress proteins (e.g., DnaK and GroEL), glycolysis-related machinery (e.g., enolase and glyceraldehyde-3-phosphate dehydrogenase), and other regulatory proteins or factors (e.g., DNA-binding protein II and ATP-dependent protease). Most of these proteins have been found to play a role in the mechanisms of heat stress adaptation in other bacteria.     

Adaptive Potential of Wheat Ribosomes toward Heat Depends on the Large Ribosomal Subunit and Ribosomal Protein Phosphorylation

In a study of the translational efficiency of ribosomal subunits as a function of an in vivo temperature pretreatment, ribosomes were isolated from heat-pretreated (36°C) and reference (20°C) wheat seedlings (Triticum aestivum L.). The efficiency of recombined subunits in translating polyuridylic acid was assessed. A threefold increase in the rate of incorporation of phenylalanine by ribosomes from heat-pretreated plants was due to the large ribosomal subunit. This adaptive temperature effect was not correlated with a higher thermal stability of ribosomes or subunits from heat-pretreated seedlings, and two-dimensional gel electrophoresis failed to detect structural alterations of ribosomal proteins. Phosphorylation of ribosomal proteins in vitro showed no differences between ribosomes or subunits from heat-pretreated and reference plants. Incubation with [³²P]orthophosphate in vivo led to twice the amount of phosphate in ribosomal proteins from heat-pretreated wheat seedlings. This result is important with respect to the evaluation of the molecular basis of enhanced translational efficiency of ribosomes isolated from heat-pretreated wheat seedlings.