Membrane properties of isolated winter wheat cells in relation to icing stress

A loss in the plasma membrane ATPase activity and its recovery coincides with incipient freeze-thaw injury and postthaw recovery in onion bulb scale tissue

Plasma membrane ATPase has been proposed to be functionally altered during early stages of injury caused by a freeze-thaw stress. Complete recovery from freezing injury in onion cells during the postthaw period provided evidence in support of this proposal. During recovery, a simultaneous decrease in ion leakage and disappearance of water soaking (symptoms of freeze-thaw injury) has been noted. Since reabsorption of ions during recovery must be an active process, recovery of plasma membrane ATPase (active transport system) functions has been implicated. In the present study, onion (Allium cepa L. cv Downing Yellow Globe) bulbs were subjected to a freeze-thaw stress which resulted in a reversible (recoverable) injury. Plasma membrane ATPase activity in the microsomes (isolated from the bulb scales) and ion leakage rate (efflux/hour) from the same scale tissue were measured immediately following thawing and after complete recovery. In injured tissue (30-40% water soaking), plasma membrane ATPase activity was reduced by about 30% and this was paralleled by about 25% higher ion leakage rate. As water soaking disappeared during recovery, the plasma membrane ATPase activity and the ion leakage rate returned to about the same level as the respective controls. Treatment of freeze-thaw injured tissue with vanadate, a specific inhibitor of plasma membrane ATPase, during postthaw prevented the recovery process. These results indicate that recovery of freeze-injured tissue depends on the functional activity of plasma membrane ATPase.

Plasma Membrane ATPase Activity following Reversible and Irreversible Freezing Injury

Plasma membrane ATPase has been proposed as a site of functional alteration during early stages of freezing injury. To test this, plasma membrane was purified from Solanum leaflets by a single step partitioning of microsomes in a dextran-polyethylene glycol two phase system. Addition of lysolecithin in the ATPase assay produced up to 10-fold increase in ATPase activity. ATPase activity was specific for ATP with a K(m) around 0.4 millimolar. Presence of the ATPase enzyme was identified by immunoblotting with oat ATPase antibodies. Using the phase partitioning method, plasma membrane was isolated from Solanum commersonii leaflets which had four different degrees of freezing damage, namely, slight (reversible), partial (partially reversible), substantial and total (irreversible). With slight (reversible) damage the plasma membrane ATPase specific activity increased 1.5- to 2-fold and its K(m) was decreased by about 3-fold, whereas the specific activity of cytochrome c reductase and cytochrome c oxidase in the microsomes were not different from the control. However, with substantial (lethal, irreversible) damage, there was a loss of membrane protein, decrease in plasma membrane ATPase specific activity and decrease in K(m), while cytochrome c oxidase and cytochrome c reductase were unaffected. These results support the hypothesis that plasma membrane ATPase is altered by slight freeze-thaw stress.

Ice-Encasement Injury to Microsomal Membranes Isolated from Winter Wheat Crowns : II. Changes in Membrane Lipids during Ice Encasement

The physical properties and chemical composition of microsomal membranes were examined during a 7 day period of ice encasement in crown tissue of winter wheat (Triticum aestivum L. cv Norstar). Membrane damage, detected as an increase in microviscosity and electrolyte leakage, began between 1 and 3 days of icing, and was associated with a reduction in the recovery of microsomal membranes from stressed tissue, an increase in the microsomal free fatty acid:total fatty acid ratio, and a decrease in the phospholipid:total fatty acid ratio. These trends were amplified between 3 and 7 days of ice encasement. Examination of the free and total fatty acid fractions showed there was a slight, but not statistically significant (P = 0.05) reduction in the degree of unsaturation of the total fatty acid fraction. The composition of the free and total fatty acid fractions were very similar during ice encasement. Furthermore, analysis of phospholipid classes revealed no significant change in the relative amounts of phosphatidylcholine, phosphatidylethanolamine, phosphatidic acid, or lysophospholipids in microsomal membranes during icing. Membrane injury during ice encasement apparently involves hydrolysis of the ester bond between glycerol and the acyl groups of the phospholipid resulting in loss of the phosphate-containing polar head group and a concomitant accumulation of free fatty acids in the bilayer.

Ice Encasement Injury to Microsomal Membranes from Winter Wheat Crowns : I. Comparison of Membrane Properties after Lethal Ice Encasement and during a Post-Thaw Period

The functional and physical properties of cellular membranes isolated from Triticum aestivum, cvs Norstar and Fredrick, were altered coincident with changes in composition after a lethal ice-encasement stress and further during a 6 hour post-thaw period. Crowns encased in ice for a duration which inhibited regrowth, exhibited enhanced rates of electrolyte leakage. Furthermore, the recovery of total microsomal protein and phospholipid declined, suggesting that some membrane degradation had been induced during the anoxic stress. The microviscosity of microsomes and liposomes prepared from such membranes increased during stress, and this was correlated with a 2- to 4-fold increase in the free fatty acid levels in the microsomal fraction. There was, however, only a relatively minor change in fatty acid unsaturation during the ice-encasement stress. The process continued during a 6 hour aerobic post-thaw treatment, but the pattern was somewhat different. During this phase, the leakage of electrolytes was further increased and the recovery of microsomal protein and phospholipid continued to decline, indicating general degradation; but, in contrast to the anoxic phase, the degree of fatty acid unsaturation declined markedly, indicating lipid peroxidation.

Changes in adenine nucleotides and energy charge in isolated winter wheat cells during low temperature stress

Adenylate energy charge (AEC) and adenine nucleotide levels of isolated winter wheat (Triticum aestivum L. cv Kharkov 22 MC) cells exposed to various low temperature stresses were determined. During ice encasement at -1 degrees C, nucleotide levels decreased gradually in approximate relation to a decline in cell viability. AEC values remained high even after 5 weeks of icing when cell viability was severely reduced. When isolated cell suspensions were exposed to various cooling and freezing regimes ranging from -10 to -30 degrees C, cell damage was dependent on the minimum temperature imposed and the duration of exposure to the freezing stress. The levels of all three adenine nucleotides declined with increasing severity of the imposed stress, but AEC values remained high even at -30 degrees C when nearly all of the cells were killed. The addition of 10 millimolar Ca(2+) to cell suspensions enhanced survival during low temperature stresses, but did not influence nucleotide levels other than through its effect on cell viability. These results indicate that impairment of the ion transport system during the early stages of ice encasement prior to a detectable decline in cell viability cannot be attributed to changes in the adenylate energy charge system of the cell.

Effect of low temperature and calcium on survival and membrane properties of isolated winter wheat cells

Isolated cells obtained by enzymic digestion of young primary leaves of cold-hardened, dark-grown Kharkov winter wheat (Triticum aestivum L.) were exposed to various low temperature stresses. The initial uptake of (86)Rb was generally decreased by increasing concentrations of Ca(2+), but after longer periods of incubation, the inhibiting effect of high Ca(2+) levels diminished. Viability of isolated cells suspended in water declined rapidly when ice encased at -1 degrees C, while in the presence of 10 millimolar Ca(2+) viability declined only gradually over a 5-week period. Ice encasement markedly reduced (86)Rb uptake prior to a significant decline in cell viability or increased ion efflux. Cell damage increased progressively when the icing temperature was reduced from -1 to -2 and -3 degrees C, but the presence of Ca(2+) in the suspending medium reduced injury. Cell viability and ion uptake were reduced to a greater extent following slow cooling than after rapid cooling to subfreezing temperatures ranging from -10 to -30 degrees C. The results from this study support the view that an early change in cellular properties due to prolonged ice encasement at -1 degrees C involves the ion transport system, whereas cooling to lower subfreezing temperatures for only a few hours results in more general membrane damage, including loss of semipermeability of the plasma membrane.

Physiological and metabolic responses of winter wheat to prolonged freezing stress

Survival and cold hardiness declined gradually when cold-hardened Fredrick winter wheat (Triticum aestivum L.) was maintained at -6 degrees C for several weeks. Moisture content of crown and root tissue did not change significantly during this period. Uptake of O(2) and accumulation of (86)Rb by root tissue declined abruptly upon exposure to -6 degrees C, whereas a concomitant negative effect of freezing on these metabolic processes was not observed in crown tissue. Electron spin resonance spectroscopic analysis of microsomal membrane preparations from crown tissue revealed no evidence of gross changes in the physical properties of the bulk lipids even when seedlings were killed. The results provide biochemical evidence that seedling damage due to prolonged exposure to a mild freezing stress is due to disruption of key metabolic process in the root while cells within the crown remain viable.