ABSTRACrThe effect of ozone was studied on the peroxidase activity from various compartments of Sedam album leaves (epidermis, intercellular fluid, residual cell material, and total cell material). The greatest increase following a 2-hour ozone exposure (OA microliters 03 per liter) was observed in extracellular peroxidases. Most of the main bands of peroxidase activity separated by isoelectric focusing exhibited an increase upon exposure to ozone. Incubation experiments with isolated peeled or unpeeled leaves showed that leaves from ozone-treated plants release much more peroxidases in the medium than untreated leaves. The withdrawal of Ca" ions reduced the level of extracellular peroxidase activity either in whole plants or in incubation experiments. This reduction and the activation obtained after addition of Ca2" resulted from a direct requirement of Ca2" by the enzyme and from an effect of Ca"2 on peroxidase secretion. The ionophore A23187 promoted an increase of extracellular peroxidase activity only in untreated plants. The release of peroxidases by untreated and ozone-treated leaves is considerably lowered by metabolic inhibitors (3-(3,4-dichlorophenyl)-1,1-dimethylurea and sodium azide) and by puromycin.Peroxidase activity increases in plants in response to a great variety ofstresses, including viral, microbial, or fungal infections, salt stress, wounding, or air pollution (9). Several pollutants such as ozone (5,6,22), SO2 (14, 16), or NO2 (13) are known to induce an enhancement ofthe total peroxidase activity of plants. The peroxidase increase following an exposure to ozone is different in different species and is a function of the resistance of the plant to ozone. Ozone-tolerant and ozone-sensitive cultivars have been described, the peroxidase activity of the former being less affected by ozone (5).In a previous work (4), it was shown that there is a parallelism between the level of air pollution and the peroxidase activity measured in Sedum album leaves. This was demonstrated in plants grown in diversely polluted areas. It appeared that S. album is a suitable plant material for the study ofthe mechanism leading plants exposed to a pollutant to increase their peroxidase activity.The present work was performed in the laboratory, under controlled environmental conditions and using standardized amounts of ozone as air pollutant. The dose applied to S. album (0.4 ,ul 1-' for 2 h) is known to increase the total peroxidase activity in many plant species (21,22). Peroxidase activity was measured in several leaf compartments. The data obtained showed that after an exposure to ozone the most significant increase of peroxidase activity occurred in the extracellular compartment. As the release of peroxidase was reported to be a calcium-dependent process in spinach cell suspensions (18), the possible involvement ofthis ion in the control ofthe extracellular peroxidases of S. album leaves was also investigated in relation with the response to ozone.
MATERIALS AND METHODSPlant Material and Growing Conditions. Sedu...
A generalized two‐step and interdependent control of basic and acidic peroxidases (EC 1.11.1.7) is observed in plant responses to different physical and chemical stimuli. An interpretative model consisting of a pathway of reactions is presented on the basis of our own data and the literature. Stress‐induced membrane depolarization would generate different species of free radicals and peroxides, which in turn initiate lipid peroxidation. The degradation of cell membranes is suggested to bring about rapid changes in ionic fluxes (especially release of K+ which would result in an enhanced endogenous Ca/K ratio) and in leakage of solutes (among them electron donors such as ascorbic acid and phenolic substances). The increased intracellular relative calcium level results in: 1) activated secretion of basic peroxidases into the free space where, in association with the electron donors and maybe with the circulating indole‐3‐acetic acid (IAA), they eliminate the peroxides; and 2) facilitated binding of basic peroxidases to membrane structures allowing a role as 1‐aminocyclopropane‐1‐carboxylic acid (ACC)‐oxidases. The resulting IAA and ACC oxidase‐mediated changes in ethylene production would further induce (this time through the protein synthesis machinery) an increase in activity of phenylalanine ammonia‐lyase (EC 4.31.5) and acidic peroxidases. The resulting lignification and cell wall rigidification determines the growth and/or the developmental response to the initial stress.
Concurrent resection of hepatic and EHD in well-selected patients may provide the possibility of long-term survival. The risk of recurrence, however, remains high, and a worse outcome is associated with both number of metastases and location of EHD.
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