PLANT ADAPTATION TO ANTHROPOGENIC FACTORS - 2017

4. THE EFFECT OF IONIZING RADIATION ON PLANT ORGANISMS

4.5. Modification of Radiobiological Effects and Plant Protection against Radiation Injury

Structure/149.html">The problem of modifying radiation injury occupies a central place in modern radiobiology, as its resolution contributes to uncovering The Nature of organismal Radioresistance and leads to The Development of Methods for protecting Cells and Multicellular Organisms from the damaging effects of ionizing radiation.

Radiation injury in plants, originating from radiation-chemical processes, is not necessarily restricted to THE CELLULAR LEVEL; it involves numerous cells within formative Tissues, altering both morphogenesis and METABOLISM. At each level of radiation response development, appropriate treatments can alter the severity of the constituent radiation damage, ultimately modifying the final integral reaction of the Organism.

Classification of factors modifying the radiation effect. There are various levels of action for factors that modify the radiation effect, and their nature, as well as Mechanisms of action, can be quite diverse. The term "modification of radiation injury" encompasses all phenomena that influence the development of a plant's radiation response. In a narrower yet frequently used sense, it refers to the Modification of the radioresistance of a Cell or organism, whereby a specific Treatment alters their physiological state, upon which the manifestation of primary radiation damage depends.

The variety of factors modifying the radiobiological effect in plants can be classified According to the following criteria: the level or object of modification, the yields of primary radiation-Chemical Reactions, the sign of modification (enhancement or mitigation of radiation injury), the timing of the most effective treatment (pre-irradiation or post-irradiation), and the Nature of the modifying factor (chemical compounds, radiation and non-radiation factors such as light, Temperature, etc.). Hence, concepts such as chemical modification, prophylactic and therapeutic treatments, and post-irradiation modification naturally arise.

Based on the levels of modification, one can distinguish the modification of individual radiation-chemical reactions, DNA damage, membranes, and other cellular structures, followed by the repopulation and regenerative recovery of The Cell.

If modification results in A change in radiosensitivity, the latter is assessed by cell survival rates. According to the sign of action, one distinguishes between radioresistance modification that either enhances or mitigates radiation injury. When radioresistance increases, it is referred to as radioprotection. Factors that increase the organism's radioresistance are called radioprotectors. The term radiosensitization denotes a modification of radioresistance in which the latter is weakened. Factors that increase radiosensitivity are termed radiosensitizers. In some studies, radioprotectors and radiosensitizers are considered independent classes of factors not classified as modifiers of radiation injury.

Depending on the timing of application, a distinction is made between prophylactic treatments, which are effective prior to irradiation, and therapeutic treatments, which are effective after irradiation.

Modifiers can be chemical substances or physical factors. Chemical modifiers can interact directly with the products of radiation-Chemical transformations of substances or affect other cellular systems, altering their behavior during radiation injury. Radioprotectors and radiosensitizers typically interact directly with the products of radiation-chemical reactions, free radicals, and ions. Physical factors can also directly influence the course of radiation-chemical reactions or indirectly affect the physiological state of cells or the organism as a whole, thereby determining the nature of the development of radiation injury.

Finally, a distinction should be made between modifiers that influence the development of radiation injury and those affecting The activity of the cell's repair systems, as well as the intensity of repopulation and regeneration.

Plant protection is associated with the action of radioprotectors and modifiers that mitigate the manifestation of radiation injury. Therefore, when discussing the classification of plant protection factors against radiation injury, one can adopt the classification proposed for modifiers.

Oxygen as a modifier of radiation injury. One of the most effective modifiers that enhance radiation injury across all organisms is oxygen. The development of radiation-chemical reactions during the irradiation of many compounds—both in solutions and in the form of powders, films, etc.—depends significantly on its presence in the reaction medium. This effect is driven by the interaction of oxygen with free radicals induced in molecules by irradiation. In complex biological systems, the action of oxygen on the yield of macromolecular radiation damage dictates the development of radiobiological effects. As a rule, The Effect of oxygen consists in the enhancement of radiation injury, a phenomenon known as the oxygen effect. The oxygen effect manifests in the reactions of radiation inactivation of Enzymes, damage to macromolecules, living cells, and tissues. It is observed when biological objects are irradiated in an atmosphere containing oxygen.

The oxygen effect is caused by interaction with free radicals R, leading to The formation of peroxy radicals ROO^-. In Water, oxygen reacts with water radiolysis products to generate a series of highly active radicals. However, the oxygen effect is also observed during the irradiation of anhydrous objects. In the latter case, its direct participation in these reactions apparently influences the course of radiation-chemical processes.

Chemical modification of radiation injury. Chemical modification of radiation injury is caused by the action of various chemical compounds which, by participating in radiation-chemical reactions or influencing the physicochemical state of cells, alter the manifestation of the radiobiological effect. Naturally, chemical modifiers at concentrations affecting radiobiological effects must not be toxic to cells. Substances that modify radioresistance are of the greatest interest, since it is generally simpler to prevent the development of radiation injury altogether than to remedy the consequences of damage to biologically vital macromolecules and cellular structures. Likewise, if it is necessary to enhance radiation injury, it is easier to influence the yields of primary molecular lesions than to subsequently counteract massive reactions of post-irradiation recovery.

The theoretical basis for the modification of radiation effects and the radioresistance of cells and multicellular organisms is a profound understanding of the mechanisms underlying The Diversity of radioresistance levels.

The well-known Belgian radiobiologists Z. Bacq and A. Alexander proposed the "biochemical Shock" hypothesis. "Biological shock" refers to a severe disruption of organismal Functions caused by the action of a modifying factor. If this "shock" halts DNA and RNA Synthesis—for example, due to the impairment of nuclear energy phosphorylation or other causes—the radioresistance of the cell may increase because the "biochemical Amplification" of radiation injury is blocked, allowing the repair of potentially lethal cell damage to manifest more fully.

The sulfhydryl hypothesis postulates the existence of a universal mechanism for the radiomodifying effect. According to this hypothesis, the modification of radiosensitivity is caused by Changes in the intracellular content of endogenous thiols, which react at the Initial Stages of radiation injury with radicals generated during irradiation. This hypothesis is supported experimentally by research demonstrating a close correlation between the RADIORESISTANCE OF ORGANISMS and the content of native thiols in cells. The hypothesis is further corroborated by evidence regarding the radioprotective properties of sulfhydryl-containing compounds and radioprotectors whose action is mediated by the intensification of endogenous thiol synthesis.

The hypothesis of the endogenous Background of radioresistance was formulated by Yu. B. Kudryashov and E. N. Goncharenko, who linked the radioresistance of organisms to the presence within cells of substances that influence the development of primary radiation reactions. In addition to thiols, the substances constituting the endogenous background of radioresistance include histamine, serotonin, dopamine, adrenaline, and noradrenaline, which act as agents inhibiting the development of primary radio-biochemical processes. Lipid Peroxidation products, hydroperoxides, and peroxides of higher Unsaturated Fatty acids act as factors that enhance the development of radiation damage. The modification of radioresistance is determined by influencing the levels of these endogenous radioprotective and radiosensitizing substances.

The membrane hypothesis attributes cell death to radiation damage of membranes. Disruption of the outer membrane (Plasmalemma) and the membranes of intracellular structures (Nucleus, Plastids) is accompanied by an increase in their permeability, leading to impairments in the intracellular Organization of metabolism and the release of enzymes from subcellular structures. This hypothesis was developed by B. M. Tarusov on The basis of the high radiosensitivity of membranes caused by lipid peroxidation chain reactions.

Another hypothesis is based on the assumption that during the post-irradiation period, a discoordination of individual Stages of DNA repair may occur, such as excessive DNA Hydrolysis under The Influence of endonucleases and exonucleases. In this case, modification may consist of correcting the repair process to align with normal repair dynamics.

Such a significant number of hypotheses regarding the nature of radioresistance and its modification indicates, on the one hand, the persistent experimental difficulties in investigating modification mechanisms and, on the other hand, the obvious multiplicity of these mechanisms.

Radioprotective effects. Experiments with plants have investigated the action of numerous substances exhibiting radioprotective properties with respect to microorganisms, animal cells, and human cells. Among such compounds, organic molecules containing a sulfhydryl group are the most effective. Among these substances, aminoethylisothiouronium (AET), British Anti-Lewisite (BAL), dithiopropanol, Glutathione, thiourea, cystamine, cysteamine ($eta$-mercaptoethylamine), Cysteine, and cystine exhibit radioprotective effects on plants.

The action of sulfhydryl compounds on plants was discovered as early as the early 1950s in experiments on onion roots treated with cysteine during irradiation. The radioprotective effect of glutathione is manifested in the yield of Chromosomal aberrations in Tradescantia ROOT apices, with its most effective concentration being $3 \cdot 10^{-4}$ M. Cysteine and thiourea also exhibit a strong protective effect in this plant species. For instance, the yield of chromosomal aberrations in the Cells of the root apical meristem decreased by 60% upon treatment with glutathione.

The radioprotective effect depends on the timing of radioprotector administration relative to irradiation. The strongest radioprotective action is observed when the radioprotector is present inside the cell during its irradiation. At the same time, sulfhydryl compounds exhibit not only prophylactic but also therapeutic radioprotective effects. The radioprotective effect is dose-dependent. The radioprotective action of sulfhydryl compounds is undoubtedly linked to their participation in radiation-Chemical Reactions Involving oxygen, which explains the absence of radioprotective effects for these compounds when irradiation is carried out in an oxygen-free environment. Therefore, there is good reason to associate the radioprotective effect of sulfhydryl compounds with a reduction in the oxygen enhancement ratio.

In general terms, the radioprotective effect can be described by the following reactions:

— Reduction reaction

— unimolecular reaction leading to non-reparable damage.

Here, M represents the target molecule; R-SH is the radioprotector; N is the irreversibly damaged molecule.

The free radical scavenging mechanism is fully supported by research on model systems, in which sulfhydryl compounds exert a protective effect by decreasing the concentration of free radicals of molecules.

Mechanisms of radioprotective action of sulfhydryl compounds other than free radical scavenging cannot be excluded. A hypothesis is considered according to which sulfhydryl compounds, by binding to alkaline and acidic membrane-bound Phosphatases, prevent their release during irradiation, thereby preventing the DNA hydrolysis that typically accompanies high-dose organism irradiation.

Other reducing agents also exhibit radioprotective activity: sodium hyposulfite, metabisulfite, and hydrosulfite. The radioprotective action of ascorbic acid and other reducing agents is evidently due to several mechanisms: direct participation as electron and proton Donors in radiation-chemical reactions, influence on the level of endogenous thiols, and through "biochemical shock." Antioxidants exhibit pronounced radioprotective properties, which is most likely due to the slowing down of The oxidation of respective products by molecular oxygen or the suppression of lipid autoxidation chain reactions. Effective radioprotector antioxidants include carotene, esters of gallic acid (gallates), and tocopherols.

Among radioprotectors, mitotic poisons and mutagens can be found. It would seem that these substances enhance radiation damage, as their action generally resembles effects characteristic of the radiation response. However, under certain conditions, these substances exert a radioprotective effect caused by the slowing down of cell progression through the mitotic cycle and the induction of repair processes. Therefore, the manifestation of radioprotective properties in A number of compounds is combined with their mutagenicity or antimutagenicity.

Natural radioprotective substances in plants. Many substances with radioprotective properties are present in plants as natural components of their biochemical systems. These substances can function as native radioprotectors or impart radioprotective properties to the plant Organs that contain them. Indeed, free Amino Acids in plants include cysteine, cystine, and Methionine; among Peptides is glutathione; and they also contain certain alcohols, Lipoic Acid, free NUCLEOTIDES which also exhibit radioprotective action, nucleotide triphosphates, ascorbic acid, and Other Compounds. Consequently, extracts from certain plants exhibit radioprotective properties. Yeast extract quite noticeably increases the radioresistance of root meristem cells, as it contains numerous substances with radioprotective characteristics: glutathione, certain aromatic amino acids, uracil, guanine, and other precursors of Nucleic Acids and ATP.

Radioprotective properties have also been detected in tobacco leaf extracts. An Histology/2.html">EXTRACT FROM THE radioresistant alga Plectonema boryanum provides substantial protection against radiation damage, as demonstrated in isolated plant roots. Aqueous extracts from radioresistant seeds of plants belonging to the cabbage family (Brassicaceae) exhibited radioprotective effects on irradiated broad bean seedlings. One of the Factors Determining the high radioresistance of cruciferous plants is believed to be the presence of sinaptin in their tissues (Fig. 16). This substance is widely distributed among Representatives of the cabbage family, where its content reaches 1% and higher.

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Fig. 16 - Natural radiosensitizer/radioprotector sinaptin

The radioprotective action of sinaptin is manifested in the localization of the electronically excited state on the radioprotector. Evidently, The Study of radioresistant plants may reveal novel radioprotectors.

It is possible that a certain role in the radioprotective effects exhibited by plant tissue extracts is due to the presence of certain enzymes (catalase, peroxidase), antioxidants, and some Metal Ions. Naturally, compounds that induce a hypoxic state in cells can also play a radioprotective role. Leghemoglobin, for example, possesses such properties. The radioprotective action of phytohemagglutinin has also been revealed. A number of metabolic Cofactors, by facilitating various biosynthetic processes, positively influence DNA Repair or increase the concentration of native thiols.

Metal ions as modifiers of radiation damage. Distinct modification that weakens the manifestation of the radiation response in plants is exhibited by ions of a number of metals. When iron (II) and (III), manganese (II), nickel, sodium, potassium, strontium, magnesium, and calcium salts are introduced into plant seeds during soaking prior to irradiation, there is an attenuation of gamma-ray-induced Inhibition of Growth and morphogenesis in pea seedlings. Concentrations of these metal chlorides ranging from 0.01 to 0.0001 M proved sufficient for the radioprotective effect to manifest. Metal ions exert an effect that, although weaker than that of cysteamine, is nevertheless quite noticeable. Furthermore, the effect was more pronounced when seeds were treated prior to irradiation; as the time interval between seed treatment and irradiation increased, the effect diminished.

Radiomodifying action of phytohormones. Phytohormones and their artificial analogues are distinguished by high radiomodifying activity. The action of phytohormones on irradiated plants should be viewed from two aspects. First, under the influence of irradiation, the content of endogenous growth regulators in the plant may decrease, and The addition of exogenous phytohormones to the medium compensates for the damage inflicted by irradiation, restoring the phytohormonal status and thereby facilitating post-radiation recovery through repopulation and regeneration. In addition, exogenous phytohormones alter Correlative Growth relationships within the plant, which also reinforces tissue post-radiation recovery processes.

Second, when phytohormones act prior to irradiation, the plant's radioresistance is weakened due to the activation of Meristems. Thus, depending on the timing of phytohormone action and the nature of the plant's response, both positive and negative modification of radiobiological effects can be observed.

Consequently, the action of phytohormones on the manifestation of the radiobiological response in plants can be broken down into the following components:

1) a component associated with The chemical properties of phytohormone molecules, through which they can exhibit radioprotective or radiosensitizing action;

2) a component caused by the indirect action of phytohormones on Chromatin status and the accumulation of native radioprotectors;

3) a component reflecting the direct phytohormonal Influence of the compound, mediated by changes in Cell Cycle structures within meristems.

Questions for independent study and self-control

1. Provide a characterization of radioactive emissions. Which of these radiations are non-corpuscular, and which are corpuscular?

2. What quantitative characteristics of ionizing radiation do you know? Provide their description.

3. Characterize the effect of ionizing radiation on plants.

4. What are the immediate and long-term effects of radiation? At which levels of the organism do they manifest?

5. Describe the modifying factors of radiation action.

6. Which of the radiation modifiers can enhance plant damage? Provide Examples.

7. Characterize the radiation modifiers that reduce radiation effects.

8. Characterize natural radiation modifiers.