Lichens are extremophilic symbiotic associations possessing phenomenal resistance to abiotic stress-factors. In this regard, melanization of thalli in response to UV is one of the mechanisms protecting lichens from excessive insolation. However, microstructure and biochemical properties of the melanized thalli are still poorly investigated. In the present study, morphological, nanomechanical, and physiological, and biochemical traits of naturally melanized thalli of the Cetraria islandica (L.) Ach. lichen were examined. In the upper cortex of its thallus, the nature of the pigment layer was verified using typical qualitative reactions for melanins. It was found that melanization leads to changes in microstructure of the upper cortex of the mycobiont, in particular, thickening of the cell walls and extension of the interhyphal space. The melanized and pale (nonmelanized) thalli were found to differ from each other in their nanomechanical properties, including the parameters of adhesion and rigidity. This implies the possible formation of complex associates of melanin with cell wall components in the melanized mycobiont. In addition, higher antioxidant activity and lower respiratory activity were found in the melanized thalli of C. islandica in comparison with the pale thalli. Presumably, the found modifications in the microstructure and nanomechanical, physiological, and biochemical properties of thalli occurring in the course of melanization make lichens more resistant to intense insolation.

Currently, due to the increasing impact of anthropogenic factors and changes in solar activity, the temperature on Earth is rising, posing a threat to biodiversity. Lichens are among the most sensitive organisms to climate change. Elevated ambient temperatures can have a significant impact on lichens, resulting in more frequent and intense drying events that can impede metabolic activity. It has been suggested that the possession of a diverse sterol composition may contribute to the tolerance of lichens to adverse temperatures and other biotic and abiotic stresses. The major sterol found in lichens is ergosterol (ERG); however, the regulation of the ERG biosynthetic pathway, specifically the step of epoxidation of squalene to 2,3-oxidosqualene catalyzed by squalene epoxidase during stress, has not been extensively studied. In this study, we used lichen Lobaria pulmonaria as a model species that is well known to be sensitive to air pollution and habitat loss. Using in silico analysis, we identified cDNAs encoding squalene epoxidase from L. pulmonaria, designating them as LpSQE1 for the mycobiont and SrSQE1 for the photobiont Symbiochloris reticulata. Our results showed that compared with a control kept at room temperature (+20 °C), mild temperatures (+4 °C and +30 °C) did not affect the physiology of L. pulmonaria, assessed by changes in membrane integrity, respiration rates, and PSII activity. An extreme negative temperature (−20 °C) noticeably inhibited respiration but did not affect membrane stability. In contrast, treating lichen with a high positive temperature (+40 °C) significantly reduced all physiological parameters. Quantitative PCR analysis revealed that exposing thalli to −20 °C, +4 °C, +30 °C, and +40 °C stimulated the expression levels of LpSQE1 and SrSQE1 and led to a significant upregulation of Hsps. These data provide new information regarding the roles of sterols and Hsps in the response of lichens to climate change.

Currently, special attention is paid to the study of the mechanisms of stress resistance of extremophile organisms that can survive in extreme conditions. Such organisms include lichens, which are symbiotic associations of fungi and algae and/or cyanobacteria. The high stress resistance of lichens is due to the presence of a wide range of biologically active metabolites, including sterols. It is known that lichens have a diverse and unique sterol composition, different from that of fungi and algae. Sterol-mediated biochemical mechanisms of stress resistance in lichens have not been fully studied and not systematized. Temperature stress is quite common for lichens, which often grow in unfavorable conditions. It is known that dry lichen thalli are able to withstand temperature changes over large ranges, while hydrated thalli are much more sensitive to unfavorable temperatures. In this work, stress-induced changes in respiratory activity and membrane stability index (MSI), as well as the sterol profile of hydrated lichen thalli, of Peltigera canina (L.) Willd. under the influence of elevated (+40°С) and low (–20°С) temperatures was investigated. It was shown that unfavorable temperatures caused a suppression of respiration rate and a decrease in the MSI of lichen thalli. Chromatomass spectrometric analysis showed the presence of P. canina ergosterol, dehydroergosterol, episterol, lichesterol, and fungisterol. Under the influence of both stress factors, there was a decrease in the level of ergosterol and an increase in the proportion of episterol. Under cold stress conditions, the proportion of dehydroergosterol also increased, the proportion of lichesterol decreased, and the relative content of the more saturated sterol fungisterol remained at the control level. It can be assumed that stress-induced changes in the sterol profile of lichens under low-temperature exposure create an optimal balance of sterols in membranes, which provides conditions for the deployment of a successful strategy leading to the adaptation of the lichen to the action of a stressor.

Currently, special attention is paid to the study of the mechanisms of stress resistance of extremophile organisms that can survive in extreme conditions. Such organisms include lichens, which are symbiotic associations of fungi and algae and/or cyanobacteria. The high stress resistance of lichens is due to the presence of a wide range of biologically active metabolites, including sterols. It is known that lichens have a diverse and unique sterol composition, different from that of fungi and algae. Sterol-mediated biochemical mechanisms of stress resistance in lichens have not been fully studied and not systematized. Temperature stress is quite common for lichens, which often grow in unfavorable conditions. It is known that dry lichen thalli are able to withstand temperature changes over large ranges, while hydrated thalli are much more sensitive to unfavorable temperatures. In this work, stress-induced changes in respiratory activity and membrane stability index (MSI), as well as the sterol profile of hydrated lichen thalli, of Peltigera canina (L.) Willd. under the influence of elevated (+40°С) and low (–20°С) temperatures was investigated. It was shown that unfavorable temperatures caused a suppression of respiration rate and a decrease in the MSI of lichen thalli. Chromatomass spectrometric analysis showed the presence of P. canina ergosterol, dehydroergosterol, episterol, lichesterol, and fungisterol. Under the influence of both stress factors, there was a decrease in the level of ergosterol and an increase in the proportion of episterol. Under cold stress conditions, the proportion of dehydroergosterol also increased, the proportion of lichesterol decreased, and the relative content of the more saturated sterol fungisterol remained at the control level. It can be assumed that stress-induced changes in the sterol profile of lichens under low-temperature exposure create an optimal balance of sterols in membranes, which provides conditions for the deployment of a successful strategy leading to the adaptation of the lichen to the action of a stressor.

Mosses are an attractive model for studying the mechanisms of plant adaptation to unfavorable environmental conditions. There is little information in the literature on the pigment composition and its role in stress resistance of mosses, although this topic is quite widely covered for higher vascular plants. Unfavorable environmental temperature is one stress factors with the strongest effect on plants, since photosynthesis, the main energy-supplying producing process in plants, is sensitive to temperature and often inhibited before other cellular functions are impaired. Chlorophyll is known to play a central role in the process of photosynthesis. In this regard, the aim of the present work was to study the response of the moss Hylocomium splendens Hedw. to temperature stress. The chlorophyll content and chlorophyll a fluorescence parameters in H. splendens shoots were investigated. Results showed that both elevated and negative temperatures decreased the content of chlorophyll a significantly, while, on the contrary, the content of chlorophyll b increased. It is interesting to note that the decrease in chlorophyll a under temperature stress was accompanied by an increase in the content of pheophytin a. The content of chlorophylls in the post-stress period did not recover to the control level. Temperature stress did not affect the maximum photochemical efficiency of photosynthesis, but the rate of electron transfer significantly decreased under the influence of low temperature. Thus, significant changes in the pigment composition of chlorophylls are not always accompanied by changes in fluorescence parameters to the same extent.

Extremophile organisms can survive under extreme conditions through tolerance mechanisms. Such organisms include lichens, which are symbiotic associations of fungi and algae and/or cyanobacteria. Among other factors, the high stress tolerance of lichens can be attributed to their ability to synthesize a wide range of metabolites, including chemically diverse lipid compounds. Despite their obvious relevance, the biochemical mechanisms of stress tolerance in lichens that are mediated by changes in the lipid profile remain understudied. Peltigerous lichens constitute a separate division of lichens characterized by high growth rates and metabolic activity indices. Taking into account the temperate climate in which these lichens grow, it can be assumed that they may be highly sensitive to temperature fluctuations. These factors led to the choice of Peltigera canina, epigeic lichen, as the subject matter of this study. The present work examined stress-induced changes in the redox status of P. canina, as well as changes in its lipid composition at an elevated temperature (40 °С). The exposure of hydrated lichen thalli to an unfavorable temperature led to an increase in the level of hydrogen peroxide, phenoloxidase activity, and lipid peroxidation, which constitutes an important part of stress response in lichen. The stress-induced changes in the redox status of lichen thalli stimulated an increase in lipophilic antioxidant agents: a dramatic increase in the level of α-tocopherol and an increase in carotenoids, specifically β-carotene. Thus, the accumulation of lipophilic antioxidants constitutes an important part of the lipid-mediated stress response of P. canina to temperature elevation.

В последнее время много внимания уделяется образованию и физиологическому действию оксида азота (NO) на растения.Было показано, что митохондрии являются одним из компартментов клетки, где генерируется эта биоактивная молекула

The effects of nitric oxide (NO) on oxygen consumption, heat generation, and cell ultrastructure were investigated in the seedlings of wheat ( Triticum aestivum L.). The experiments were conducted with the excised roots of 5-day-old seedlings grown in the solution of CaCl 2 (2.5 × 10 −4 M). The source of NO was NaNO 2 (5 × 10 −3 M) where the roots were incubated. Production of NO was determined by means of EPR, respiration — gasometrically, heat generation — using a microcalorimeter. The results showed that NO was formed in the presence of NaNO 2 . This was accompanied by a decrease in the respiration rate by about 30%, which lasted for 5–6 h. Apparently, NO inhibited mitochondrial oxidation because stimulation of oxygen consumption induced by 2,4-DNP was completely removed in the presence of NaNO 2 . When the cells were affected by succinic acid in the presence of NaNO 2 , respiration was strongly inhibited. The effects of succinic acid and NaNO 2 were negated by ascorbic acid. A decrease in the rate of respiration was accompanied by a reduction in heat generation. Moreover, the efflux of potassium ions to the root incubation medium was stimulated, which may point to changes in ionic membrane permeability. The observed changes in energy exchange were accompanied by disturbances in the cell ultrastructure. Nitric oxide induced a clarification of the mitochondrial matrix and a reduction in the number of cristae. It was concluded that NO excess in plant tissues brings about a deceleration of energy exchange, disturbance of the ultrastructural organization, and cell death.

Changes in respiration and cell ultrastructure induced by long-term incubation with dexamethasone (DM) in excised roots of 5-day old wheat (Triticum aestivum L.) seedlings were investigated. During 5 h incubation of roots with DM, oxygen consumption was inhibited by 20-30%, while respiratory coefficient did not change and its value was about 1. DM prevented from glucose-induced activation of respiration, which indicated blockade of glycolysis and decrease in oxygen uptake by this apoptotic inductor. It has been suggested that the respiratory inhibition by DM might be also connected with the influence of DM on the 1st segment ofmitochondrial electron transport chains. This suggestion is supported by the fact that succinate prevented DM-induced inhibition of respiration. Furthermore, stabilization of intracellular pH by dipeptide carnosine abolished inhibitory effect of DM on respiration. Probably depression of oxygen consumption by DM is also due to acidification of cytoplasm. Strong vacuolization of cytoplasm, one of the characteristics of cell death, occurred in 5 h after treatment of roots with DM. Vacuolization was to a great extent prevented by carnosine. The ultrastructure of root cells after long-term (23 h) treatment with DM was disturbed, and oxygen consumption was also dramatically decreased. These effects of DM were in part prevented by carnosine. The data obtained suggest that DM causes acidification of cytoplasm, disturbance of energy exchange and cytoplasm vacuolization in root cells, and induces death of these cells.

The influence of exogenous ascorbic acid (AsA) on oxidative phosphorylation was studied using wheat seedling roots. Treatment of them with AsA stimulated the rates of oxygen consumption and the heat production and caused a decrease of the respiratory coefficient. The increase in respiration was prevented by inhibitors of ascorbate oxidase, diethyldithiocarbamate (DEDTC), and of cytochrome oxidase, cyanide (KCN). Exogenous AsA sharply stimulated the rate of oxygen consumption of roots when complexes I and III of the mitochondrial electron transport chain were inhibited by rotenone and antimycin A, respectively, while the rates of heat production did not change significantly. It is concluded that AsA is a potent energy substrate, which can be used in conditions of failing I and III complexes in the mitochondrial electron transport chain.

Respiration of excised roots of 5 day old wheat seedlings with blocked mitochondrial oxidation under simultaneous action of rotenone and antimycine A was studied. A reduced rate of oxygen uptake was observed within the first hour of root treatment inhibitors. However, after a 5 h exposure there was an increase in oxygen uptake, which was prevented by KCN but amplified by malate and ascorbate. The application of inhibitors caused a considerable increase in the respiratory coefficient (RC) up to 2.1, that suggests a significant CO2 release, when the initial sites of mitochondrial electron transport chain were inhibited. RC did not raise, when ascorbate was added in the presence of inhibitors. We assume that inhibition of mitochondrial oxidation at I and III sites of electron transport chain facilitates switching on the alternative paths of reductant translocation to oxygen. Participation of ATPases and redox system of plasma membrane in the response reactions of respiration directed to the restoration of ion, particularly, proton homeostasis in conditions of inhibited mitochondrial oxidation is discussed.

This paper reports changes in ion transport and energy metabolism of plant cells during short- and long-term expositions, resp., to antibiotic nystatin, which is known to specifically bind with plasma membrane sterols to form channels. The excised roots of 5 days old wheat seedlings were used as a model system in this research. It has been shown that treatment of excised roots with nystatin leads to activation of energy metabolism expressed as an increase of respiration and heat production by root cells. Furthermore, in the presence of nystatin increased pH of incubation medium, plasma membrane depolarization and a significant loss of potassium ions were observed. Nystatin-induced stimulation of respiration was prevented by malonate, an inhibitor of succinate dehydrogenase, electron acceptor dichlorophenolindophenol, and AgNO3, an inhibitor of H(+)-ATPase. Based on the data obtained it can be suggested that nystatin-induced stimulation of respiration is related to electron transport activation via mitochondrial respiratory chain, and is connected with activation of plasmalemma proton pump. Moreover, nystatin-induced increase of oxygen consumption was prevented by cerulenin, an inhibitor of fatty acid and sterol synthesis. This indicates that additional sterols and phospholipids may be synthesized in root cells to heal nystatin-caused damage of plasma membrane. A supposed chain of events of cell response to nystatin action may by as following: formation of nystatin channels-influx of protons--depolarization of plasmalemma-efflux of potassium ions-disturbance of ion homeostasis--activation of H(+)-ATPase work-increase in energy requests for H(+)-ATPase function--increase in the rate of oxygen consumption and heat production. The increased energy production under the action of nystatin, may provide the work of proton pump and synthesis of sterols and phospholipids, which are necessary for membrane regeneration.

Salicylic acid (SA) results in an increase in oxygen consumption and heat production by excised wheat roots. Activation of respiration and heat production can be caused by protonophoric abilities of SA, which can acidify cell cytoplasm. This is accompanied by increased H+-ATPase activity of the plasma membrane. Oxygen consumption and heat production by root cells in the presence of SA are sensitive to KCN, an inhibitor of cytochrome oxidase. As cytoplasm acidification leads to electron transfer via the outer mitochondrial membrane, it is believed that these electrons are transferred to cytochrome oxidase. Malate is a unique substrate that may be oxidized by the outer mitochondrial membrane. In the presence of SA, malate causes an increase in respiration, which is sensitive to KCN. Ascorbate, as the electron donor for cytochrome c and cytochrome oxidase, stimulates respiration of roots in the presence of SA as well, and this stimulation is also sensitive to KCN. Unike SA, malate and ascorbate do not combine with protons in the membrane and thus do not disrupt the energy accumulation due to ion translocations, that do not lead to the extra heat production by roots. However, as for SA, respiration of roots treated with malate and ascorbate is stimulated because their ions remain dissociated in the aqueous phase and can be transported through the outer mitochondrial membrane via cytochrome/redox reactions.

The dissipation of ion gradients across plasma membranes contributes to the heat production by live cells. The aim of the present research is to determine the dependence of the rate of heat production by plant tissues on ion balance shifts, supposing a priori that the shifts are the beginning mechanism in adaptation to various stresses. Excised wheat roots subjected to prolonged incubation in different solutions served as objects for investigation. Two ion transporters (K+H+-transporter nigericin and Ca2+-ionophore A23187) shifted the ion homeostasis and induced distinct changes in the energy metabolism of plant tissues. Nigericin, which considerably increased the plasmalemma conductivity for protons and potassium, enhanced production of heat by root cells throughout the exposure. Prolonged incubation with A23187 was required to display its ion-transporting properties which were accompanied by a rise in the rate of heat release. The high rate of heat release and respiration in root cells exposed to ion transporters is a reflection of the ion-gradient dissipation and the increased energy expenditure for the activation of ATPase systems necessary for the restoration of ionophore-disturbed ion homeostasis.