Comparison of the functions of plasma membrane and vacuolar membrane lipids in plant cell protection against hyperosmotic stress.

MAIN CONCLUSION: The comparison of the changes of the lipid content in plant cell boundary membranes demonstrates a substantial role of the vacuolar membrane in response to hyperosmotic stress. Comparison of variations in the lipid content of plant cell boundary membranes (vacuolar and plasma membranes) isolated from beet root tissues (Beta vulgaris L.) was conducted after the effect of hyperosmotic stress. Both types of membranes participate in the formation of protective mechanisms, but the role of the vacuolar membrane was considered as more essential. This conclusion was connected with more significant adaptive variations in the content and composition of sterols and fatty acids in the vacuolar membrane (although some of the adaptive variations, especially, in the composition of phospholipids and glycoglycerolipids were similar for both types of membranes). In the plasma membrane under hyperosmotic stress, the increase in the content of sphingolipids was noted that was not observed in the tonoplast.

The microdomains (rafts) of plasmalemma in the protection of the plant cell under oxidative stress.

The investigation of the lipid-protein microdomains of the plasmalemma isolated with the aid of the non-detergent technique in the zones of the sucrose density gradient after high-speed centrifugation from the tissue pieces of beet roots, which underwent oxidative stress, was conducted. The microdomains, whose lipid composition - according to the definition - allowed us to classify them as rafts, were studied. After the exposure to oxidative stress (100 mM hydrogen peroxide), the variations in the composition of membrane lipids bound up mainly with the elevations of the content of raft-forming lipids (sterols, sterol esters). Oxidative stress provoked redistribution in the composition of sterols, which led to an elevation in the content of campesterol and in the ratio of stigmasterol/sitosterol. Furthermore, the variations were registered in the content of phospholipids and phosphoglycerolipids, which are capable of stabilizing the lamellar structure of membranes. The results obtained allow one to assume that under the oxidative stress, variations in the composition of lipids in microdomains of the plasma membrane can take place. These variations may influence the functioning of the membranes, and the membranes may participate in the protection of the plant cell.

Role of tonoplast microdomains in plant cell protection against osmotic stress.

MAIN CONCLUSION: Variations in the content of tonoplast microdomains, isolated with the aid of a non-detergent technique, are induced by osmotic stress and may take part in plant cell adaptive mechanisms. Investigation of tonoplast microdomain lipids isolated with the aid of the non-detergent technique from beetroots (Beta vulgaris L.) subjected to either hyperosmotic or hypoosmotic stress was conducted. Earlier, an important role of tonoplast lipids in the protection of plant cells from stress was demonstrated (Ozolina et al. 2020a). In the present paper, we have put forward a hypothesis that lipids of microdomains of raft nature present in the tonoplast are responsible for this protective function. The variations in the content of lipids of the studied nondetergent-isolated microdomains (NIMs) under hyperosmotic and hypoosmotic stresses were different. Under hyperosmotic stress, in the scrutinized microdomains, some variations in the content of lipids were registered, which were characteristic of the already known protective anti-stress mechanisms. These variations were represented by an increase in sterols and polar lipids capable of stabilizing the bilayer structure of the membranes. The found variations in the content of sterols may be bound up with some intensification of the autophagy process under stress because sterols foster the formation of new membrane contacts necessary for this process. Under hypoosmotic stress, the pattern of redistribution of the lipids in the scrutinized membrane structures was different: the largest part of the lipids appeared to be represented by hydrocarbons, which fulfilled mainly a protective function in plants and could prevent the excess water influx into the vacuole. The results obtained not only demonstrate the possible functions of the vacuolar membrane microdomains but also put forward an assumption on the role of any membrane microdomain in the protection mechanisms of the plant cell.

Changes in the fatty acid composition of pine needle lipids under the aluminum smelter emissions.

Changes in the fatty acid (FA) composition of total lipids of Pinus sylvestris needles at different pollution levels caused by emissions from a large aluminum smelter (BrAS) have been studied. In the needles of trees from unpolluted (background) territories, the FA spectrum is represented by 24 acids with prevalence of unsaturated FAs (71.6%). The main unsaturated FA are represented by oleic (C18: 1ω9), linoleic (C18: 2ω6), and α-linolenic (C18: 3ω3) acids. Under the influence of BrAS emissions, the total amount of identified FAs in the needles and the proportion of unsaturated FAs decrease, while the fraction of saturated FAs, on the contrary, increases from 25.4% in unpolluted needles to 33.2% in polluted ones. The content of palmitic FA (C16:0) in the needles exceeds background values by 1.5 times, behenic acid (C22:0) - by 1.6-2.5 times, arachidic acid (C20:0) - by 1.5 times, palmitic margaric acid (C17:0) - by 1.5-2.3 times. These FAs play the important role in the protection of plant membranes from the effects of abiotic stress factors, making them less permeable. The sum of short-chain saturated FAs (C12:0, C14:0, C15:0) increase by 4.8 times in needles of trees that are highly polluted. Pentadecanoic (C15:0) acid is found in the needles only in the background areas and at the low pollution level. With a more severe pollution, C15:0 is not identified, but lauric acid with the cis-configuration of double bonds in the structure (izo-C12:0) appears. The presence of "relict" ∆5-polymethylene FAs in the composition of pine needle membrane lipids is determined. In the background areas, they account for 12.9% of the total FAs. With the industrial pollution intensification, their total content increases and reaches 14.1%. ∆5-polymethylene FAs are also able to protect membranes against negative influences. Thus, changes in the quantitative and qualitative FA composition of pine needle total lipids indicate the activation of the stabilization mechanisms of membrane lipids due to their tight packing in a bilayer. It is one of the adaptive reactions of Pinus sylvestris in response to the impact of the aluminum industry emissions.

Non-detergent Isolation of Membrane Structures from Beet Plasmalemma and Tonoplast Having Lipid Composition Characteristic of Rafts.

Vacuolar and plasma membranes were isolated by a detergent-free method from beet roots (Beta vulgaris L.), and were fractionated in a sucrose density gradient of 15-60% by high-speed centrifugation at 200,000×g during 18 h. The membrane material distributed over the sucrose density gradient was analyzed for the presence of lipids characteristic of raft structures in different zones of the gradient. The quantitative and qualitative content of lipids and sterols, and the composition of fatty acids were analyzed. Some membrane structures differing in their biochemical characteristics were revealed to be located in different zones of the sucrose gradient. The results of the analysis allowed us to identify three zones in the sucrose gradient after the vacuolar membrane fractionation and two zones in the plasma membrane where membrane structures, which may be defined as rafts for their lipid composition, were presented.

Variations in the content of tonoplast lipids under abiotic stress.

MAIN CONCLUSION: The vacuolar membrane is an essential component in protecting the plant cell from stress factors. Different variations in the tonoplast lipid content, which depend on the type of stress, have been reviewed. The lipid content of vacuolar membranes of beet roots (Beta vulgaris L.) under hypoosmotic, hyperosmotic and oxidative types of stress has been studied. These types of stress induce variations in the content of almost all the classes of studied lipids (phospholipids, glycoglycerolipids, sterols and fatty acids). The variations, which are characteristic of a single stress, include the variations (i) in the content of individual glycoglycerolipids and in their total content, (ii) in the total content of sterols, and (iii) in the ratio of content of phosphatidylcholine/phosphatidylethanolamine in the scope of tonoplast phospholipids. Variations observed under all of the types of stress under scrutiny include (i) variations in the content of fatty acids of tonoplast lipids, (ii) some decrease in the content of phosphatidic acid and phosphatidylethanolamine, and (iii) variations in the content of individual sterols. Stigmasterol, campesterol, as well as the stigmasterol/sitosterol ratio increased in varying degrees under all of the types of stress. The most substantial variations have been observed in the content of sterols under abiotic stress. This is probably due to role of sterols in regulation of such membrane characteristics as permeability and microviscosity. In our opinion, sterols may represent one of the main components of tonoplast adaptive mechanisms.

Effect of cadmium on the roots of beetroot (Beta vulgaris L.).

The article dwells upon identifying the effect of cadmium on the roots of beetroot. The exposure effects of various concentrations of cadmium were studied at different levels of the plant organization (tissue pieces, organelles, membrane vesicles). The effect was noted only at a concentration of 100 µm. The negative effect of cadmium on the roots tissues of beetroot appeared with an increase in permeability and a decrease in the stability of cell membranes due to a change in the composition of fatty acids of membrane lipids and an increase in oxidation processes. The effect of cadmium in model experiments on the activity of the proton pumps of the vacuolar membrane has been evaluated. The pumps provide for the transport of heavy metals into the vacuole, which is one of the effective mechanisms for phytoremediation. The influence of cadmium in model experiments on the activity of the proton pump of a vacuolar membrane was evaluated. Under the influence of cadmium, a decrease in the activity of V-ATPase was observed, while the activity of V-PPase did not change. The results obtained complement our understanding of the damaging effects that occur in plant cells under cadmium stress.