Tolerance of Plant Cell Wall to Environment

Abstract

Drought and flooding of soil are negatively factors for growth and development of plants. Exogenous factors, including moisture of soil, intensity of sun light, temperature, salinization, the content and diffusion rate of CO2 and O2 is main that influence terrestrial and flood plants. Cell walls actively participate in the mechanisms of plant adaptation to drought and flooding. It has been established that the resistance of plants to unfavorable environmental conditions is due to the plasticity of the structural, biochemical and functional characteristics of plant cell walls, that manifests itself in a change of ultrastructure cell walls, density of stomata and wax in leaf epidermis, compacting or loosening of cell walls, presence of cuticle pores, change of content of crystalline and amorphous cellulose, hemicellulose, callose and lignin and change in a ratio of syringyl/quajacyl monolignols and also expression of the specific genes.

1. Introduction

It is known that external environmental conditions provoke to the phenotypic and genetic plasticity of plant during vegetative and generative growth and lead to change of duration of ontogenesis of both individual species and populations [1]. Given that exposed to a specific exogenous factor, some plants experience stress, and for other plant species this factor is the optimal condition for life, the definition of stress for the plant is quite complex and problematic. According to many definitions, stress is a harmful adverse force or condition that inhibits the normal functioning of a biological system, such as a plant. According to [2], stress for a plant is its response to the action of adverse or even detrimental to growth and development of plant. For the plant, stress is measured by both signs of survival and signs of adaptation, yield, growth parameters and assimilation. External signals of the environment, such as light, temperature, water status of the soil–these are the most important signals that affect the growth of the plant. The perception of these signals and the plant’s response to them affects a whole cascade of events that require knowledge of the signal and its transduction into a physiological response [3]. In the perception of signals of adverse abiotic stresses, primarily involved protein receptors of the cell wall, that send this signal to the transport system into the cytoplasm. Such receptors of ell wall appear to be arabinogalactan protein molecules that bind the cell wall to the plasmalemma, cytoskeleton elements, and apoplast components. In addition to these proteins, stress receptors can be mitogen-activated protein, numerous kinases, and several transcription factors [4]. Stress is first perceived by cell wall receptors, which send a signal to the receptors of the cytoplasmic membrane, then the signal is reformed and reduced, and the result of this transformation is the participation of secondary mediators [3, 4, 5, 6].

Determination of the plant state in a changing environment in conditions of increased anthropogenic pressure and global climate changes is becoming one of the main problems of plant biology and ecology. In natural conditions plants can be influenced by a complex of unfavorable environmental factors. Despite the long list of abiotic and biotic stresses, including: cold, high temperature, salinity, drought, floods, radiation, air and soil pollutants, pathogens and others, we will consider the most significant adverse environmental factors: drought and flooding, which negatively affect plants’ growth, up to their death. The search for universal biomarkers that would make it possible to determine the state of plants regardless of nature and number of stress factors is urgent. The cell wall of plants can be such a marker, since it is the growth and differentiation of the cell wall during primary and secondary growth that undergoes significant changes under conditions of changes in the water balance of the plant. The basis of this section is the idea that the stability of ontogenesis under conditions of unfavorable climatic and anthropogenic changes in the environment is due to the plasticity of the structural and functional organization of plant cell walls. We put forward a hypothesis about the existence of a coordinated response of the structural and functional systems of the cell wall and the cytoplasm of plant cells, which is involved in the adaptation of the plant to the action of extreme natural factors—drought and flooding.

2. Drought

Drought is a deficit of water in the soil, which affects the growth and development of the plant. Drought stress is seen as a condition in which water potential and turgor of a cell are reduced, although the plant can function normally. Water stress is considered as the loss of water by the plant, which leads to the closure of the stomata and restriction of gas exchange by the plant. Wilting of plants is characterized by an intensive loss of water, which leads to next changes, including of plant metabolism and cell structure, to change of activation of catalytic enzymatic reactions, to inhibiting the process of photosynthesis and destructed metabolism, which can lead to cell death [7, 8]. Drought can be chronic or temporary. The latter is observed when the weather changes rapidly and unpredictably. Moderate drought is a phenomenon in which the plant begins to feel the effects of drought. Under such conditions, plants have developed specific mechanisms of acclimatization and adaptation in response to the short-term or long-term action of the factor [9, 10].

In this respect, the reaction of plants to drought is well studied in psammophytes growing on sand dunes has been better studied. Psammophytes develop mechanisms and specific features that ensure not only a normal state of life, but also functioning under stressful conditions. These mechanisms are reflected in the morpho-anatomical changes in the vegetative organs of plants [7, 11] that help psammophytes to adapt to environmental conditions, and manifested in a decrease in the size of leaf blades, the formation of water-retaining parenchyma, a change in the size of the leaf conducting system, twisting of leaf blades, a change in the cell wall structure, change of density of stomata, an optimization of transpiration, enhanced synthesis of wax and lignin, the formation of trichomes and silicon inclusions in cell walls and formation a thick cuticle [7, 12].

2.1 Growth processes during drought

It is showed that even with a slight drought, the growth rate of plant organs decreases: roots and aboveground organs react very strongly to such stress, their growth reduce [13, 14] that connected with structural-functional changes of cell walls [15]. Drought cell growth decreases have been described for leaves for psammophytes, including Zygophyllum album and Nitraria retusa [16], Tragopogon borysthenicus and T. orientalis [11], Helichrysum arenarium and H. corymbiforme [17]. The mechanisms of this phenomenon in psammophytes have not been studied. However, in mesophytes and halophytes, cell and organ sizes have been shown to be mediated by the rate of cell division and stretching [7]. That is, the decrease in cell growth is mediated by changes in the synthesis of polysaccharides of cell walls. This phenomenon has been studied in Sium latifolium leaves [18]. When studying the development of the ephemeral desert plant Gymnarrhena micranatha for 50 days, it was found that its inflorescence is located in the depth of the soil. The apical bud is drawn from the soil surface into the soil to a depth of 10 mm. It is shown that the direct retraction of the main apex of the stem occurs from the soil surface into its depth, while the stem rotates, changes its direction of growth by 180^o, continues to grow and develop. What processes take place in cells’ walls? There was compression of cells in the endoderm, pericycle and primary phloem of the root, which decreased in length (by ½ initial lengths). This was due to the formation of radial cell walls. The shrunken and compressed cell walls had a wavy appearance. Partial disintegration of polysaccharides was observed in cell walls, including lysis of cellulose wavy-like micro fibrils [19].

Under drought conditions in the roots there revealed a decrease in the size of the parenchyma [20]; in the endoderm, cell walls thicken, and additional layers of cells were with strongly suberinized cell walls are formed around the stele [21]. In the periderm, cell walls were also impregnated with suberin, which reduces the penetration of water through the cells of the cortex. Special lacunae for water storage were formed in root [22]. Whereas in leaves the effect of drought is manifested in the reduction of sugars in the fraction of cell walls, which should certainly be reflected in the composition of polysaccharides in the walls. Studies of the effects of drought on crops have shown that the cell walls of aboveground photosynthetic organs are also sensitive to this factor. Studies of polysaccharides of cell wall matrix in reduced coleoptiles of wheat seedlings under drought from 6 to 15 weeks shown that during the first week of drought exposure, drought-sensitive varieties showed a decrease sugar in the fractions of wall matrix: rhamnose, mannose, galactose, arabinose, xylose, and glucose and uronic acids [23]. In addition, in the hemicellulose fraction of drought-resistant variety was shown decrease in arabinose, mannose, galactose and increase in rhamnose, xylose, glucose, uronic acids in comparison with drought-sensitive variety. These changes were accompanied by an increase in the activity of glucoside hydrolysing enzymes: α-galactosidase, α-L-arabinofuranosidase and 1.3–1.4-β-glucanase in drought-resistant varieties. The observed changes in the matrix of cell wall of coleoptiles of two varieties of wheat under the action of drought reflect changes in cell metabolism, which directly affected the growth rate [23]. Similar changes in the content of sugars (glucose, fructose and sucrose) and the activity of 1.3–1.4-β-glucanase have been previously noted by other researchers in studying the effects of water and salt stress on wheat stalks [24].

2.2 The role of wax and cuticle

Wax and cutin are involved in the regulation of water and lipids transport through the cell wall [25]. Plant’ wax is a mixture of aliphatic and cyclic hydrocarbons and their derivatives. The composition of waxes varies depending on the species and organs’ plant. Cutin is involved in the regulation of the diffusion of gases and moisture in the main cells of the epidermis and the stomata. It is known that the cuticular membrane can be both hydrophobic and hydrophilic. If the cuticular membrane is hydrophobic, the functions of the cuticle are to reduce water loss by the organs; and if the cuticle is hydrophilic, then the function is to transport water, aqueous solutions, and lipids (waxes) [26]. It is known that the aboveground organs of plants that grow in dry climates synthesize a significant amount of wax and cuticle, which are a barrier to transpiration [27]. Wax and cuticle are the main barriers against “uncontrolled” water loss by leaves. Therefore, in the adaptive responses of above-ground bodies, to action of a drought, the strengthened synthesis of these two components of cellular components of epidermis plays a certain role.

Wax can be located the inside cutin layer, or be situated on top of the cuticle. A two-year study of the long-term effects of drought on pine needles (Pinus pinaster) showed that pine needles activated the synthesis of cuticular wax, which was accompanied by the expression of transcription factor (SHINE), which is involved in cuticle synthesis [28]. It was found that the content of newly deposited wax under such conditions depends on the duration of exposure and the plant state [29]. Thus, a study of the effects of drought on tobacco plants (Nicotiana glauca L.) showed an increase of one and a half to two times the wax content in the leaves during 3 days of drought compared with unstressed plants [30]. Under conditions of enhanced drought, Arabidopsis thaliana plants increase not only the wax and cutin content, but it was accompanied by an increase of almost 50% in the thickness of cell walls. Wax deposition and extension of the cuticle layer in the epidermis increased plant tolerance to prolonged drought. A similar phenomenon has been noted by other researchers in studying the effects of drought on the leaves [31]. Even in temperate climates (under conditions of natural water deficit in the soil) there is an acceleration of wax synthesis on the surface of leaf blades. Carrying out a comparative study of the ultrastructure of the epidermis of leaves Alisma plantago-aquatuca, which grew on the river bank and in conditions of moderate soil drought, the researchers found an increase in the density of wax on the upper epidermis 2.5 times, on the lower epidermis—eight times [32]. In roots, drought or salt stress causes an increase in the content of suberin in the cell walls of the exo- and endoderm, which was shown in the species Pistacia integerrima and a hybrid, P. atlantica x Plagiochila integerrima [33].

For plants that grow in drought conditions is characterized by the participation of cell walls of the epidermis of the leaves in the water intake. It is known that the above ground organs of desert plants can absorb water from the leaf surface, intercept precipitation and absorb fog, using an atmosphere saturated with water [34, 35]. To do this, plants use trichomes [36], the specialized glands [37], and also form a hydrophilic surface in specialized epidermal cells that contain water pores [38]. It is shown that the leaves of Reaumuria soongorica, a super-xerophytic desert plant, are characterized by the presence of water-absorbing cells in the epidermis, which are closed by scales. Such scales cover water-absorbing cells during the day and open water-absorbing pore channels at night, rising above the surface of epidermis. During the day, the valves of the water-absorbing structure of the scales are compressed, leaving a small central hole. At night, when the humidity rises, the basal cells raise the upper cells of the lid; after which the cells of the porous channels (capillaries) expand, forming a hole through which they begin to absorb atmospheric water [39]. The depth of the stomata is also an important structural feature of plants that have adapted to drought. Slightly sunken, submerged stomata are a typical feature of numerous psammophytes growing on the Mediterranean coast, including the coasts of Crete, Lampione, Tavolari and Malta, where extreme values of annual temperature have been recorded [40]. The results indicate that modifications in leaf architecture, including deepening the stomata into the epidermis, are important anatomical and physiological strategies that help psammophytes reduce water consumption.

Another feature of leaf structure to optimize water balance is twisting and/or folding of leaves. Leaf twisting is designed to maintain the optimal water balance of plants growing in inadequate water supply conditions [40, 41]. The twisting of the leaves of many psammophyte grasses is due to the specialized structure of the epidermis of the leaf blades and the presence of bulliform (motor) cells, the cell walls of which function to enter and exit water, reducing leaf area affected by drought [41, 42]. The cell walls of the bulliform cells of the epidermis synthesize guajacyl monolignol and callose, which helps to quickly change the entry or exit of water from these cells [43]. Twisting preserves optimal heat transfer and optimal water-vapor density in leaf tissues [41].

The presence of trichomes and increased cuticle density in cell walls are typical features of the leaf blades of psammophyte plants growing on coastal dunes [44]. Psammophytes have two types of trichomes: glandular and non-glandular. Glandular trichomes were found in the leaves and stems of psammophyte Silene thymifolia, which grew in Romania [45], in the leaves of Tragopogon borysthenicus and T. orienthalis, which grew in the Black Sea Reserve (Ukraine) and sand terraces of the Desna River, Ukraine [11].

2.3 Role of lignin, pectin and cellulose

Lignin is a branched biopolymer that, together with hemicellulose and pectin, acts as an adhesive matrix for cellulose microfibrils. Lignin provides mechanical strength of tissues and organs, impermeability of water and aqueous solutions through the cell walls. Lignin is a complex of phenylpropanoids (monolignols) [46]. Early work (Barnett, 1976) on the effect of drought on wood lignification showed that the tracheid rings stuck together because the secondary walls of young trees did not contain lignin. The formation of false rings in drought-stricken trees is a well-known phenomenon [47]. According to Lloyd Donald [48], who studied the anatomy of wood and the characteristics of cell walls in Pinus radiate under conditions of water stress (drought), false growth rings with bundles of weakly lignified tracheids, were found. It has been shown that wood exfoliation is due to poor adhesion between the tracheids due to a decrease in lignin content in the middle plate. The author explains this phenomenon in such a way that, apparently, there is an abnormal lignification due to dehydration of the outer cell walls. Since the formation of lignin occurs with the movement of water, the emergence of water or its insufficiency in the cell walls should interfere with or prevent both the transport and inclusion of lignin precursors and the process of lignification of the wall.

It has been shown that even a slight drought (up to 12 days) caused an increase in lignin precursors (coumaric and caffeic acids) in xylem maize juice, and this was due to a decrease in anionic peroxidase activity, indicating the effect of drought on lignin biosynthesis [49]. Different areas of the corn root respond differently to drought: in the basal part of the roots, growth is inhibited compared to the apical part of the roots, which is associated with the expression of two genes involved in lignin biosynthesis: shinamyl-CoA reductase-1 and -2. Such decrease in growth is due to an increase in lignin deposits, which increase the stiffness of the cell wall and reduce the growth rate, which may also be due to changes in factors such as water, minerals and sugars.

It was shown that after 28 days of drought, Trifolium repens reduced growth and increased lignin synthesis in the leaves, accompanied by increased activity of guajacol peroxidase, syringaldazine peroxidase and coniferol alcohol peroxidase [50]. It has been found that the activity of enzymes associated with lignin synthesis changes in plant leaves during drought. Thus, in the study of prolonged (35 and 47 days) drought on the leaves of Ctenanthe setosa, it was found that in parallel with the increase in lignin activity of enzymes involved in its synthesis also increased, in particular: phenylalanine ammonium lyase, soluble covalently bound peroxidase and polyphenol oxidase [51].

It is established that the impact of drought depends on the duration of its action, the species of plants and the growth stage. It has been shown that even a slight drought (up to 12 days) caused an increase in lignin precursors (coumaric and caffeic acids) in xylem maize juice, and this was due to a decrease in anionic peroxidase activity, indicating the effect of drought on lignin biosynthesis [49]. Roig-Oliver et al. showed for the first time that during long-term water deficiency, changes in the content of lignin, cellulose and hemicellulose in the cell walls of Helianthus annuus were accompanied by a significant decrease in phenols associated with the wall (coumaric, ferulic and caffeic acids) and with a negative correlation with photosynthesis (conductivity of the mesophyll to CO₂), and with a positive correlation with palisade mesophyll thickness [52, 53].

Abiotic stress, including drought, cause a change in the mechanical strength of the cell wall due to the synthesis of lignin and activation of several the types of reactive oxidative species (ROS). Cell walls become stiffer and the overall mechanical stability of tissues and cells increases provided of an increase of wall peroxidases activity, increase in H₂O₂ concentration and/or an excess of peroxidase substrates [54]. The resulting increase in mechanical strength of the cell wall is occurred the change of cell’s turgor that enable plant cells to endure the osmotic stress caused by drought [55].

Cell walls not only change their structure in response to drought, to reduce water evaporation by cells, but also act as structures that, accumulate water for the needs of the cell. In particular, plants increase the content of pectins as a wet absorbing structure. This has been shown in the laboratory in the study of roots and stems of wheat seedlings (Triticum durum) of two varieties (drought-resistant and drought-resistant) [14]. Comparison between the two genotypes showed some differences in the content of polysaccharides of the wall matrix and the content of α-cellulose. It was found that the residues of xylose, glucose and arabinose in the matrix are more than 90 mole%; the level of xyloglucans—was 23–39 mole%, arabinoxylans—38–48 mol%, while the content of pectins and 1–3,1–4 β-D-glucans—was very low. It has been shown that in drought-stable plants the content of rhamno-galacturonans I and II significantly increased under conditions of water stress, while in the second genotype such an increase was not observed [14]. The obtained results indicate that in drought-resistant wheat varieties adaptation to drought occurs due to the increase of pectin chains, which leads to an increase in pectin gel that is the wet absorbing structure of the wall.

The study of the effect of water deficiency on the content of pectins in sunflower leaves showed that this polysaccharide is the most sensitive to water stress, it is the first to react to stress, reducing its content after a short exposure to stress (5 hours), while hemicellulose and lignin changed its contents only after 24 hours of stressful influence [56]. Early was established that pectins are crucial to determine wall characteristics. Changes in pectin physicochemical properties during stress induce the rearrangement of cell wall compounds, thus, modifying wall architecture and influencing on photosynthetic characterization of leaves of A. thaliana atpme17.2 (SALK_059908) and atpae11.1 (GK 505H02) mutants from wild-type Columbia (Col-0) [57]. This study provides insights on how different cell wall architecture could influence the photosynthetic efficiency in A. thaliana atpme17.2 and atpae11.1 mutants in comparison to wild-type Col-0. Thus, it was established that cell wall composition modification could lead to reduced photosynthetic traits in atpme17.2 and atpae11.1 mutants maybe because of alterations in However, there was a strong reduction in the cell wall pectin fraction, expressed by the amounts of galacturonic acid in both atpae11.1 and atpme17.2 mutants.

The use of a model object, in particular Penium margaritaceum, which is a unicellular zygnematophyte to study the effect of a hyperosmotic environment on the formation of cell walls, showed that rhamnogalacturan-1 is one of the first components of the wall, which take part in acclimatization to hyper osmotic stress. This phenomenon was established using the method the labeled antibody [33, 58]. The study of cell walls composition in six Antarctic mosses (Brachytecium austrosalebrosum, Warnstorfia sarmentosa, Bryum pseudotriquetrum, Polytrichum juniperinum, P. alpinum, and Sanionia uncinata) showed a clear positive correlation between cell wall thickness and pectin content, which apparently determines the porosity of cell walls in arctic species, and contributes to adaptation and optimal photosynthesis in such conditions. The investigators found that the less pectin in the leaves, the higher the values of CO₂ assimilation [59].

2.4 Role of silicon

It is established that the resistance of plants to drought is due not only to changes in the structure of cell walls of epidermal tissue, but also the deposition of silica in cell walls in the form of amorphous or crystalline inclusions [60]. According to Wang [61] silicon inclusions in epidermal cells reduce the influence of thermal effect on the leaves by reflecting the heat flow in the far infrared region of the sun light flux. This provides a passive mechanism for cooling the leaves in high sunlight. Although the mechanism of this action is not yet known, these issues need further to study. Silicon can deposit in leaf epidermis trichomes giving these structures are hardness and rigidity, making the leaves inedible to animals [62]. As a rule, most silicon is contained in cell wall protopectin, a water-soluble pectin fraction [63].

It has been established that silicon decrease the cuticle transpiration of aboveground organs. This chemical element, which accumulates in the cells of the epidermis of leaves and stems, forms a thickened cuticle-silicon wall, which protects the plant from excessive moisture consumption by reducing the cuticle transpiration. In addition, the plant’s walls can form hydrophilic silicate-galactose complexes that bind free water, thereby increasing the water retention capacity as in specific cells, as and in different tissues and in the organs of plant [64, 65].

Because of the density of cell walls and their ability to retain moisture, silicon compounds can significantly increase plant resistance to drought and protect plants from being lodged (fallen) [66]. Silicon reduces of water evaporation on the leaf surface, as has been shown, for example, on rice seedlings [67], on other crops, in particular in drought-resistant wheat [68] and sorghum [69]. Silicon can also influence water transport by regulating the osmotic potential of cells by increasing synthesis and accumulation of osmotic active substances (e.g., proline, sugars and inorganic ions) [70, 71].

2.5 Involvement of genes in adaptation process to drought

Over the years, significant progress has been made in discovering the cell wall-specific genes related to drought tolerance [72, 73]. These researches were carried out at rice in vegetative and reproductive stages [72]. In the reviews [72, 73] shown the major candidate genes underlying the function of quantitative trait loci directly or indirectly associated with the cell wall plasticization-mediated under drought tolerance or salinity stress of plants. On rice plant during of drought stress was identifying series genes, which take part in tolerance of this species to both drought or salinity stress: 1) drought inducible AP2/ERF family TF gene OsERF48, including cell wall related genes such as OsXTH9, OsAGP24, OsEXPA4, OsEXPA8, OsEXPB2, OsEXPB3, OsEXPB6 and OsAGP3, which associated with cell expansion and cell wall plasticization-mediated root growth under abiotic stress; 2) a lignin biosynthesis gene OsCCR10 (Oryza sativa CINNAMOYL-COA REDUCTASE 10) is also highly induced by drought in the roots of rice/ and 3) genes are associated with cell wall loosening (OsEXP1, OsEXP2, EGase, and two XETs), with lignin biosynthesis (PAL, C3H, 4CL, CCoAOMT, CAD, and peroxidase), and with the metabolism of cell wall proteins (GRP and UDP-GlcNAc pyrophosphorylase) and polysaccharides (OsCslF2, GMPase, xylose isomerase, and beta-1,3-glucanase), and 4) genes, including drought responsive, ABA-responsive, superoxide dismutase and cell wall-related genes (LOC_Os01g64860; LOC_Os01g72510; LOC_Os05g35320; LOC_Os12g36810, etc. [72].

Molecular methods have shown that during drought, increased wax and cutin synthesis is accompanied by activation of genes (Ltps and WAX9) that express proteins involved in the synthesis and deposition of wax and cutin, as well as the synthesis of transport proteins [74]. In a study of prolonged drought on the model plant A. thaliana [75, 76] was identified the presence of 11 genes, which were divided into groups according to the functions of the proteins they encode:

1. Transcription factor that regulates the biosynthesis of the cuticle (SHINE1

2. Genes (CER6 / KCS6, KCS4, KCR1, ECR / CER10), which regulate protein synthesis in VLCFA (very long-chain fatty acids);

3. Genes involved in the reduction and decarboxylation pathways (CER1, CER2, CER3);

4. Gene involved in the biosynthesis of wax and cuticle (LACS3).

An early response of the pDr (mDr- drought) genes to stress drought in A. thaliana seedlings was detected and it was shown that drought reduces the expression of four expansin genes on the first day of stress. The fifth gene, EXLB1, has been deducting since the second day of stress [76]. It was shown that these genes take part in an early response to drought. Other researchers have found the expression of expansin genes (Exp1, Exp5 and ExpB8) by reducing water potential in the apex of roots, corn leaves, and in the apex of tomato stems [77, 78, 79].

The physical properties of the cell wall are also play an important role in water deficiency [80]. Analysis of the pDr transcript showed repression of numerous genes involved in wall elongation stretching [81], whereas the action of mean osmotic stress induced elongating genes [82]. Microray analysis (qRT-PCR) revealed the regulation of cell stretching genes during mDr treatment. It has been established that most drought genes reduce their regulation during drought. The repression of four EXLB genes begins on the first day, while the fifth EXLB1 gene peaks on the first day and decreases on the second day; it is on the first day that acclimatization to stress may begin, depending on the organ, species and tissue [29, 83]. These studies were performed on A. thaliana [76].

The study of physical properties, stiffness in particular, cell wall from the root elongation zone using atomic force microscopy in A. thaliana Columbia-0 (Col-0) wild-type and mutant plants (with TETRATRICOPEPTIDE THIOREDOXIN-LIKE 1, ttl1 gene, cause root swelling and root growth arrest under NaCl and osmotic stress) revealed that root of mutant (with ttl1 gene) increase of the stiffness of the cell wall in root elongation zone [84].

A study of the effects of drought on Arabidopsis mutant plants (with cellulose synthase genes—AtCesA8 / IRX1), which were resistant to drought, NaCl, mannitol and other osmotic stresses, showed that cellulose synthesis under drought and osmotic stress is due to the expression of cellulose-synthase [85]. These researchers showed the effect of drought, by the absence of watering for 2 weeks, on the genetic traits of the cellulose-synthesizing complex in the leaves of A. thaliana. It was found the next following physiological and molecular changes in the leaves of two mutants Arabidopsis (with genes withering, leaf wilting 2–1 and leaf wilting 2–2; genes: lew2–1 and lew 2–2), which were resistant to drought, salt salinity, mannitol and other osmotic stresses: the lew2 mutant was shown to accumulate more ABA, proline, and soluble sugars compared to the control (wild type). New alleles of the AtCesA8 / IRX1 gene encoding subunits of the cellulose-synthesizing complex have been discovered in this mutant. The data obtained suggest that cellulose synthesis is quite important for the response to osmotic stress and drought [85].

Rui and Finneny [86] proposed a model for regulating the cell wall response to stress; according to this model, certain aspects of the wall itself can act as growth-regulating signals. The molecular components of the signaling pathways that determine and maintain cell wall integrity are shown, including sensors that detect changes on the cell surface and downstream signal transduction modules. There are several cell wall receptors that sense stress, including drought or salinity. Such receptors, according to the authors, may be the receptor-like kinase THESEUS1 (THE1) and FERONIA (FER) localized on the plasma membrane or Ca²⁺.ATPase. Kinase THE1 has been identified by suppressor screening in a cellulose-deficient mutant background; and FER is widely expressed and serves as a signaling node that functions in a wide range of processes, including plant growth, vacuole morphology, mechanosensing, hormonal signaling, and others. In contrast, the FER protein exhibits defects in growth recovery under salt stress as a result of failure to reverse salt-induced softening of the wall and increased frequency of cell rupture.

Summarizing the above material of numerous experimental works, we can propose the following scheme of response of cell walls of plants growing in drought or deserts: perception of drought signal (high air temperature and low soil moisture) leaves and roots → stopping or inhibiting growth of root and leaves → reduction of cell size → closure of stomata in leaves → reduction of stomatal conductivity for CO₂ (or cessation of stomata and shedding of leaves) → in the roots of the formation of water lacunae; in stems of succulents (during leaf shedding) water storage in specialized lacunae of the parenchyma → thickening of cell walls, their lignification and suberinization, intensified synthesis of wax, expression of genes associated with the synthesis of extensins, dehydrins and cellulose, activation of enzymes for synthesis of lignin, suberin due to changes in the expression of the corresponding genes (Figure 1).

Figure 1.

Schematic representation of the main functional changes of plant cell wall during adaptation to drought.

3. Flooding

Flooding is a potentially detrimental stress for many terrestrial plants; flooding occur when water covers the area, caused by both natural (river floods, heavy rainfall, tides) and artificial causes (construction of reservoirs, ponds); it can be short-term, intermittent (during river floods) or long-term, in which many species may die. Peculiarity of flooding as a stress factor is a combination of significant changes in water availability of plant and oxygen respiration in the root system, and as a result there is inhibition of aerobic processes, impaired absorption of ions and nutrients, changes in metabolism and growth processes [87]. The next factors are affected on the flood plant: a decrease in illumination and change in the light spectrum, a lack of acidity and CO₂. It is known that water absorb flow of light and disperse of light [88, 89]. In flood conditions, the diffusion of gases _is much slower than in air, and this is what limits normal photosynthesis and aerobic respiration [87]. Some plants that are resistant to flooding use the acceleration of stem growth to get out of the water and such a stem rises above the flooded part of the plant. The part of the plant that emerges from the water begins to come into contact with the air environment, renovating aerobic metabolism and photosynthesis [90]. Hydrophytes and wetland plants, which have adapted to both the lack of oxygen in the soil and the constant aquatic environment, have for millennia developed certain mechanisms of adaptation at different levels of the organization. The main signs of rearrangement are a decrease in the thickness of the leaf blade, rearrangement of the mesophyll, the presence of chloroplasts in the epidermis and changes in the structure of cell walls [91]. Cell wall of the epidermis of flooding plants is the first to react to the water environment, changing their structural- functional characteristics to optimize the water balance of plants. Therefore, the analysis of comparative structural and functional studies of flooded and above-water leaves is important for understanding the role of cell wall in the adaptation of plants to the aquatic environment.

3.1 Structural and functional changes of cell walls

The greatest stress for under-water plants is the weakening of gas exchange, which causes a decrease in oxygen in the stem and root, and also [92, 93] can induce enhanced growth by elongation, which promotes the release of leaves from the water to the surface and accelerates their contact with air [94]. Modification of cell walls for underwater growth and elongation requires energy, but, as a rule, such plants are characterized by limited aerobic metabolism. It is studied the structural changes in Rumex palustris stems and leaves that were induced by flooding [95]; authors have shown the decrease of size leaf and cuticle thickness in comparison with that in above-water leaves.

In cell walls of flooding leaves is occurred in protein synthesis. Under-water growth of rice is characterized by more elastic cell walls, which are usually characteristic of walls with increased synthesis of expansin [96, 97, 98]. In the cell wall noted protein modification, including expansins, which are activated at acidic pH [99, 100]. Rapid regulation of apoplastic pH provides a rapid way to regulate and modify apoplast expansin activity. The association between decreased cell wall elongations has been attributed to decreased tissue sensitivity to expansins [101]. Changes in the composition and nature of cross-links between cell wall polysaccharides may be limited by the mobility of expansins or their availability to the substrate polymer. The study showed a change in the ability of expansions to bind to cellulose depending on the properties of the hemicelluloses that cover the microfibrils [102].

In low-growing rice, flood resistance is explained by the activation of two genes: SUB1A-1 gene and gene ERF (ethylene response factor) [103]. The SUB1A-1 gene exhibits prolonged resistance to flooding associated with anaerobic metabolism and suppression of genes associated with high-energy processes, such as growth involving expansin [104]. The expression of SUB1A-1e is induced by ethylene, which is synthesized in a plant growing under water. Restriction of expansin transcripts by induction of SUB1A-1 occurs as a result of suppression of stem tension and dormancy of the plant. Modification of the growth rate is mediated by the modification of the cell wall in the plant organs that is not only under water but also above water. In particular, when the internode of rice came out of the water into the air, the rate of growth decreased.

The composition of the wall can also determine the effectiveness of expansins to elongation of a wall under conditions of flooding the plant. The decrease in the elongation of cell walls in the segments of underwater rice stalks at the exit from the water to the air correlates with the changes in the composition of walls: an increase in xylose and pectic acids, such as ferule acid [101], which has the ability to form cross-links between polysaccharides of a cell wall [105]. Deposits of xylose-enriched polysaccharides can change the composition of the cell wall by limiting the action of expansin. It was found that the composition of polysaccharides of flooded plants differs from that of surface organs, as shown by Little [106] in stems of Ludwigia repens: along with a significant decrease in cellulose content, the content of hemicelluloses and lignin in underwater stems increased.

The outer cell walls of the epidermis of submerging and the above-water leaves are the first barrier, the first transport route of CO₂ and water, as well as the point of contact of plant organs with the environment. Cell walls of flooding leaves became thinner and their structure is characterized by loosening. Regarding the loosening of the cell wall, there are many models of this process. The first hypothesis about the acid-induced loosening mechanism was proposed by Cleland [107, 108]. It was later shown that the hydrolysis of polysaccharides during loosening is a complex process in which the enzymatic hydrolysis of polysaccharides of the wall matrix occurs with the participation of endoglucanases and expansins. The latter shown that hydrolyse polysaccharides induce cell expansion and increase the plasticity of the wall depending on the pH of the apoplast [109, 110]. The mechanism of formation of thin cell walls in various plant tissues is explained by changes in cell turgor and a decrease in the activity of enzymes involved in the synthesis of wall polysaccharides [109].

In submerged plants in the epidermis is also synthesized and deposited cuticle in the periclinal walls, and cuticle structure change [111]. In the cuticle of leaves and stems, which grow rapidly by elongation under water, there is an accelerated hydrolysis of cutin polymers [112]. In aquatic plants (hydrophytes) the cuticle of the epidermis of leaves and stems causes the presence of super hydrophobicity; it is this property that prevents the formation of an aqueous film on the surface of the organs submerged in water, which greatly reduces the gas exchange between the surface of the leaf and the gases dissolved in water. Despite the fact that CO₂ absorption for photosynthesis is reduced in flooded plants, the air layer or gas film on the surface of underwater leaves continues to exchange O₂ and CO₂ through the cuticle from the surrounding water layer, and therefore underwater photosynthesis and underwater respiration occur in epidermal cells [113].

Most underwater leaves of hydrophytes have no stomata. Transport functions mainly fall on the cell walls of the epidermis and pores in epidermis. Cuticular pores were revealed on the cross-sections of epidermal cells of underwater Sagittaria sagittifolia, Trapa natans, Myriophyllum spicatum, Potamogeton pectinatus and Potamogeton perfoliatus [113, 114]. It was shown that average high of the cuticular pores in the cells of the epidermis of M. spicatum was about 130 nm; pore density ~ 12–15 per 1 μm of cuticle length; in P. pectinatus cells—the pore length depending on the plane of the section is ranged from 80 to 100 nm, the average pore density ~ 20 per 1 μm of cuticle length; in P. perfoliatus, the cuticular pores had low contrast and were barely visible in the form of rounded electronically transparent structures with a diameter near ~4–5 nm. Besides, author shown the absence of stomata on both surfaces of flooding leaves, the decrease of high of cuticular ridges in anticlinal walls and absent of wax in cell walls. The mechanism of the absence of wax on the surface of the periclinal wall of the underwater leaves can be explained by the next, it is established that genes (Ltp, LTPs and WAX9) that are responsible for the transcription of lipid-transporting proteins have recently been identified in wax-enriched epidermal cells. In the case of expression of the corresponding genes in the cells of the epidermis, the effect of the accumulation of the corresponding mRNA was found [115, 116], and possible that absent of wax in the epidermis of underwater leaves may be a consequence of genetic plasticity, which lead to the inhibition of the synthesis of precursors of wax (C12-, C14- and C16-ω-hydroxy fatty acids) and wax synthase activity (fatty alcohol acyltransferase) [117, 118].

3.2 The role of cellulose

Under-water leaves are characterized by the increase of amorphous cellulose and the decrease of its crystalline form. It is known that water is adsorbed by amorphous zones of cellulose, which are dominated by hydrogen bonds [119]. The crystalline component of cellulose micro fibrils is not involved in the transport or absorption of water molecules [120]. Given the above literature, we hypothesized that one of the adaptive features of the plant to flooding should be not only differences in cellulose content, but also advantages in the synthesis of its amorphous form. An optimal example of such adaptation to flooding can be the data of comparative structural and functional studies of cellulose in plants with underwater and above-water (surface) leaves, in particular in Sium latifolium, T. natans and S. sagittifolia, as well as leaves of hydrophytes growing only under water: Potamogeton perfoliat, P. pectinatus and Myriiophyllum spicatum. The above-water leaves of studied species, particular S. latifolium, T. natans and S. sagittifolia contained twice more of crystalline cellulose [18, 121, 122]. Considering the data on the identification of PhEXP1 gene (in Petunia hybrida mutant) responsible for the synthesis of amorphous cellulose [123], we believe that increased content of amorphous cellulose in underwater leaves, due to genetic differences, is an adaptive change of flooded plants. Literature data on the cellulose content in flooded plants are different. Métraux and Kende [124] found no differences in cellulose content in flooded and above-water internodes of rice stems, whereas Little [106] showed a 1.5-fold decrease in cellulose content in flooded shoots of L. repens compared to above-water shoots. It is possible, that it depends on the species, tissue and stage of development of the plant. We hypothesize that the decrease in cellulose content in the underwater leaves of the studied species may be due to inhibition of thegene CesA (cellulose synthase catalytic subunit) encoding cellulose synthesis enzymes in both primary [125] and secondary cell walls [126].

3.3 The role callose, lignin and pectin

Callose—a polysaccharide of the cell walls, formed by glucose residues, connected at the base of β-1-3-glucoside bonds and in the lateral branches—1-6 connections. It is known that β—1,3—glucan plays a key role in intercellular water transport, cell growth and differentiation, osmotic stretching of cells, plant protection under biotic and abiotic stresses [127] and increases the elasticity and flexibility of leaves and stems [128, 129, 130]. It was established the effects of natural flooding on callose content in Alisma plantago-aquatica leaves with laser confocal microscopy. It was shown that the content of callose in the cell walls of submerged leaves was more in three times in epidermis walls and in 8 times—in mesophyll cell walls in comparison with above-water leaves [131]. Increase of callose content in cell walls of other species was noted also: in submerged leaves of S. sagittifolia—in six times, in floating leaves of T. natans- in 1.4 times in comparison with above-water leaves. It was established that increase of callose content in submerged leaves are accompanied by change of calcium ions content in walls [132].

Lignin is a polymer of aromatic alcohols, which is synthesized in the cell walls, is completed the growth by tension, and it is involved in the adaptation of plants to flooding and in the change of the structure of the matrix of cell wall, providing obstruction of water and aqueous solutions through the cell walls and also form the barrier for pathogens. Lignin is a complex of monolignols formed from p-hydroxyphenyl, guajacyl, syringyl and H-phenylpropanoids components [133], which are involved in the polymerization of lignin, and they differ in the degree of methoxylation [46]. Flooding and siltation affect the lignification of cell walls. The study of mechanisms of adaptation of the root system of rice to flooding and siltation shown the main effect is the deficiency of oxygen, resulting in roots forming aerenchyma for storing of oxygen [134]. Lignin deposition, which counteracts the penetration of ions such as Fe²⁺, Cu²⁺ and NaCl [135] has been observed during of flooding roots. It is considered that lignin and suberin can form a barrier to the penetration of oxygen and ions.

The effect of flooding on the lignification of rice stems was found by comparing the stems of three varieties of rice. It was found that the lignin content in rice stems and the activity of two enzymes of the lignification (coniferol alcohol dehydrogenase (CAD) and phenylalanine ammonium lyase (PAL) were reduced after flooding in the flood-sensitive variety and in control. Lignin and the activities of the studied enzymes were interrelated. According to researchers [136, 137], underwater plant organs are stressed due to the tension of the water column and the mechanical action of waves, which should cause stress in flooded organs. Lignin of dicotyledonous plants consists of guajacyl (G), syringyl (S) and phenylpropanoids (H) components, Lignin of most monocotyledons have G and S units, the content of which is almost the same, they may also contain H units [138].

The question of the distribution of lignin in various tissues of submerged plant organs, the role of monolignols and their ratio in cell walls in the process of natural adaptation of plants to flooding has remained open until recently. Recently it was established that in floating leaf walls underwater leaves of T. natans the level of lignin fluorescence intensity increased 1.52 times in the anticlinal walls of the epidermis and 1.2 times—in the periclinal walls, and decreased 1.6 times in the cell walls of photosynthetic parenchyma compared to the corresponding cell walls of floating leaf [18]. Cytochemical studies of monolignols, their localization and content in the leaves of the four studied species of hydrophytes (Sagittaria sagitifolia, T. natans, M. spicatum; and P. perfoliatus) showed both common and different features. Common features were: (1) the presence of syringyl and guajacyl in the studied species, regardless of the conditions of leaf growth; (2) almost identical (low) values of the S/G ratio in the cells of the vessels of the above-water leaves of Sagittaria sagitifolia and water nut; (3) the highest S/G values for M. spicatum; and (4) a certain polarity S/G, which is characteristic of each species. Distinctive features were: (1) the relative content of syringyl and guajacyl, as well as the ratio of S/G in the cell walls of underwater and above-water leaves of arrowroot and water nut; and (2) high S/G ratio in the periclinal walls of floating leaves compared to those in submerged leaves of water nut [132].

Similar to the increase in lignin in flooded stems Ludviga repens was established [106]. Little S. showed that when L. repens stems were flooded, the lignin content became 1.6 times higher than in stems that came out (from) of the water. Why is this happening? Lignin is known to be a highly branched polymer of phenylpropanoid components synthesized in a complex cycle [139], the passage of which depends on numerous endogenous (phytohormones) and exogenous factors, including exposure to light, temperature, different gases and biotic stresses [140]. It is believed that the functional value of lignin is the mechanical support of tissues, which allows the plant to stay upright relative to the Earth and not fall [141]. At the same time, underwater organs of flooded plants (stems and leaves) must withstand water pressure (its weight) and the action of waves [136, 137] which affects the growth and structural and functional parameters of tissues and cells as this is described for flooded leaves of Veronica anagallis-aquatica [141]. On the other hand, it is known that underwater leaves are devoid of trichomes, stomata and thick cuticle) [132], so the surface of submerged plant organs becomes more sensitive to pathogen invasion. It has been shown that leaves and stems synthesize lignin in response to attack by pathogens (bacteria and fungi) [142]. It is possible that the above exogenous parameters and some endogenous factors cause increased synthesis of lignin in the underwater leaves of the studied species of T. natans and S. sagittifolia [132].

At flooding of terrestrial plants leads to the formation of aerenchyma in roots, nodules, stem or submerged leaves. Aerenchyma helps the plant to survive in conditions of hypoxia by reducing the number of oxygen-consuming cells in vegetative organs [143, 144]. It was established that at lysigenous type of formation of an aerenchyma occurred the lysis not only of cytoplasmic organelles in tissues, but also lysis of their cell walls. The increases in aerenchyma air volume may enable prolonged functioning of aerobic metabolic processes in tissues exposed to low-oxygen conditions. Cellulose, hemicellulose and pectin lysis are occurs during aerenchyma formation. Probably, that modification of the pectin homogalacturonan backbone structure through de-methyl-esterification appears to be one mechanism by which cell walls and middle lamella of tissues is degradate of pectin and enable cavity formation of aerenchyma in roots [143]. Additionally, presence of fully and partially de-methyl-esterified homogalacturonan residues in cell walls of forming tylose-like cells suggests these pectin structures are essential to development of the cells that occlude aerenchyma of P. sativum, P. coccineus and C. arietinum. The investigators think that aerenchyma formation may depend on activity of cellulase, xylanase working together to achieve cell wall degradation. Specifically, xylanases and cellulases may degrade xylan and cellulose polysaccharides in advance of de-methyl-esterification of pectin by PME enzymes and subsequent degradation by pectinases [143].

That is, the constant aquatic environment is one of the main exogenous factors of increased synthesis of lignin in the studied hydrophytes. In addition, we see that the presence of syringyl and guajacyl monolignols, as well as their relationship in the cell walls of the epidermis, mesophyll and leaf vessels of hydrophytes is similar to that described for dicotyledonous angiosperms [138, 145]. We do not rule out that the cell walls of the underwater leaves of the studied plants contain a third monolignol—phenylpropanoid (p-hydroxyphenyl) which will need to be investigated by other methods. Summarizing the whole section on the impact of flooding on the structural and functional changes of cell walls, in particular on the synthesis of cellulose, callose and lignin, we can schematically present the course of major events occurring in the apoplast of most cells (Figure 2).

Figure 2.

Schematic representation of the main functional changes of plant cell wall during adaptation to flooding.

4. Concluding remarks

The results of researches concerning on the role of cell walls in plant response to natural unfavorable conditions influences show that cell wall is one of the compartments of a plant cell that responds to drought and flooding. In most wild species and in cultivated species, cell walls stand a marker of such influence. The inhibition of plant growth, the change of plant morphological and anatomical signs, change of cell wall ultrastructure, its composition is occurred under prolonged drought or flooding. Changes in the structural and functional characteristics of cell walls allow plants to survive. Plant adaptation to these factors is depended on species, stage of growth plant and influence duration. Numerous studies have shown that drought effects negatively on cell walls. The main mechanisms of plant adaptation to the effects of drought involve a decrease in the intensity of transpiration, an increase in the synthesis of wax, suberin, and lignin, as well as the compaction of the walls of the epidermis tissues for preservation of optimal water balance. Upon exposure to flooding, adaptation mechanisms are expressed in the next: decrease stomata density and wax in leaf epidermis; a loosening of cellulose micro fibrils in walls of epidermal tissue and a present of cuticle pores; the decrease of common cellulose content and crystalline form of cellulose; an increase of content of amorphous cellulose, hemicelluloses in a cell wall; an intensification of callose synthesis; the change of a ratio of monolignols (syringyl and quajacyl) in walls; the activation of peroxidase and expansin, an intensification of ethylene synthesis and a change of calcium balance in apoplast. However, the sequence of these processes has not been fully disclosed. The question of the launch of adaptative processes also remains open. These issues require further research. The question of the relationship between the water balance of the cell, photosynthesis and the values of energy of light photons on the surface of the leaves, which launch an adaptive response in the plant under adverse natural changes or under stress, also remains open.

Competing interests

The author declares that there is no conflict interest.