Wall stress relaxation

Last updated September 18, 2023

The plant cell wall is made up of hydrated polymetric material, allowing it to have viscoelastic properties.^[1] The primary cell wall of a plant consists of cellulose fibers, hemicellulose, and xyloglucans.^[2] This load bearing network is also surrounded by pectins and glycoproteins.

Wall stress relaxation is an important factor in cell wall expansion. Wall stress (measured in force per unit area) is created in response to the plant cell's turgor pressure.^[2] Turgor pressure creates tension in the cell walls of plants, fungi, and bacteria, as it opposes the pressure of the cell's primary cell wall; this also allows for stretching of the cell wall.^[1] The stretching of the cell wall, or the reduction of stress, occurs as a result of cell expansion and rearrangement. Cell expansion is crucial for the reshaping and rearranging of plant cells.^[1] Expansion is the result of "creep", or selective wall loosening, which is driven by turgor pressure. During this "creep", cellulose microfibers move relative to each other creating an irreversible extension ^[2]

Cell expansion

Cell expansion begins with the selective loosening of the cell wall, reducing the plant cell's turgor pressure and water potential. This allows for the influx of water, leading to cell enlargement.^[3] This enlargement is made possible by the sliding of polymers, increasing the cell wall's surface area.

In most plants, cell expansion is anisotropic. Previous experiments have confirmed that the cellulose microfibril orientation in the primary cell wall is the key for determining the direction of anisotropic growth and expansion. Cells tend to grow transversely to the cellulose microfibril orientation.^[2]

It has been found that cell walls expand faster under acidic conditions, this is called acid growth. Treating living cells with acid induces acidification of the cell wall by activating an ATPase in the cell wall's plasma membrane.^[1] In onion epidermal cells, which are used as models to study anisotropy in extension, extension is pH dependent in both directions (transverse and parallel to cellulose orientation). Extension is also nearly three times higher at a pH of 4-5 than a pH of 6. This is a strong indication of acid growth in these cell wall samples.^[2] It has also been shown that the activity of expansins, a cell-wall loosening protein, is maximized at low pH conditions (around a pH of 4).^[3]

Heat inactivation reduces extension transverse to cellulose microfibril orientation but does not reduce parallel extension. This indicates that heat inactivation has a directional effect on cell wall extension. In the transverse direction, extension depends on proteins, as denatured proteins cause reduced extension. In addition, extension occurs parallel to cellulose microfibril orientation is dependent on pH. Therefore, acid induced extension in the parallel direction is not protein mediated as the proteins are denatured and extension parallel to cellulose microfibril orientation is not affected.^[2]

Expansins

Expansins are a class of proteins that act as wall-loosening agents. These proteins break hydrogen bonds between xyloglucans and cellulose as well as restore acid growth of heat-inactivated cell walls by stimulating growth.^[2] To date, three classes of expansins have been identified: α-expansins, β-expansins, and bacterial expansins.^[3] There are still numerous unanswered questions regarding expansins and their exact mechanism.

Experiments done with mechanical stress assays have shown that α-expansins do not weaken the cell wall, yet they have been shown to induce "creep" in cell walls. Additionally, α-expansins have been found to mediate acid-induced wall extension.^[3] Opposed to α-expansins, β-expansins drastically reduce the tensile strength of cell walls.^[3] Not only do β-expansins cause "creep" but it also solubilizes polysaccharides in the middle lamella, aiding the penetration of the pollen tube to the plant's ovary.^[3] β-expansins have been studied in grass pollen due to the fact that β-expansins are difficult to extract in active form from plants outside the pollen group.^[3] Expansins have even been as plant pathogenic bacteria, identified by phylogenetic analysis. Gene knockout experiments were used in discovering that bacterial expansins facilitate the colonization of plant tissue.^[3]

Xyloglucan endotransglucosylase/endohydrolases (XTHs)

Xyloglucan endotransglucosylase/endohydrolases (XTHs) are another class of enzymes that play a role in cell wall loosening. Most XTHs break and put back together xyloglucans that restrict the movement of adjacent cellulose microfibrils in the cell wall. This is referred to as xyloglucan endotransglucosylase action (XET action). XET action allows for xyloglucan restructuring which therefore allows cellulose microfibrils to move apart while still maintaining the mechanical strength of the cell wall, preventing lysis. Other XTHs use and bind to water which is referred to as xyloglucan hydrolase action (XEH action).^[2]

These XTH enzymes need to diffuse into the cell wall and form complexes with, or act on, xyloglucans in the non-load bearing outer region of the cell wall. After a lag period, the enzymes will reach a concentration threshold in the load-bearing region of the cell wall and the activity of the enzyme will then be apparent.^[2]

A specific XTH, SkXTH1 is able to perform XET activity over a large range of pHs and temperatures.^[2] Heat inactivation reduces the cell wall extension but if SkXTH1 is added, about 66% of the protein-dependent creep activity that was eliminated during heat inactivation can thus be restored.^[2] However, this restoration of extension occurred only in the transverse to the cellulose microfibril orientation. This indicates that SkXTH1 is an XTH enzyme responsible for catalyzing the movement of adjacent cellulose microfibrils relative to one another transverse to their net cellulose orientation. This further supports that extension in this transverse direction is protein dependent, whereas in the parallel direction, it is not.^[2]

Related Research Articles

A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mechanism. Cell walls are absent in many eukaryotes, including animals, but are present in some other ones like fungi, algae and plants, and in most prokaryotes. A major function is to act as pressure vessels, preventing over-expansion of the cell when water enters.

Cellulose is an organic compound with the formula (C^₆H^₁₀O^₅)^_n, a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Some species of bacteria secrete it to form biofilms. Cellulose is the most abundant organic polymer on Earth. The cellulose content of cotton fiber is 90%, that of wood is 40–50%, and that of dried hemp is approximately 57%.

<span class="mw-page-title-main">Hemicellulose</span> Class of plant cell wall polysaccharides

A hemicellulose is one of a number of heteropolymers, such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls. Cellulose is crystalline, strong, and resistant to hydrolysis. Hemicelluloses are branched, shorter in length than cellulose, and also show a propensity to crystallize. They can be hydrolyzed by dilute acid or base as well as a myriad of hemicellulase enzymes.

Fibrils are structural biological materials found in nearly all living organisms. Not to be confused with fibers or filaments, fibrils tend to have diameters ranging from 10–100 nanometers. Fibrils are not usually found alone but rather are parts of greater hierarchical structures commonly found in biological systems. Due to the prevalence of fibrils in biological systems, their study is of great importance in the fields of microbiology, biomechanics, and materials science.

A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently observed structural pattern is the 9+2 pattern in which two central protofibrils are surrounded by nine other pairs. Cellulose inside plants is one of the examples of non-protein compounds that are using this term with the same purpose. Cellulose microfibrils are laid down in the inner surface of the primary cell wall. As the cell absorbs water, its volume increases and the existing microfibrils separate and new ones are formed to help increase cell strength.

Katanin is a microtubule-severing AAA protein. It is named after the Japanese sword called a katana. Katanin is a heterodimeric protein first discovered in sea urchins. It contains a 60 kDa ATPase subunit, encoded by KATNA1, which functions to sever microtubules. This subunit requires ATP and the presence of microtubules for activation. The second 80 kDA subunit, encoded by KATNB1, regulates the activity of the ATPase and localizes the protein to centrosomes. Electron microscopy shows that katanin forms 14–16 nm rings in its active oligomerized state on the walls of microtubules.

Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall.

Xyloglucan is a hemicellulose that occurs in the primary cell wall of all vascular plants; however, all enzymes responsible for xyloglucan metabolism are found in Charophyceae algae. In many dicotyledonous plants, it is the most abundant hemicellulose in the primary cell wall. Xyloglucan binds to the surface of cellulose microfibrils and may link them together. It is the substrate of xyloglucan endotransglycosylase, which cuts and ligates xyloglucans, as a means of integrating new xyloglucans into the cell wall. It is also thought to be the substrate of alpha-expansin, which promotes cell wall enlargement.

<span class="mw-page-title-main">Pectinesterase</span> Class of enzymes

Pectinesterase (EC 3.1.1.11; systematic name pectin pectylhydrolase) is a ubiquitous cell-wall-associated enzyme that presents several isoforms that facilitate plant cell wall modification and subsequent breakdown. It catalyzes the following reaction:

<span class="mw-page-title-main">Brassinolide</span> Chemical compound

Brassinolide is a plant hormone. The first isolated brassinosteroid, it was discovered when it was shown that pollen from rapeseed could promote stem elongation and cell division. The biologically active component was isolated and named brassinolide.

Acid growth refers to the ability of plant cells and plant cell walls to elongate or expand quickly at low (acidic) pH. The cell wall needs to be modified in order to maintain the turgor pressure. This modification is controlled by plant hormones like auxin. Auxin also controls the expression of some cell wall genes. This form of growth does not involve an increase in cell number. During acid growth, plant cells enlarge rapidly because the cell walls are made more extensible by expansin, a pH-dependent wall-loosening protein. Expansin loosens the network-like connections between cellulose microfibrils within the cell wall, which allows the cell volume to increase by turgor and osmosis. A typical sequence leading up to this would involve the introduction of a plant hormone (auxin, for example) that causes protons (H⁺ ions) to be pumped out of the cell into the cell wall. As a result, the cell wall solution becomes more acidic. It was suggested by different scientist that the epidermis is a unique target of the auxin but this theory has been disapproved over time. This activates expansin activity, causing the wall to become more extensible and to undergo wall stress relaxation, which enables the cell to take up water and to expand. The acid growth theory has been very controversial in the past.

Expansins are a family of closely related nonenzymatic proteins found in the plant cell wall, with important roles in plant cell growth, fruit softening, abscission, emergence of root hairs, pollen tube invasion of the stigma and style, meristem function, and other developmental processes where cell wall loosening occurs. Expansins were originally discovered as mediators of acid growth, which refers to the widespread characteristic of growing plant cell walls to expand faster at low (acidic) pH than at neutral pH. Expansins are thus linked to auxin action. They are also linked to cell enlargement and cell wall changes induced by other plant hormones such as gibberellin, cytokinin, ethylene and brassinosteroids.

The secondary cell wall is a structure found in many plant cells, located between the primary cell wall and the plasma membrane. The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding.

The UDP-forming form of cellulose synthase is the main enzyme that produces cellulose. Systematically, it is known as UDP-glucose:(1→4)-β-D-glucan 4-β-D-glucosyltransferase in enzymology. It catalyzes the chemical reaction:

Xyloglucan endotransglucosylase (XET) is an apoplastic enzyme found across the plant kingdom. The enzyme catalyzes the endotransglucosylation of two xyloglucan polysaccharides, effectively 'stitching' them together.

Microcrystalline cellulose (MCC) is a term for refined wood pulp and is used as a texturizer, an anti-caking agent, a fat substitute, an emulsifier, an extender, and a bulking agent in food production. The most common form is used in vitamin supplements or tablets. It is also used in plaque assays for counting viruses, as an alternative to carboxymethylcellulose.

<span class="mw-page-title-main">Polygalacturonase</span>

Endo-polygalacturonase (EC 3.2.1.15, pectin depolymerase, pectolase, pectin hydrolase, and poly-α-1,4-galacturonide glycanohydrolase; systematic name (1→4)-α-D-galacturonan glycanohydrolase (endo-cleaving)) is an enzyme that hydrolyzes the α-1,4 glycosidic bonds between galacturonic acid residues:

Glucanases are enzymes that break down large polysaccharides via hydrolysis. The product of the hydrolysis reaction is called a glucan, a linear polysaccharide made of up to 1200 glucose monomers, held together with glycosidic bonds. Glucans are abundant in the endosperm cell walls of cereals such as barley, rye, sorghum, rice, and wheat. Glucanases are also referred to as lichenases, hydrolases, glycosidases, glycosyl hydrolases, and/or laminarinases. Many types of glucanases share similar amino acid sequences but vastly different substrates. Of the known endo-glucanases, 1,3-1,4-β-glucanase is considered the most active.

In molecular biology, the xyloglucan endo-transglycosylase (XET) is an enzyme that is involved in the metabolism of xyloglucan, which is a component of plant cell walls. This enzyme is part of glycoside hydrolase family 16.

The acid-growth hypothesis is a theory that explains the expansion dynamics of cells and organs in plants. It was originally proposed by Achim Hager and Robert Cleland in 1971. They hypothesized that the naturally occurring plant hormone, auxin (indole-3-acetic acid, IAA), induces H⁺ proton extrusion into the apoplast. Such derived apoplastic acidification then activates a range of enzymatic reactions which modifies the extensibility of plant cell walls. Since its formulation in 1971, the hypothesis has stimulated much research and debate. Most debates have concerned the signalling role of auxin and the molecular nature of cell wall modification. The current version holds that auxin activates small auxin-up RNA (SAUR) proteins, which in turn regulate protein phosphatases that modulate proton-pump activity. Acid growth is responsible for short-term (seconds to minutes) variation in growth rate, but many other mechanisms influence longer-term growth.

References

1 2 3 4 Taiz, Lincoln (2015). Plant Physiology and Development (Sixth ed.). Sinauer Associates, Inc.
1 2 3 4 5 6 7 8 9 10 11 12 Van Sandt, Vicky (4 October 2007). "Xyloglucan Endotransglucosylase Activity Loosens a Plant Cell Wall". Annals of Botany. 100 (7): 1467–73. doi:10.1093/aob/mcm248. PMC 2759230 . PMID 17916584.
1 2 3 4 5 6 7 8 Cosgrove, Daniel J. (2016-01-29). "Catalysts of plant cell wall loosening". F1000Research. 5: F1000 Faculty Rev–119. doi: 10.12688/f1000research.7180.1 . ISSN 2046-1402. PMC 4755413 . PMID 26918182.

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[:0-1] 1 2 3 4 Taiz, Lincoln (2015). Plant Physiology and Development (Sixth ed.). Sinauer Associates, Inc.

[:1-2] 1 2 3 4 5 6 7 8 9 10 11 12 Van Sandt, Vicky (4 October 2007). "Xyloglucan Endotransglucosylase Activity Loosens a Plant Cell Wall". Annals of Botany. 100 (7): 1467–73. doi:10.1093/aob/mcm248. PMC 2759230 . PMID 17916584.

[:2-3] 1 2 3 4 5 6 7 8 Cosgrove, Daniel J. (2016-01-29). "Catalysts of plant cell wall loosening". F1000Research. 5: F1000 Faculty Rev–119. doi: 10.12688/f1000research.7180.1 . ISSN 2046-1402. PMC 4755413 . PMID 26918182.

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