The following points highlight the top seven functions of cell wall in the life of a plant. The functions are: 1. Gives Mechanical Strength 2. Maintains Cell Shape 3. Controls Cell Expansion 4. Controls Intercellular Transport 5. Protects Against Infective Organisms 6. Cell Wall has Signalling Device against Attack of Microorganism 7. Acts as a Reservoir of Food.
Function # 1. Gives Mechanical Strength:
The non-lignified wall derives strength from the cellulose microfibrils. It is believed that the mechanical strength of plant cell walls is due to the presence of skeletal framework formed by cellulosic microfibrils. Cellulose is the major component of paper, cotton etc. Maximum strength is obtained from collenchyma and sclerenchyma.
The walls of collenchyma cells are thickened with pectin, hemicellulose, protein and cellulose. Collenchyma cells are living and retain their protoplast even when mature. Therefore, they can regulate the deposition and orientation of wall materials according to the need of developing organs.
The cellulosic walls of wood are further strengthened by lignification. The large size and structural strength of a woody plant is achieved by the cell wall. In these plants the constituents of cell wall contributes 95% of the dry weight of wood. The rigidity of the whole plant and strength is due to cell wall.
In some species where cellulose is absent, other polysaccharides assume strengthening role and may form microfibril.
Function # 2. Maintains Cell Shape:
The various shapes of cell are attributable to cell wall. Isolated protoplasts are more or less spherical whose boundary is the plasma membrane. The plasma membrane is the most common site of cellulose synthesis and cellulose is one of the components of cell walls. Additional wall component may include pectin and/or lignin. These wall components surround the plasma membrane and thus the cell wall determines the shape of cells.
Function # 3. Controls Cell Expansion (Fig. 3.2):
Growth of plant cell depends on the elastic (i.e. reversible) and plastic (i.e. nonreversible) nature of cell walls.
In turgid cell turgor pressure presses the cell wall to expand and due to elastic nature the wall regains its original position at flaccid condition. In growing cell, when the critical turgor pressure exceeds, the wall expands in a plastic manner thus resulting in a permanent increase in cell volumes and growth.
This is a very slow process. The elastic stretching of wall due to turgidity accelerates the phenomenon. Thus, the turgor pressure is the driving force behind the extension. In multicellular plants the pressure, exerted by neighbouring cells, has much control in cell extension. Probably the covalent or non-covalent bonds within the matrix of wall are weakened by acid.
The covalent bond affects the cell wall through the action of an enzyme – probably transglycosylase. This enzyme causes the transfer of polysaccharide fragments from one site of glycosidic attachment to another. The non-covalent bonds may be the calcium bridges between polygalacturonate molecules or glycoproteins or glucuronoxylan.
Another non- covalent bonding is given by lectins in the cell wall. Lectins are proteins, which are involved in cross-linking polysaccharides or glycoproteins. Later the phenolic- cross-links between polymers are regarded to control wall extensibility. Phenolic links are found in lignin.
A cell ceases to elongate when lignin is laid down over the whole area of the wall. In some primary tracheids and phloem fibres, where lignin is deposited to a part of wall only, elongation and growth continue in the non-lignified areas.
Function # 4. Controls Intercellular Transport:
The transport of substances between plant cells involves cell wall and it occurs in two ways: apoplastic and symplastic transport (Fig. 3.3). In apoplastic transport, the movement of substances between neighbouring cells occurs through the matrix of cell wall and across the plasma membrane whereas in symplastic transport the movement occurs via plasmodesmata present between cells.
In apoplastic transport the size of the substance is very significant. The microfibrils and matrix polymer of the cell wall form sieve like structure, which inhibit the entry of large molecules and microorganisms. The smaller molecules, ions, small proteins and polysaccharides move through the aqueous channels present in the matrix.
The charges of molecules also have an effect on movement. The positively charged molecules tend to bind with negatively charged polymers present in the wall and thus their movement is retarded; the negatively charged, neutral or uncharged molecules move freely. The primary wall thus provides an ease transport.
The cuticle present on epidermis, the casparian strip of endodermis and lignified walls resist the apoplastic movement. The plasmodesmata, through which symplastic movement occurs, exist in pit pairs and in the sieve pores of sieve tubes.
Function # 5. Protects Against Infective Organisms:
The epidermal cell walls of higher plants are always subjected to attack by pathogenic microorganisms like virus, bacteria and fungi. This pathogen penetrates the host either by enzymatic dissolution of part of cell wall or through the opening of stomata or opportunistic entry through breaks or wounds in the wall. So, a cell wall possesses passive and active defense mechanisms against attack by microorganisms.
The intact cell wall forms an effective and passive physical barrier. The pores on cell wall are too small to allow entry even the tiny microorganisms. The cuticle, lignin, cutin and in grasses – silica in the epidermal cell wall provide an effective barrier against fungi. To penetrate cutin and middle lamella, secretion of cutinase and pectinase respectively are required by fungi. Silicon forms a mechanical barrier and act as a toxin against fungi.
Intact cell wall also possesses active defense mechanisms. It forms an effective barrier around the point of parasitic infection by the deposition of lignin and sometime suberin thus sealing the path of penetration of the pathogen.
The death of cells around infection also prevents further penetration. Sometimes callose deposits at the point of penetration of fungal haustorium adjacent to plasma membrane thus providing an effective barrier.
Evidences suggest that due to pathogenic attack cell wall polysaccharides degrade to oligosaccharides. Oligosaccharides from injured tissues diffuse out to the neighbouring cells and there it stimulates the active defensive mechanisms.
Function # 6. Cell Wall has Signalling Device against Attack of Microorganism:
The cell wall can react in response to pathogenic attack. It triggers active defence mechanisms to prevent the entry and establishment of a pathogen. The most common example is the production of phytoalexin by higher plants.
Phytoalexins are nonspecific toxins produced by higher plants when they are stimulated by the attack of a pathogen. The chemical structure of it varies from plant genera (e.g. Pisatin, Phaseolin, hircinol etc.). This protective substance takes an active part in the defence mechanisms of plants.
Apart from phytoalexin formation, a cell produces inhibitors of microbial proteins in response to signal emanated from injured tissues. Proteinases of microbial origin degrade protein of the host and the inhibitors prevent the protein from degradation. There is evidence that pectic polysaccharides and oligosaccharides are the signals for the production of proteinase inhibitor and phytoalexin.
Due to pathogenic attack cell wall polysaccharides degrade to oligosaccharides. The latter diffuses out to the neighbouring cells to stimulate the active defensive mechanism. Oligosaccharides have also role in cell extension and cell differentiation.
The cell wall also possesses passive and active defence mechanisms against larger predators. The passive mechanism includes presence of lignin on cell wall rendering plant tissues tough and unpalatable to higher animals. Presence of silicon to the epidermal cells also decreases the palatability.
The active defence mechanism of plants is triggered when the cell wall is stimulated by the attack of insects. In response, the plant produces the same inhibitor of microbial proteinase.
Function # 7. Acts as a Reservoir of Food:
A substantial quantity of food is always present in a seed to provide nutrition to the young seedling until it can absorb and photosynthesize independently. The foods may remain stored within the cell or as a component of the cell wall. The polysaccharides (mannans, xyloglucans and galactans) of cell wall are the example of the latter type. Mannans are cell wall storage polysaccharides.
Endosperm of Ericaceae and Apiaceae contain 90% mannose approximately. Glucomannans (i.e. glucose and mannose) are present in the endosperm of Liliaceae and Iridaceae. Galactomannans (mannose and galactose) are found in the endosperm of Fabaceae. Xyloglucans are found in those families where the food is stored in cotyledons, e.g. Legumes. Galactans are present as cell wall food reserve in the seed of Lupinus.
Cell wall also plays some important roles in the activities like secretion, transpiration, absorption, translocation etc.