Plant Vacuoles -

VACUOLES

TONOPLAST MEMBRANE

Vacuoles are surrounded by vacuole membrane known as tonoplast or vacuolar membrane. It resembles the plasmalemma, but it differs in function and is often thinner. The plasmalemma controls the entry and exists of solutes into the cytoplasm, the tonoplast transports solutes into and of the vacuoles and thus controls the water potential of the cell. This is especially important for example, in the guard cells of the stomatal apparatus. Potassium and other ions are pumped in or out of guard cells to swell or shrink, closing or opening of stomates. The tonoplast is derived from the ER, but recent evidence suggest that this may be via Golgi apparatus, as in the case with the plasmalemma. In some cases, ER may small (dilate) directly to form vacuoles

FUNCTIONS OF VACUOLES

vacuoles plays several metabolic roles-

1- Storage and accumulation: vacuoles store a large variety of molecules, including inorganic ions, organic acids, sugars, enzymes, storage proteins and many types of secondary metabolites. Proteins in the tonoplast transport all of these molecules except for storage proteins into the vacuoles. The resulting accumulation of solutes in the vacuole drives the osmotic uptake of water, producing the turgor pressure needed for cell enlargement. Some vacuoles have a high concentration of pigments, which produce the color of many flowers or the red or red maple leaves. In maple leaves, the red color is so concentrated in the epidermis that they mask the green color of the chloroplast many primary metabolites can be retrieved from vacuoles.

2- Plant cell expansion is driven by a combination of osmotic uptake of water into the vacuoles and altered. Cell wall extensibility the water taken into the vacuoles generates turgor pressure which not only expands the primary cell wall but also creates stiff load-bearing structures in conjunction with the wall. Wilting and the associated softening of plant organs is caused by loss of water from the vacuoles and surrounding cytosol.

To maintain the turgor pressure of continuously expanding cells solutes must be actively transported into the growing vacuoles to maintain its osmolarity. An electrochemical gradient across the tonoplast provides the driving force for this uptake solutes. The gradient is produced and maintained by two electrogenic protons pump. V-type H⁺ ATPase and vacuolar H⁺ pyrophosphates (H-Ppase). The principal solutes in vacuoles include the ions K⁺, Na⁺, ca²⁺, Mg²⁺, Cl^–, So₄^2–, Po₄^3– and No^3– and primary metabolites, such as amino acids, organic acids, and sugar. The moment of water across the tonoplast is mediated by aquaporin channels consisting of tonoplast intrinsic proteins (TIPs).

Digestion (vacuoles as lysosomes)

The enzymes in vacuoles digest vacuous materials absorbed in them. This probably happens when the protoplast of wood cells break down and die vacuole behaves like lysosomes, a cellular organelle common in animal and some fungal and protest cells. Lysosomes contain hydrolytic enzymes that break down materials they absorb or these enzyme digest much of the protoplasm following cell death and breakdown of lysosome membranes. The importance of this role for vacuole is still begin investigated because not all enzymes that break down protein in cells are present in vacuoles. In Yeast, 90% of these enzymes are located in vacuoles.

pH and ionic homeostasis

Homeostasis is the tendency for various physiological parameters to be maintained at some relatively constant level. A good example in the plant is a relatively constant concentration of a various substance in the cytosol. The concentration of H⁺(pH) provides an example and the vacuole plays an important role in the maintenance of constant cytosolic pH provides an example and the vacuole plays an important role in the maintenance of constant cytosolic pH. Excess of hydrogen ions in the cytosol may be pumped into the vacuoles. Plant vacuoles have a pH between 5.0 and 5.5 but the range extends from 25 or so (lemon fruit vacuoles) to greater than 7.0 in inactivated protein storage vacuoles. By controlling the release of protons and other ions into the cytosol cells can regulate not only cytosolic pH but also the actively of enzymes, the assembly of cytoskeletal structures and membrane fusion events.

Ca⁺ and phosphate ions would be toxic to the cytosol if their concentration became too high. The vacuole absorbs these ions and keeps their concentration in the cytosol within limits. Phosphate and nitrate provide examples of essential ions stored in vacuoles. If cytosolic phosphate or nitrate level drops too low, these lows can move from vacuoles into the cytosol. The same is for sugar, amino acids, and many other stored materials. Therefore vacuoles may be a dumping ground, but it is a warehouse.

Defense against microbial pathogens and herbivores

Plant cells accumulate an amazing variety of toxic compound in their vacuoles, both to reduce feeding by herbivores and to destroy microbial pathogens these compounds include the following:

1. Phenolic compounds alkaloids cyanogenic glycosides and a protease inhibitor to discourage insect and animal behaviors.
2. Cell wall degrading enzymes, such as chitinase and glucanase and defense molecules such as saponins to destroy pathogenic fungi and bacteria.

Latexes would be clogging emulsions of hydrophobic polymers that possess insecticidal and fungicidal properties which serves as antiherbivory agents.

Sequestration of toxic compounds

Plants cannot escape from toxic sites nor  they can eliminate toxic materials like heavy materials like oxalate by excretion. Instead, plants sequester these compounds into vacuoles. For example to remove oxalate specific cells develops vacuoles containing an organic matrix within which oxalate is allowed to react with Ca to form Ca-oxalate crystals. In other plant cell types membrane of the ABC family of transporters are used to transport xenobiotics from the cytoplasm into the vacuoles. Accumulation of toxic compounds in leaf vacuoles is one of the reasons leaves are shed on a regular basis.

Pigmentation

Vacuoles that contain anthocyanin pigments are found in many types of plant cells. Pigmented flowers, petals and fruits are used to attract pollinators and seed disperser. Some leaf pigments screen outs uv are visible light, preventing photo-oxidation damage to the photosynthetic apparatus. This screening appears to be essential for the survival of the leaves of evergreens that grow in freezing conditions during hunters prevents the absorbed light energy being used in photosynthesis.