3. Vesicular trafficking
Vacuoles are membrane-bound organelles found in plant cells and fungi, including yeasts. They are critical organelles for plant cell function.
Vacuoles are usually large compartments that in mature cells may be up to 90 % of the total cell volume. They are the largest compartment of plant cells. The name vacuole is derived from the Latin word "vacuus", which means empty. This was clearly a misunderstanding because vacuoles are not empty, but filled with a more or less concentrated aqueous solution. The membrane of the vacuole is known as tonoplast, and it is an essential part for the function of this organelle. In plants, there are several types of vacuoles according to the role they carry out. A plant cell may contain different types of vacuoles, and a vacuole can modify its enzyme repertory and then change its function.
Vacuoles are usually rounded, but the final shape is influenced by the cell morphology. One large vacuole is often observed in mature plant cells, but sometimes the membrane of the vacuole gets deeply and profusely folded, and creates small compartments that look like many small vacuoles when observed at light microscopy, but actually is just one because there is continuity of the vacuole membrane.
The formation of new vacuoles is by fusion of vesicles released from the Golgi complex. The new compartment is known as pro-vacuole. A meristematic cell may have hundreds of pro-vacuoles. Then, during cell differentiation, pro-vacuoles fuse between each other giving rise to larger vacuoles, and the fusion process continues until a large central vacuole is formed. However, endoplasmic reticulum might be also involved in the formation and growth of vacuoles in some plant cells.
The main vacuole of most plant cells is a large compartment filled with an acidic solution containing salts (sodium, potassium), metabolites (carbohydrates, organic acids) and some pigments. Some of these molecules enter from the cytosol to the vacuole against concentration gradient. The normal pH inside the vacuole ranges between 5 and 5.5, although it can be around 2 in the lemon fruit, or even 0.6 in some algae.
Vacuoles are essential for physiology and homeostasis of plant cell, and develop different functions according to the cell type. The following are some of them:
Turgor. Cell turgor is the level of hydrostatic pressure against the cell wall of the plant cell. This pressure is under the control of vacuoles, which get different substances inside, like ions, to produce variable osmotic environments when compared with cytosol. The different osmolarity at both sides of the tonoplast makes the water cross the membrane, either inward or outward. The substances that contribute to the inner osmolarity can cross the tonoplast by ATP dependent transport mediated by ionic pumps. H(+)-ATPase and H(+)-pyrophosphatase are able to form proton gradients between both sides of the vacuole membrane, and these gradients are used to transport other molecules. The ability to store water inside the vacuole is essential for plant cell growth after mitosis. Plant cells can increase their size 10 to 20 times, which is very useful for the increase in size and modify the shape of plant organs. The growth mediated by hydrostatic pressure saves energy because it is cheaper to increase the amount of water than synthesize new molecules (animal cell growth is based in molecular synthesis). It is safer for plant cells to accumulate water in the vacuole because in this way the cytosolic molecules do not get diluted, which would compromise the cell survival.
Storage. Vacuoles are a last station for some of the vesicular traffic pathways. In some cells, they are the place to store carbohydrates and proteins. This clearly happens in seeds where vacuoles accumulate proteins needed during germination. Unlike animals, plant cells do not have an excretory system nor they can move to avoid toxic substances, so that these potentially dangerous substances are stored in vacuoles. In this way, metabolism residues and toxic substances like heavy metals, such as cadmium, zinc and nickel, are found in vacuoles. However, they also store other substances such as pigments (for example, anthocyanins) in the epidermal cells of petals, toxic substances to avoid herbivores, resins, alkaloids like opium, etcetera. Most of the taste of fruits and vegetables is because of substances stored in vacuoles.
Degradation centers. The interior of vacuoles is acid and contains lytic enzymes. This low pH is produced by proton pumps located in the vacuole membrane. Low pH and lytic enzymes allow degradative processes. Vacuoles have a similar role to lysosomes of animal cells. Furthermore, like lysosomes do, vacuoles participate in autophagia.
Apoptosis. Vacuoles are involved in plant cell apoptosis via a mechanism known as autolysis.
Vacuoles are a final station of the vesicular traffic. Actually, they may be regarded as a product of the vesicular traffic. Molecules which are going to be stored or degraded, included the lytic enzymes, are targeted via vesicles to vacuoles. Furthermore, all the molecules of the membrane vacuole arrive as part of these vesicles. Molecules can follow different vesicular pathways to arrive to vacuoles:
Endoplasmic reticulum> Golgi complex> Vacuole; Golgi complex> pre-vacuolar compartment > vacuole. This is the default pathway to move lytic enzymes toward vacuoles which behave as lytic centers. Pre-vacuolar compartments are similar to late endosomes of animal cells. Curiously, lytic enzymes are not selected in the Golgi complex by 6-phosphate-mannose moieties, but by a sequence of amino acids located in their amino acid chain. There are specific sequences of amino acids for targeting proteins to the lytic vacuoles and other sequences are specific for moving other proteins toward the storing vacuoles.
Endoplasmic reticulum> vacuole. Molecules may arrive to vacuoles directly from the endoplasmic reticulum. This pathway is prominent in seeds as a pathway for storing. However, in other plant cells, as in leaves, this pathway might be very rare. Vesicles traveling from the endoplasmic reticulum to vacuoles are independent of COP-II coats, which are needed for vesicles targeted to the Golgi complex. In the endoplasmic reticulum-vacuole pathway, there are sometimes intermediate compartments, but they are transient membrane-bound organelles where molecules are shortly retained before they arrive to the vacuole.
Plasma membrane > vacuole. Endocytic vesicles fuse directly with vacuoles, which work like early endosomes.
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