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 (Figures 1 and 2). 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 cell morphology. One large vacuole is often observed in mature plant cells. However, the membrane of the vacuole gets sometimes deeply and profusely folded and forms small compartments that look like many small vacuoles when observed at light microscopy, but they are actually just one vacuole because membrane is continuous.
New vacuoles are formed by fusion of vesicles released from the Golgi apparatus. Initially they form a new compartment known as pro-vacuole. A meristematic cell may have hundreds of pro-vacuoles. Then, during cell differentiation, pro-vacuoles fuse between each other into small vacuoles, and the fusion process continues until a large central vacuole is formed. The endoplasmic reticulum might be also involved in the formation and growth of vacuoles in some plant cells, mostly in seeds. Once a large vacuole is present, vesicles from the Golgi apparatus and plasma membrane regulate the size by adding and removing membrane.
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 the vacuole from the cytosol 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 cells, and perform different functions according to the cell type. The following are some of them:
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, including ions, to produce variable inner osmotic environments when compared with those of the cytosol. The different osmolarity at both sides of the vacuole membrane makes the water cross the membrane, either inward or outward. The substances that contribute to the vacuole osmolarity can cross the vacuole membrane 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 grow after mitosis. Plant cells can increase their size 10 to 20 times, which is very useful for the body plant to grow and for modifying the shape of plant organs. The grow 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 on 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 cell survival.
Vacuoles are the last station for some vesicular traffic pathways. In some cells, they are the compartment to store carbohydrates and proteins. This clearly happens in seeds, where vacuoles accumulate proteins needed during germination. Storage vacuoles become lytic vacuoles during cell differentiation. Unlike animals, plants do not have an excretory system, nor they can move to avoid toxic substances. In plants, potentially dangerous substances are stored in vacuoles. In this way, metabolism residues and toxic substances like heavy metals (cadmium, zinc and nickel) are found in vacuoles. In addition, they also store other substances such as pigments (for example, anthocyanins) in the epidermal cells of petals, toxic substances against herbivores, resins, alkaloids like opium, etcetera. Most of the taste of fruits and vegetables is the result of substances stored in vacuoles.
Lytic vacuoles can be found in vegetative tissues, so they are also known as vegetative vacuoles. They contain enzymes like proteases and nucleases, as well as a number of proteins involved in the defense against pathogens. Proton pumps inserted in the vacuole membrane enter protons into the vacuole and acidify the interior content. The low pH and the lytic enzymes allow degradation processes. Vacuoles have a similar role to lysosomes of animal cells. Furthermore, like lysosomes, vacuoles participate in autophagy. Vacuolar processing enzymes are proteins also found in vacuoles. They transform molecular precursors arriving to the vacuole as inactive molecules into active molecules.
Vacuoles are involved in plant cell apoptosis via a mechanism known as autolysis. In addition, a type of cell death known as hypersensitive cell death occurs in plant cells when the vacuole membrane gets broken.
There are specialized vacuoles in different plant tissues. For example, in the seed internal teguments, vacuoles accumulate flavonoids for protection against ultraviolet light. Flavonoids are synthesized in the cytosolic surface of the endoplasmic reticulum membranes and then translocated to the interior of vacuoles for a final chemical processing. In the vacuole membrane there are transporters to carry out this translocation.
Some plant species, like brassicas, have vacuoles in their vegetative tissues for repealing herbivores. These vacuoles store proteins, such as myrosinases. Once released by the herbivore activity, these enzymes degrade molecular compounds of the leaves that become toxic for the animal. Cells storing myrosine are known as myrosine cells and can be found near the vascular bundles of leaves.
Plants lack immune system so that each cell has their own defense system. Defense proteins and enzymes can be found in vacuoles. There are two defense mechanisms that vacuoles can perform (Figure 3): vacuole membrane collapses and fusion of membrane vacuole with plasma membrane. Viral infections lead to vacuole membrane breakage and release enzymes into the cytosol, where they can attack viruses. The fusion of vacuole membrane and plasma membrane releases vacuole enzymes to the extracellular space where they can kill pathogens like bacteria.
3. Vesicular trafficking
Vacuoles are part of the vesicular traffic. Actually, they may be regarded as an end-product of the vesicular trafficking since their formation and maintaining depends on the incoming vesicles. Molecules which are going to be stored or degraded, included hydrolytic enzymes, as well as all membrane molecules are targeted to vacuoles via vesicles. Molecules can follow different vesicular pathways to get to vacuoles:
Endoplasmic reticulum> Golgi apparatus> Vacuole; Golgi apparatus> pre-vacuolar compartment > vacuole. This is the default pathway to transport hydrolytic enzymes toward vacuoles. Pre-vacuolar compartments are similar to multivesicular bodies/ late endosomes of animal cells. Curiously, hydrolytic enzymes are not selected in the Golgi apparatus 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 hydrolytic vacuoles and other sequences are specific for moving other proteins toward the storing vacuoles. All proteins targeted to vacuoles have a signal sequence, and they need to be specifically recognized by receptors.
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 apparatus. 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. This vesicular pathway may be derived from autophagy cellular components.
Plasma membrane > vacuole. Endocytic vesicles fuse directly with vacuoles, which work like early endosomes.
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