The Vacuole

by Chris Trobacher & Alison Sinclair

Department of Molecular & Cellular Biology, College of Biological Sciences,
University of Guelph. ON N1G 2W1 Canada.

Plant vacuoles are bound by a single membrane, the tonoplast, and participate in a wide variety of cellular functions [1]. Several types of vacuoles exist in plants, and any given cell may contain multiple types [2, 3]. These dynamic structures are able to alter their luminal pH, convert to a different vacuolar class, or fuse together to form a large central vacuole [4, 5, 6]. They participate in growth through regulation of turgor, maintain cellular homeostasis, function as storage organelles, sequester toxic materials, may contain anti-fungal enzymes and anti-herbivory compounds, degrade old organelles (autophagy), and participate in programmed cell death via auto-lysis. Vacuoles are classified primarily by their function; however, molecular markers such as Tonoplast Intrinsic Proteins (TIPs) and constituent enzymes such as proteases provide new tools for vacuolar classification.
The Central Vacuole
The central vacuole of many plant cells is perhaps the most conspicuous organelle occupying up to 90% of a cell’s volume [1]. Meristematic cells contain numerous small provacuoles (resembling animal lysosomes) that arise from fusion of trans-golgi-derived vesicles. As a cell differentiates and expands, the pro-vacuoles fuse, forming a large acidic central vacuole. The well recognized role of the central vacuole is to increase cell size via changes in turgor pressure in concert with alteration of cytoskeletal and cell wall architecture [1]. The vacuole generates this turgor pressure by sequestering osmotic agents such as K+ [1, 7]. Vacuoles can sequester a variety of materials that are toxic to plants. Excess metals such as Cd, Zn, and Ni are stored within the central vacuole of various leaf cells including mesophyll, epidermis, and trichomes [8, 9]. The central vacuole of specialized crystal idioblast cells in developing soybean leaves accumulates calcium oxalate crystals as a mechanism to sequester excess calcium from other developing cells [10]. Crystal idioblasts in species such as Pistia and Tragia participate in plant defence. These cells produce needle-like calcium oxalate crystals that are ejected from thin-walled cells upon mechanical contact, piercing the dermis of unsuspecting animals causing irritation, presumably an effective deterrent against herbivory [10]. Other vacuole mediated defences include antimicrobial pathogenesis-related (PR) proteins. Five groups of PR proteins have two forms, the basic form stored in the central vacuole, and the acidic form found in the apoplast [11].
Vacuoles also participate in autophagy, the process by which the cell digests its own organelles. Upon Tobacco Mosaic Virus infection, mesophyll cells produce autophagosomes, double-membrane bound structures derived from the endoplasmic reticulum. The autophagosomes sequester cytoplasm, organelles, and/or pathogens and subsequently fuse with the central vacuole where the contents are degraded by hydrolytic enzymes [12]. A similar process is induced in sucrose-starved Arabidopsis cell cultures, as well as during reserve mobilization in Vigna mungo cotyledons, however, a lytic vacuole is the site of degradation rather than a central vacuole [13, 14]. Leaf vacuolar anthocyanins impart a reddish colour to leaves at discrete developmental stages, eg. leaf senescence in deciduous trees, or in response to stress. The current paradigm suggests that this pigmentation protects the photosynthetic apparatus against both photoinhibition and photooxidation [15]. Pigments also accumulate in the central vacuole of petal epidermal cells [16, 1], likely serving to attract pollinators.
The central vacuole also plays an important role in programmed cell death through a process known as auto-lysis. After autophagic processes have reclaimed much of the cell’s resources, the tonoplast is permeabilized, or ruptures completely, lowering cytosolic pH and releasing hydrolytic enzymes into the cytoplasm. This leads to bulk degradation of the cell corpse and has been observed in differentiating tracheary elements, in dying endosperm and aleurone cells during seedling establishment, and in dying nucellar cells during seed development [17, 18, 19, 20].
Protein Storage Vacuoles
Protein storage vacuoles (PSVs) in mature seeds are filled with storage proteins during seed development and have several characteristic ultrastructural features. These organelles contain a matrix of soluble storage proteins, crystalloid protein deposits, and globoids of phytic acid or oxalate crystals [21, 22]. The PSV tonoplast is typically marked by alpha and delta tonoplast intrinsic proteins (TIPs), and the crystalloid can be immunolabelled with anti-Dark Intrinsic Protein (DIP) antibodies [22]. Some vegetative tissues contain small neutral vacuoles, or vegetative PSVs, in addition to the lytic central vacuole; these compartments are not well-studied to date [23].
Senescence Associated Vacuoles
During leaf senescence, chloroplast-containing cells of Arabidopsis and soybean form small, acidic, 550-700 nm in diameter senescence associated vacuoles (SAVs). These vacuoles have both a lower luminal pH, and different tonoplast markers, than the central vacuole. The SAVs of Arabidopsis label with a senescence-associated cysteine proteinase-GFP fusion (SAG12-GFP), and contain multiple proteases [24].
List of FP probes for the Vacuole
Probes for Vacuoles

References

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