The Vacuole

by  Lorenzo Frigerio1, Chris Trobacher2 & Alison Sinclair2

1. Department of Biological Sciences, University of Warwick, Coventry CV4 7AL. United Kingdom
2. Department of Molecular & Cellular Biology, College of Biological Sciences, University of Guelph. ON N1G 2W1 Canada.

Version 2. by L. Frigerio . May 2010. 

Vacuoles in Arabidopsis

Left panel: Central (lytic) vacuoles in Arabidopsis leaf epidermal cells. TIP1;1-YFP (green) marks the tonoplast, spRFP-AFVY (red) highlights the lumen [24]. Right panel: Protein storage vacuoles (PSV) in mature Arabidopsis embryo cotyledonary cells. TIP3;1-YFP (green) marks the tonoplast, and spRFP-AFVY (red) highlights the lumen [24].

Plant vacuoles are organelles bound by a single membrane, the tonoplast, which participate in a wide variety of functions [1, 2]. Several types of vacuoles exist in plant tissues, and in some species cells may contain multiple vacuolar types [3-5]. Vacuoles are the intracellular endpoint of the secretory pathway. They are dynamic structures able to alter their luminal pH, converting to a different class of vacuole, or fuse together forming a large central vacuole [6-8]. They participate in growth through regulation of turgor, maintain homeostasis, function as storage organelles, sequester toxic materials, contain both 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 function, and molecular markers such as tonoplast intrinsic proteins (TIPs) and constituent enzymes such as proteases are useful tools to highlight different vacuolar types.  The biogenesis of vacuoles and the routes that transport proteins to both their lumen and the tonoplast are key areas of current research (for a recent review see [9]).
Central Vacuole: The central vacuole of many plant cells is perhaps the most conspicuous organelle occupying up to 90% of a cell’s volume [2]. Meristematic cells contain numerous small provacuoles (resembling animal lysosomes) that arise from fusion of trans-Golgi-derived vesicles [1]. As a cell differentiates and expands, the pro-vacuoles fuse, forming a large acidic central vacuole. The primary 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 [2]. The vacuole generates this turgor pressure by sequestering osmotic agents such as K+ [10]. The secondary function(s) of the central vacuole is dependent on the plant and the tissue. 
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 [11, 12]. 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 [13]. 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 [13]. Other vacuolar 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 [14].
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 [15]. 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 [16, 17].
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 [18]. 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 autolysis. 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 [19-21].
Protein Storage Vacuoles: PSVs in mature seeds are filled with storage proteins during seed maturation 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 [22, 23]. The PSV tonoplast is typically marked by alpha-tonoplast intrinsic proteins (TIP3;1)[24, 25], and the crystalloid can be immunolabelled with anti-Dark Intrinsic Protein (DIP) antibodies [22, 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 [26].
List of FP probes for the Vacuole
Probes for Vacuoles

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