Plant Cells PDF

▲ Plant Cells 5 FIGURE 1.3 (A) The outer epidermis (dermal tissue) of a THE PLANT CELL leaf of welwischia mirabilis (120×). Diagrammatic representa- tion...

▲ Plant Cells 5 FIGURE 1.3 (A) The outer epidermis (dermal tissue) of a THE PLANT CELL leaf of welwischia mirabilis (120×). Diagrammatic representa- tions of three types of ground tissue: (B) parenchyma, (C) Plants are multicellular organisms composed of millions of collenchyma, (D) sclerenchyma cells, and (E) conducting cells with specialized functions. At maturity, such special- cells of the xylem and phloem. (A © Meckes/Ottawa/Photo ized cells may differ greatly from one another in their struc- Researchers, Inc.) tures. However, all plant cells have the same basic eukary- otic organization: They contain a nucleus, a cytoplasm, and subcellular organelles, and they are enclosed in a mem- brane that defines their boundaries (Figure 1.4). Certain sues are illustrated and briefly chacterized in Figure 1.3. structures, including the nucleus, can be lost during cell For further details and characterizations of these plant tis- maturation, but all plant cells begin with a similar comple- sues, see Web Topic 1.3. ment of organelles. Vacuole Tonoplast Nucleus Nuclear Peroxisome envelope Nucleolus Chromatin Ribosomes Rough Compound endoplasmic middle reticulum lamella Smooth endoplasmic Mitochondrion reticulum Primary cell wall Plasma membrane Middle lamella Cell wall Golgi body Primary cell wall Chloroplast Intercellular air space FIGURE 1.4 Diagrammatic representation of a plant cell. Various intracellular com- partments are defined by their respective membranes, such as the tonoplast, the nuclear envelope, and the membranes of the other organelles. The two adjacent pri- mary walls, along with the middle lamella, form a composite structure called the compound middle lamella. 6 Chapter 1 An additional characteristic feature of plant cells is that the bilayer. As a result, the fluidity of the membrane is they are surrounded by a cellulosic cell wall. The following increased. The fluidity of the membrane, in turn, plays a sections provide an overview of the membranes and critical role in many membrane functions. Membrane flu- organelles of plant cells. The structure and function of the idity is also strongly influenced by temperature. Because cell wall will be treated in detail in Chapter 15. plants generally cannot regulate their body temperatures, they are often faced with the problem of maintaining mem- Biological Membranes Are Phospholipid Bilayers brane fluidity under conditions of low temperature, which That Contain Proteins tends to decrease membrane fluidity. Thus, plant phos- All cells are enclosed in a membrane that serves as their pholipids have a high percentage of unsaturated fatty outer boundary, separating the cytoplasm from the exter- acids, such as oleic acid (one double bond), linoleic acid nal environment. This plasma membrane (also called plas- (two double bonds) and α-linolenic acid (three double malemma) allows the cell to take up and retain certain sub- bonds), which increase the fluidity of their membranes. stances while excluding others. Various transport proteins embedded in the plasma membrane are responsible for this Proteins. The proteins associated with the lipid bilayer selective traffic of solutes across the membrane. The accu- are of three types: integral, peripheral, and anchored. Inte- mulation of ions or molecules in the cytosol through the gral proteins are embedded in the lipid bilayer. Most inte- action of transport proteins consumes metabolic energy. gral proteins span the entire width of the phospholipid Membranes also delimit the boundaries of the specialized bilayer, so one part of the protein interacts with the outside internal organelles of the cell and regulate the fluxes of ions of the cell, another part interacts with the hydrophobic core and metabolites into and out of these compartments. of the membrane, and a third part interacts with the inte- According to the fluid-mosaic model, all biological rior of the cell, the cytosol. Proteins that serve as ion chan- membranes have the same basic molecular organization. nels (see Chapter 6) are always integral membrane pro- They consist of a double layer (bilayer) of either phospho- teins, as are certain receptors that participate in signal lipids or, in the case of chloroplasts, glycosylglycerides, in transduction pathways (see Chapter 14). Some receptor-like which proteins are embedded (Figure 1.5A and B). In most proteins on the outer surface of the plasma membrane rec- membranes, proteins make up about half of the mem- ognize and bind tightly to cell wall consituents, effectively brane’s mass. However, the composition of the lipid com- cross-linking the membrane to the cell wall. ponents and the properties of the proteins vary from mem- Peripheral proteins are bound to the membrane surface brane to membrane, conferring on each membrane its by noncovalent bonds, such as ionic bonds or hydrogen unique functional characteristics. bonds, and can be dissociated from the membrane with high salt solutions or chaotropic agents, which break ionic Phospholipids. Phospholipids are a class of lipids in and hydrogen bonds, respectively. Peripheral proteins which two fatty acids are covalently linked to glycerol, serve a variety of functions in the cell. For example, some which is covalently linked to a phosphate group. Also are involved in interactions between the plasma membrane attached to this phosphate group is a variable component, and components of the cytoskeleton, such as microtubules called the head group, such as serine, choline, glycerol, or and actin microfilaments, which are discussed later in this inositol (Figure 1.5C). In contrast to the fatty acids, the head chapter. groups are highly polar; consequently, phospholipid mol- Anchored proteins are bound to the membrane surface ecules display both hydrophilic and hydrophobic proper- via lipid molecules, to which they are covalently attached. ties (i.e., they are amphipathic). The nonpolar hydrocarbon These lipids include fatty acids (myristic acid and palmitic chains of the fatty acids form a region that is exclusively acid), prenyl groups derived from the isoprenoid pathway hydrophobic—that is, that excludes water. (farnesyl and geranylgeranyl groups), and glycosylphos- Plastid membranes are unique in that their lipid com- phatidylinositol (GPI)-anchored proteins (Figure 1.6) ponent consists almost entirely of glycosylglycerides (Buchanan et al. 2000). rather than phospholipids. In glycosylglycerides, the polar head group consists of galactose, digalactose, or sulfated The Nucleus Contains Most of the Genetic galactose, without a phosphate group (see Web Topic 1.4). Material of the Cell The fatty acid chains of phospholipids and glycosyl- The nucleus (plural nuclei) is the organelle that contains the glycerides are variable in length, but they usually consist genetic information primarily responsible for regulating the of 14 to 24 carbons. One of the fatty acids is typically satu- metabolism, growth, and differentiation of the cell. Collec- rated (i.e., it contains no double bonds); the other fatty acid tively, these genes and their intervening sequences are chain usually has one or more cis double bonds (i.e., it is referred to as the nuclear genome. The size of the nuclear unsaturated). genome in plants is highly variable, ranging from about 1.2 The presence of cis double bonds creates a kink in the × 108 base pairs for the diminutive dicot Arabidopsis thaliana chain that prevents tight packing of the phospholipids in to 1 × 1011 base pairs for the lily Fritillaria assyriaca. The  Plant Cells 7 (A) (C) H3C Choline N+ H H3C C H C C O Phosphate H O Hydrophilic H O P region O Cell wall H H Glycerol C H H Plasma C H C membrane O O O C O C H H C C H H H H C C H H H H C C H H H H C C Carbohydrates H H H H C C H H H H Outside of cell C C H H H H C C H H Hydrophobic H H C H C H region H H H Hydrophilic C H H C C H region C H Phospholipid H H H C H C C H bilayer H H H C H Hydrophobic H H H C C H region C H H H C H C H H H H C Hydrophilic H H region Phosphatidylcholine Cytoplasm Integral Peripheral protein protein Choline (B) O –O O Galactose P Plasma membranes O O H2C CH CH2 H 2C CH CH2 Adjoining O O O O primary walls C O C O C O C O CH2 CH2 CH2 CH2 1 mm FIGURE 1.5 (A) The plasma membrane, endoplasmic retic- ulum, and other endomembranes of plant cells consist of proteins embedded in a phospholipid bilayer. (B) This trans- Phosphatidylcholine Galactosylglyceride mission electron micrograph shows plasma membranes in cells from the meristematic region of a root tip of cress (Lepidium sativum). The overall thickness of the plasma mem- brane, viewed as two dense lines and an intervening space, is 8 nm. (C) Chemical structures and space-filling models of typical phospholipids: phosphatidylcholine and galactosyl- glyceride. (B from Gunning and Steer 1996.) 8 Chapter 1 OUTSIDE OF CELL Glycosylphosphatidylinositol (GPI)– anchored protein Ethanolamine P Galactose Glucosamine Mannose Inositol P Lipid bilayer NH HO OH O Myristic acid (C14) Palmitic acid (C16) Farnesyl (C15) Geranylgeranyl (C20) Ceramide C O S S S Amide bond HN CH2 CH2 CH2 Gly Cys H C C O CH3 H C C O CH3 C N O N O N C Fatty acid–anchored proteins N N Prenyl lipid–anchored proteins CYTOPLASM FIGURE 1.6 Different types of anchored membrane proteins that are attached to the membrane via fatty acids, prenyl groups, or phosphatidylinositol. (From Buchanan et al. 2000.) remainder of the genetic information of the cell is contained ure 1.8). There can be very few to many thousands of in the two semiautonomous organelles—the chloroplasts nuclear pore complexes on an individual nuclear envelope. and mitochondria—which we will discuss a little later in The central “plug” of the complex acts as an active (ATP- this chapter. driven) transporter that facilitates the movement of macro- The nucleus is surrounded by a double membrane molecules and ribosomal subunits both into and out of the called the nuclear envelope (Figure 1.7A). The space nucleus. (Active transport will be discussed in detail in between the two membranes of the nuclear envelope is Chapter 6.) A specific amino acid sequence called the called the perinuclear space, and the two membranes of nuclear localization signal is required for a protein to gain the nuclear envelope join at sites called nuclear pores (Fig- entry into the nucleus. ure 1.7B). The nuclear “pore” is actually an elaborate struc- The nucleus is the site of storage and replication of the ture composed of more than a hundred different proteins chromosomes, composed of DNA and its associated pro- arranged octagonally to form a nuclear pore complex (Fig- teins. Collectively, this DNA–protein complex is known as  Plant Cells 9 (A) (B) Nuclear envelope Nucleolus Chromatin FIGURE 1.7 (A) Transmission electron micrograph of a plant cell, showing the nucleolus and the nuclear envelope. (B) Freeze-etched preparation of nuclear pores from a cell of an onion root. (A courtesy of R. Evert; B cour- tesy of D. Branton.) chromatin. The linear length of all the DNA within any nucleus, segments of the linear double helix of DNA are plant genome is usually millions of times greater than the coiled twice around a solid cylinder of eight histone pro- diameter of the nucleus in which it is found. To solve the tein molecules, forming a nucleosome. Nucleosomes are problem of packaging this chromosomal DNA within the arranged like beads on a string along the length of each chromosome. During mitosis, the chromatin condenses, first by coil- ing tightly into a 30 nm chromatin fiber, with six nucleo- somes per turn, followed by further folding and packing CYTOPLASM Nuclear pore complex processes that depend on interactions between proteins and nucleic acids (Figure 1.9). At interphase, two types of 120 nm chromatin are visible: heterochromatin and euchromatin. Cytoplasmic ring Cytoplasmic About 10% of the DNA consists of heterochromatin, a Outer nuclear filament highly compact and transcriptionally inactive form of chro- membrane matin. The rest of the DNA consists of euchromatin, the dispersed, transcriptionally active form. Only about 10% of Spoke-ring assembly the euchromatin is transcriptionally active at any given time. The remainder exists in an intermediate state of con- densation, between heterochromatin and transcriptionally active euchromatin. Nuclei contain a densely granular region, called the nucleolus (plural nucleoli), that is the site of ribosome syn- thesis (see Figure 1.7A). The nucleolus includes portions of Nuclear Inner nuclear one or more chromosomes where ribosomal RNA (rRNA) ring membrane genes are clustered to form a structure called the nucleolar organizer. Typical cells have one or more nucleoli per Nuclear Central nucleus. Each 80S ribosome is made of a large and a small basket transporter subunit, and each subunit is a complex aggregate of rRNA and specific proteins. The two subunits exit the nucleus NUCLEOPLASM separately, through the nuclear pore, and then unite in the cytoplasm to form a complete ribosome (Figure 1.10A). FIGURE 1.8 Schematic model of the structure of the nuclear Ribosomes are the sites of protein synthesis. pore complex. Parallel rings composed of eight subunits each are arranged octagonally near the inner and outer Protein Synthesis Involves membranes of the nuclear envelope. Various proteins form Transcription and Translation the other structures, such as the nuclear ring, the spoke- ring assembly, the central transporter, the cytoplasmic fila- The complex process of protein synthesis starts with tran- ments, and the nuclear basket. scription—the synthesis of an RNA polymer bearing a base 10 Chapter 1 FIGURE 1.9 Packaging of DNA in a metaphase chromo- 2 nm some. The DNA is first aggregated into nucleosomes and then wound to form the 30 nm chromatin fibers. Further coiling leads to the condensed metaphase chromosome. DNA double helix (After Alberts et al. 2002.) Linker DNA Translation is the process whereby a specific protein is 11 nm synthesized from amino acids, according to the sequence information encoded by the mRNA. The ribosome travels Histones Nucleosome the entire length of the mRNA and serves as the site for the sequential bonding of amino acids as specified by the base Nucleosomes ( “beads on a string”) sequence of the mRNA (Figure 1.10B). The Endoplasmic Reticulum Is a Network of Internal Membranes Cells have an elaborate network of internal membranes called the endoplasmic reticulum (ER). The membranes of 30 nm the ER are typical lipid bilayers with interspersed integral and peripheral proteins. These membranes form flattened or tubular sacs known as cisternae (singular cisterna). Nucleosome Ultrastructural studies have shown that the ER is con- tinuous with the outer membrane of the nuclear envelope. 30 nm chromatin fiber There are two types of ER—smooth and rough (Figure 1.11)—and the two types are interconnected. Rough ER (RER) differs from smooth ER in that it is covered with ribosomes that are actively engaged in protein synthesis; in addition, rough ER tends to be lamellar (a flat sheet com- 300 nm posed of two unit membranes), while smooth ER tends to be tubular, although a gradation for each type can be Looped domains observed in almost any cell. The structural differences between the two forms of ER are accompanied by functional differences. Smooth ER functions as a major site of lipid synthesis and membrane 700 nm assembly. Rough ER is the site of synthesis of membrane proteins and proteins to be secreted outside the cell or into Condensed chromatin the vacuoles. Secretion of Proteins from Cells Begins with the Chromatids Rough ER Proteins destined for secretion cross the RER membrane and enter the lumen of the ER. This is the first step in the 1400 nm ▲ Highly condensed, duplicated FIGURE 1.10 (A) Basic steps in gene expression, including metaphase chromosome transcription, processing, export to the cytoplasm, and of a dividing cell translation. Proteins may be synthesized on free or bound ribosomes. Secretory proteins containing a hydrophobic signal sequence bind to the signal recognition particle (SRP) in the cytosol. The SRP–ribosome complex then moves to the endoplasmic reticulum, where it attaches to the SRP sequence that is complementary to a specific gene. The receptor. Translation proceeds, and the elongating polypep- RNA transcript is processed to become messenger RNA tide is inserted into the lumen of the endoplasmic reticu- (mRNA), which moves from the nucleus to the cytoplasm. lum. The signal peptide is cleaved off, sugars are added, and the glycoprotein is transported via vesicles to the The mRNA in the cytoplasm attaches first to the small ribo- Golgi. (B) Amino acids are polymerized on the ribosome, somal subunit and then to the large subunit to initiate with the help of tRNA, to form the elongating polypeptide translation. chain.  Plant Cells 11 (A) Nucleus Cytoplasm DNA Nuclear pore Transcription Nuclear Intron Exon envelope RNA transcript Processing Poly-A Cap RNA rRNA mRNA tRNA (B) Amino Arg Polypeptide acids chain Gly Ser Cap tRNA Val Ser Poly-A P Phe mRNA tRNA site Ribsomal CAG A AGG E subunits site AAA site 5’ m7G AGC GUC UUU UCC GCC UGA 3’ Ribosome mRNA Poly-A Ribosome Translation Protein synthesis on Protein synthesis on ribosomes ribosomes free in attached to endoplasmic reticulum; cytoplasm polypeptide enters lumen of ER Processing and glycosylation in Golgi body; Poly-A Cap Poly-A Cap sequestering and secretion of proteins Signal Signal sequence recognition particle (SRP) Polypeptide Transport vesicle Polypeptides free in Release of SRP Poly-A Cap cytoplasm Poly-A Cap SRP receptor Cleavage of Rough signal sequence endoplasmic Carbohydrate side chain reticulum 12 Chapter 1 Polyribosome Ribosomes (C) Smooth ER (A) Rough ER (surface view) FIGURE 1.11 The endoplasmic reticulum. (A) Rough ER can be seen in surface view in this micrograph from the alga Bulbochaete. The polyribosomes (strings of ribosomes attached to messenger RNA) in the rough ER are clearly visible. Polyribosomes are also present on the outer surface of the nuclear envelope (N-nucleus). (75,000×) (B) Stacks of regularly arranged rough endoplasmic reticulum (white arrow) in glandular trichomes of Coleus blumei. The plasma (B) Rough ER (cross section) membrane is indicated by the black arrow, and the material outside the plasma membrane is the cell wall. (75,000×) (C) Smooth ER often forms a tubular network, as shown in this transmission electron transfer of the elongating polypeptide across the ER mem- micrograph from a young petal of Primula kewensis. brane into the lumen. (In the case of integral membrane (45,000×) (Photos from Gunning and Steer 1996.) proteins, a portion of the completed polypeptide remains embedded in the membrane.) Once inside the lumen of the ER, the signal sequence is secretion pathway that involves the Golgi body and vesi- cleaved off by a signal peptidase. In some cases, a branched cles that fuse with the plasma membrane. oligosaccharide chain made up of N-acetylglucosamine The mechanism of transport across the membrane is (GlcNac), mannose (Man), and glucose (Glc), having the complex, involving the ribosomes, the mRNA that codes stoichiometry GlcNac2Man9Glc3, is attached to the free for the secretory protein, and a special receptor in the ER amino group of a specific asparagine side chain. This car- membrane. All secretory proteins and most integral mem- bohydrate assembly is called an N-linked glycan (Faye et al. brane proteins have been shown to have a hydrophobic 1992). The three terminal glucose residues are then sequence of 18 to 30 amino acid residues at the amino-ter- removed by specific glucosidases, and the processed gly- minal end of the chain. During translation, this hydropho- coprotein (i.e., a protein with covalently attached sugars) bic leader, called the signal peptide sequence, is recognized is ready for transport to the Golgi apparatus. The so-called by a signal recognition particle (SRP), made up of protein N-linked glycoproteins are then transported to the Golgi and RNA, which facilitates binding of the free ribosome to apparatus via small vesicles. The vesicles move through the SRP receptor proteins (or “docking proteins”) on the ER cytosol and fuse with cisternae on the cis face of the Golgi (see Figure 1.10A). The signal peptide then mediates the apparatus (Figure 1.12).  Plant Cells 13 be no direct membrane continuity between successive cisternae, the con- tents of one cisterna are transferred to trans Golgi the next cisterna via small vesicles network (TGN) budding off from the margins, as occurs in the Golgi apparatus of ani- trans cisternae mals. In some cases, however, entire cisternae may progress through the medial Golgi body and emerge from the cisternae trans face. Within the lumens of the Golgi cis- cis cisternae ternae, the glycoproteins are enzy- matically modified. Certain sugars, such as mannose, are removed from the oligosaccharide chains, and other sugars are added. In addition to these FIGURE 1.12 Electron micrograph of a Golgi apparatus in a tobacco (Nicotiana modifications, glycosylation of the tabacum) root cap cell. The cis, medial, and trans cisternae are indicated. The trans —OH groups of hydroxyproline, ser- Golgi network is associated with the trans cisterna. (60,000×) (From Gunning and ine, threonine, and tyrosine residues Steer 1996.) (O-linked oligosaccharides) also occurs in the Golgi. After being processed within the Golgi, the gly- Proteins and Polysaccharides for Secretion Are coproteins leave the organelle in other vesicles, usually Processed in the Golgi Apparatus from the trans side of the stack. All of this processing The Golgi apparatus (also called Golgi complex) of plant appears to confer on each protein a specific tag or marker cells is a dynamic structure consisting of one or more stacks that specifies the ultimate destination of that protein inside of three to ten flattened membrane sacs, or cisternae, and or outside the cell. an irregular network of tubules and vesicles called the In plant cells, the Golgi body plays an important role in trans Golgi network (TGN) (see Figure 1.12). Each indi- cell wall formation (see Chapter 15). Noncellulosic cell wall vidual stack is called a Golgi body or dictyosome. polysaccharides (hemicellulose and pectin) are synthesized, As Figure 1.12 shows, the Golgi body has distinct func- and a variety of glycoproteins, including hydroxyproline- tional regions: The cisternae closest to the plasma membrane rich glycoproteins, are processed within the Golgi. are called the trans face, and the cisternae closest to the cen- Secretory vesicles derived from the Golgi carry the poly- ter of the cell are called the cis face. The medial cisternae are saccharides and glycoproteins to the plasma membrane, between the trans and cis cisternae. The trans Golgi network where the vesicles fuse with the plasma membrane and is located on the trans face. The entire structure is stabilized empty their contents into the region of the cell wall. Secre- by the presence of intercisternal elements, protein cross- tory vesicles may either be smooth or have a protein coat. links that hold the cisternae together. Whereas in animal cells Vesicles budding from the ER are generally smooth. Most Golgi bodies tend to be clustered in one part of the cell and vesicles budding from the Golgi have protein coats of some are interconnected via tubules, plant cells contain up to sev- type. These proteins aid in the budding process during vesi- eral hundred apparently separate Golgi bodies dispersed cle formation. Vesicles involved in traffic from the ER to the throughout the cytoplasm (Driouich et al. 1994). Golgi, between Golgi compartments, and from the Golgi to The Golgi apparatus plays a key role in the synthesis and the TGN have protein coats. Clathrin-coated vesicles (Fig- secretion of complex polysaccharides (polymers composed ure 1.13) are involved in the transport of storage proteins of different types of sugars) and in the assembly of the from the Golgi to specialized protein-storing vacuoles. They oligosaccharide side chains of glycoproteins (Driouich et al. also participate in endocytosis, the process that brings sol- 1994). As noted already, the polypeptide chains of future gly- uble and membrane-bound proteins into the cell. coproteins are first synthesized on the rough ER, then trans- ferred across the ER membrane, and glycosylated on the The Central Vacuole Contains Water and Solutes —NH2 groups of asparagine residues. Further modifications Mature living plant cells contain large, water-filled central of, and additions to, the oligosaccharide side chains are car- vacuoles that can occupy 80 to 90% of the total volume of ried out in the Golgi. Glycoproteins destined for secretion the cell (see Figure 1.4). Each vacuole is surrounded by a reach the Golgi via vesicles that bud off from the RER. vacuolar membrane, or tonoplast. Many cells also have The exact pathway of glycoproteins through the plant cytoplasmic strands that run through the vacuole, but each Golgi apparatus is not yet known. Since there appears to transvacuolar strand is surrounded by the tonoplast. 14 Chapter 1 Protein body FIGURE 1.13 Preparation of clathrin-coated vesicles isolated from bean leaves. (102,000×) (Photo courtesy of D. G. Robinson.) Lytic vacuole In meristematic tissue, vacuoles are less prominent, though they are always present as small provacuoles. Provacuoles are produced by the trans Golgi network (see FIGURE 1.14 Light micrograph of a protoplast prepared Figure 1.12). As the cell begins to mature, the provacuoles from the aleurone layer of seeds. The fluorescent stain reveals two types of vacuoles: the larger protein bodies (V1) fuse to produce the large central vacuoles that are charac- and the smaller lytic vacuoles (V2). (Photo courtesy of P. teristic of most mature plant cells. In such cells, the cyto- Bethke and R. L. Jones.) plasm is restricted to a thin layer surrounding the vacuole. The vacuole contains water and dissolved inorganic ions, organic acids, sugars, enzymes, and a variety of secondary metabolites (see Chapter 13), which often play roles in plant outer and an inner membrane). Mitochondria (singular defense. Active solute accumulation provides the osmotic mitochondrion) are the cellular sites of respiration, a process driving force for water uptake by the vacuole, which is in which the energy released from sugar metabolism is required for plant cell enlargement. The turgor pressure used for the synthesis of ATP (adenosine triphosphate) generated by this water uptake provides the structural from ADP (adenosine diphosphate) and inorganic phos- rigidity needed to keep herbaceous plants upright, since phate (Pi) (see Chapter 11). they lack the lignified support tissues of woody plants. Mitochondria can vary in shape from spherical to tubu- Like animal lysosomes, plant vacuoles contain hydro- lar, but they all have a smooth outer membrane and a highly lytic enzymes, including proteases, ribonucleases, and gly- convoluted inner membrane (Figure 1.15). The infoldings cosidases. Unlike animal lysosomes, however, plant vac- of the inner membrane are called cristae (singular crista). uoles do not participate in the turnover of macromolecules The compartment enclosed by the inner membrane, the throughout the life of the cell. Instead, their degradative mitochondrial matrix, contains the enzymes of the path- enzymes leak out into the cytosol as the cell undergoes way of intermediary metabolism called the Krebs cycle. senescence, thereby helping to recycle valuable nutrients In contrast to the mitochondrial outer membrane and all to the living portion of the plant. other membranes in the cell, the inner membrane of a mito- Specialized protein-storing vacuoles, called protein bod- chondrion is almost 70% protein and contains some phos- ies, are abundant in seeds. During germination the storage pholipids that are unique to the organelle (e.g., cardiolipin). proteins in the protein bodies are hydrolyzed to amino The proteins in and on the inner membrane have special acids and exported to the cytosol for use in protein syn- enzymatic and transport capacities. thesis. The hydrolytic enzymes are stored in specialized The inner membrane is highly impermeable to the pas- lytic vacuoles, which fuse with the protein bodies to ini- sage of H+; that is, it serves as a barrier to the movement of tiate the breakdown process (Figure 1.14). protons. This important feature allows the formation of electrochemical gradients. Dissipation of such gradients by Mitochondria and Chloroplasts Are Sites of Energy the controlled movement of H+ ions through the trans- Conversion membrane enzyme ATP synthase is coupled to the phos- A typical plant cell has two types of energy-producing phorylation of ADP to produce ATP. ATP can then be organelles: mitochondria and chloroplasts. Both types are released to other cellular sites where energy is needed to separated from the cytosol by a double membrane (an drive specific reactions.  Plant Cells 15 (A) (B) Intermembrane space H+ H+ H+ Outer membrane H+ H+ Inner membrane ADP + ATP Pi H+ Matrix Cristae FIGURE 1.15 (A) Diagrammatic representation of a mito- chondrion, including the location of the H+-ATPases involved in ATP synthesis on the inner membrane. (B) An electron micrograph of mitochondria from a leaf cell of Bermuda grass, Cynodon dactylon. (26,000×) (Photo by S. result in a proton gradient across the thylakoid membrane. E. Frederick, courtesy of E. H. Newcomb.) As in the mitochondria, ATP is synthesized when the pro- ton gradient is dissipated via the ATP synthase. Plastids that contain high concentrations of carotenoid Chloroplasts (Figure 1.16A) belong to another group of pigments rather than chlorophyll are called chromoplasts. double membrane–enclosed organelles called plastids. They are one of the causes of the yellow, orange, or red col- Chloroplast membranes are rich in glycosylglycerides (see ors of many fruits and flowers, as well as of autumn leaves Web Topic 1.4). Chloroplast membranes contain chlorophyll (Figure 1.17). and its associated proteins and are the sites of photosynthe- Nonpigmented plastids are called leucoplasts. The most sis. In addition to their inner and outer envelope mem- important type of leucoplast is the amyloplast, a starch- branes, chloroplasts possess a third system of membranes storing plastid. Amyloplasts are abundant in storage tis- called thylakoids. A stack of thylakoids forms a granum sues of the shoot and root, and in seeds. Specialized amy- (plural grana) (Figure 1.16B). Proteins and pigments (chloro- loplasts in the root cap also serve as gravity sensors that phylls and carotenoids) that function in the photochemical direct root growth downward into the soil (see Chapter 19). events of photosynthesis are embedded in the thylakoid membrane. The fluid compartment surrounding the thy- Mitochondria and Chloroplasts Are lakoids, called the stroma, is analogous to the matrix of the Semiautonomous Organelles mitochondrion. Adjacent grana are connected by unstacked Both mitochondria and chloroplasts contain their own membranes called stroma lamellae (singular lamella). DNA and protein-synthesizing machinery (ribosomes, The different components of the photosynthetic appa- transfer RNAs, and other components) and are believed to ratus are localized in different areas of the grana and the have evolved from endosymbiotic bacteria. Both plastids stroma lamellae. The ATP synthases of the chloroplast are and mitochondria divide by fission, and mitochondria can located on the thylakoid membranes (Figure 1.16C). Dur- also undergo extensive fusion to form elongated structures ing photosynthesis, light-driven electron transfer reactions or networks. (A) Outer and Inner Stroma membranes Grana Stroma lamellae (B) Thylakoid Granum Stroma Stroma lamellae (C) Outer membrane Inner membrane Thylakoids Stroma Granum (stack of Thylakoid thylakoids) Thylakoid lumen membrane FIGURE 1.16 (A) Electron micrograph of a (D) chloroplast from a leaf of timothy grass, Stroma Phleum pratense. (18,000×) (B) The same H+ H+ H+ preparation at higher magnification. H+ H+ (52,000×) (C) A three-dimensional view of H+ H+ H+ grana stacks and stroma lamellae, showing the complexity of the organization. (D) Diagrammatic representation of a chloro- plast, showing the location of the H+- ADP ATPases on the thylakoid membranes. ATP + (Micrographs by W. P. Wergin, courtesy of Pi H+ E. H. Newcomb.)  Plant Cells 17 Vacuole Tonoplast Grana stack FIGURE 1.17 Electron micro- graph of a chromoplast from tomato (Lycopersicon esculen- tum) fruit at an early stage in the transition from chloroplast to chromoplast. Small grana stacks are still visible. Crystals of the carotenoid lycopene are indicated by the stars. (27,000×) (From Gunning and Steer 1996.) Lycopene crystals The DNA of these organelles is in the form of circular eral of the proteins that participate in photosynthesis. Nev- chromosomes, similar to those of bacteria and very differ- ertheless, the majority of chloroplast proteins, like those of ent from the linear chromosomes in the nucleus. These DNA mitochondria, are encoded by nuclear genes, synthesized circles are localized in specific regions of the mitochondrial in the cytosol, and transported to the organelle. Although matrix or plastid stroma called nucleoids. DNA replication mitochondria and chloroplasts have their own genomes in both mitochondria and chloroplasts is independent of and can divide independently of the cell, they are charac- DNA replication in the nucleus. On the other hand, the num- terized as semiautonomous organelles because they depend bers of these organelles within a given cell type remain on the nucleus for the majority of their proteins. approximately constant, suggesting that some aspects of organelle replication are under cellular regulation. Different Plastid Types Are Interconvertible The mitochondrial genome of plants consists of about Meristem cells contain proplastids, which have few or no 200 kilobase pairs (200,000 base pairs), a size considerably internal membranes, no chlorophyll, and an incomplete com- larger than that of most animal mitochondria. The mito- plement of the enzymes necessary to carry out photosynthe- chondria of meristematic cells are typically polyploid; that sis (Figure 1.18A). In angiosperms and some gymnosperms, is, they contain multiple copies of the circular chromosome. chloroplast development from proplastids is triggered by However, the number of copies per mitochondrion gradu- light. Upon illumination, enzymes are formed inside the pro- ally decreases as cells mature because the mitochondria plastid or imported from the cytosol, light-absorbing pig- continue to divide in the absence of DNA synthesis. ments are produced, and membranes proliferate rapidly, giv- Most of the proteins encoded by the mitochondrial ing rise to stroma lamellae and grana stacks (Figure 1.18B). genome are prokaryotic-type 70S ribosomal proteins and Seeds usually germinate in the soil away from light, and components of the electron transfer system. The majority of chloroplasts develop only when the young shoot is mitochondrial proteins, including Krebs cycle enzymes, are exposed to light. If seeds are germinated in the dark, the encoded by nuclear genes and are imported from the cytosol. proplastids differentiate into etioplasts, which contain The chloroplast genome is smaller than the mitochon- semicrystalline tubular arrays of membrane known as pro- drial genome, about 145 kilobase pairs (145,000 base pairs). lamellar bodies (Figure 1.18C). Instead of chlorophyll, the Whereas mitochondria are polyploid only in the meris- etioplast contains a pale yellow green precursor pigment, tems, chloroplasts become polyploid during cell matura- protochlorophyllide.. tion. Thus the average amount of DNA per chloroplast in Within minutes after exposure to light, the etioplast dif- the plant is much greater than that of the mitochondria. ferentiates, converting the prolamellar body into thylakoids The total amount of DNA from the mitochondria and plas- and stroma lamellae, and the protochlorophyll into chloro- tids combined is about one-third of the nuclear genome phyll. The maintenance of chloroplast structure depends (Gunning and Steer 1996). on the presence of light, and mature chloroplasts can revert Chloroplast DNA encodes rRNA; transfer RNA (tRNA); to etioplasts during extended periods of darkness. the large subunit of the enzyme that fixes CO2, ribulose-1,5- Chloroplasts can be converted to chromoplasts, as in the bisphosphate carboxylase/oxygenase (rubisco); and sev- case of autumn leaves and ripening fruit, and in some cases 18 Chapter 1 (A) (B) (C) Plastids Etioplasts Prolamellar bodies FIGURE 1.18 Electron micrographs illustrating several stages of plastid development. (A) A higher-magnification view of a proplastid from the root apical meristem of the broad bean (Vicia faba). The internal membrane system is rudimentary, and grana are absent. (47,000×) (B) A meso- phyll cell of a young oat leaf at an early stage of differentia- tion in the light. The plastids are developing grana stacks. (C) A cell from a young oat leaf from a seedling grown in the dark. The plastids have developed as etioplasts, with elaborate semicrystalline lattices of membrane tubules called prolamellar bodies. When exposed to light, the etio- plast can convert to a chloroplast by the disassembly of the prolamellar body and the formation of grana stacks. (7,200×) (From Gunning and Steer 1996.) Crystalline core Microbody Mitochondrion this process is reversible. And amyloplasts can be con- verted to chloroplasts, which explains why exposure of roots to light often results in greening of the roots. Microbodies Play Specialized Metabolic Roles in Leaves and Seeds Plant cells also contain microbodies, a class of spherical organelles surrounded by a single membrane and special- ized for one of several metabolic functions. The two main types of microbodies are peroxisomes and glyoxysomes. Peroxisomes are found in all eukaryotic organisms, and in plants they are present in photosynthetic cells (Figure 1.19). Peroxisomes function both in the removal of hydro- gens from organic substrates, consuming O2 in the process, according to the following reaction: RH2 + O2 → R + H2O2 where R is the organic substrate. The potentially harmful peroxide produced in these reactions is broken down in peroxisomes by the enzyme catalase, according to the fol- lowing reaction: FIGURE 1.19 Electron micrograph of a peroxisome from a mesophyll cell, showing a crystalline core. (27,000×) This H2O2 → H2O + 1⁄ 2O2 peroxisome is seen in close association with two chloro- plasts and a mitochondrion, probably reflecting the cooper- Although some oxygen is regenerated during the catalase ative role of these three organelles in photorespiration. reaction, there is a net consumption of oxygen overall. (From Huang 1987.)  Plant Cells 19 Another type of microbody, the glyoxysome, is present preventing fusion. Oleosins may also help other proteins in oil-storing seeds. Glyoxysomes contain the glyoxylate bind to the organelle surface. As noted earlier, during seed cycle enzymes, which help convert stored fatty acids into germination the lipids in the oleosomes are broken down sugars that can be translocated throughout the young and converted to sucrose with the help of the glyoxysome. plant to provide energy for growth (see Chapter 11). The first step in the process is the hydrolysis of the fatty acid Because both types of microbodies carry out oxidative chains from the glycerol backbone by the enzyme lipase. reactions, it has been suggested they may have evolved Lipase is tightly associated with the surface of the half–unit from primitive respiratory organelles that were super- membrane and may be attached to the oleosins. seded by mitochondria. Oleosomes Are Lipid-Storing Organelles THE CYTOSKELETON In addition to starch and protein, many plants synthesize The cytosol is organized into a three-dimensional network and store large quantities of triacylglycerol in the form of of filamentous proteins called the cytoskeleton. This net- oil during seed development. These oils accumulate in work provides the spatial organization for the organelles organelles called oleosomes, also referred to as lipid bod- and serves as a scaffolding for the movements of organelles ies or spherosomes (Figure 1.20A). and other cytoskeletal components. It also plays funda- Oleosomes are unique among the organelles in that they mental roles in mitosis, meiosis, cytokinesis, wall deposi- are surrounded by a “half–unit membrane”—that is, a tion, the maintenance of cell shape, and cell differentiation. phospholipid monolayer—derived from the ER (Harwood 1997). The phospholipids in the half–unit membrane are Plant Cells Contain Microtubules, Microfilaments, oriented with their polar head groups toward the aqueous and Intermediate Filaments phase and their hydrophobic fatty acid tails facing the Three types of cytoskeletal elements have been demon- lumen, dissolved in the stored lipid. Oleosomes are strated in plant cells: microtubules, microfilaments, and thought to arise from the deposition of lipids within the intermediate filament–like structures. Each type is fila- bilayer itself (Figure 1.20B). mentous, having a fixed diameter and a variable length, up Proteins called oleosins are present in the half–unit mem- to many micrometers. brane (see Figure 1.20B). One of the functions of the oleosins Microtubules and microfilaments are macromolecular may be to maintain each oleosome as a discrete organelle by assemblies of globular proteins. Microtubules are hollow (A) (B) Oleosome Peroxisome Smooth endoplasmic reticulum Oil FIGURE 1.20 (A) Electron micrograph of an oleosome beside a peroxisome. (B) Diagram showing the formation of oleosomes by the synthesis and deposition of oil within the Oil body Oleosin phospholipid bilayer of the ER. After budding off from the ER, the oleosome is surrounded by a phospholipid mono- layer containing the protein oleosin. (A from Huang 1987; B after Buchanan et al. 2000.) 20 Chapter 1 cylinders with an outer diameter of 25 nm; they are com- (A) Dimer posed of polymers of the protein tubulin. The tubulin NH2 COOH monomer of microtubules is a heterodimer composed of two similar polypeptide chains (α- and β-tubulin), each NH2 COOH having an apparent molecular mass of 55,000 daltons (Fig- ure 1.21A). A single microtubule consists of hundreds of thousands of tubulin monomers arranged in 13 columns (B) Tetramer called protofilaments. NH2 COOH Microfilaments are solid, with a diameter of 7 nm; they COOH NH2 are composed of a special form of the protein found in NH2 COOH muscle: globular actin, or G-actin. Each actin molecule is COOH composed of a single polypeptide with a molecular mass NH2 of approximately 42,000 daltons. A microfilament consists (C) Protofilament of two chains of polymerized actin subunits that intertwine in a helical fashion (Figure 1.21B). Intermediate filaments are a diverse group of helically wound fibrous elements, 10 nm in diameter. Intermediate filaments are composed of linear polypeptide monomers (D) Filament of various types. In animal cells, for example, the nuclear lamins are composed of a specific polypeptide monomer, while the keratins, a type of intermediate filament found in the cytoplasm, are composed of a different polypeptide monomer. In animal intermediate filaments, pairs of parallel FIGURE 1.22 The current model for the assembly of inter- monomers (i.e., aligned with their —NH2 groups at the mediate filaments from protein monomers. (A) Coiled-coil same ends) are helically wound around each other in a dimer in parallel orientation (i.e., with amino and carboxyl coiled coil. Two coiled-coil dimers then align in an antipar- termini at the same ends). (B) A tetramer of two dimers. allel fashion (i.e., with their —NH2 groups at opposite Note that the dimers are arranged in an antiparallel fash- ion, and that one is slightly offset from the other. (C) Two ends) to form a tetrameric unit. The tetrameric units then tetramers. (D) Tetramers packed together to form the 10 nm assemble into the final intermediate filament (Figure 1.22). intermediate filament. (After Alberts et al. 2002.) Although nuclear lamins appear to be present in plant cells, there is as yet no convincing evidence for plant ker- atin intermediate filaments in the cytosol. As noted earlier, integral proteins cross-link the plasma membrane of plant undoubtedly stabilize the protoplast and help maintain cell cells to the rigid cell wall. Such connections to the wall shape. The plant cell wall thus serves as a kind of cellular exoskeleton, perhaps obviating the need for keratin-type (A) (B) intermediate filaments for structural support. Microtubules and Microfilaments Can Assemble and Disassemble a Tubulin G-actin In the cell, actin and tubulin monomers exist as pools of b subunits subunit a (a and b) free proteins that are in dynamic equilibrium with the poly- b merized forms. Polymerization requires energy: ATP is a Protofilament required for microfilament polymerization, GTP (guano- b sine triphosphate) for microtubule polymerization. The a attachments between subunits in the polymer are nonco- b 8 nm valent, but they are strong enough to render the structure a stable under cellular conditions. Both microtubules and microfilaments are polarized; 25 nm 7 nm that is, the two ends are different. In microtubules, the polarity arises from the polarity of the α- and β-tubulin het- FIGURE 1.21 (A) Drawing of a microtubule in longitudinal erodimer; in microfilaments, the polarity arises from the view. Each microtubule is composed of 13 protofilaments. The organization of the α and β subunits is shown. (B) polarity of the actin monomer itself. The opposite ends of Diagrammatic representation of a microfilament, showing microtubules and microfilaments are termed plus and two strands of G-actin subunits. minus, and polymerization is more rapid at the positive end.  Plant Cells 21 Once formed, microtubules and microfilaments can dis- will form after the completion of mitosis, and it is thought assemble. The overall rate of assembly and disassembly of to be involved in regulating the plane of cell division. these structures is affected by the relative concentrations of During prophase, microtubules begin to assemble at free or assembled subunits. In general, microtubules are two foci on opposite sides of the nucleus, forming the more unstable than microfilaments. In animal cells, the prophase spindle (Figure 1.24). Although not associated half-life of an individual microtubule is about 10 minutes. with any specific structure, these foci serve the same func- Thus microtubules are said to exist in a state of dynamic tion as animal cell centrosomes in organizing and assem- instability. bling microtubules. In contrast to microtubules and microfilaments, inter- In early metaphase the nuclear envelope breaks down, mediate filaments lack polarity because of the antiparallel the PPB disassembles, and new microtubules polymerize orientation of the dimers that make up the tetramers. In to form the mitotic spindle. In animal cells the spindle addition, intermediate filaments appear to be much more microtubules radiate toward each other from two discrete stable than either microtubules or microfilaments. Although foci at the poles (the centrosomes), resulting in an ellip- very little is known about intermediate filament–like struc- soidal, or football-shaped, array of microtubules. The tures in plant cells, in animal cells nearly all of the interme- mitotic spindle of plant cells, which lack centrosomes, is diate-filament protein exists in the polymerized state. more boxlike in shape because the spindle microtubules arise from a diffuse zone consisting of multiple foci at Microtubules Function in Mitosis and Cytokinesis opposite ends of the cell and extend toward the middle in Mitosis is the process by which previously replicated chro- nearly parallel arrays (see Figure 1.24). mosomes are aligned, separated, and distributed in an Some of the microtubules of the spindle apparatus orderly fashion to daughter cells (Figure 1.23). Micro- become attached to the chromosomes at their kinetochores, tubules are an integral part of mitosis. Before mitosis while others remain unattached. The kinetochores are located begins, microtubules in the cortical (outer) cytoplasm in the centromeric regions of the chromosomes. Some of the depolymerize, breaking down into their constituent sub- unattached microtubules overlap with microtubules from the units. The subunits then repolymerize before the start of opposite polar region in the spindle midzone. prophase to form the preprophase band (PPB), a ring of Cytokinesis is the process whereby a cell is partitioned microtubules encircling the nucleus (see Figure 1.23C–F). into two progeny cells. Cytokinesis usually begins late in The PPB appears in the region where the future cell wall mitosis. The precursor of the new wall, the cell plate that (A) (B) (C) (D) (E) (F) (G) (H) (I) (J) (K) FIGURE 1.23 Fluorescence micrograph taken with a confocal microscope showing changes in microtubule arrangements at different stages in the cell cycle of wheat root meristem cells. Microtubules stain green and yellow; DNA is blue. (A–D) Cortical microtubules disappear and the preprophase band is formed around the nucleus at the site of the future cell plate. (E–H) The prophase spindle forms from foci of microtubules at the poles. (G, H) The preprophase band disappears in late prophase. (I–K) The nuclear membrane breaks down, and the two poles become more diffuse. The mitotic spindle forms in parallel arrays and the kinetochores bind to spindle microtubules. (From Gunning and Steer 1996.)  Prophase Prometaphase Metaphase Diffuse spindle Cell wall pole Condensing Plasma chromosomes Chromosomes membrane (sister chromatids align at held together metaphase Nucleus at centromere) plate (nucleolus disappears) Cytoplasm Preprophase Kinetochore band microtubules Prophase disappears spindle Nuclear Polar Spindle pole envelope microtubules develops fragment Anaphase Telophase Cytokinesis Kinetochore Decondensing microtubules chromosomes shorten Two cells Nuclear formed envelope re-forms Separated Cell plate Phragmoplast chromatids grows are pulled toward Endoplasmic Nucleolus poles reticulum FIGURE 1.24 Diagram of mitosis in plants. forms between incipient daughter cells, is rich in pectins Myosins are proteins that have the ability to hydrolyze (Figure 1.25). Cell plate formation in higher plants is a mul- ATP to ADP and Pi when activated by binding to an actin tistep process (see Web Topic 1.5). Vesicle aggregation in the microfilament. The energy released by ATP hydrolysis pro- spindle midzone is organized by the phragmoplast, a com- pels myosin molecules along the actin microfilament from plex of microtubules and ER that forms during late anaphase the minus end to the plus end. Thus, myosins belong to the or early telophase from dissociated spindle subunits. general class of motor proteins that drive cytoplasmic streaming and the movements of organelles within the cell. Microfilaments Are Involved in Cytoplasmic Examples of other motor proteins include the kinesins and Streaming and in Tip Growth dyneins, which drive movements of organelles and other Cytoplasmic streaming is the coordinated movement of par- cytoskeletal components along the surfaces of microtubules. ticles and organelles through the cytosol in a helical path Actin microfilaments also participate in the growth of down one side of a cell and up the other side. Cytoplasmic the pollen tube. Upon germination, a pollen grain forms a streaming occurs in most plant cells and has been studied tubular extension that grows down the style toward the extensively in the giant cells of the green algae Chara and embryo sac. As the tip of the pollen tube extends, new cell Nitella, in which speeds up to 75 µm s–1 have been measured. wall material is continually deposited to maintain the The mechanism of cytoplasmic streaming involves bun- integrity of the wall. dles of microfilaments that are arranged parallel to the lon- A network of microfilaments appears to guide vesicles gitudinal direction of particle movement. The forces nec- containing wall precursors from their site of formation in essary for movement may be generated by an interaction the Golgi through the cytosol to the site of new wall for- of the microfilament protein actin with the protein myosin mation at the tip. Fusion of these vesicles with the plasma in a fashion comparable to that of the protein interaction membrane deposits wall precursors outside the cell, where that occurs during muscle contraction in animals. they are assembled into wall material.  Plant Cells 23 FIGURE 1.25 Electron micrograph of a cell plate forming in a maple seedling (10,000×). (© E. H. Newcomb and B. A. Palevitz/Biological Photo Service.) Nuclear envelope the two chromatids of each replicated chromosome, Vesicles which were held together at their kinetochores, are Microtubule separated and the daughter chromosomes are pulled to opposite poles by spindle fibers. At a key regulatory point early in G1 of the cell cycle, the cell becomes committed to the initiation of DNA synthesis. In yeasts, this point is called START. Once a cell has passed START, it is irre- versibly committed to initiating DNA synthesis and Nucleus completing the cell cycle through mitosis and cytokinesis. After the cell has completed mitosis, it may initiate another complete cycle (G1 through mitosis), or it may leave the cell cycle and differen- tiate. This choice is made at the critical G1 point, before the cell begins to replicate its DNA. Intermediate Filaments Occur in the Cytosol and DNA replication and mitosis are linked in mammalian Nucleus of Plant Cells cells. Often mammalian cells that have stopped dividing Relatively little is known about plant intermediate fila- can be stimulated to reenter the cell cycle by a variety of ments. Intermediate filament–like structures have been hormones and growth factors. When they do so, they reen- identified in the cytoplasm of plant cells (Yang et al. 1995), ter the cell cycle at the critical point in early G1. In contrast, but these may not be based on keratin, as in animal cells, plant cells can leave the cell division cycle either before or since as yet no plant keratin genes have been found. after replicating their DNA (i.e., during G1 or G2). As a con- Nuclear lamins, intermediate filaments of another type that sequence, whereas most animal cells are diploid (having form a dense network on the inner surface of the nuclear two sets of chromosomes), plant cells frequently are membrane, have also been identified in plant cells (Fred- tetraploid (having four sets of chromosomes), or even poly- erick et al. 1992), and genes encoding laminlike proteins are ploid (having many sets of chromosomes), after going present in the Arabidopsis genome. Presumably, plant through additional cycles of nuclear DNA replication with- lamins perform functions similar to those in animal cells as out mitosis. a structural component of the nuclear envelope. The Cell Cycle Is Regulated by Protein Kinases The mechanism regulating the progression of cells through CELL CYCLE REGULATION their division cycle is highly conserved in evolution, and The cell division cycle, or cell cycle, is the process by which plants have retained the basic components of this mecha- cells reproduce themselves and their genetic material, the nism (Renaudin et al. 1996). The key enzymes that control nuclear DNA. The four phases of the cell cycle are desig- the transitions between the different states of the cell cycle, nated G1, S, G2, and M (Figure 1.26A). and the entry of nondividing cells into the cell cycle, are the cyclin-dependent protein kinases, or CDKs (Figure 1.26B). Each Phase of the Cell Cycle Has a Specific Set of Protein kinases are enzymes that phosphorylate proteins Biochemical and Cellular Activities using ATP. Most multicellular eukaryotes use several pro- Nuclear DNA is prepared for replication in G1 by the tein kinases that are active in different phases of the cell assembly of a prereplication complex at the origins of repli- cycle. All depend on regulatory subunits called cyclins for cation along the chromatin. DNA is replicated during the their activities. The regulated activity of CDKs is essential S phase, and G2 cells prepare for mitosis. for the transitions from G1 to S and from G2 to M, and for The whole architecture of the cell is altered as cells enter the entry of nondividing cells into the cell cycle. mitosis: The nuclear envelope breaks down, chromatin con- CDK activity can be regulated in various ways, but two denses to form recognizable chromosomes, the mitotic of the most important mechanisms are (1) cyclin synthe- spindle forms, and the replicated chromosomes attach to sis and destruction and (2) the phosphorylation and the spindle fibers. The transition from metaphase to dephosphorylation of key amino acid residues within the anaphase of mitosis marks a major transition point when CDK protein. CDKs are inactive unless they are associated 24 Chapter 1 (A) Mitotic p (B) Active CDK has stimulates mitosis e phase se M cyclin Anapha se se degradation ha ha P Meta P p lop is Pro CDK es Te n P ki Mitosis to