Bio 013: Plant Cells

Organ Growth

• Altering division plane increases length, girth and cell specialization •(A1) Without changing the division plane, cytokinesis and cell expansion result in a creation of a single file of cells •(A2) Cells that alter their division plane between successive divisions. This results in the formation of a set of cube formed cells. -Such a pattern of cytokinesis and expansion results in increases in either the thickness/radius of an organ. •(B) Changing division plane results in the formation of highly specialized cells. (In this case the development of guard cells of the stomata.) -This cell undergoes an asymmetric cell division, creating a large and a smaller cell - the smaller cell then under goes a subsequent division, cutting that cell into 2 smaller cells. (The division plane has gone from vertical to horizontal) - this second division results in the formation of the developing guard cells

Plant Cell Expansion

- All plant cells expand by the same general mechanism. 1. Turgor pressure 2. Wall weakening 3. Addition of new wall and membrane -Expansion can take 2 different forms •Isotropic expansion (a) --> Cell expands equally in all directions, leading to generally spherical cells •Anisotropic expansion (b) --> Cell expands in a specific direction, leading to elongated cells -->There are 2 mechanisms for anisotropic growth (What differs is the location where expansion occurs.) 1. Diffuse growth involves the incorporation of new wall and membrane material in all regions of the cell, with a greater amount added to the elongating axis of the cell 2. Tip growth involves the incorporation of new wall and membrane material to a specific location of the cell, resulting in expansion only in a specific region, which becomes the growing tip.

Unifying theme

- Cell division consists of 3 components: 1. Replication of DNA into 2 identical copies 2. Mitosis (division of genetic material into 2 identical masses) 3. Cytokinesis (division of cytoplasm into 2 masses) -Mitosis in plant cells is similar, yet different from mitosis in animal cells • Similarities --> Accomplish 3 major tasks of cell division --> Mitotic events (prophase, metaphase, anaphase, telophase) are the same --> Role of spindle fibers (kinetochore and non-kinetochore MTs) the same • Differences --> No centrioles in plant cells --> Spindle forms differently --> Spindles of plant and animal cells have a distinctly different shapes - Cytokinesis is very different between plant and animal cells • Presence of cell wall requires specialized process for cytokinesis

Cell structure

- Different organelles include the Golgi, chloroplast, nucleus, peroxisomes, cell wall, middle lamella, and intercellular space - plant specific components such as the -tonoplast: membrane that surrounds the large central vacuole. - also in the cell walls there are pit fields that are filled with clusters of plasmodesmata - the large central vacuole is traversed with trans-vacuolar strands (in diag labeled as cytoplasmic strands): these strands are filled with cytosol, and are invaginations of the tonoplast membrane. These strands allow cell components to move from one part of the cell to another via cytoplasmic streaming.

Mechanisms for controlling plant cell expansion

- For a cell to expand in specific directions there needs to be specific deposition of wall material - Controlled cell expansion needs to occur, along with controlled division planes - Because cells have walls, the expansion properties of the cell are determined by the expansion characteristics of the cell wall. - The expansion characteristics of the cell wall are controlled by 1. Selective weakening of wall strength to allow stretching of wall by turgor pressure 2. Addition of new cell wall and cell membrane components to reinforce wall and prevent membrane rupture 3. Selective patterning of cellulose microfibrils (cmfs) in the developing cell wall to regulate the direction of wall stretching -The expansion characteristics of the wall can be changed during cell growth and development by changing the patterning of the most recently deposited cellulose MFs.

Plasmodesmata

- Functionally similar to gap junctions in animal cells - Allow passage of ions and molecules btwn adjacent cells - plasmodesmata are complex array of membrane connections - Different in that they are open channels where the cell membrane of one cell is continuous with adjacent cell membrane - Desmotubule connects ER membrane of adjacent cells - 4 distinct transport pathways for cells to communicate with each other 1. Plasma membrane 2. ER membrane 3. Cytoplasm 4. Lumen of ER

Cytoskeletal Dynamics Control Cell Growth and Development

- Plant cells contain dynamic, three dimensional arrays both microtubules (MTs) and microfilaments (MFs) which are involved in the regulation of cell growth and development. -Specific reorganizations of the cytoskeleton are required for proper plant cell growth and division -Microtubules (MTs) • Involved in Mitosis • involved in Cytokinesis: division of cytoplasm (direction that the cell divides (horizontal, vertical, asymmetric, etc.) • Control diffuse cell growth (direction of cell expansion) - when cells develop specific shapes, those shapes are predicted by the patterns of microtubules -Actin Microfilaments (MFs) • they are involved in Cytoplasmic streaming (long distance transport in plant cells) • Directed tip synthesis (specialized form of cell expansion) • Mitosis • cytokinesis

Contents of the Cytpolasm

- Same components as animal cells, except for a few deletions and additions - Deletions • Animal cells have centrioles: which function to organize arrays of cytoplasmic tubules. Plants do this wo centrioles - Additions • Plant cells have Plastids, that animal cells lack 3 major types of plastids (usually contain 1 or 2 plastids) 1. Chloroplasts- commonly 3 sets of membranes function in photosynthesis 2. Chromoplasts- Single membrane bound organelles found in fruits, flowers, roots, and stressed and aging leaves, and are responsible for their distinctive colors 3. Leucoplasts- Single membrane bound organelle. Colorless plastids found in endosperm, tubers, and cotyledons. They serve in storage and perform biosynthetic functions • Vacuoles

Balance btwn Turgor pressure and wall pressure results in tip growth

- The large black arrows indicate turgor pressure pushing out in all directions of the cell - In the cell wall, along the length of the tip growing cell are cMFs and hemicellulose molecules generating a relatively strong rigid cell wall - at the growing tip cMFs are absent- only non cellulosic components are present - vesicles are transported along actin filaments and bind to the plasma memb. at the growing tip - this process of binding to the memb. and dumping vesicle contents into the cell wall is an ex of exocytosis

Plant Cell Mitosis

- The nucleus soon develops a clear textured appearance, and the chromosomes are visible in the cell center - The chromosomes are drawn rapidly towards the spindle poles, and begin to shorten -As they shorten they reveal a collection of granuals in the center of the cell, btwn the two masses of chromosomes - These granuals develop into the cell pate and ultimately the new cell wall - At first the cell plate has contortions, but as it reaches the parent cell wall, the growing cell plate begins to stiffen and finally when it attaches, it becomes a stiff new wall

Organelles exclusive to plant cells

- These are ex of plastids - The micrograph on the top right is of a potato is showing large cells with lots of small single membrane bound organelles called leucoplasts. There are also large starch grains in potatoes. - The bottom right pic shows the development of plastids • In embryonic tissue the cells contain proplastids: they are membrane bound organelles and do not have a particular function at the moment • given the appropriate env stimulus, they can develop into either; Chloroplasts, or Leucoplasts... depending on which tissues they are located. • If chloroplasts are deprived of light develop into Etioplasts • If the Etioplasts are given light they develop back into chloroplasts • From Etioplasts can develop chromoplasts. • There are several diff types of Leucoplasts, defined by the molecules stored inside them • Amyloplast store starch • Elaiopplast store lipids • Proteinoplast store proteins

Plant Vauoles

- Vacuoles of plant cells are multifunctional organelles - Derived from fusion of Golgi vesicles - Dividing cells have many small vesicles, whereas expanding mature cells have large central vacuole (+ small vesicles) - Most evident in non-dividing, mature plant cells with specific functions. - Can make up 95% of cell volume - central role in plant development. - They can function as 1. lytic compartment filled with digestive enzymes for breakdown of macromolecules 2. crucial to processes of detoxification and general cell homeostasis. 3. reservoirs for ions and metabolites, including pigments ( stored in central vacuole) for ex. in red onion, the red color is stored in the central vac not in the chromoplast 4. provide turgor pressure to drive cell expansion- due to all the ions and metabolites in the central vacuole the water concentration is low compared to the conc in the env. By osmosis water enters into the central vacuole and creates an outward pressure (turgor pressure). This turgor pressure pushes on the cytoplasm and ultimately on the cell wall causing the cell to expand.

Secondary Plant Cell Wall

- Walls deposited as cell stop expanding - Secondary walls are deposited in between plasmalemma and the primary cell wall •--> as cell produces successive layers of secondary cell wall-->wall thickness increases--> as a result the volume of cytoplasm decreases. - There may be one, several, or multiple layers of secondary cell wall - Change in cell wall composition from primary cell wall •Secondary cell walls have much higher proportion of cellulose •Cellulose microfibrils themselves contain a more highly polymerized chain of glucose. Sec. cell wall- cellulose chains have ab 14,000 glucose units. In prim. cell wall- 2,000-to 6,000. •Less non-cellulose components(the pectins, hemicellulose proteins, etc.) • Addition of lignin: Complex non-carbohydrate polymer, which provides high strength and rigidity to the cell wall (not in prim cell wall) -These differences in wall organization and chemistry result in different physical properties of the wall • they are stronger • more rigid -Help define cell function -Sec Cell wall provide the rigidity needed to prevent above ground parts of the plant from falling over to due gravity or windstorm

Cytoplasmic Streaming and Trans- vacuolar Strands

- different sized cellular components are moving through the cell in one direction, indicating it is an active process (not diffusion). This is cytoplasmic streaming. - The streams in the cytoplasm are trans-vacuolar strands with various organelles moving along cables of actin filaments.

Microtubule dynamics during cytokinesis and early interphase

- image D Telophase: reforming of the daughter nuclei after mitosis and early cytokinesis: forming of the cell plate btwn the 2 daughter nuclei - image E cytokinesis shows later stage of cell plate formation - fragmoplast highlighted in the bracket consists of the MTs, and the golgi vesicles transported along those MT's and developing cell plate - the fragmoplast does not include the golgi bodies that produce those vesicles - in the center the cell plate is well formed and the vesicles are attaching to the periphery of that plate and will expand until it reaches the parent cell membrane and cell wall -once fused, the MT of fragmoplast disassemble and disappear - image F Early interphase the nuclear envelope has formed around each of the new daughter nuclei. From that nuclear envelope grow arrays of MTs radiating out into the cytoplasm. - image G interphase Nuclear associated MTs disassemble and are replaced by MTs in the cortical cytoplasm: cytoplasm in close proximity to the plasma membrane. The MT are highly organized around the circumference of the cell however, at the ends of the cell the MT are far less organized - image H Cell enlargement The cell is starting to elongate. The MT continue to be well organized along the length of the cell, but at the two ends of the cell the MTs are less organized.

Sub-Cellular Organization in Tip growing cells

- top cartoon shows the distribution of organelles - this cartoon there are no tip growing cells in plants that contain chloroplasts! - lower image displays the organization of the cytoskeleton; the actin filaments and MTs. - the MTs are located in the cortical cytoplasm, near the plasma membrane and are oriented along the length of the cell. - Actin filaments are organized into 2 structures • large actin filament bundles- (running down the length of the cell but stop just short of the tip) • fine arrays of actin filaments (formed in the tip of tip growing cells )

Mitosis in barley cells (immunofluorescence)

-Examine mitosis using aminocytochemistry: the use of specific antibodies that bind specific cellular components -The cells were stained with anti-tubulin (in red) and with anti-CENH3 (a kinetochore protein, in green), also stained with hooks dye resulting in Chromosomes shown in blue. Prophase- kinetochore and microtubules present Prometaphase- kinetochore start to move towards the center of spindle, each set of kinetochores has MTs attached Metaphase- all kinetochores are lined up in center of spindle, forming the metaphase plate, and MTs are radiating in each direction away from the kinetochore towards the spindle pole Anaphase- kinetochores are being drawn in opposite directions, MTs get shorter and more constricted towards poles Late Anaphase- kinetochore MTs are very short and kinetochores have been drawn to the spindle poles. Non kinetochore MTs help push two spindle poles apart. Late anaphase spindle is wider than anaphase spindle Telophase- spindle MTs have disappeared, all kinetochores are associated with poles of spindle, chromosomes are now contracting towards those poles. A new population of MTs forms the new cell wall and membrane - There is no evidence of aster MTs radiating from the spindle pole in plant cells

Diffuse (Intercalary) Growth

-Growth in all directions: isotropic growth -Growth predominantly in one orientation (length or girth) :anisotropic growth -In both cases new wall and membrane material is added throughout the cell surface. - New material is produced by the Golgi and delivered to the membrane by the golgi vesicles -Membrane bound receptors determine the sites where vesicle attach and fuse, adding new cell wall and membrane. -Fine network of actin filaments guides vesicles to membrane receptors -Direction of growth controlled by patterns of MTs

Tip Synthesis (growth)

-Specialized form of anisotropic growth -Expansion only in one region of the cell, which becomes the growing tip of the cell -Cytoskeletal components direct and control fusion of Golgi vesicle to a specific site on the cell •MTs may or may not be involved in tip growth (depending on the system) -->In angiosperms, loss of MT has no effect on tip growth -->In gymnosperms, loss of MTs inhibits tip growth (alters organelle and actin patterns) -->May play a role in defining the site of tip growth •Actin filaments regulate the binding of vesicles to the plasmalemma.

Primary Plant Cell Wall

-The cartoon on top left and the electron micrograph show adjacent plant cells fusing together by the middle lamella. -the pattern of the various cell wall components are shown. The large linear structures are microfibrils. The smaller structures appear to connect the microfibrils to each other. -the small structures are various polysaccharide polymers such as pectins and hemicelluloses - Large cellulose microfibrils provide structural integrity and strength for the prim. cell wall -extensin: is an enzymatic protein. It functions to break the polymers such as pectins and hemicelluloses that hold the cellulose microfibrils in place. Breaking these connections allows the cellulose microfibrils to slide past one another in response to turgor pressure during the process of cell expansion -Molecular structure of Cellulose: complex polymer of chains of glucose that are hydrogen bonded together to provide rigidity and strength to the microfibril. -Connecting the microfibrils together are various pectins and hemicellulose polymers forming a complex 3 dem. network

Role of the cytoskeleton in Diffuse Cell Growth

1. New wall and membrane are added long the length of the cell and at the ends (i.e. all six sides) 2. Direction of cell expansion is controlled by the patterning of the cortical microtubules 3. Vesicles are transported from the trans-Golgi network and the trafficking requires the involvement of fine actin filaments. 4. Because the actin network is evenly distributed through out the cortical cytoplasm, these vesicles are evenly distributed through out the cell. 5. Vesicles contain cell wall components, cellulose synthase, wall loosening enzymes and new membrane 6. Wall loosening enzymes weaken the wall and allow for turgor pressure-driven cell expansion 7. Newly deposited cellulose synthases enzymes begin to make cellulose microfibrils in a pattern that is controlled by the patterning of cortical microtubules 8. The new microfibrils restrict expansion that is parallel to the orientation of the microfibrils, thus expansion is perpendicular to the orientation of the newly deposited cellulose. 9. Both microtubules and actin microfilaments are essential for controlled cell expansion during diffuse cell growth. The actin microfilaments determine the locations(s) where new wall and membrane will form and the microtubules control the direction of wall expansion.

Cell Differentiation: interplay btwn cytokinesis and cell expansion

MT cycling determines: 1. Direction of cell expansion a) Resulting in cells elongating in different directions 2. Plane of cell division a) Resulting is cells of similar size and shape b) Resulting in cells with different sizes/shapes 3.Location of wall deposition a)Dispersed along the length of cells b)Localized into discrete locations in the cell.

Plant Cell Division and Development

Mitosis and Cytokinesis in plant cells - This is a light micrograph of the embryonic region of an onion root tip. The cells are stained so the DNA appears black - many cells are in some stage of mitosis (prophase, metaphase, anaphase, telophase, and interphase) - most of the cells are in interphase

Review: Animal Cell Spindle

Mitosis in animal cells -Aster MTs: population of MT that extend from the centriole away from the mitotic spindle, out into the cytoplasm -Kinetochore MT: attach to the kinetochores on chromosome -Non-Kinetochore MT: elongate past the mid region of the spindle and interact with non-kinetochore MTs extending from the opposite spindle pole

Role of the cytoskeleton in Tip Synthesis

Only root hairs and pollen tubes exhibit this type of growth in higher plants 1. The concentration of Ca2+ is higher in the tip and dec as one moves away from the tip 2. PI4,5P2 (receptors for vesicles) are enriched at the apical plasma membrane. 3. Vesicles are transported from the Golgi to the growing tip along actin filaments. Inserted into the membranes of these vesicles are myosin molecules interacting with actin filaments. This results in the movement of vesicle towards the tip 4. Actin filament cables become sparse and unorganized at the sub-apical cytoplasmic region in tip-growing cells. Fine network of actin filaments formed just below the tip. 5. These vesicles accumulate in the apical region, which is also designated the vesicle rich zone. 6. Cell wall deposited in the tips of root hairs typically contain short, randomly oriented cellulose microfibrils. (resulting in relatively weak cell wall in that region) 7. Behind the actively growing tip are arrays of more highly organized cellulose microfibrils (cmfs), observed and the orientation of the cellulose is longitudinal or helical along the length of root hairs and pollen tubes, respectively. The organization of cmfs in this region may be controlled by cortical MTs. 8. While microtubules are essential for determining the polarity of root hairs, vesicle delivery to the apical region was not blocked by depolymerization of microtubules, indicating they are not required for tip growth

Plant Cell Walls

Primary Cell Wall: - The vast majority of plant cells have a primary cell wall - it is made and deposited during cell division and subsequent cell expansion Secondary Cell Wall: -Some plant cells produce an additional cell wall -has a diff chemical composition and structural organization - it is made and deposited after cell expansion stops and continues after cells seize any further expansion. - Other parts of the plant cell wall include -Middle Lamella: is a layer of polysaccharide polymers (mostly Pectins) that "glues" plant cells together. This allows plant tissues to maintain their integrity. If removed, plant cells would come away from one another and organ will disassemble into individual cells -Plasmodesma: are cytoplasmic channels through the cell wall that allow adjacent cells to communicate. Allows for various components like ions, small organic molec, hormones, large organic molec, proteins, nucleic acids to pass from one cell to another

**Comparison of primary and secondary cell walls**

Primary walls: - Deposited as cell increases in size - Found in all cells - Composition of wall • Cellulose microfibrils (50%) --> shorter, more flexible • Pectin and hemicellulose (30-40%) • Lignin - absent • Proteins (~10%) -Organization of wall • Generally thin • Microfibrils loosely packed, wavy patterns • connected by non-cellulose polymers • Microfibrils in single layer --> outer region more random --> inner region more organized Secondary walls: -Deposited as cell stops increasing in size - Found in some mature, non-dividing cells -Composition of wall •cellulose microfibrils (80-90%) --> Longer, more rigid • Pectin and hemicellulose (5-10%) • Lignin (10-20%) •Proteins (<5%) - Organization of wall •Generally thicker than primary wall • Microfibrils densely packed, highly parallel arrays • Connected by hydrogen bonds btwn microfibrils • Microfibrils in multiple layers --> each layer is highly org --> each layer has specific parallel arrangement of microfibrils

Evidence for MTs controlling where the cellulose synthase is located

Q: Is it the cellulose MFs that control the pattern of the MTs? Or vice versa? In a set of experiments this was answered. Green: Cellulose synthase enzymes Red: location of MTs • MT patterns change over time • MT appears first and later cellulose synthase enzymes paralleling the MTs--> clearly indicates that the pattern of the MTs that controls the precise pattern of cellulose MFs by controlling where the cellulose synthase enzymes are located

Dynamics of Microtubule arrays during the plant cell cycle

These illustrations show microtubule (MT) arrays through the plant cell cycle. A• A pre-prophase band is linked to the nucleus by phragmosome MTs, which marks the future division site. B• Metaphase spindle with a dispersed polar region. C• In late telophase, spindle MTs disappear and the phragmoplast forms as a concentrated cylinder of MTs between the daughter nuclei. D• The cytokinetic cell plate and associated phragmoplast MTs expands centripetally (from center, outwards to parent cell wall) towards attachment sites previously established by the pre-prophase band E• Once cytokinesis is complete, MTs of the phragmoplast disappear and are replace by new MTs extending from the nucleus toward the cytoplasm and plasma membrane-associated MTs appear. F• In anaphase, the cell has begun to elongate and the MTs are oriented perpendicular to the long axis of the cell and found predominantly in the cortical cytoplasm.

The plant cell

electron micrograph and a cartoon that represent a typical plant cell found in many different plants. -plant cells can be 10-100 x's larger than the typical animal cell

Compare Cytokinesis in Animal and Plant Cells

• Animal Cells- formation of contractile ring of actin and myosin filaments that draws membrane together, pinching the cell into two smaller cells Bc plant cells have a cell wall, they differ from animal cells in cytokinesis • Plant Cells- make new cell wall and plasmalemma in golgi bodies and transfer that material to the site where the parent cell will form a new cell wall. Golgi vesicles contain wall materials or enzymes to make wall materials, the membranes of these vesicles become components of the new cell membrane. Golgi vesicles accumulate and fuse together forming a cell plate with developing cell wall in the middle surrounded by membrane. Cell plate forms in the center of cell and expands towards parent cell wall and membrane as new golgi vesicles fuse to the periphery of the developing cell plate

How Plant Cells Acquire Their Shape

• Expansion properties of the wall are influenced by cmf orientation which is controlled by the patterns of MTs in the cytosol • In all cases thus far examined, patterns of cortical MTs predict patterns of most recently deposited cmfs • In all cases thus far examined, changes in the direction of cell expansion are predicted by changes in MT patterns • Cellulose Synthase Interacting (CSI) proteins move along MTs, via kinesin proteins, moving cellulose synthase complexes along MTs.

Relationship between PPB and cell plate

• The location of PPB predicts exactly where the new cell wall plate will fuse with the parent cell wall, this location is called the cortical division site (CDS) • PPB function is to insert proteins into the cell wall/membrane that remain after the PPB has disappears, which act as signal molecules so that when the cell plate recognizes or binds to these signal molecules, that's where the plate fuses. • Cell plate forms and fuses where the PPB used to be • Over time, band of MT gets tighter and tighter to form a tight ring around the cell periphery • Condensing of MT is due to the function of the actin filaments •Tan and RanGAP proteins, forming a tighter and tighter band around the cell periphery, this helps focus the MTs and thus predicts exactly where the cell plate will fuse

Plant Cell expansion: Weakening of the wall (Occurs through the process of Acid Growth Hypothesis)

• the primary cell wall is made up of cellulose microfibrils that are cross-linked together with pectins and hemicellulose molecules • Pumping protons into the cell wall making it acidic (by hydrogen ion pump proteins embedded in the membrane powered by ATP hydrolysis) which activates enzymes (like Expansin). • Enzymes break bonds between wall components (binding of pectins and hemicelluloses to cellulose), allowing existing cmfs to slide past one another, allowing wall to stretch. •Cmfs passively reorient in the direction of expansion •Deposition of new cmfs, using cellulose synthase enzymes deposited into the plasmalemma, determines direction of stretch

**Summary: Plant cell anisotropic growth mechanisms**

•A schematic drawing of (a) diffuse anisotropic growth and (b) tip growth in plant cells. •Similarities: 1. Roles of turgor pressure and wall loosening 2. Roles for both actin cables and fine filaments 3. Roles for receptor molecules 4. Function of Golgi •Differences In diffuse growth, 1. No clear-cut zonation of organelles 2. MTs are oriented transversely to the growth axis 3. MTs function to determine cell shape 4. Secretory vesicles dispersed in cell 5. No Ca+ gradients in cells In tip growth, 1. there is a zonation of mitochondria, Golgi and ER in the sub-apical regions. 2. MTs are arranged in bundles along the longitudinal axis of the cell. 3. No clear role of MTs in determining cells shape 4. High concentration of secretory vesicles in apical region of cells 5. High levels of Ca+2 in apical region of cells

Early Spindle Assembly in Plant Cells

•Before spindle assembly, during interphase and up to prophase all MTs and the proteins to assemble MTs are restricted to the cytoplasm. •However, in the nucleus there are various MT assembly factors (RanGTP, NuMA). These proteins are associated with nuclear envelope and are believed to be responsible for the assembly of MT's radiating away from the nuclear envelope. • In nucleus, there are DNA binding proteins such as histones and nucleosomes, • In nucleus, there are no MT subunits, no tubulin proteins, so while assembly factors are there, MT do not form in the nucleus • In late prophase, Nuclear Env disassembles, which results in the diffusion of MT assembly proteins into the cytoplasm allowing them to mix with MT subunits. Mixing of the assembly factors with MT proteins, induces MTs to form. • MTs assemble and disassemble because of dynamic instability, however, bc these MTs bind to kinetochores they become stabilized. • As more stabilized MTs form, they elongate and interact with one another, gradually forming the plant mitotic spindle • PPB- pre-prophase band of MTs. These MTs encircle the cell just before prophase. This band is not involved in MT assembly. Bridging MTs connect PPB to nucleus • Phragmosome- a collection of MTs that help center nucleus in the cell just before mitosis begins *Difference between spindle formation in plant and animal cells

How do cells regulate the direction of expansion?

•Cells/walls expand due to turgor pressure, powered by osmosis •Previously deposited cmfs become reorganized as the old wall stretches. •The direction of wall expansion is determined by the pattern of new cmfs that are deposited during cell expansion. •These newly deposited cmfs still have their connections with pectins and hemicelluloses and thus form a relatively ridged structure that does not stretch due to turgor pressure •Newly deposited cmfs restrict cell expansion in the direction that parallels the orientation of the new cmfs • As a result, the direction of expansion is transverse to the orientation of newly deposited cmfs

Division Plane Controls Morphogenesis

•The orientation of division planes results in increases in; organ thickness (Z- axis), organ girth (y-axis), organ length (x- axis)