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PLANT IMMUNOLOGY Lecture 5, 6 PLANT CELL • The cell is the basic unit of life. • Plant cells are eukaryotic cells or cells with membrane bound nucleus. • Generally, plant cells are larger than animal cells and are mostly similar in size and are rectangular or cube shaped. • Plant cells are similar to animal cells in being eukaryotic and they have similar cell organelles. COMPONENTS OF PLANT CELL COMPONENTS OF PLANT CELL • Outer-layer of cell is composed of: Cell wall Plasma membrane Plasmodesmata • Inside the cell, it has following components: Cytoplasm Cell nucleus Vacuole Nucleolus Nuclear membrane Ribosomes Rough endoplasmic reticulum Smooth endoplasmic reticulum Mitochondrian Chloroplast Golgi body Microtubules CELL WALL • Plant cells have cell walls. The most composition of a plant cell wall is cellulose. • Long cellulose molecules grouped into bundles called microfibrils are twisted into rope-like macrofibrils. • Sometimes there is also another layer component of cell wall e.g lignin - which gives strength, e.g. strengthens wood (xylem cells) in trees. • suberin - which helps prevent water from penetrating, e.g. suberin in mangroves minimize salt intake from their habitat important chemical FUNCTION(S) OF PLANT CELL WALLS • The main functions of plant cell walls are mechanical. • Plant cell walls form part of a transport system called the apoplast system via which water and some solutes can pass through plant tissue via apoplastic pathways (along / through cell walls) symplastic pathways (i.e. through the cytoplasm of a series of adjacent cells). PLASMA MEMBRANE • The plasma membrane is flexible enough to move closer to or away from the cell wall - according to changes in the water content of the cytoplasm within the cell. • As a differentially-permeable surface, the plasma membrane controls movement of solutes in and out of the cell. • Synthesis and assembly of cell wall components PLASMODESMATA • Plasmodesmata are tiny strands of cytoplasm that pass through pores in plant cell walls, forming "connections" or "pathways" between adjacent cells. • Specifically, plasmodesmata form the symplast pathway for the movement of water and solutes through plant structures. • These cell-cell connections are especially important for the survival of plant cells during conditions of drought. CYTOPLASM • The cytoplasm is the part of a plant cell that includes all the contents of the cell within the cell membrane but outside of the nucleus of the cell. • It therefore includes the cytosol (i.e. the semi-fluid part of a cell's cytoplasm) as well as the plant cell organelles incl. mitochondria, chloroplasts, etc.. • Also located within the cytoplasm is the cytoskeleton, which is a network of fibres whose function is to provide mechanical support to the cell, including helping to maintain the cell's shape. • Cytoplasm consists mainly of water and contains enzymes, salts, organelles, and various organic molecules. • It has a clear appearance and a gel-like texture. • The cytoplasm helps to move materials around the cell and also dissolves cellular waste VACUOLE • A cell's vacuole can occupy a large proportion of the total volume of the cell - e.g. 90% of the volume of some mature plant cells. • Each vacuole is enclosed by a vacuolar membrane called the tonoplast. • Contents of the vacuole: cell sap, which is a solution of salts, sugars and organic acids. Enzymes needed for recycling components of cells, e.g. chloroplasts. Anthocyanins are sometimes present in cell vacuoles. These are chemical pigments responsible for some of the (non-green) colours of glowers, e.g. reds, blues, purples. • Helps maintain turgor pressure (turgidity) inside the cell. This pressure pushes the plasma membrane against the cell wall. Plants need turgidity to maintain rigidity. CELL NUCLEUS • The nucleus is the "control center" of a eukaryotic cell (i.e. plant cells and animal cells) • Each cell nucleus is surrounded by a nuclear membrane that is also known as the "nuclear envelope". • The contents of the nucleus - so, inside the nuclear membrane includes DNA (genetic material) in the form of genes and a nucleolus. • The cell nucleus controls the activity of the cell by regulating protein synthesis within the cell. NUCLEOLUS & NUCLEAR MEMBRANE • The nucleolus is located within the nucleus and is the site of synthesis of: Transfer RNA ribosomal RNA ribosomal subunits • The nuclear membrane is also known as the nuclear envelope and encloses the contents of the nucleus of the cell - separating the contents of the nucleus from the rest of the cell. • Nuclear pores in the nuclear membrane enable various substances, such as nutrients and waste products, to pass into and out of the nucleus. ENDOPLASMIC RETICULUM • Rough endoplasmic reticulum is the site of protein synthesis (which takes place within the ribosomes attached to the surface of the RER) as well as storage of proteins and preparation for secretion of those proteins. • Smooth endoplasmic reticulum is the site of lipid synthesis and secretion within cells MITOCHONDRIAN • Mitochondria are structures found in both plant and animal cells. • They are bounded by double membranes, the inner of which is folded inwards, forming projections (called cristae). • Their function of mitochondria is energy production. • Mitochondria contain enzyme systems needed to synthesize adenosine triphosphate (ATP) by oxidative phosphorylation. • The quantity of mitochondria within cells varies with the type of cell. • In the case of plant cells, mitochondria may be particularly abundant in sieve tube cells (also called sieve tube members), root epidermal cells and dividing meristematic cells. CHLOROPLASTS • Chloroplasts are the sites of photosynthesis within plant cells. • Chloroplasts are very important parts of plant cells. • Some cells include up to 50 chloroplasts. • The number of chloroplasts per cell varies according to the type of cell and its function. • They are plentiful in leaf cells that receive sunlight - as opposed to root cells that do not receive light. • Chloroplasts are a type of plastid. There are also other types of plastids (not all of which are present in all plant cells but all of which are derived from proplastids. GOLGI APPARATUS • The Golgi apparatus of a cell is sometimes called the "packaging organelle" because it plays a role in transporting proteins. • It's structure and appearance takes the form of a stack of tiny pancake-like shapes, each of which is enclosed by a single membrane and contains fluid and biochemicals such as proteins, sugars and enzyme. FUNCTIONS OF GOLGI APPARATUS • Modifies some newly-synthesized biomolecules before storing them in granules, sometimes called vesicles - ready for transport later. • Forms lysosomes - which are tiny sacs filled with enzymes that enable the cell to utilize its nutrients, so are sometimes described as "cell digestion machines". Lysosomes also destroy the cell after it has died. • Transports the proteins produced in the ER: After a protein has been synthesized in the ER, a transition vesicle (or "sac") is formed then floats through the cytoplasm to the Golgi apparatus, into which it is absorbed. • After processing the molecules inside the sac, a secretory vesicle is formed and released into the cytoplasm, moves to the cell membrane, then releases the molecules from the cell MICROTUBULES • Microtubules are hollow rope-like structures composed of the protein Tubulin . They can be as long* as 25μm and have a diameter of approx 25 nm. • Functions facilitate addition of cellulose to cell wall form the spindles and cell plates of dividing cells play a role in cytoplasmic streaming (i.e. moving the fluid cytoplasm within the cell) e.g. to/from chloroplasts. RIBOSOMES • Ribosomes take part in the synthesis of some proteins by catalyzing the formation of those proteins from individual amino acids (using messenger RNA as a template). • Examples of proteins catalyzed by ribosomes include glycoproteins, lysosome proteins, membrane proteins and some organelle proteins. • In general, ribosomes can be either "free" or "membrane-bound", which in the cases of plant and animal cells means attached to the rough endoplasmic reticulum (RER). • An individual ribosome could be "membrane-bound" while it is making one protein, then "free" while making a different protein. ORGANIZATION OF PLANT TISSUES • Plants are made up of meristematic and permanent tissues and are supported by shoot and root organ systems. The primary plant is made up of 4 tissue types: Tissue type Functional roles meristematic division of new cells for new growth or repair ground tissue bulk tissue; storage, processing, physical support dermal tissue protection and sometimes nutrient absorption vascular tissue movement of fluids/food and physical support PLANT ORGANS • Shoot: stem and leaf • Node/internode • Root • Meristems: Apical & Lateral PLANT TISSUES MERISTEMATIC TISSUES • Meristematic tissues are responsible for the division of new cells... they are zones of actively dividing cells. • Cell division occurs solely in expansion may occur anywhere. • Thus in a single plant there are zones of young dividing cells, maturing cells, and mature cells. • There are 3 meristematic regions in the plant: Apical meristems Lateral meristem Intercalary meristem meristematic regions, while APICAL MERISTEMS • Apical meristems are located at the apices or tips - at root and shoot tips and are directly involved in their elongation • They create derivatives which form primary growth. The protoderm which forms the outer dermal layer of tissues, The ground meristem which forms the cortical cells and The procambium which forms the vascular tissue. ROOT APICAL MERISTEM LATERAL MERISTEM • Lateral meristem are responsible for horizontal expansion. It has following parts: Vascular meristem- internal growth in girth which involves secondary tissues (xylem and phloem). In the fasicular region the cambial cells which divide toward the center form xylem tissue and towards the outside phloem tissue. • Interfasicular indicates the cambium between the 'fasciles of xylem & phloem. Cork cambium- external girth growth beyond the phloem area. They form the characteristic corky layer as well as an internal layer. INTERCALARY MERISTEM • occurs between mature tissues sections in the vicinity of the nodes or leaf attachment. • common in grasses (occur at bases of nodes) • helps regenerate parts removed (by lawnmowers, herbivores, etc.) GROUND TISSUES • Ground Tissue is simple non-meristematic tissue (nondividing tissue) made up 3 cell types: Paranenchyma Collenchyma sclerenchyma. • This tissue generally forms either the pith, cortex or bulk of leaf ( mesophyll). PARENCHYMA CELLS • Most abundant cells in plants • Spherical cells which flatten at point of contact; alive at maturity; pliable, primary cell walls • large vacuoles for storage of starch, fats, and tannins (denature proteins) • Primary sites of the metabolic functions such as photosynthesis, respiration, and protein synthesis; • They are "ready reserves" from which a plant makes specialized cells to meet its changing needs. COLLENCHYMA CELLS • Living protoplasm • Unevenly thickened primary cell walls; elongate cells • Longer than wide; just beneath epidermis • Function to support growing organs, grass, floral parts, and border veins; their non-lignified cell walls can stretch. SCLERENCHYMA CELLS • Most are dead at maturity • Rigid, thick, lignified, nonstretchable secondary cell walls. • There are 2 types of sclerenchyma cells: Sclereids or stone cells- short; variable shape; form hard layers such as the shells of nuts and seed coats; produce the gritty texture of pears Fibers- long, slender; occur in strands or bundles; tiny cavity or lumen; the different hardnesses of fibers are used to make coarse rope, linen cloth, etc. DERMAL TISSUE • Epidermal tissue is a single layer of cells, (except in velamen, pepperomia and rubber plants) that covers the plant body • Functions: Acts as a buffer between the environment and the internal plant tissues Absorption of water and minerals primarily in the root region On the stem and leaves generally covered with a cutin which prevents evaporation, secretion of cuticle (waterproof; made of cutin= fatty substances) Can form a barrier resistant to bacteria and fungi Can prevent leaching of materials in or out of the surface • Leaf hairs (trichomes)- 1-2 cells; important in boundary layer and in defense; they reflect light to protect against overheating and excessive water loss. • This is an incredibly important function for plants in dry regions where excess light may lead to photobleaching of pigments and excess absorption of light would overheat the tissue. • The layer acts to hold in a layer of humidity 'trapped' between the epidermis and the tips of the trichomes. • This layer also prevent air moving directly against the stomata which would encourage water loss. Its' rough surface breaks up wind currents. • When in high density they act to deter herbivory by small animals. STOMATA • Created by guard cells; most abundant on underside of leaves • Regulate diffusion of CO2 into the leaf for photosynthesis as well as regulate loss of water from the leaf • Stomata respond to turgor pressure • Stomata open when water enter, thereby increasing turgor pressure in guard cells, thus pushing them apart • Stomata close when water leave the guard cells, thus leaving them flaccid and closing upon each other. SALT GLANDS • Dump sites for the excess salt absorbed in water from the soil; help plants adapt to life in saline environments • A crust of salt forms on leaves which tastes bad and the white surfaces act to reflect light Salt Glands VASCULAR TISSUE • Complex tissues: involve more than one kind of cell type. • Vascular tissue is complex mix of parenchyma, sclerenchyma, fiber cells, all non-transporting cells and those cells involved in transport: vessels, tracheids or phloem tube, and ray cells. XYLEM • Xylem functions to: Transport water and dissolved substances Support the physical structure Can act in food storage • Specific cells- Vessels: • predominate in angiosperms; • have short, wide, thick secondary cell walls; • are dead, hollow cells; lack end walls; • have a large diameter, therefore water movement through them is rapid; • the wall thickenings in vessels are pitted as you can see from the image on the right • note the annular thickenings in cells below on left and the helical thickenings in cells below on right which act to help support the walls TRACHEIDS • Predominate in conifers • long, slender cells with tapered, overlapping ends; • water moves upward from tracheid to tracheid through pit pairs, thus preventing large gas bubbles from forming and thus no cavitation during freezing/defrosting periods. • Water can move through the plant with little seasonal disruption allowing water flow and photosynthesis to occur year round. • No need for loss of leaves in the winter as in most angiosperms • Ray cells: regulate lateral transfer or storage. PHLOEM • Transports dissolved organic materials • Living protoplasts at maturity; no nuclei; most organelles gone; vacuole disappears; little protoplasm; • An accessory companion cell= controls metabolic regulation as it contains functional DNA. Note the narrow cell to the left of the one with the arrow. • Contain P-protein along longitudinal walls • Sieve elements are the conducting cells of phloem. • There are two kind of sieve elements: Sieve Cells Sieve Tube Members • Sieve cells- occur in nonflowering plants; long with tapered overlapping ends; associated with albuminous cells, which help regulate the sieve cells' activities • Sieve tube members- arranged end to end in sieve tubes; larger pores than sieve cells; concentrated along contacting end walls of adjacent sieve tube members SPECIALIZED CELLS • Root hairs- absorb water and the material it carries (minerals); they increase absorptive surface area of roots. • Although single celled, they can be viewed by the eye on rapidly growing radishes.