Growth and Development
I. Terminology: Growth, Differentiation, and Development A. Growth: an irreversible increase in size 1. Fresh Mass (Weight) 2. Dry Mass (Weight) B. Differentiation: process of cellular specialization C. Development or Morphogenesis: formation of tissues, organs, and organisms II. Patterns of Growth and Development A. Terminology 1. Growth Regions in Plants a. Apical Meristems: Root Tip, Apical Bud, Axillary Buds) (1) Primary Meristems: root tips, apical and Axillary buds. Produce and increase in vertical growth of a plant (shoots grow up, root grow down) (2) Primary Growth: tissues produced from cell divisions of the primary meristems. b. Lateral Meristems (Cork Cambium, Vascular Cambium) (1) Secondary Meristems: cambiums that give an increase in the girth of a plant. Typically found in dicots but not in monocots. Cork cambium produces cells that add to the phellem (cork) on the outer surface of the plant. The vascular cambium adds cells to the xylem and phloem. (2) Secondary Growth: those tissue that are derived from the cork and vascular cambiums. 2. Plant Growth Patterns a. Determinate: Having definite limits. A type of growth in which the plant stops growing after it reaches a certain size. b. Indeterminate: Having no set limits. A type of growth characteristic of plants in which the organism continues to grow as long as it lives. 3. Temporal Growth Patterns: a. Monocarpic species (single reproductive event), typical of many annual and biennial plants that flower once, then die. Polycarpic species (many reproductive events), typical of perennial plants that grow for many years and flower each year. b. Annual: plants that have seed germination, growth and reproduction in a one year cycle. Biennial: plant that have seed germination, growth, and reproduction in a two year cycle. Typically the first years growth is a rosette of leaves around an underground stem. Flowering occurs during the second year. Perennial: plant grow and flower over several years. B. Steps in Cell Growth and Development 1. Cell division: mitosis, cytokinesis, and the cell cycle (review these topics from Bgy 11, Bgy 30, and Bgy 33) a. Cells divide in different plants: (1) Periclinal: parallel to the perimeter (closest surface) of the plant (2) Anticlinal: perpendicular to the perimeter (closest surface) of the plant b. Cell Plate formation in dividing cells (1) Vesicles pinched off from the Golgi vesicles. Contain pectins that make up the middle lamella. Middle lamella cements the walls of adjacent cells together. (2) New cell wall formed. Primary Cell wall (3) Vesicles migrate along the rodlike microtubules that extend toward opposite poles of the dividing cell 2. Cell Enlargement: a. In part controlled by the plane of cell division and the formation of the primary wall. b. Cellulose laid down like a spring around the new cell. Allows elongation of the cell as it takes up water in the central vacuole 3. Cell Differentiation: specialization of the new cells. Controlled in part by growth hormones. C. Primary Wall Changes During Growth 1. Primary Walls: cellulose microfibrils embedded in an amorphous matrix of noncellulosic polysaccharides and some protein. 2. Cellulose microfibrils behave like a multistranded cable that minimizes extension in the direction of the long axis. 3. Growth can occur at right angles to the orientation of the microfibril axis. 4. The plant cell wall retains a near-uniform thickness during growth. The newest deposited cell wall has the greatest control over cell growth. 5. If new microfibril orientation is random, growth will be equal in all directions. 6. If new microfibril orientation is not random, but occurs along one axis, growth will be favored in a direction perpendicular to that axis. This is seen in primary tissues of elongating roots, stems, and petioles. 7. Cellulose formation is controlled by enzymes located on the plasma membrane (plasmalemma): globules on the outer face, rosettes on the inner face of the membrane. D. Physics of Growth: Water Potentials and Yield Points 1. Uptake of water stretches the cell walls. New cell wall and membrane materials are synthesized so that the wall does not become thinner. 2. Water pressure (turgor) drives growth by forcing the wall and membranes to expand. Rate of water movement into the cell is governed by two factors: the water potential gradient and the permeability of the membrane to water. The rate of cell enlargement is also proportional to these factors. 3. Y = Ys + Yp = Ys + P Y = water potential Ys = osmotic or solute potential Yp = pressure potential which is a real pressure, P, called turgor pressure Difference of water potential (DY) inside and outside a cell is: DY = (Yse + Pe) - (Ysi + Pi) e = external i = internal Equation for the rate of cell enlargement: (1/V)(dV/dt) = L(DY) = L(Yse - Ysi- Pi) L = hydraulic conductivity of the membrane to water V = cell volume dV = incremental change in the volume of the cell dt = infinitesimal increment of time dV/Dt is the rate of cell increase in volume, or the rate of growth. Growth is a geometric or logarithmic function of time. To express this, divide dV/dt by the cell volume to give a relative growth rate of the cell which is defined as r. 4. These two equations show that there are two ways for water potential inside a cell to be more negative than water potential outside the cell. This makes water uptake and growth possible a. Solutes inside the cell may increase, making the osmotic potential inside the cell more negative However, solute concentrations inside growing cells remain constant. Therefor, the driving force for growth must be the turgor pressure. b. Pressure inside the cell can decrease. Pressure in the cell is caused by the mechanical resistance of the cell wall to being stretched. If this mechanical resistance is decreased (the wall relaxes due to stretching) it results in lower pressure, which reduces cell water potential, leading to a larger water potential gradient (DY) and movement of water into the cell. 5. Elastic Growth: cell walls stretch Plastic Growth: cell walls do not return to original dimensions. Plastic stretching increased by auxin. Extensibility: both elastic and plastic components. 6. Plastic wall stretching is achieved as the cell wall is loosened, so cellulose microfibrils can slide past each other (shear) by breaking bonds between adjacent microfibrils. 7. Plastic and elastic elements in series with each other. As the plastic elements relax and stretch, they allow the elastic elements to shorten. This only happens if cell wall stress and turgor pressure are reduced. If water enters as pressure decreases in response to relaxation of the plastic element, the elastic elements shorten only slightly, resulting in pressure decreases only slightly. This is called creep: a steady-state, growth process, where wall stress and turgor remain constant. 8. Relative cell growth rate (r) is proportional to turgor pressure (P) when it exceeds a yield threshold or yield point (Y). r=F(P-Y) for P>Y Proportionality factor F is the wall extensibility. Yield point (Y) is expressed as the minimum pressure necessary to cause cell growth. Relative cell growth rate depends upon five interrelated factors: a. Conductivity of walls and membranes to water around the cells b. Difference in osmotic (solute) potential inside and outside the cell c. Cell turgor pressure d. Two wall-yielding properties: extensibility and yield threshold 9. Wall relaxation is the reduction in wall stress at constant cell-wall dimensions. 10. As wall relaxation occurs, turgor pressure drops exponentially until the yield point (Y) is reached at some value above zero. The rate of turgor-pressure decrease can be used to evaluate the wall extensibility (F). 11. In vivo stress relaxation uses the natural stresses on the wall (cell turgor). Turgor applies hydraulic stress in all directions from within the cell. 12. In enlarging cells, water uptake dilutes the solute and lowers Ysi. However, solutes are absorbed from the surroundings or synthesized in the tissue. III. Growth Kinetics: Growth Through Time A. Whole Organs: The S-Shaped Growth Curve 1. Phases of a Sigmoidal Growth Curve: a. Logarithmic phase: size (V) increases exponentially with time (t). b. Growth rate (dV/dt) is slow at first, but the rate continuously increases. Rate is proportional to the size of the organism; the larger the organism, the faster it grows. c. Linear phase: increase in size continues at a constant, usually maximum rate for some time. d. Senescence phase: decreasing growth rate as the plant reaches maturity and begins to senesce. 2. Typically, elongation is more rapid during the long days of late spring and early summer but there are exceptions. 3. Growth in stem diameter continues at a decreasing rate, until well after growth in height stops. 4. Dormancy does not occur in the roots. B. The Flow Analogy of Plant Growth 1. Dividing cells are found in the apical meristems. Elongating cells are found farther from the tip. Differentiating cells are found even farther from the tip. 2. Phyllotaxy: history of leaf production can be understood by examining pattern formed by leaves or leaf scales along the stem. Future leaf development of a leaf primordium at the stem tip can be determined by examining older leaves farther down the stem. 3. These characteristics of a stem indicate that indeterminate growth in plants is a flow process. 4. New cells being produced in the meristem pass through the hook at the top of the developing stem. These cells enlarge, elongate, and eventually differentiate below the hook a. Cell divisions stop before the cells reach the hook b. Hook must form when cells on the outside elongate more than those on the inside. 5. Hook straightening is controlled by light of different irradiance and wavelength. IV. Plant Organs: How They Grow A. Roots 1. Organization of the Young Root. a. Seed germination begins with protrusion of the radicle. In some species, cytokinesis occurs in the radicle before germination is complete. In other species few mitotic divisions occur before radicle protrusion. Elongation occurs by growth of cells formed when the embryo developed. b. Oldest root cap cells are distal (farthest from the meristem). Proximal (closer to meristem) root cap cells are younger. Root cap protects the meristem and is important in the perception of gravity. The root cap also exudes a polysaccharide rich slime or mucigel that lubricates the root to help in soil penetration. The root cap may also harbor microorganisms, influence formation of mycorrhizae, root nodules, and ion uptake. c. Cells divide in the apical meristem. Develop into epidermis, cortex, endodermis, pericycle, phloem, and xylem. In-between the root cap and the apical meristem of the root is the quiescent center where cell division seldom occurs. 2. Formation of Lateral Roots begins several millimeters to a few centimeters distal to the root tip. The pericycle is the tissue responsible for initiation of lateral root formation. Thus, a lateral root must penetrate the cortex of the root before emerging into the soil. 3. Radial Growth of Roots a. In the root-hair zone, the vascular cambium between the primary phloem and primary xylem produces new xylem and phloem cell. These tissues increase with of the root. Most monocots do not form a vascular cambium and thus do not have secondary growth. b. After the vascular cambium initiates secondary growth, a cork cambium (phellogen) is initiated in the pericycle. Cork (phellem) is produced toward the outside and secondary cortex (phelloderm) is produced to the inside of the cork cambium. The new cell become suberinized. c. Suberinized roots can absorb water and mineral salts through: (1) lenticles, (2) tiny crevices formed by penetration of branch roots, (3) holes left when branch roots die. B. Stems 1. Apical meristems of shoots form in the embryo and are responsible for origination of new leaves, branches, and floral parts. 2. Intercalary meristems: intercalated (inserted) between regions of older, nondividing cells. C. Leaves 1. Leaf Primordium: do not develop randomly a. Phyllotaxis b. Recent model: one substance diffuses from the apex and another from the ends of the vascular system; the second substance is consumed by formation of a leaf. 2. Leaf Development: outward extension occurs by both periclinal and anticlinal divisions at the primordium tip. Meristematic activity begins throughout the length of the leaf. a. Grass leaf and conifer needle development stops first at the distal end and eventually stays at the leaf base. b. Angiosperms: meristems produce new cells along margins of the leaf axis; cease activity before the leaf matures. c. Grasses: meristem is intercalary: active for long periods, stimulated by defoliation (i.e. mowing the grass!) 3. Cell division in a leaf is complete when the leaf has reached less than 1/5 its final area. Final 80% of leaf expansion is caused by growth of performed cells. Growth occurs over the entire leaf area, but not uniformly. Cells in the young leaf are compact. As the leaf expands, the mesophyll cells stop growing before the epidermal cells. The expanding epidermis pulls the mesophyll cells apart, causing development of intercellular air spaces in the mesophyll. D. Flowers: 1. Perfect: containing female and male parts Imperfect: staminate (male) or pistillate (female) flowers Dioecious: separate male and female plants Monoecious: staminate and pistillate flowers at different positions along a single stem Cones (Strobili): most conifers are monoecious; Junipers and others are dioecious. 2. Anthesis: opening of flowers: a. Some flowers remain open from anthesis until abscission. Other flowers open and close at specific times for several days. Permanent opening is caused by differential growth of the inner verses the outer part of the petal. Continued opening and closing is a response to temporary changes in turgor pressure across the two sides. Influenced by temperature, atmospheric vapor pressure, internal clock. b. Withering is associated with extensive transport of solutes from the flowers to other parts of the plant, often to the ovary, with rapid water loss. E. Seeds and Fruits 1. Chemical changes in growing seeds and fruits. a. The zygote, embryo sac, and ovule develop into the seed. The ovary develops into the fruit (pericarp). Sucrose, glucose, and fructose accumulate in ovules until the endosperm nuclei are surrounded by cell walls. Sugar content decreases as they are used in cell-wall formation and starch or fat synthesis. Nitrogen is present in proteins, amino acids, glutamine and asparargine. Amino acids and amides decrease in concentration as storage proteins are formed in the protein bodies. b. Role of enzymes and nucleic acids in developing seeds: must posses enzymes necessary for germination. (1) Enzymes essential to germination are produced in a stable form during seed development (2) Translated from stable messenger RNA, tRNA, and rRNA molecules synthesized during seed maturation (3) Other proteins are formed from newly transcribed RNA molecules after the seed germinates. c. Dehydration: loss of water during seed maturation is critical d. Chemical composition of edible fruits and transformation of carbohydrates during ripening changes. e. Transformation of chloroplasts to carotenoid-rich chromoplasts, accumulation of anthocyanin pigments, and accumulation of flavoring components. 2. Importance of seed for fruit growth a. Development of fruits usually depends on germination of pollen grains on the stigma (pollination) or on pollination plus fertilization. Developing seeds are essential for normal fruit growth. b. Normal production of fruits lacking seeds is called Parthenocarpic Fruit Development. Ovary development without pollination. Fruit growth stimulated by pollination without fertilization or form fertilization followed by abortion of the embryos. 3. Relations between vegetative and reproductive growth a. Topping: removal of flowers and fruits. b. Competition for nutrients among vegetative and reproductive organs. Mineral salts, sugar, and amino acids Nutrient accumulation in developing flowers, fruits, or tubers occurs largely at the expense of materials in nearby leaves. c. Mechanism by which fruits can divert nutrients out of leaves and into their on tissues. Against concentration gradients. Controlled by phloem unloading. Various hormones, cytokinins, involved. d. Factor that stimulate shoot growth retard flower, tuber, and fruit development. High nitrogen fertilization causes stem and leaf growth of tomato plants, may reduce fruit development. There is no real evidence that high nitrogen inhibits fruit production per plant is other nutrients, phosphorus and potassium are adequate. e. Heavy pruning, drought , typing branches to the ground, or various other mutilation procedures stimulate flowering. Commercial growth retardant inhibit growth of stems, stunting is sometimes accompanied by early appearance of flower buds or a greater number of flowers per plant. V. Morphogenesis: Juvenility A. After seed germination, annual and perennial seedlings enter a rapid growing phase in which flowering cannot be induced. Characteristic morphology established. Juvenile phase as opposed to the mature or adult phase. B. Juvenile phase with respect to flowering varies in perennials. One year to 40 years. "Ripeness to respond": long juvenile phases in conifers and other trees can pose obstacles to genetic programs designed to improve quality. Typically stem cuttings are used to form adventitious roots in juvenile trees. In the adult phase, the rooting ability is reduced and sometime lost. C. Heterophylly: juvenile and adult morphologies of leaves. Balance of gibberellins and ABA may be involved in the transition from one state to another. VI. Morphogenesis: Totipotency A. Totipotent: a nonembryonic cell has the potential to dedifferentiate into an embryonic cell and then to develop into a complete new plant. B. Totipotency illustrated by development of cultured callus tissues into new plants. Partial totipotency occurs when adventitious roots develop from stem cells and when xylem and phloem are regenerated from wounded cortex cells. C. Several cells cooperate to form primordia from which the while plant arises. 1. Frederick Stewart, Cornell university 1950's: work on cytokinins 2. Vasil and Hildebrandt 1965: produced entire plants from isolated single cells. D. Haploid pollen grains develop into callus tissues and then whole plants. 1. Endoreduplication (doubling of chromosomes in mitosis, with lack of subsequent cytokinesis) E. Somatic embryogenesis: somatic haploid or diploid cells develop into differentiated plants through characteristic embryological stages. Process occurs from cells associated with seed development, such as nucellus or synergid cells. Apomixis: embryo in a seed is not formed from a union of gametes F. Somatic embryogenesis: more easily occurs in tissues that are embryonic. G. Isolation of a cell from its surroundings can predispose the cell to embryogenesis. The role of growth hormones in this process is not well understood. Auxins can induce callus formation on many stems, and specific growth regulators, particularly cytokinins, often must be added to culture media to induce embryogenesis.