Plant Growth and Development

Cheatsheet Content

1. Introduction to Plant Growth & Development Definition: Development is the sum of two processes: growth and differentiation. Growth: Irreversible permanent increase in size of an organ or its parts, or even of an individual cell. Accompanied by metabolic processes (anabolic and catabolic) at the expense of energy. Factors: Governed and controlled by both intrinsic (internal, e.g., genetic, plant growth regulators) and extrinsic (external, e.g., light, temperature, water, oxygen, nutrition) factors. Seed Germination: First step in plant growth. Occurs when favorable conditions exist; otherwise, seeds enter a period of suspended growth or rest. 2. Plant Growth 2.1 General Characteristics Indeterminate Growth: Plants retain the capacity for unlimited growth throughout their life due to meristems. Meristems: Cells with capacity to divide and self-perpetuate. Products of division eventually lose capacity to divide and form the plant body. Open Form of Growth: New cells are continuously added to the plant body by meristem activity. Apical Meristems: Root apical meristem and shoot apical meristem; responsible for primary growth (elongation along axis). Lateral Meristems: Vascular cambium and cork-cambium (in dicots and gymnosperms); responsible for secondary growth (increase in girth). 2.2 Growth Measurement Growth is principally an increase in protoplasm, which is difficult to measure directly. Parameters for Measurement: Increase in fresh weight Dry weight Length (e.g., pollen tube) Area (e.g., dorsiventral leaf) Volume Cell number (e.g., maize root apical meristem) Increase in size (e.g., watermelon cells) 2.3 Growth Rates Increased growth per unit time. Can be expressed mathematically. Arithmetic Growth: Only one daughter cell continues to divide, the other differentiates. Example: Root elongating at a constant rate. Equation: $L_t = L_0 + rt$ $L_t$: length at time 't' $L_0$: length at time 'zero' $r$: growth rate / elongation per unit time Produces a linear curve when length is plotted against time. Geometric Growth: Both progeny cells retain the ability to divide and continue to do so. Initial growth is slow (lag phase), then rapid (exponential phase), followed by a stationary phase due to limited nutrient supply. Equation: $W_1 = W_0 e^{rt}$ $W_1$: final size (weight, height, number, etc.) $W_0$: initial size at the beginning of the period $r$: growth rate (relative growth rate or efficiency index) $t$: time of growth $e$: base of natural logarithms Produces a sigmoid (S-curve) when growth parameter is plotted against time. Typical for living organisms in natural environments, cells, tissues, and organs. Quantitative Comparisons: Absolute Growth Rate: Measurement and comparison of total growth per unit time. Relative Growth Rate: Growth of the given system per unit time expressed on a common basis (e.g., per unit initial parameter). 2.4 Phases of Growth Observed in root tips: Meristematic Phase: Located at root and shoot apices. Cells are constantly dividing, rich in protoplasm, possess large conspicuous nuclei. Cell walls are primary, thin, cellulosic, with abundant plasmodesmatal connections. Elongation Phase: Cells proximal to the meristematic zone. Characteristics: Increased vacuolation, cell enlargement, new cell wall deposition. Maturation Phase: Further away from the apex, proximal to the elongation phase. Cells attain maximal size with wall thickening and protoplasmic modifications. Most tissues and cell types studied in earlier classes represent this phase. 2.5 Conditions for Growth Water: Essential for cell enlargement (turgidity), provides medium for enzymatic activities. Oxygen: Helps release metabolic energy for growth activities. Nutrients: Macro and micro essential elements for protoplasm synthesis and energy. Temperature: Optimum range required; deviations can be detrimental. Environmental Signals: Light and gravity affect certain phases/stages of growth. 3. Differentiation, Dedifferentiation, and Redifferentiation Differentiation: The process where cells derived from meristems mature to perform specific functions. Involves structural changes in cell walls and protoplasm. Example: Tracheary elements lose protoplasm and develop strong, elastic, lignocellulosic secondary cell walls for water transport. Differentiation in plants is 'open' – cells/tissues from the same meristem can have different structures at maturity, influenced by their location. Example: Cells away from root apical meristems differentiate as root-cap cells, while peripheral cells mature as epidermis. Dedifferentiation: The phenomenon where living differentiated cells, having lost the capacity to divide, regain the capacity of division under certain conditions. Example: Formation of meristems (interfascicular cambium, cork cambium) from fully differentiated parenchyma cells. Redifferentiation: Meristems/tissues formed by dedifferentiation divide and produce cells that again lose the capacity to divide but mature to perform specific functions. 4. Development Definition: All changes an organism goes through during its life cycle, from seed germination to senescence. Sequence in a plant cell: Cell Division $\rightarrow$ Plasmatic growth $\rightarrow$ Differentiation $\rightarrow$ Expansion (Elongation) $\rightarrow$ Maturation $\rightarrow$ Mature Cell $\rightarrow$ Senescence $\rightarrow$ Death. Plasticity: The ability of plants to follow different pathways in response to environment or phases of life to form different kinds of structures (e.g., heterophylly). Heterophylly: Different leaf shapes on the same plant depending on the growth stage (juvenile vs. mature, e.g., cotton, coriander, larkspur) or environment (air vs. water, e.g., buttercup). Development is the sum of growth and differentiation. 5. Plant Growth Regulators (PGRs) / Phytohormones 5.1 Characteristics Small, simple molecules of diverse chemical composition. Also called plant growth substances or plant hormones. Synthesized in various parts of the plant, controlling different developmental events. Can be broadly divided into two groups based on function: promoters and inhibitors. Their roles can be complementary or antagonistic, individualistic or synergistic. 5.2 Discovery of PGRs Auxins: Charles and Francis Darwin observed phototropism in canary grass coleoptiles. Concluded the tip of coleoptile was the site of transmittable influence causing bending. F.W. Went isolated auxin from oat seedling coleoptile tips. Gibberellins: 'Bakanae' (foolish seedling) disease in rice caused by Gibberella fujikuroi . E. Kurosawa (1926) reported disease symptoms in rice seedlings treated with sterile fungal filtrates. Active substances later identified as gibberellic acid. Cytokinins: F. Skoog and co-workers observed callus proliferation in tobacco stem internodal segments only with auxins and extracts of vascular tissues, yeast extract, coconut milk, or DNA. Miller et al. (1955) identified and crystallized the cytokinesis-promoting active substance as kinetin. Abscisic Acid (ABA): Mid-1960s, three independent researches reported purification of inhibitors: inhibitor-B, abscission II, and dormin. All three proved chemically identical and named abscisic acid. Ethylene: H.H. Cousins (1910) confirmed release of a volatile substance from ripened oranges hastened ripening of stored unripened bananas. Identified as ethylene, a gaseous PGR. 5.3 Major Groups of PGRs and Their Physiological Effects 5.3.1 Plant Growth Promoters Involved in growth-promoting activities like cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation. Examples: Auxins, Gibberellins, Cytokinins. 5.3.1.1 Auxins (Indole-3-acetic acid - IAA) Natural: IAA, Indole Butyric Acid (IBA). Synthetic: Naphthalene Acetic Acid (NAA), 2,4-D (2,4-dichlorophenoxyacetic acid). Production Site: Growing apices of stems and roots. Key Functions: Initiate rooting in stem cuttings (plant propagation). Promote flowering (e.g., pineapples). Prevent fruit and leaf drop at early stages. Promote abscission of older mature leaves and fruits. Apical Dominance: Growing apical bud inhibits lateral (axillary) bud growth. Removal of shoot tip (decapitation) promotes lateral bud growth (used in tea plantations and hedge-making). Induce parthenocarpy (e.g., tomatoes). Herbicides: 2,4-D is used to kill dicotyledonous weeds without affecting monocots (used in weed-free lawns). Controls xylem differentiation and helps in cell division. 5.3.1.2 Gibberellins (GAs) Over 100 types (GA$_1$, GA$_2$, GA$_3$, etc.), GA$_3$ is the most studied. All GAs are acidic. Key Functions: Increase length of axis (e.g., grape stalks). Elongate and improve shape of fruits (e.g., apple). Delay senescence (extends market period for fruits). Speed up malting process in brewing industry (GA$_3$). Increase stem length in sugarcane, increasing yield (up to 20 tonnes/acre). Hastens maturity period in juvenile conifers (early seed production). Promotes bolting (internode elongation just prior to flowering) in rosette plants like beet, cabbages. 5.3.1.3 Cytokinins Specific effects on cytokinesis. Natural: Zeatin (from corn-kernels, coconut milk), others. Synthetic: Kinetin (modified adenine, not naturally occurring). Production Site: Regions of rapid cell division (root apices, developing shoot buds, young fruits). Key Functions: Produce new leaves, chloroplasts in leaves. Promote lateral shoot growth and adventitious shoot formation. Help overcome apical dominance. Promote nutrient mobilization (delays leaf senescence). 5.3.2 Plant Growth Inhibitors Play important roles in plant responses to wounds, stresses (biotic & abiotic), dormancy, and abscission. Example: Abscisic Acid (ABA). 5.3.2.1 Ethylene ($C_2H_4$) Gaseous PGR. Production Site: Synthesized in large amounts by tissues undergoing senescence and ripening fruits. Key Functions: Horizontal growth of seedlings, swelling of axis, apical hook formation in dicot seedlings. Promotes senescence and abscission of plant organs (leaves, flowers). Highly effective in fruit ripening, enhances respiration rate during ripening (respiratory climactic). Breaks seed and bud dormancy, initiates germination (peanut seeds), sprouting (potato tubers). Promotes rapid internode/petiole elongation in deep water rice (helps leaves/upper shoot parts remain above water). Promotes root growth and root hair formation (increases absorption surface). Initiates flowering and synchronizes fruit-set in pineapples. Induces flowering in mango. Ethephon (source of ethylene) hastens fruit ripening (tomatoes, apples) and accelerates abscission (thinning cotton, cherry, walnut), promotes female flowers in cucumbers (increasing yield). 5.3.2.2 Abscisic Acid (ABA) Role in regulating abscission and dormancy. General plant growth inhibitor and inhibitor of plant metabolism. Key Functions: Inhibits seed germination. Stimulates stomatal closure. Increases plant tolerance to various kinds of stresses (hence called stress hormone ). Plays important role in seed development, maturation, and dormancy. Induces dormancy, helping seeds withstand desiccation and unfavorable growth factors. Acts as an antagonist to Gibberellins.