Plant Development PDF

Patterns in Plant Development Growth, Differentiation & Development Development Sum of two processes: growth & differentiation Sum of all changes that an organism goes through in its life cycle Germination – Growth – Maturation – Flowering – Seed Formation – Se...

Patterns in Plant Development Growth, Differentiation & Development Development Sum of two processes: growth & differentiation Sum of all changes that an organism goes through in its life cycle Germination – Growth – Maturation – Flowering – Seed Formation – Senescence An expression of the genetic program that directs the activities and interactions of individual cells Growth A quantitative term Related to increases in size / volume and mass Fresh weight, dry weight, length, width Differentiation A series of qualitative changes in the cell, tissue, or organ Differences other than size A dividing cell becomes two daughter cells Eg: zygote → cells that become root and shoot Unspecialized parenchyma cells → xylem vessels or phloem sieve tubes Most plant cells are totipotent: except for the most highly differentiated cells, they retain a complete genetic program and the capacity to renew division, growth, and differentiation Control of development The orderly development of a complex multicellular organism is coordinated by Intrinsic controls Extrinsic controls Intrinsic controls are expressed at both Intracellular level Intercellular level Intracellular controls Primarily genetic Requiring a programmed sequence of gene expression Intercellular controls Primarily hormonal, chemical messengers that allow cells to communicate with one another Extrinsic controls are environmental cues such as light, temperature, gravity, nutrient status, soil moisture Most environmental cues appear to operate at least in part by modifying gene expression or hormonal activities Signal Perception & Transduction All developmental stimuli are characterized by a sequence of signal perception, transduction, and response Signal perception requires a receptor molecule – normally a protein Receptors are known for Red-light receptor phytochrome Blue-light receptor crytochrome Hormones auxin and ethylene Receptors play a particularly important role for two reasons: Presence & absence of the appropriate receptor determines which cells are able to respond to a particular signal Different cell types may possess different receptors that in turn elicit different response to the same signal Signal transduction A diverse array of biochemical events that ultimately determines the cell’s response to that signal Involves a variety of small, mobile second messengers Second messengers To relay information from the receptor to the biochemical machinery inside the cell May also amplify the original signal by initiating a cascade of biochemical events Include the G-proteins, protein kinases, cyclic nucleotides, calcium ion, and inositol triphosphate Signal Transduction Mechanism of Auxin: Stimulation of Cell Elongation Loosening of Cell Wall – Cell Elongation Cell walls and Cell growth The driving force for cell enlargement is water uptake To resist rupturing in the face of high turgor pressure, plant cells are surrounded by a very strong and relatively rigid wall For a cell to enlarge, the strength and rigidity of the wall must be modified In a turgid cell, the force of water pressing against the wall generates stress within the extensively cross-linked wall components Growth is initiated when these stresses are relieved by wall-loosening events, which causes the load-bearing cross links between wall polymers to yield Relieved of stress, the wall expands, turgor is reduced, and more water moves in until both turgor and wall stresses are restored Wall loosening and cell expansion is stimulated by Low pH Expansins – induce stress relaxation & extension of isolated cell walls at low pH Cell division Some, but not all, cells are able to divide and produce new cells A plant cell about to divide has a large nucleus and no vacuole The nucleus divides first The middle lamella develops and separates the two cells The cytoplasm lays down a primary wall and layers of cellulose on each side of the middle lamella One of the cells develops a vacuole and enlarges The other cell retains the ability to divide again Although each cell has it own primary wall, these often give the appearance of a single, intermediate wall between cells Plant development The beginning of a plant: seed germination Shoot & root development Flower evocation & development Flower & fruit development Seed A quiescent structure Containing a food supply (usually an endosperm) An embryo that contains the rudimentary organs of a young plant – the root and shoot Seed germination Begins with the imbibition of water to hydrate the relatively dry seed tissues Followed by the mobilization of stored carbon reserves and the onset of respiration Radicle emerges from the seed coat Rupture of the seed coat and protrusion of the radicle allows it to make contact with water and nutrient to support further growth of the young seedling Shoot development Elongation of shoot axis The hypocotyl (below the cotyledons) is the first to elongate, pulling the cotyledons and the enclosed first foliage leaves up through the soil – epigeal germination Bean  In other dicots & most monocots, the epicotyl (above the cotyledons) undergoes extensive elongation to bring the first leaves above the soil – hypogeal germination Pea Corn Shoot Development epigeal germination hypogeal germination Shoot apical meristem (SAM) Located at the apex of the stem A dome-shaped structure usually surrounded by clasping leaf primordia Two principal functions: To form primordia that give rise to lateral organs To perpetuate itself by maintaining a small population of undifferentiated dividing cells Central zone Contains the stem cells, which divide slowly but are the ultimate source of the tissues that make up the plant body Peripheral zone Cells divide rapidly, surrounds the central zone and produces the leaf primordia Rib zone Lies below the central zone and generates the central tissues of the stem Root development Roots grow and develop from their distal ends The boundaries are not sharp Four developmental zones in a root tip: root cap meristematic zone elongation zone maturation zone Elongation of the roots and shoot leads to the establishment of a young seedling, floral evocation, and the development of flowers and fruit Stages of plant differentiation Growth & Developmental Stages in Rice