Transcript Document
Vegetative Plant Development Chapter 37 Embryo Development Begins once the egg cell is fertilized -The growing pollen tube enters angiosperm embryo sac and releases two sperm cells -One sperm fertilizes central cell and initiates endosperm development (nutrients for embryo) -Other sperm fertilizes the egg to produce a zygote -Cell division soon follows, creating the embryo 2 Embryo Development The first zygote division is asymmetrical, resulting in two unequal daughter cells -Small cell divides repeatedly forming a ball of cells, which will form the embryo -Large cell divides repeatedly forming an elongated structure called a suspensor -Transports nutrients to embryo The root-shoot axis also forms at this time -Roots near suspensor cells and shoots at other end. 3 How do plants divide assymetrically in the first place? Sperm entry point, light, gravity Main body = thallus Algae “anchor” = Rhizoid This is all information gathered from brown algae!! 4 Development of Body Plan In plants, three-dimensional shape and form arise by regulating cell divisions -The vertical axis (root-shoot axis) becomes established at a very early stage -Cells soon begin dividing in different directions producing a solid ball of cells -Apical meristems establish the root-shoot axis in the globular stage Root–shoot axis Radial axis Cell wall forming parallel to embryo surface Embryo Suspensor 5 Development of Body Plan The radial axis (inner-outer axis) is created when cells alternate between synchronous cell divisions -Produce cell walls parallel to and perpendicular to the embryo’s surface The 3 basic tissue systems arise at this stage -Dermal, ground and vascular tissue established Cell wall forming perpendicular to embryo surface Vascular tissue system (procambium) Ground tissue system (ground meristem) Shoot apical meristem Root–shoot axis Dermal tissue system (protoderm) Multiple parallel and perpendicular divisions, accompanied by apical growth divisions lengthening the root– 6 shoot axis Root apical meristem Some Genes Involved in RootShoot Formation Both shoot and root meristems are apical meristems, but are independently controlled -Shootmeristemless stm mutant (STM) is necessary for shoot formation, but not root development -STM encodes a transcription factor STM wild type 7 Cotyledons not mature leaves with homeobox region are shown Some Genes Involved in RootShoot Formation The HOBBIT gene is required for root meristem, but not shoot meristem formation Hobbit is a protein that inhibits another protein that stops the gene expression of the genes that Auxin 8 causes to be made!!!! Auxin and Monopteros Promote Root Development One way that auxin induces gene expression is by activating the MONOPTEROS (MP) protein -Auxin releases the repressor from MP -MP then activates the transcription of a root development gene 9 Two Internal Proteins Responsible for the Development of a Structure Cause Similar Phenotypes if their corresponding genes are mutated Abnormal cell division create stub rather than a root Has a basal peg not a root 10 Developmental Changes During the Globular Stage Primary meristems differentiate while the plant embryo is still at the globular stage...THERE ARE 3 PRIMARY MERISTEMS -No cell movements are involved The outer protoderm develops into dermal tissue that protects the plant The ground meristem develops into ground tissue that stores food and water The inner procambium develops into vascular tissue that transports water & nutrients 11 Morphogenesis The heart-shaped globular stage gives rise to bulges called cotyledons -Two in eudicots and one in monocots These bulges are produced by embryonic cells, and not by the shoot apical meristem -This process is called morphogenesis -Results from changes in planes and rates of cell division 12 More Morphogenesis Changes -As development proceeds, the cells with multiple potentials are restricted to the meristem regions -Many meristems have been established by the time embryogenesis ends and the seed becomes dormant During embryogenesis, angiosperms (Flowering plants) undergo three other critical events: -Storage of food in the cotyledons or endosperm -Differentiation of ovule tissue to form a seed coat -Development of carpel wall into a fruit 13 Endosperm Information Endosperm varies between plants -In coconuts it is liquid -In corn it is solid -In peas and beans it is used up during embryogenesis -Nutrients are stored in thick, fleshy cotyledons 14 Seeds -In many angiosperms, development of the embryo is arrested soon after meristems and cotyledons differentiate -The integuments develop into a relatively impermeable seed coat -Encloses the seed with its dormant embryo and stored food 15 Seeds Seeds are an important adaptation because: 1. They maintain dormancy under unfavorable conditions 2. They protect the young plant when it is most vulnerable 3. They provide food for the embryo until it can produce its own food 4. They facilitate dispersal of the embryo SEED FACTS Once a seed coat forms, most of the embryo’s metabolic activities cease Germination cannot take place until water and oxygen reach the embryo Seeds of some plants have been known to remain viable for thousands of years 16 Styles of Seed Germination Specific adaptations ensure that seeds will germinate only under appropriate conditions -Some seeds lie within tough cones that do not open until exposed to fire 17 Styles of Seed Germination -Some seeds only germinate when sufficient water is available to leach inhibitory chemicals from the seed coat -Still other seeds germinate only after they pass through the intestines of birds or mammals 18 Fruits = Pistils...During Seed Formation Flower Develops into Fruit Pistils = One or more Carpels Carpels = Stigma, Style & Ovary The ovary wall is termed the pericarp -Has three layers: exocarp, mesocarp and endocarp -One, some, or all of these layers develop to recognized fruit Fruits can be: -Dry or fleshy -Simple (single carpel), -Aggregate (multiple carpels), -Multiple (multiple flowers) 19 Fruits have seeds!! (in general) In Fleshy Fruits Like Tomatoes Pericarp Is Thicker In Dry Fruit (Like Legumes) the Pericarp Is Dry @ Maturity 20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Aggregate Fruits Derived from many ovaries of a single flower; strawberries, blackberries. Unlike tomato, these ovaries are not fused and covered by a continuous pericarp. Sepals of a single flower Seed Ovary Multiple Fruits Individual flowers form fruits around a single stem. The fruits fuse as seen with pineapple. Pericarp of individual flower 21 Main stem Fruits Developmentally, fruits are fascinating organs that contain 3 genotypes in one package: -The fruit and seed coat are from the prior sporophyte generation -The developing seed contains remnants of the gametophyte generation (????) -The embryo represents the next sporophyte generation 22 Fruit Dispersal -Ingestion and transportation by birds or other vertebrates -Hitching a ride with hooked spines on birds and mammals -Blowing in the wind -Floating and drifting on water 23 Germination Germination is defined as the emergence of the radicle (first root) from the seed coat Germination begins when a seed absorbs water & oxygen is available for metabolism -Often requires an additional environmental signal such as specific wavelength of light 24 Releasing Sugars From Cotyledon...So the Embryo Can Grow 25 Releasing Sugars From Cotyledon...So the Embryo Can Grow Embryo produces gibberellic acid -This hormone signals the aleurone (outer endosperm layer) to produce a-amylase -Breaks down the endosperm’s starch into sugars that are passed to embryo 26 Germination New growth comes from delicate meristems As the sporophyte pushes through the seed coat, it orients with the environment such that the root grows down & shoot grows up -Usually, the root emerges before the shoot -The shoot becomes photosynthetic, and the postembryonic phase is under way Cotyledons may be held above or below the ground -May become photosynthetic or shrivel 27 Germination 28