Vegetative Plant Development Chapter 37

2 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

3 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.

4 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!!

5 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 Embryo Suspensor Root–shoot axis Radial axis Cell wall forming parallel to embryo surface

6 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 Multiple parallel and perpendicular divisions, accompanied by apical growth divisions lengthening the root– shoot axis Vascular tissue system (procambium) Ground tissue system (ground meristem) Dermal tissue system (protoderm) Shoot apical meristem Root apical meristem Root–shoot axis

7 Some Genes Involved in Root- Shoot Formation Both shoot and root meristems are apical meristems, but are independently controlled -Shootmeristemless (STM) is necessary for shoot formation, but not root development STM wild type stm mutant -STM encodes a transcription factor with homeobox region Cotyledons not mature leaves are shown

8 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 causes to be made!!!! Some Genes Involved in Root- Shoot Formation

9 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

10 Two Internal Proteins Responsible for the Development of a Structure Cause Similar Phenotypes if their corresponding genes are mutated Has a basal peg not a root Abnormal cell division create stub rather than a root

11 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

12 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

13 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

14 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

15 -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 Seeds

16 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

17 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

18 -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 Styles of Seed Germination

19 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) Fruits = Pistils...During Seed Formation Flower Develops into Fruit

20 In Dry Fruit (Like Legumes) the Pericarp Is Maturity Fruits have seeds!! (in general) In Fleshy Fruits Like Tomatoes Pericarp Is Thicker

21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ovary Sepals of a single flower Derived from many ovaries of a single flower; strawberries, blackberries. Unlike tomato, these ovaries are not fused and covered by a continuous pericarp. Individual flowers form fruits around a single stem. The fruits fuse as seen with pineapple. Seed Aggregate Fruits Main stem Pericarp of individual flower Multiple Fruits

22 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

23 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

24 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

25 Releasing Sugars From Cotyledon...So the Embryo Can Grow

26 Embryo produces gibberellic acid -This hormone signals the aleurone (outer endosperm layer) to produce  -amylase -Breaks down the endosperm’s starch into sugars that are passed to embryo Releasing Sugars From Cotyledon...So the Embryo Can Grow

27 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

28 Germination