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 -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 3 4 Embryo Development Embryo Development (Cont.) 5 6 1
Embryo Development Embryo Development Asymmetrical cell division is also observed in the zygote of the brown alga Fucus -Unequal material distribution forms a bulge -Cell division occurs there, resulting in: -A smaller cell that develops into a rhizoid that anchors the alga -A larger cell that develops into the thallus, or main algal body Fate of two cells is held in memory by cell wall components 7 8 Embryo Development Arabidopsis mutants have revealed the normal developmental mechanisms -Suspensor mutants undergo aberrant development in the embryo followed by embryo-like development of the suspensor -Thus, the embryo normally prevents embryo development in suspensor 9 10 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 11 The radial axis (inner-outer axis) is created when cells alternate between synchronous cell divisions -Produce cells walls parallel to and perpendicular to the embryo s surface The 3 basic tissue systems arise at this stage -Dermal, ground and vascular 12 2
(Cont.) 13 14 Both shoot and root meristems are apical meristems, but are independently controlled -Shootmeristemless (STM) is necessary for shoot formation, but not root development -STM encodes a transcription factor with homeobox region 15 The HOBBIT gene is required for root meristem, but not shoot meristem formation 16 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 17 18 3
(Cont.) 19 20 Formation of Tissue Systems Primary meristems differentiate while the plant embryo is still at the globular stage -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 21 22 23 24 4
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 25 26 Morphogenesis The form of a plant body is largely determined by the plane in which its cells divide -Based on the position of the cell plate -Determined by microtubule orientation Microtubules also guide cellulose deposition as the cell wall forms around the new cell -Cells expand in the directions of the two sides with the least cellulose reinforcement 27 28 Morphogenesis Early in embryonic development, most cells can give rise to a wide range of cell and organ types, including leaves -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 29 30 5
Morphogenesis During embryogenesis, angiosperms 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 Morphogenesis 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 31 32 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 33 34 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 35 36 6
Seeds 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 Seeds 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 37 38 Seeds -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 Fruits Fruits are most simply defined as mature ovaries (carpels) -During seed formation, the flower ovary begins to develop into fruit -It is possible, however, for fruits to develop without seed development -Bananas are propagated asexually 39 40 Fruits The ovary wall is termed the pericarp -Has three layers: exocarp, mesocarp and endocarp -Their fate determines the fruit type Fruits can be: -Dry or fleshy -Simple (single carpel), aggregate (multiple carpels), or multiple (multiple flowers) 41 42 7
43 44 45 46 Fruits Fruit Dispersal 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 47 Occurs through a wide array of methods -Ingestion and transportation by birds or other vertebrates -Hitching a ride with hooked spines on birds and mammals -Burial in caches by herbivores -Blowing in the wind -Floating and drifting on water 48 8
49 50 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 -Or appropriate temperature -Or stratification (period of low Germination Germination can occur over a wide temperature range (5 o -30 o C) Some seeds will not germinate even under he best conditions -The presence of ungerminated seeds in the soil of an area is termed the seed bank temperature exposure 51 52 Germination Germination requires energy sources such as: -Starch stored in amyloplasts, proteins, or fats and oils In cereal grain kernels, the single cotyledon is modified into a massive scutellum -Its abundant food is used first during germination -Later it serves as a conduit from the Germination Embryo produces gibberellic acid -This hormone signals the aleurone (outer endosperm layer) to produce α-amylase -Breaks down the endosperm s starch into sugars that are passed to embryo Abscisic acid, another hormone, can inhibit starch breakdown -Establishes dormancy endosperm to the rest of the embryo 53 54 9
55 56 Germination 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 57 58 59 60 10