Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 37 LECTURE SLIDES

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 3

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First zygote division is asymmetrical, resulting in cells with 2 different fates –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 –Cells near suspensor become root –Cells at the other end become shoot 5

Polar nuclei Egg Micropyle Sperm Pollen tube 3n endosperm2n zygote Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Asymmetrical cell division is also observed in the zygote of the brown alga Fucus –Bulge develops on one side of embryo –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 –Axis is first established by the point of sperm entry, but it can be changed by environmental signals 8

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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 10

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Body Plan In plants, three-dimensional shape and form arise by regulating amount and pattern of cell divisions –Vertical axis (root–shoot axis) becomes established at a very early stage –Same is true for establishment of a radial axis (inner–outer axis) First cells divide producing a solid ball of cells 12

Apical meristems establish the root–shoot axis in the globular stage, from which the three basic tissue systems arise –Dermal –Ground –Vascular tissue These tissues are organized radially around the root–shoot axis 13

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16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. embryo suspensor a. Kindly provided by Prof. Chun-ming Liu, Institute of Botany, Chinese Academy of Sciences

17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. embryo suspensor a. a-d: Kindly provided by Prof. Chun-ming Liu, Institute of Botany, Chinese Academy of Sciences b.

18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. a-d: Kindly provided by Prof. Chun-ming Liu, Institute of Botany, Chinese Academy of Sciences b. c. embryo suspensor

19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a-d: Kindly provided by Prof. Chun-ming Liu, Institute of Botany, Chinese Academy of Sciences b. Cotyledon Ground meristem Protoderm Procambium a. d.c. embryo suspensor

Both shoot and root meristems are apical meristems, but are independently controlled SHOOTMERISTEMLESS (STM) –Necessary for shoot formation –Plants that do not make STM protein fail to produce viable shoots, but produce roots 20

HOBBIT gene is required for root formation –Hobbit mutants form shoot meristems, but no root meristems form –HOBBIT protein allows auxin to induce the expression of a gene or genes needed for correct cell division to make a root meristem 21

One way that auxin induces gene expression is by activating a transcription factor –MONOPTEROS (MP) is a gene that codes for an auxin-induced transcription factor Necessary for root formation, but not shoot –Once activated, MP protein binds to the promoter of another gene, leading to transcription of a gene or genes needed for root meristem formation 22

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c. A wild-type seedling depends on auxin-induced genes for normal root initiation during embryogenesis d. The hobbit seedling has a stub rather than a root because abnormal cell divisions prevent root meristem formation e. The monopteros seedling also fails to develop a root 24

Formation of Tissue Systems Primary meristems differentiate while the plant embryo is still at the globular stage –No cell movements are involved Outer protoderm develops into dermal tissue that protects the plant Ground meristem develops into ground tissue that stores food and water Inner procambium develops into vascular tissue that transports water and nutrients 25

Morphogenesis The globular stage gives rise to heart- shaped embryo with 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 26

Form of a plant body is largely determined by the plane in which its cells divide –Also controlled by changes in cell shape as cells expand osmotically after they form Based on the position of the cell plate –Determined by microtubules and actin –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

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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 After germination, apical meristems continue adding cells to the growing root and shoot tips 30

During embryogenesis, angiosperms undergo three other critical events 1.Development of a food supply 2.Development of seed coat 3.Development of fruit surrounding seed 31

Endosperm varies between plants –In coconuts it includes the liquid “milk” –In corn it is solid –In peas and beans it is used up during embryogenesis Nutrients are stored in thick, fleshy cotyledons 32

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Seeds In many angiosperms, development of the embryo is arrested soon after meristems and cotyledons differentiate Integuments develop into a relatively impermeable seed coat Encloses the seed with its dormant embryo and stored food 34

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Seeds are an important adaptation 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 36

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 37

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 38

Fruits Most simply defined as mature ovaries (carpels) During seed formation, the flower ovary begins to develop into fruit It is possible for fruits to develop without seed development –Bananas are propagated asexually 39

The ovary wall is termed the pericarp –3 layers: exocarp, mesocarp and endocarp –Their fate determines the fruit type Fruits contain 3 genotypes in 1 package –Fruits and seed coat from prior sporophyte generation –Remnants of gametophyte generation produced egg –Embryo represents next sporophyte generation 40

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Fruit Dispersal 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 46

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Germination Defined as the emergence of the radicle (first root) from the seed coat Germination begins when a seed absorbs water and metabolism resumes –Oxygen must be available –May requires additional environmental signals such as specific wavelength of light, appropriate temperature, or stratification (period of low temperature exposure) 49

Germination can occur over a wide temperature range (5 o –30 o C) Some seeds will not germinate even under the best conditions –Presence of ungerminated seeds in the soil of an area is termed the seed bank 50

Germination requires energy sources –Starch stored in amyloplasts, proteins, or fats and oils –Food sources can readily be digested during germination, producing glycerol and fatty acids, which yield energy through cellular respiration 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 endosperm to the rest of the embryo 51

Embryo produces gibberellic acid –Hormone signals the aleurone (outer endosperm layer) to produce  -amylase –Breaks down the endosperm’s starch into sugars that are passed to embryo –Abscisic acid, another hormone, can inhibit starch breakdown Establishes dormancy 52

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As the sporophyte pushes through the seed coat, it orients with the environment such that the root grows down and shoot grows up –Shoot becomes photosynthetic –Postembryonic phase is under way In monocots –Coleoptile protect shoot –Coleorhiza surrounds radicle 56

Emergence of the embryonic root and shoot from the seed during germination varies widely from species to species –In most plants, the root emerges before the shoot appears –Cotyledons may be held below or above ground –Cotyledons may become green or may shrivel quickly 57

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