Chapter 40

Question 1

Reproductive Development

Answer 1

Angiosperms represent an evolutionary innovation with their production of flowers and fruits​

Plants go through developmental changes leading to reproductive maturity by adding structures to existing ones with meristems

Question 2

Initiation of Flowering

Answer 2

Once plants are competent to reproduce, a combination of factors – including light, temperature, and both promotive and inhibitory internal signals – determines when a flower is produced​

Undergo phase change – subtle or obvious

Question 3

Phase Change

Answer 3

Internal developmental changes allow plants to obtain competence to respond to external or internal signals that trigger flower formation​

May be morphologically obvious or very subtle

Question 4

Examples of Phase Changes

Answer 4

The lower branches in the oak tree retain their leaves in the winter, because these lower branches were initiated by juvenile meristems and have not made a phase change. Juvenile ivy makes adventitious roots that can cling to walls, but after a phase change mature ivy lacks the ability to produce adventitious roots.

Question 5

Delay of Flowering

Answer 5

In Arabidopsis, the gene embryonic flower (emf) prevents early flowering​

emf mutants flower immediately.​

Flowering is the default state​

Many mechanisms have evolved to delay flowering

Question 6

Inducing Flowering

Answer 6

The juvenile-to-adult transition can be induced by overexpressing a flowering gene called LEAFY​

LEAFY (L F Y) was cloned in Arabidopsis and replaced with a viral promoter that results in constant, high levels of L F Y transcription​

Overexpression of L F Y in aspen causes flowering to occur in weeks instead of years

Question 7

Accelerated Phase Change

Answer 7

Normally aspen trees grow for several years before producing flowers. Overexpression of the Arabidopsis flowering gene LFY causes rapid flowering in transgeneic aspen.

Question 8

Four genetically regulated pathways to flowering have been identified

Answer 8

-The light-dependent pathway​

-The temperature-dependent pathway​

-The gibberellin-dependent pathway​

-The autonomous pathway​

Plants can rely primarily on one pathway, but all four pathways can be present

Question 9

Light-Dependent Pathway

Answer 9

Also termed the photoperiodic pathway​

Keyed to changes in the proportion of light to dark in the daily 24-hr cycle (day length)​

Short-day plants flower when daylight becomes shorter than a critical length​

Long-day plants flower when daylight becomes longer than a critical length​

Day-neutral plants flower when mature regardless of day length

Question 10

Obligate and Facultative Plants

Answer 10

In obligate long- or short-day plants there is a sharp distinction between short and long nights, respectively​

In facultative long- or short-day plants, the photoperiodic requirement is not absolute​

Flowering occurs more rapidly or slowly depending on the length of day.

Question 11

Manipulation of Photoperiod

Answer 11

Using light as a cue allows plants to flower when abiotic conditions are optimal​

Manipulation of photoperiod in greenhouses ensures that short-day poinsettias flower in time for the winter holidays

Question 12

Phytochrome and Cryptochrome

Answer 12

Conformational change in a phytochrome (red-light sensitive) or cryptochrome (blue-light sensitive) light-receptor molecule triggers a cascade of events that leads to the production of a flower​

In Arabidopsis, regulate via the gene CONSTANS (C O) which encodes a transcription factor that turns on genes that are needed for flowering​

This signaling cascade leads to expression of L F Y​

Phytochrome regulates the transcription of C O

Question 13

CONSTANS

Answer 13

C O protein is produced day and night​

The levels of C O are maintained in accordance with the circadian clock​

Levels of C O mR N A are lower at night because of targeted protein degradation by ubiquitin​

Phytochrome causes an increase in transcription at daybreak​

Cryptochrome prevents degradation by the ubiquitin-dependent pathway during the day

Question 14

Temperature-Dependent Pathway

Answer 14

Some plants require a period of chilling before flowering – vernalization​

Described in the 1930s by Ukrainian scientist T.D. Lysenko.​

Winter wheat would not flower without a period of chilling.​

Seeds could be chilled and planted in the spring.

Question 15

Gibberellin-Dependent Pathway

Answer 15

Decreased levels of gibberellins have been shown to delay flowering in some species.​

Gibberellin has been shown to bind to the promoter of the L F Y gene, which supports a model where gibberellin induces an increase in L F Y gene expression.​

This would directly affect flowering.

Question 16

Autonomous Pathway

Answer 16

Does not depend on external cues except for basic nutrition​

Presumably delays flowering​

A balance between floral promoting and inhibiting signals may regulate when flowering occurs​

Can test determination for flowering by changing the environment and ascertaining whether developmental fate has changed

Question 17

Flowering Pathways

Answer 17

The four flowering pathways lead to an adult meristem becoming a floral meristem​

Activate or repress the inhibition of floral meristem identity genes.​

The floral meristem identity genes: L F Y and AP1​

Turn on floral organ identity genes.​

Define the four concentric whorls.​

Sepal, petal, stamen, and carpel.

Question 18

Explains how 3 classes of floral organ identity genes can specify 4 distinct organ types

Answer 18

-Class A genes alone – Sepals​

-Class A and B genes together – Petals​

-Class B and C genes together – Stamens​

-Class C genes alone – Carpels​

When any one class is missing, aberrant floral organs occur in predictable positions

Question 19

A B C model cannot fully explain specification of floral meristem identity

Answer 19

Class D genes are essential for carpel formation​

But does not explain why plants lacking A, B, and C gene function produce four whorls of sepals rather than leaves​

Class E genes SEPALATA (S E P)​

S E P proteins interact with class A, B, and C proteins that are needed for the development of floral organs.​

Modified A B C D E model was proposed

Question 20

Flower Structure

Answer 20

Floral organs are thought to have evolved from leaves​

A complete flower has four whorls​

Calyx, corolla, androecium, and gynoecium.​

An incomplete flower lacks one or more of the whorls

Question 21

Calyx

Answer 21

Consists of flattened sepals

Question 22

Corolla

Answer 22

Consists of petals

Question 23

Androecium

Answer 23

Collective term for all the stamens (male structures) of a flower​

Stamen consists of a filament and an anther.

Question 24

Gynoecium

Answer 24

Collective term for all carpels (female structures) of a flower​

Carpel consists of ovary, style, and stigma.​

Ovules produced in ovary.

Question 25

Trends in Floral Specialization

Answer 25

2 major trends​

Floral parts have grouped together​

Floral parts lost or reduced​

Modifications often relate to pollination mechanisms

Question 26

Trends in Floral Symmetry

Answer 26

Primitive flowers are radially symmetrical​

Advanced flowers are bilaterally symmetrical

Question 27

Genetic Regulation of Asymmetry

Answer 27

Snapdragon flowers normally have bilateral symmetry, like the one shown here on the left. The CYCLOIDEA gene regulates floral symmetry, and cycloidea mutant snapdragons, like the one on the right, have radially symmetrical flowers.

Question 28

Gamete Production

Answer 28

Alternation of generations​

Diploid sporophyte → haploid gametophyte​

In angiosperms, the gametophyte generation is very small and is completely enclosed within the tissues of the parent sporophyte​

Male gametophyte – pollen grains.​

Female gametophyte – embryo sac.

Question 29

Reproductive Organs of Angiosperms

Answer 29

Gametes are produced in separate, specialized structures of the flower​

Reproductive organs of angiosperms differ from those of animals in two ways​

Both male and female structures usually occur together in the same individual​

Reproductive structures are not permanent parts of the adult individual

Question 30

Pollen Formation

Answer 30

Anthers contain four microsporangia which produce microspore mother cells (2n)​

Microspore mother cells produce microspores (n) through meiosis​

Microspore develops by mitosis into pollen​

Generative cell in the pollen grain will later divide to form two sperm cells

Question 31

Pollen Grains

Answer 31

In the Easter lily, Lilium candidum, the pollen tube emerges from the pollen grain through the groove or furrow that occurs on one side of the grain. In a plant of the sunflower family, Hyoseris longiloba, three pores are hidden among the ornamentation of the pollen grain. The pollen tube may grow out through any one of them.

Question 32

Embryo Sac Formation

Answer 32

Within each ovule, a diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores​

Usually only one megaspore survives​

Enlarges and undergoes repeated mitotic divisions to produce eight haploid nuclei​

Enclosed within a seven-celled embryo sac

Question 33

Self-pollination

Answer 33

Pollen from a flower’s anther pollinates stigma of the same flower.

Question 34

Cross-pollination

Answer 34

Pollen from anther of one flower pollinates another flower’s stigma.​

Also termed outcrossing.

Question 35

Successful Pollination

Answer 35

Successful pollination in many angiosperms depends on regular attraction of pollinators​

Floral morphology has coevolved with pollinators​

Early seed plants wind pollinated​

Among insect-pollinated angiosperms, the most numerous groups are those pollinated by bees

Question 36

Bee Pollination

Answer 36

Bees typically visit yellow or blue flowers​

Many have stripes or lines of dots that indicate the location of the nectaries

Question 37

How a Bee Sees a Flower

Answer 37

The yellow flower of Ludwigia peruviana (Peruvian primrose) photographed in normal light (on the left) appears yellow, but under ultraviolet light it has a conspicuous central bull’s-eye. This is because the outer sections of the petals reflect both yellow and ultraviolet, while the inner portions reflect yellow only and therefore appear dark in the photograph that emphasizes ultraviolet reflection.

Question 38

Other Insect Pollinators

Answer 38

Flowers that are visited regularly by butterflies often have flat “landing platforms”​

Flowers that are visited regularly by moths are often white or pale in color​

Also tend to be heavily scented, making them easy to locate at night

Question 39

Bird Pollination

Answer 39

Flowers that are visited regularly by birds must produce large amounts of nectar​

Often have a red color​

Conspicuous to birds, but usually inconspicuous to insects.

Question 40

Other Animal Pollinators

Answer 40

Signals are species-specific​

Small rodents may pollinate plants​

Bats, bird, and insects pollinate saguaro cacti

Question 41

Wind Pollination

Answer 41

Some angiosperms are wind-pollinated​

Characteristic of early seed plants.​

Flowers are small, green, and odorless, with reduced or absent corollas​

Often grouped and hanging down in tassels​

Stamen- and carpel-containing flowers are usually separated between individuals​

Strategy that greatly promotes outcrossing.

Question 42

Staminate and Pistillate Flowers

Answer 42

Birches (Betula sp) are monoecious; their staminate flowers hang down in long, yellowish tassels, and their pistillate flowers mature into clusters of small, brownish, conelike structures.

Question 43

Wind-pollinated Flowers

Answer 43

The large yellow anthers, dangling on very slender filaments, are hanging out, about to shed their pollen to the wind. Later, these flowers will become pistillate, with long, feathery stigmas – well-suited for trapping windblown pollen – sticking far out of them. Many grasses, like this one, are therefore dichogamous.

Question 44

Self-Pollination

Answer 44

Outcrossing is generally advantageous for plants and for eukaryotic organisms​

Nevertheless, self-pollination also occurs in some angiosperms​

2 basic reasons for frequency of self-pollination​

-Self-pollination is favored in environments where pollinators are scarce​

-Offspring are more uniform and probably better adapted to their environment, which is favored in stable environments

Question 45

Promotion of Outcrossing

Answer 45

Several evolutionary strategies promote outcrossing​

Separation of stamens and pistils in space.​

Dioecious plants produce only ovules or only pollen.​

Monoecious plants produce male and female flowers on the same plant, but they may mature at different times (dichogamy).​

Self-incompatibility that prevents self-fertilization.

Question 46

Self-incompatibility

Answer 46

Self-incompatibility increases outcrossing​

Pollen and stigma recognize each other as being genetically related and pollen tube growth is blocked​

Controlled by alleles at the S locus​

2 types of self-incompatibility​

-Gametophytic self-incompatibility​

Depends on the haploid S locus of the pollen and the diploid S locus of the stigma.​

-Sporophytic self-incompatibility​

If the alleles in the stigma match either of the pollen parent’s S alleles, the haploid pollen will not germinate.

Question 47

Double Fertilization

Answer 47

Only in angiosperms​

Requires two sperm cells​

Double fertilization results in two key developments​

Fertilization of the egg.​

Formation of endosperm that nourishes the embryo.

Question 48

Embryo Development

Answer 48

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 with its polar nuclei and initiates endosperm development.​

Other sperm fertilizes the egg to produce a zygote.​

Cell division soon follows, creating the embryo.

Question 49

First Zygotic Division

Answer 49

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.

Question 50

Zygote Development in Fucus

Answer 50

Asymmetrical cell division 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.

Question 51

The Suspensor (sus) Mutant

Answer 51

This suspensor (sus) mutant of Arabidopsis has a defect in embryo development. Inhibition of embryo development in the suspensor is blocked, resulting in embryo-like development of the suspensor. S U S, an allele normally present in the embryo, is required to suppress embryo development in suspensor cells

Question 52

Body Plan

Answer 52

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 cell division gives rise to a single row of cells, cells soon begin dividing in different directions, producing a solid ball of cells

Question 53

Apical Meristems

Answer 53

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

Question 54

SHOOTMERISTEMLESS

Answer 54

SHOOTMERISTEMLESS (S T M) needed for shoot formation​

S T M gene codes for a transcription factor with a homeobox region​

The stm mutant of Arabidopsis has a normal root meristem but fails to produced a shoot meristem between its two cotyledons.

Question 55

Auxin

Answer 55

One way that auxin induces gene expression is by activating a transcription factor​

MONOPTEROS (M P) is a gene that codes for an auxin-induced transcription factor.​

Necessary for root formation, but not shoot.​

Once activated, M P protein binds to the promoter of another gene, leading to transcription of a gene or genes needed for root meristem formation.

Question 56

HOBBIT Gene Action

Answer 56

HOBBIT (H O B) mutation affects root development

Question 57

MONOPTEROS Gene Action

Answer 57

MONOPTEROS mutation affects root development

Question 58

Formation of Tissue Systems

Answer 58

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

Question 59

Morphogenesis

Answer 59

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.

Question 60

Plant Body Form

Answer 60

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 due to turgor pressure 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.

Question 61

WOODEN LEG (W O L)

Answer 61

wol mutant (right) has less vascular tissue than wild-type Arabidopsis (left), but all of it is xylem​

The WOODEN LEG gene is needed for phloem development

Question 62

Early Embryonic Development

Answer 62

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

Question 63

Critical Developmental Events

Answer 63

During embryogenesis, angiosperms undergo three other critical events​

Development of a food supply​

Development of seed coat​

Development of fruit surrounding seed

Question 64

Endosperm Variation

Answer 64

The sporophyte transfers nutrients via the suspensor to the endosperm in angiosperms​

Endosperm varies between plants​

In coconuts it includes the liquid “milk”.​

In corn it is solid.​

it expands with heat to form the white edible part of popcorn.​

In peas and beans it is used up during embryogenesis.​

Nutrients are stored in thick, fleshy cotyledons.

Question 65

Seeds

Answer 65

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

Question 66

Initiation of Germination

Answer 66

Germination: the emergence of the radicle (first root) through the seed coat​

Germination cannot take place until water and oxygen reach the embryo​

Stratification: some seeds require periods of time at low temperatures before germination

Question 67

Food Storage in the Seed

Answer 67

Germination and early seeding growth require the utilization of metabolic reserves stored as starch in amyloplasts and protein bodies​

Depending on the kind of plant, these reserves may be stored in the embryo or in the endosperm​

Scutellum: in kernels of cereal grains, the single cotyledon is modified into this structure, which transfers nutrients from the endosperm to the embryo

Question 68

Hormonal Effects

Answer 68

In response to the absorption of water by a seed, the embryo produces gibberellic acid​

This signals the aleurone (the outer layer of the endosperm) to produce α-amylase​

This enzyme is responsible for breaking down starch.​

Levels of abscisic acid, which inhibits starch breakdown, may be reduced when a seed absorbs water

Question 69

Asexual Reproduction

Answer 69

Produces genetically identical individuals because only mitosis occurs​

More common in harsh, unchanging environments​

All clones are adapted.​

Variations may not be adapted.​

Should conditions change dramatically, there will be less variation in the population for natural selection to act on, and the species may be less likely to survive.​

Used in agriculture to propagate a particularly desirable plant with traits that would be altered by sexual reproduction

Question 70

Apomixis

Answer 70

Apomixis – asexual development of a diploid embryo in the ovule​

Offspring is genetically identical to the sporophyte that produced it​

Gain advantage of seed dispersal usually associated with sexual reproduction.

Question 71

Vegetative Reproduction

Answer 71

New plant individuals are cloned from parts of adults​

Comes in many and varied forms​

Runners or stolons.​

Rhizomes.​

Suckers.​

Adventitious plantlets.

Question 72

Cloning Plants

Answer 72

Whole plants can be cloned by regenerating plant cells or tissues on nutrient medium with growth hormones​

Individual cells can be isolated from tissues with enzymes that break down cell walls​

What is left behind is a protoplast – a plant cell enclosed only by a plasma membrane.​

Many, but not all, cell types in plants maintain the ability to generate organs or an entire organism in culture​

Cells divide in culture, regenerating cell walls, to form an undifferentiated mass of cells called a callus

Question 73

Plant Life Spans

Answer 73

Once established, plants live for variable periods of time, depending on the species​

Woody plants, which have extensive secondary growth, typically live longer than herbaceous plants, which don’t​

Bristlecone pine, for example, can live upward of 4,000 years.​

Depending on the length of their life cycles, herbaceous plants may be annual, biennial, or perennial

Question 74

Annual, Biennial, or Perennial

Answer 74

Annual plants grow, flower, and form fruits and seeds within one growing season​

They then die when the process is complete.​

Biennial plants have life cycles that take two years to complete​

Store energy in year one, flower in year two.​

Perennial plants continue to grow year after year​

They may be herbaceous or woody.