Campbell: Chapter 38 (Plant Reproduction and Biotechnology)

Chapter 38
Plant Reproduction and Biotechnology

ALTERNATION OF GENERATIONS
– Gametophyte (N) and sporophyte (2N) take turns producing each other
– Sporophyte is dominant generation
– Over the course of evolution, gametophytes became reduced in size and dependent upon their sporophyte parents

1. sporophyte undergoes meiosis
2. 4 spores (N) are produced
3. mitosis of spores
4. gametophyte (N) – multicellular male and female plants
5. mitosis and cellular differentiation
6. gametophytes develop and produce gametes
• sperm
• egg
7. fertilization
8. zygote (2N)
9. zygote divides by mitosis
10. formation of new sporophytes

SEXUAL REPRODUCTION IN ANGIOSPERMS
– sporophyte develops a flower
• reproductive structure of angiosperms
• unique to angiosperms
– male and female gametophytes develop within anthers and ovaries respectively, of a sporophyte flower
– pollination
• by wind or animals
• brings a male gametophyte to a female gametophyte
– fertilization takes place within the ovary
– devt of seed
• occurs within the ovary
• contains sporophyte embryos
– ovary becomes a fruit

Structures unique to angiosperms
1. flower
2. fruit

FLOWER
– specialized shoots bearing the reproductive organs of sporophyte
– varies in size, shape, and color
– typically composed of 4 whorls of floral organs
• highly modified leaves
• separated by short internodes
– are determinate shoots  cease growing once flower and fruit are formed
– attached to the stem by receptacle

FLORAL ORGANS

1. Sepals
• nonreproductive organs
• enclose and protect the floral bud before it opens
• green and leaflike in appearance

2. Petals
• nonreproductive organs
• brightly colored
• advertise the flower to pollinators

3. Stamen
• male reproductive organ

a. filament
 stalk
b. anther
 terminal structure
 contains pollen sacs – where pollen is produced

4. Carpel (Pistil)
• female reproductive organ
• flowers may have multiple carpels
• for some species, several carpels are fused into a single structure – ovary with two or more chambers, each containing ovule(s)

a. style
 slender neck
 stigma
 found at the tip
 sticky structure
 landing platform for pollen
b. ovary
 located at the base of the carpel
 ovule(s)
 contain a single sporangium
 form within chambers of ovary

Sporangia
– found in stamens and carpels
– where spores and gametophytes develop

Pollen Grains
– develops into the male gametophyte
– form within pollen sacs of anthers

Embryo Sacs
– female gametophytes
– egg-producing structures
– form within ovules in ovaries

TYPES OF FLOWERS

Based on Floral Organs
1. Complete
• have all 4 organs
2. Incomplete
• lacking one or more floral organs
• ex. most grasses (no petals)

Based on Sexuality of Flower
1. Bisexual (Perfect)
• has both stamens and pistils
• an incomplete flower that lacks sepals or petals may also be bisexual
2. Unisexual (Imperfect)
• missing either stamens or carpels
• called staminate or carpellate

Types of Plants (Sexuality)
1. Monoecious
• staminate and carpellate flowers are located on same plant
2. Dioecious
• staminate flowers and carpellate flowers on separate plants
• ex. date palms

FORMATION OF MALE AND FEMALE GAMETOPHYTES

MICROSPOROGENESIS
– development of male gametophyte
– occurs within the anthers

1. Microsporocytes (2N)
• within sporangia in pollen sacs
2. Meiosis
3. 4 microspores (N)
4. Mitosis
5. 2 cells (for each microspore)
a. generative cell
 eventually produce sperm
b. tube cell
 encloses generative cell
 will produce pollen tube
6. encapsulation of 2-celled structure
• thick, resistant wall
7. pollen grain
• 2-celled structure + wall
• an immature male gametophyte
8. generative cell divides by mitosis
9. result: 2 sperm cells (from each generative cell)
• sperm cell = male gamete
• germinated pollen grain = mature male gametophyte
• in most species, this occurs after pollen tube begins to form

MEGASPOROGENESIS

1. Megasporocyte (2N)
• one cell in the sporangium in the ovule
• grows
2. Meiosis
3. 4 Megaspores (N)
4. only one megaspore survives (for most angiosperms)
5. megaspore continues to grow
6. nucleus of megaspore divides by mitosis 3x  1 cell with 8 (N) nuclei
7. membranes partition this mass into embryo sac – multicellular female gametophyte
a. 1 egg cell
 female gamete
 located at one end of the embryo sac
b. 2 synergids
 flank the egg cell
 function in guidance and attraction of pollen tube
c. 3 antipodal cells
 opposite end of embryo sac
 unknown function
d. 2 polar nuclei
 not partitioned into separate cells
 share cytoplasm of large central cell of embryo sac
8. ovule = embryo sac + surrounding integuments (protective layers of sporophytic tissue)

POLLINATION
– occurs when pollen released from anthers lands on a stigma

TYPES OF POLLINATING AGENTS
1. Wind
• to compensate for randomness, wind-pollinated plants release enormous quantities of pollen grains
2. Animals
• transfer pollen directly between flowers

STEPS
1. pollen grain lands on receptive stigma
2. pollen grain absorbs moisture and germinates
3. each pollen grain produces a pollen tube  essential for sperm delivery
4. pollen tube grows down into the ovary via the style
5. generative cell mitotically divides to form 2 sperm
6. pollen tube enters ovary
7. pollen tube probes through micropyle – gap in ovule integuments
8. pollen tube discharges sperm into embryo sac
9. fertilization

MECHANISMS THAT PREVENT SELF-FERTILIZATION
– contributes to genetic variation by ensuring that sperm and eggs come from diff plarents
– dioecious plants cannot self-fertilize – unisexual

For bisexual flowers
1. stamens and carpels mature at different times

2. stamens and carpels are arranged in such a wa that it is unlikely that an animal pollinator could transfer pollen from anthers to stigma of same flower

3. self-incompatibility
• most common strategy
• ability of plant to reject its own pollen and pollen of closely related individuals
• biochemical block prevents pollen form completing devt
• analogous to immune response of humans – based on ability to distinguish self from nonself
• recognition of self pollen is based on S-genes – genes for self-incompatibility
• if pollen grain and stigma have same alleles at S-locus, then pollen grain fails to complete formation

a. gametophytic self-incompatibility
 block occurs in pollen grain
 bean, tobacco, rose
b. sporophytic self-incompatibility
 block response by cells in stigma
 mustard family

NOTE: many agriculturally impt plants are self-compatible

DOUBLE FERTILIZATION
– one sperm fertilizes egg = zygote
– other sperm combines with 2 polar nuclei – (3N) nucleus in large central cell of embryo sac
– large cell will give rise to endosperm
• food storing tissue of seed
• double fertilization ensures that endosperm will only develop in ovules where egg has been fertilized

Similarity to Fertilization in Animals
1. increase in cytoplasmic Ca+2 levels of the egg after gamete fusion
2. establishment of block to polyspermy

DEVELOPMENT OF THE SEED
– ovule develops into seed
– ovary develops into fruit enclosing the seed
– embryo develops from zygote – seed stockpiles proteins, oils, and starch
• nutrients are initially stored in endosperm
• cotyledons assume to storage function later on

ENDOSPERM DEVELOPMENT
– NOTE: endosperm devt usually precedes embryo devt
– After double fert – 3N nucleus of ovule’s central cell divides
– Mutinucleate cell with milky consistency
– Endosperm – liquid mass
– Multinucleate cell becomes multicellular when membrane forms between nuclei
– “naked” cells produce cell walls
– endosperm becomes solid
– ex. coconut milk – liquid endosperm
coconut meat – solid endosperm
– endosperm is rich in nutrients
– most monocots and some dicots – endosperm contains nutrients that can be used by the seedling after germination
– many dicots – food reserves of endosperm are completely transferred to cotyledons

EMBRYO DEVELOPMENT
– first mitotic division of zygote – transverse
a. Basal cell
 continues to divide transversely
 produces the suspensor
 thread of cells
 anchors embryo to parent plant and in some, the endosperm
 functions in transfer of nutrients to the embryo from the parent and endosperm
b. Terminal cell
 divides several times
 forms spherical proembryo attached to suspensor
– cotyledons begin to form as bumps on proembryo
– embryo elongates
– meristems on two ends of the embryo – sustains primary growth indefinitely after seed germinates
– 3 primary meristems are present in the embryo
a. protoderm  dermal
b. ground meristem  ground
c. procambium  vascular tissues

2 Features of Plant Form
– established during embryo devt

1. root-shoot axis – meristems at opposite ends
2. radial pattern of 3 primary meristems set to give rise to 3 tissue systems

STRUCTURE OF MATURE SEED
– last stages of maturation – seed dehydrates
– embryo stops growing until seed germinates
– embryo and food supply are enclosed by seed coat
• protection
• from integuments of ovule

Dicot Seed
1. hypocotyl
• embryonic axis below the point where cotyledons are attached
• terminates in radicle
2. epicotyl
• embryonic axis above cotyledons
• tip: plumule – shoot tip + pair of miniature leaves
3. cotyledons
4. endosperm
• Castor bean
• Contains food supply

Monocot Seed
1. Scutellum
• specialized cotyledon
• thin
• large surface area pressed against endosperm
• absorbs nutrients from endosperm
2. sheaths
a. coleoptile
 covers the young shoot
b. coleorhiza
 covers the young root
3. endosperm

DEVELOPMENT OF FRUIT
– simultaneous with seed devt
– pollination triggers hormonal changes that cause the ovary to begin its transformation
– wall of ovary becomes the pericarp
– other parts of flower are shed
– in some angiosperms, floral parts contribute to fruit
– ripens at about the same time that its seeds are completing their devt

Ripening of Dry Fruit
– aging of fruit tissues
– aging allows fruit to open and release the seeds

Ripening of Fleshy Fruit
– controlled by complex interaction of homrmones
– pulp becomes softer as a result of enzymes digesting cell walls
– color change
– fruit becomes sweeter – organic acids/starch molecules are converted to sugar

Function of the Fruit
1. protects the enclosed seeds
2. aids in seed dispersal

SUMMARY
– formation of gametes
– dispersal of pollen grains
– pollination – pollen lands on stigma
– pollen grain produces pollen tube
– formation of 2 sperm cells
– pollen tube discharges sperm into ovule
– double fertilization
– zygote gives rise to embryo
– ovule that contains the embryo develops into a seed
– entire ovary develops into a fruit – may contain one or more seeds, depending on the species
– seeds are dispersed
– if seeds are deposited in sufficiently moist soil, they germinate

EVOLUTIONARY ADAPTATIONS FOR SEED SURVIVAL

Seed Dormancy
– phase of extremely low metabolic rate and suspension of growth and devt
– conditions to break dormancy vary
– increases the chance that germination will occur at a time and place most advantageous for growth

Seed to Seedling
(see Campbell)

ASEXUAL REPRODUCTION

1. Fragmentation
• separation of parent plant into parts that reform the whole plant

2. Apomixis
• produce seeds without flowers being fertilized
• diploid cell in ovule give rise to embryo  ovules mature into seeds

3. Grafting
• makes it possible to combine the best qualities of different species or varieties into a single plant
• usually done when plant is young
• stock – plant that provides the root
• scion – grafted plant part
• quality of fruit is determined by genes of scion

4. Cloning

Advantages of Sexual Reproduction
1. genetic variation
2. seed
• can disperse to new locations
• can wait to grow until envtl conditions improve

Advantages of Asexual Reproduction
1. a plant well-suited to an envt can make many copies of itself
2. offspring are not as frail as seedlings