Plant reproduction and development

Human diets are based on consuming plant reproductive structure that have their seeds and fruits derived from
flowers

- Flower: reproductive structure in angiosperms that produce gametes, attracts pollinators, receives
gametes from other individual and nourishes embryos
- Seeds: consists of an embryo and nutrient store surrounded by a protective coat
- Fruits: develop from the flower’s seed-producing organ and contain seeds
- Asexual reproduction: does not involve fertiliztion and results in the production of clones
o Shoots and root emerging from a horizontal stem are called rhizomes, which grows
underground

- The gladiolus plant (part b) has propagated itself with modified stems called corms, which grow under
the surface of soil
- The Kalanchoe (part c) produces “plantlets” (small plants) from meristematic tissue located along the
margins of its leaves.
 o When the plantlets mature, they drop off the parent plant and grow into independent
individuals.
- Dandelion has matured seeds that can form without fertilization occurring known as apomixis
o Apomixis result in seeds that are identical to the parent
- key characteristic of asexual reproduction is efficiency.
o If a disease wipes out other plants that surround a grass plant, the grass can quickly send out
rhizomes.
o Its asexually produced offspring fill the unoccupied space before seeds from competitors grow.
o The parent plant can also nourish these progeny as they become established.
- An individual in a diploid phase of the life cycle is called Sporophyte
o Sporophytes produce spores by meiosis.
- An individual in a haploid phase of the life cycle is called Gametophyte
o Gametophytes produce gametes by mitosis.

1. Meiosis occurs in sporophytes and results in the production of haploid spores

a. Meiosis and spore production occur inside structures called sporangia.
2. Spores undergo mitosis and develop into multicellular, haploid gametophytes.
3. Gametophytes produce sperm and eggs by mitosis
4. Fertilization occurs when two gametes fuse to form a diploid zygote
5. The zygote undergoes mitosis and grows into a multicellular, diploid embryo (sporophyte).

Reproductive structure
- They are made up of four basic organs that are essentially modified leaves
o (1) sepals
o (2) petals
o (3) stamens
o (4) one or more carpels
 - Sepals are leaf-like structures that make up the outermost parts of a flower
o Sepals form an outer, protective Whorl
o Green and photosynthetic and are thick compared with other parts of the flower
o Sepals enclose the flower bud as it develops and grows
 protecting young buds from damage by desiccation or disease-causing agents
o Entire group of sepals in the flowers is called Calyx
- Petals are arranged around the receptacle in a whorl
o the colour of the petals correlates with the visual abilities of animals
 ex: responds to wavelength, colours attract bees
o the base of the petals contains a gland called nectary
 Nectary produces the sugar-rich fluid nectar
o Research done that coated flowers with sunscreen to decrease the UV-reflecting and UV-
absorbing areas and found they were less visited by bees
- The entire group of petals in a flower is collectively called the corolla
- The petals within the corolla vary in size, shape, and function:
o Flattened petals provide a landing pad for flying insects.
o Elongated, tube-like petals have a nectary at their base that can be reached only by animals with
a long beak or tongue-like proboscis
o Some petals protect the reproductive organs located inside the corolla.
o Specialized cells in petals synthesize and release molecules that provide a scent attractive to
certain species of pollinating animals
- In contrast, wind-pollinated angiosperms such as oaks, birches, pecans, and grasses have flowers that
have small petals or no petals at all, and they lack nectarines.

Pollen and ovules

Stamens: reproductive structures that produce male gametophyte known as pollen grains and contains:

o Slender filament
o Pollen-producing organs called anthers
- The anther is the business end of the stamen—where meiosis and pollen formation take place.
- The filament holds the stamen in a place where insects can contact the pollen grains produced in the
anther.

Fourth reproductive structure in flowers is carpel, which produces female gametophytes. A carpel consists of
three regions:

o Stigma: the sticky top that receives pollen

o Style: slender silk
o Ovary: enlarged structure at the base of carpel
- Inside the ovary, female gametophytes are produced in structures called ovules
o contain more than one ovule
o When the female gametophytes that are produced inside ovules mature, they produce eggs

Flowers that contain both stamens and carpels are referred to as perfect
- Flowers can be imperfect: they contain either stamens or carpels, but not both
o separate stamen- or carpel-producing flowers occur on the same individual.
 They are called Monoecious (one house)
- Each ovule contains a structure called the megasporangium, inside which there is a diploid cell called the
megasporocyte.

1. The megasporocyte divides by meiosis.

2. Four Haploid megaspores result from meiosis, but three degenerate.
3. The surviving megaspore divides by mitosis to produce a structure with haploid nuclei.
a. The female gametophyte is called embryo sac
4. The haploid nuclei segregate to different positions in the embryo sac, and cell walls form around them
a. One of these cells becomes the haploid egg
- In the carpel, a diploid megasporocyte divides by meiosis to form a megaspore, which then divides by
mitosis to form the female gametophyte.
o Female gametophytes are encased in an ovary, are retained in the flower, and produce an egg.
- In angiosperms: the embryo sac contains eight haploid nuclei and seven cells
o Two polar nuclei (nuclei that fuse with one sperm nucleus to produce endosperm) stay together
within one central cell. the largest cell in the ovule
- The egg cell is located at one end of the female gametophyte, near an opening in the ovule called
micropyle (little gate)
o It’s where sperm will enter the ovule before fertilization
o Two haploid cells called synergids lie close to the egg
 1. Microsporocytes undergo meiosis.
2. Each haploid cell results in microspore
a. All microspores survive and divide by mitosis
3. The two nuclei that result from mitotic division in a microspore form a haploid, immature male
gametophyte called pollen grain
- Each diploid microsporocyte ultimately yields four haploid pollen grains
- before it has produced sperm, the male gametophyte consists of two cells:
o a small generative cell enclosed within a larger tube cell
- its considered mature when the haploid cell undergoes mitosis and produces two sperm cells
o maturation step occurs while pollen is still in the anther
o other times, maturation and sperm production don’t occur until after the pollen grain lands on
a stigma and begins to grow
- The wall of a pollen grain develops a tough outer coat that includes the watertight compound called
sporopollenin
o This coat protects the male gametophyte when the pollen is released from the parent plant

Pollination and Fertilization

Pollination is the transfer of pollen grains from an anther to a stigma; fertilization occurs when a sperm and an
egg unite to form a diploid zygote

- Self-fertilization or selfing, occurs when a sperm and an egg from the same individual combine to
produce an offspring
o Advantage: successful pollination is virtually assured—it doesn’t depend on other agents
o Disadvantage: Selfed offspring are usually much less diverse genetically than outcrossed
offspring are
- Plants outcross: a sperm and an egg from different individuals combine to form an offspring
o Outcross is the result of cross-pollination: when pollen is carried from the anther of one
individual to the stigma of a different individual
- Plants elaborate mechanism to prevent selfing:
o Temporal avoidance: In species that have perfect flowers, male and female gametophytes
mature at different times. Thus, selfing does not occur.
o Spatial avoidance: Selfing isn’t possible in dioecious species unless pollinators transfer pollen
between “different-sexed” flowers on the same individual.
  species with perfect flowers, the anthers and stigma are so far apart that self-pollination
is unlikely—if pollen falls inside the flower, it has almost no chance of landing on the
stigma.
o Molecular matching: In species that produce both pollen and ovules, molecular interactions
occur to prevent pollen grains from delivering sperm to the female gametophytes produced on
the same plant
 It is called self-incompatible: unable to form viable seed when carpels are pollinated
- Animal pollination is an example of mutualism: a mutually beneficial relationship between two species
o Where they receive food and the plant gets pollinated

Pollination syndromes: are suites of flower characters that are associated with certain types of pollinators

o Ex: bats are active at night and feed on white flowers

- Directed-pollination hypothesis: natural selection has favoured flower colours, shapes, and scents to
attract specific pollinators
- Structures are considered adaptation:
o Flowers and pollinators have adaptations that increase pollination frequency and feeding
efficiency
o Coevolution

In most mosses and ferns, they do not form pollen, but in gnetophytes and angiosperms, they produces pollen
due to being pollinated by animals

1. Pollination evolved late in land plant evolution. Conifers and other groups that are strictly or primarily
wind pollinated evolved later but before angiosperms.
2. Seed plants do not need external water for sexual reproduction to occur.
a. As a result, the evolution of pollen allowed these species to be much less dependent on wet
habitats.

Pollination become a more precise process when animals are involved

- Wind-borne pollen grains have a low probability of landing successfully on a flower stigma
o Invest into making large number of pollen grains
o Free, but not always go to the desired place
- Animal-borne pollen, in contrast, is much more likely to be successfully transferred to flowers of the
same species.
o Make fewer pollen grains, but invest un structures that attract animals
o Cost where they reward animals with nectar
- Pollination by animals encourage speciation
o Ex: alpine skypilots with two different characteristics:
 Grown in forested habits or below timberline have small flowers with short stalks and
smell “skunky”
 Grown in tundra habitats above timberline have large flowers with long stalks and sweet
smell
o Different insects pollinate each
  Small flies are abundant at slightly lower elevations, are attracted to skunky odours
 large bumblebees are abundant at higher elevations, are attracted to sweet odours
 Bumblebees prefer big flowers because larger flowers support their mass
- evolutionary changes in the size or food-finding habits of a pollinator affect the angiosperm populations
they pollinate

Fertilization

Step 1: After landing on the stigma of a mature flower, a pollen grain absorbs water and germinates.

o Germination: resumption of growth and development

o This step is blocked in self-incompatible species because it’s from the same plant

Step 2: When the male gametophyte germinates, a long tubular cell called pollen tube grows through the stigma

o Direction of growth is affected by chemical attracts called LUREs: small proteins released by
synergids
o Sometimes, it can travel down the length of the tube and the cell divides to form two sperms
o Others, the generative cells from sperm before the pollen is shed

Step 3: When the pollen tube reaches the micropyle of the ovule, it grows through and enters a synergid within
the female gametophyte. The synergid degenerates and two sperm are released

Step 4: Double fertilization takes place. One sperm unites with the egg to form the zygote. The other sperm
moves through the female gametophyte and fuses with the polar nuclei in the central cell.

o Two polar nuclei are present and a large triploid (3n) cell forms.
- The result endosperm is triploid and stores nutrients for the embryo, including starch or oils (lipids) plus
proteins and other nutrients
 - After fertilization, embryogenesis ensues and seeds develop into mature, formant structures that are
adapted for dispersal

Seeds and fruits

Fertilization triggers the development of a young, diploid sporophyte

- As a seed matures, the embryo and endosperm develop and become surrounded by a covering
called seed coat.
- Also, the ovary around the ovule develops into a fruit, which encloses and protects the seed
o Fruits often aid in dispersing seeds away
- With in the seed, they store nutrients that the embryo will use for growth and development for once
the seed germinates
o It allows offspring to be much more successful in colonizing habitats that are crowded with
competitors than offspring produced from spores, which are single cells

Before, in most eudicots, the cotyledons take up the nutrients that were initially stored in the endosperm and
store them again

- But in these species (seeds), there is no endosperm left by the time the seed matures—instead, the
cotyledons function as the nutrient storage organ in mature seeds.
- When it matures, the roots and shoot system with the seed leaves have formed
- Loss of water is an adaptation that prevents seeds from germinating until after they are dispersed.
o The dry condition of seeds ensures that they will not germinate until water is available
- When water is isolated, the membranes disintegrate and the proteins denature
o As water leaves the seed during drying, sugars become concentrated and maintain the integrity
of plasma membranes and proteins.

After pollination and fertilization, the ovary begins to develop into the fruit while the ovules develop into seeds.

- As the fruit matures, the wall of the ovary thicken to from the pericarp: part of the fruit that surrounds
and protects the seed
 - Simple fruits: cherry develop from a single flower that contains a single carpel or several carpels that are
fused together.
- Aggregate fruits: single flower that contains a single carpel or several carpels that are fused together
(blackberry)
- Multiple fruits: develop from many flowers and thus many carpels (pineapple)
- Accessory fruits: developed not from ovaries, but from modified floral tissues (strawberry)

Fruit function
Fruits have two functions: aid in seed dispersal, and they protect seeds from physical damage and seed
predators.

- Fruits sometimes split open and release seeds to be dispersed directly

- Dry fruits would be fall to the ground to be dispersed
o Some plants actually disperse dry fruits via propulsion.
 It produces a seed pod that shrinks as it dries. It will split apart violently, spraying seeds
in all directions and is called dynamite tree
 - Mammals are often active at night and use their well-developed sense of smell to locate fruits
o This was tested as each animal was offered three kinds of fruit: hackberries, fruits from a strain
of chilies that can’t synthesize capsaicin.
o the researchers recorded the percentage of each fruit eaten during a specific time interval
o found that capsaicin appears to be an effective deterrent to seed predation

once seeds are dispersed, seeds may not germinate in a condition called dormancy

- a feature of seeds from species that inhabit seasonal environments, where for periods conditions may
be too cold or dry for seedlings to thrive
o dormancy is interpreted as an adaptation that allows seeds to remain viable until conditions
improve.
- Mutants cannot make or respond to ABA(hormone abscisic acid) because it exhibit a property called
viviparity, it will germinate on the parent plant as they are mature. (ABA is affected by temperature)
o ABA triggers the accumulation of storage compounds, desiccation tolerance, and the prevention
of germination
- After it has been dispersed, may remain dormant in the soil for years before it germinates

The coats of some seeds are thick enough to prevent water and oxygen from physically reaching the
embryo. For germination to occur, these seed coats must be disrupted, or scarified (damage)

- Dormancy can be broken in response to a wide variety of habitat-specific environmental cues

- They’re place in large drums with prices of sandpaper that abrade and scarify the seeds.
o When planted, the scarified seeds germinate quickly and uniformly
o Species native to high latitudes or high elevations often produce seeds that must undergo cool,
wet conditions before they will germinate
- Small seeds have few nutrient reserves in their cotyledons or endosperm and need to germinate near
the soil, where they are exposed to light.
o Ex: lettuce seed must be exposed to red light before they break dormancy and germinate
 Red light is an environmental cue, because wavelengths in the red portion of the light
spectrum are used for photosynthesis
- Many of the seeds produced by species native to habitats where wildfires are frequent have a chemical
requirement to break dormancy
o They must be exposed to fire and it is advantageous for seeds to germinate after the fire cleared

The formation of a mature embryo is one requirement to ensure the development of viable seed

- Even if dormancy is broken, seeds need to germinate with water.

- Once the seed coat allows water penetration, water enters by moving along a water-potential gradient,
because the seed is so dry
- Water uptake in a typical angiosperm seed has three distinct phases:
o Phase 1: Germination begins with a rapid influx of water. Oxygen consumption and protein
synthesis in the seed increase, but no new messenger RNAs are transcribed
 germination are driven by mRNAs that are stored in the seed before maturation
o Phase 2: An extended period when water uptake slows or stops. New mRNAs are transcribed
and translated into protein products.
  Mitochondria also begin to multiply because it supports growth due to the water uptake
in phase 1 to hydrate their existing proteins and membranes
o Phase 3: Water uptake resumes as growth begins. This renewed phase of water uptake enables
cells to develop enough turgor pressure to enlarge.
 Eventually, the seedling bursts from the seed coat.
- The radicle (root of plant) emerges first, then develops into the mature root system
o Leaves are less important because the nutrients stores in the seed

- In eudicots, the shoot system with its cotyledons usually emerges shortly after the radicle appears.
o In eudicots the stem has a hook shape and is used protect the apical meristem from damage as
the shoot works its way upward through rough soil particles
- In monocots such as corn, the emerging shoot is covered by the protective coleoptile
- The seedling is said to be mature when the young plant no longer relies on food reserves in its
endosperm or cotyledons.
o Instead, it receives all of its nourishment on its own, from the compounds produced by
photosynthesis.

Embryogenesis and Vegetative development

Plant development occurs as vegetative or reproductive development

- Vegetative development: produces the nonreproductive portions of the plant body—the roots, leaves,
and stems
- Reproductive development: mature plants with shoots meristems that produce reproductive structure
- Embryogenesis: developmental process by which a single-celled zygote becomes a multicellular embryo
 - After fertilization, the zygote divides and produces daughter cells that are different in size, content, and
fate.
o This cell division is called an asymmetric division
- The bottom, or basal, cell is large and gives rise to a column of cells called suspensor
o Suspensor provides a route for nutrient transfer from the parent plant to the developing
embryo
- The asymmetry between the basal and apical cells helps establish one of the primary axes (directions) of
the plant body: Apical-basal axis
o Apical refers to the tip
o Basal refer to the base
o The basal cell divides parallel to the future apical–basal axis to produce the suspensor
o The apical cell produces daughter cells that divide both perpendicularly to the apical–basal axis
and parallel to it to produce a group of cells called the globular stage embryo
- Asymmetry arises as cells of globular embryo continue to divide
o Cells in the interior of the structure are surrounded by other cells of the embryo
- Compare to, cells in the outermost layer contact surrounding tissues in the seed in addition to
underlying embryo cells
- This creates aa second body axis: radial axis
o The radial axis extends from the interior of the plant body out to the exterior
- The fate of a plant cell can be summed up in the old quip about location
- As the embryo continues to develop, the long axis of the plant begins to emerge and several important
structures take shape:
o (1) cotyledons
 absorb nutrients from the endosperm and supply them to the rest of the embryo
o (2) Hypocotyl (under cotyledons)
 embryonic stem
o (3) Radicle
 embryonic root that have epicotyl (above cotyledon): a portion of the embryonic stem
that extends above the cotyledons

As the cotyledons, hypocotyl, and root begin to take shape, groups of cells called Shoot apical meristem (SAM)
and root apical meristem (RAM)
 - Within each meristem, the rate of cell division is dictated by cell–cell signals produced in response to
environmental cues (ex: amount of light, abundance of water)
- below and the periphery of meristem, daughter cells produced by mitosis and cytokinesis in the
meristem grow in specific directions and differentiate into epidermal, ground, or vascular tissue
- During embryogenesis, these tissues are produced and arranged along the radial axis
o The epidermis (over skin) is an outer covering of specialized cells that protects the individual
o Inside the epiderma layer of cells if ground tissue: a mass of cells that may later differentiate
into cells that are specialized for photosynthesis and food storage
o Vascular tissue in the centre of the plant will eventually differentiate into specialized cells that
transport food and water between root and shoot.
- The root meristem can form all the underground portions of the plant, and the shoot meristem can form
all the aerial portions, including reproductive structures
- For the cotyledons and other embryonic structures to take shape, cell divisions need to occur in precise
orientations
- Some cells grow larger than others, and the direction of cell expansion along the apical–basal or radial
axes is tightly controlled and often radically different.

Research on genetics was started to identify genes that are transcribed in the zygote or embryo
of Arabidopsis and that are responsible for establishing the apical–basal axis of the plant body

- Their initial goal was to identify individuals with developmental defects at the seedling stage
o Looking for patterning mutants that lacked regions along the apical–basal axis of the body
o Apical mutants lacked the first leaves, or cotyledons.
o Central mutants lacked the embryonic stem, or hypocotyl.
o Dubbed basal mutants lacked both hypocotyls and roots.
- Each type of Arabidopsis mutant had a defect in a different gene and played a role in specifying the
position of cells along the apical–basal axis of the body
- Monopteros (DNA-binding domain) are responsible for the basal mutants
o encode a transcription factor that regulates the activity of target genes
o activated in response to signals from auxin (cell-to-cell signal molecule)
 Auxin is produced in the shoot apical meristem and transported toward the basal parts
o Resulting in a auxin [ ] gradient along the apical–basal axis
- Auxin acts as a morphogen to trigger the production of the regulatory transcription factor: monopteros
o In turn, monopteros unleashes a regulatory cascade that determines which cells in the basal
portion of the embryo will form hypocotyl and roots.

Once a leaf begins to grow, it develops along three structural axes:

- proximal: towards the main body

o distal: away from main body
- mediolateral (middle to side): runs from the middle of a leaf towards its margin
 - Adaxial-Abaxial (upper-lower)
- Gene responsible for the three leaf axes is phantastica (PHAN)
o Regulatory transcription factor
o controls the expression of genes that cause cells to form the upper surface of leaves
o regulatory cascade that begins with auxin and cell–cell signals and ends with the growth of a
normal leaf

Reproductive development
Potential to form sperm or egg is called the germ line

- In plants, there is no predetermined germ line

o Instead shoot meristems have the potential to switch from vegetative to reproductive
development in response to environmental conditions
- A shoot apical meristem (SAM) transitions to a floral meristem instead of vegetative structure
- The flower contain four organs: sepals, petals, stamen and carpels
- Floral meristem produce these four organs with different characteristic due to homeotic mutation
o That regulate the development of various body parts in plants and animals
o In the mutant, one kind of floral organ was replaced by another
- There were three groups of mutants where one has only carpels and stamens, others had only sepals
and carpels and those only with petals and sepals

- Each caused by a defect in a single gene

- Tested if three genes set up the pattern of a flower, the mutant phenotypes suggested a hypothesis for
how the three gene products interact with A,B,C genes
o Called the ABC model
ABC model
Three key points:

- Each of the three genes is expressed in two adjacent whorls.

- Because each gene is expressed in two adjacent whorls, a total of four different combinations of gene
products can occur.
- Each of these four combinations of gene products triggers the development of a different floral organ.
- If these classes occurs then:
o The A protein inhibits production of the C protein
o The C protein inhibits production of the A protein

The hypnosis were

- (1) the A protein alone causes cells to form sepals

- (2) a combination of A and B proteins sets up the formation of petals
- (3) B and C combined specify stamens
- (4) the C protein alone designates cells as the precursors of carpels

It was tested and they found that groups of A,B,C class genes were expressed in predicted region by the ABC
model

- plants have a genetic tool kit for development

o Differences in not the type of tools, but the form
o Ex: plants use a group of proteins called MADS-box transcription factors to specify structures
 plants use graded concentrations of auxin to specify position within the embryo
o Ex: animals use Hox transcription factors
 animals use graded concentrations of other molecules