BIO 131 Chapter 38: Plant Reproduction and Development

38.1-38.5

Flower

* reproductive structure in angiosperms
* produces gametes, attracts pollinators, receives gametes from other individuals, nourishes embryos, and develops seeds and fruits

Seeds

* embryo and nutrient stores surrounded by a protective seed coat

Fruits

* develop from the ovary
* contains seeds

Asexual reproduction

* does not involve fertilization
* results in the production of clones
* based on mitosis
* rhizomes, corms, plantlets, aoximis
* key advantage: efficiency; may populate unoccupied space before competitors can establish
* key disadvantage: lack of genetic diversity, susceptibility to the same pathogens

Aoximis

* phenomenon in which mature seeds can form without fertilization occurring
* results in seeds that are genetically identical to the parent

Sexual reproduction

* based on meiosis and fertilization
* advantages: genetic diversity

Alternation of generations

* alternate phases of life cycle
* characterized by two distinct multicellular forms—one diploid, one haploid

Sporophyte

* an individual in the diploid phase of the life cycle
* produces spores by meiosis

Gametophyte

* an individual in the haploid phase of the life cycle
* produces gametes by mitosis (rather than meiosis, as in animals)

5 key processes in land plant life cycles

* sporophytes undergo meiosis in sporangia to produce haploid spores
* spores undergo mitosis to develop into multicellular haploid gametophytes
* gametophytes produce sperm and eggs by mitosis
* fertilization—two gametes fuse to form a diploid zygote
* zygote undergoes mitosis to grow into a multicellular, diploid embryo (sporophyte)

General flower structure

* four basic organs (essentially modified leaves) attached to a compressed portion of the stem called the receptacle
* sepals, petals, stamens, carpels
* not all four organs are present in all flowers

Sepals

* leaflike structures that make up the outermost parts of the flower
* usually green and photosynthetic
* relatively thick compared to other parts of the flower
* encloses and protects the flower bud as it develops

Calux

* the entire group of sepals in the flower

Petals

* arranged around the receptacle in a whorl
* often brightly colored and scented to attract pollinators—color and scent may correlate with pollinators’ visual abilities and habits
* very small or nonexistent in wind-pollinated angiosperms

Corolla

* entire group of petals in a flower
* petals within the same corolla can vary in size, shape, and function

Stamens

* reproductive structures that produce male gametophytes
* composed of filaments and anthers

Filament

* the slender stalk supporting the anthers
* holds the stamen in a place where pollinators can make contact with pollen grains

Anthers

* pollen-producing organs (contains microsporangia)
* business end of the stamen

Carpels

* reproductive structures which produce female gametophytes
* composed of three regions—stigma, style, and ovary

Stigma

* sticky tip of the carpel which receives pollen

Style

* slender stalk connecting the stigma and ovary

Ovary

* enlarged structure at the base of the carpel
* contains ovules, which contain megasporangium, which contains a diploid megasporocyte, which produce female gametophytes (mature into eggs)

Perfect flowers

* flowers that contain both stamens and carpels

Imperfect flowers

* refers to flowers which contain either stamens or carpels, but not both

Monoecious plants

* refers to plants which separate imperfect carpel- and stamen-bearing flowers within the same plant
* e.g. corn

Dioecious plants

* refers to species where individual plants produce either stamen-bearing or carpel-producing flowers only

Production of female gametophytes

* megasporocyte divides by meiosis, resulting in four haploid megaspores
* three of the megaspores degenerate
* surviving megaspore divides by mitosis to produce a structure with 8 haploid nuclei—the female gametophyte (embryo sac)
* haploid nuclei segregate to different positions in the embryo sac and cell walls form 7 cells—one of these cells becomes the haploid egg

Polar nuclei

* typically 2 nuclei within the central cell (largest cell in the ovule)
* plays an important role in seed development after pollination and fertilization take place

Mature female gametophyte structure

* egg cells at one end near the micropyle
* 2 haploid cells called synergids close to the eggs
* large central cell

Micropyle

* opening in the ovule where sperm will enter before fertilization

Production of male gametophytes

* microsporocytes (diploid cells within the anther microsporangia) undergo mitosis
* resulting 4 microspores usually all survive and divide by mitosis
* the two resulting nuclei form a haploid, immature male gametophyte

Immature male gametophyte structure

* small generative cell enclosed in larger tube cell
* tough, waterproof, sporopollenin-containing outer coat which protects the gametophyte when it is released by the parent plant

Mature male gametophyte

* considered mature when the haploid generative cell undergoes mitosis to form two sperm cells
* maturation may occur while the pollen is still in the anther or after it lands on a stigma and begins to grow

Pollination

* the transfer of pollen grains from an anther to a stigma
* wind, water, organisms (birds, bats, insects, etc)

Fertilization

* occurs when a sperm and an egg unite to form a diploid zygote

Self-fertilization (selfing)

* occurs when a sperm and an egg from the same individual combine to produce an offspring
* advantages: successful pollination is virtually assured
* disadvantages: offspring are usually much less genetically diverse than outcrossed offspring (even though it uses meiosis); may suffer from inbreeding depression

Cross-pollination

* occurs when pollen is carried from the anther of one individual to the stigma of another individual
* results in outcross
* advantage: produces genetically diverse offspring that may be much more successful at warding off attacks from pathogens
* disadvantage: riskier in terms of reduced chance of pollination
* much more common than selfing

Temporal avoidance (selfing prevention)

* in some species with perfect flowers, male and female gametophytes mature at different times

Spatial avoidance (selfing prevention)

* dioecious species—selfing is not possible
* monoecious species—flowers are separated along plant (such as corn); selfing is rare unless pollinators transfer pollen within the same plant
* perfect flowers—anthers and stigma may be separated far enough that self-pollination would be extremely unlikely

Molecular matching (selfing prevention)

* molecular interactions may occur that prevent pollen grains from delivering sperm to the female gametophytes on the same plants (“incompatible” species)
* based on the plant’s ability to recognize self vs. nonself

Pollination syndromes

* suites of flower characteristics that are associated with certain types of pollinators
* ex: bird-pollinated flowers tend to be red, unscented, and open during the day, while moth- and bat-pollinated flowers tend to be white, strongly scented, and open at night

Directed-Pollination Hypothesis

* proposes that natural selection has favored flower colors, shapes, and scents to attract specific pollinators
* based on observations of pollination syndromes

Phylogenetic pollination patterns

* pollination evolved late in land plant evolution
* seed plants do not need external water for sexual reproduction to occur—the evolution of pollen therefore allowed seed plants to be much less dependent on wet habitats

Animal-pollinated species

* invest in structures which attract and reward animal pollinators (e.g. nectaries) rather than large amounts of pollen, since animal-borne pollen is much more likely to be successfully transferred to flowers of the same species

Wind-pollinated species

* tend to invest in making large numbers of pollen grains, since wind is a much less “accurate” pollinator

Pollination encouraging speciation (observation)

* alpine skypilots are beginning to diverge above and below the mountain timberline
* plants above the timberline have large, sweet-smelling flowers on long stalks, which attract large bumblebees
* plants below the timberline have small, “skunky”-smelling flowers on short stalks, which attract small flies
* since the pollinators prefer to visit different types of flowers, gene flow between the populations is low and they are evolving different characteristics

Fertilization process

* pollen grain lands on stigma of mature flower from the same species, absorbs water, and germinates
* tubular cell grows through the stigma, down the style, and through the micropyle into a synergid within the female gametophyte
* generative cell divides to form two sperm, which are released when the synergid degenerates
* sperm unites with egg to form zygote; other sperm fuses with the polar nuclei in the central cell
* resulting (usually) triploid cell undergoes mitosis and cytokinesis to form the endosperm

Germination

* resumption of growth and development
* blocked in many self-incompatible species if the pollen comes from the same plant

LUREs

* small proteins released by the synergids which signal the direction of pollen tube growth

Endosperm

* inside-seed tissue
* usually triploid
* stores nutrients (starches, oils/lipids, proteins, and other nutrients) for the embryo

Mature seed

* dormant embryo with root, shoot, and leaf precursors formed
* food supply (originating from the endosperm)
* seed coat
* dry seed tissues
* fruit—protects seed and often aids in seed dispersal

Seed evolutionary importance

* allows offspring to be much more successful colonizing crowded habitats than offspring produced from spores (since they contain stored nutrients)

Cotyledons (eudicots)

* take up nutrients initially stored in the endosperm and store them again
* mature seeds have no endosperm left, with cotyledons instead functioning as the nutrient storage organ

Seed drying

* adaptation that prevents seeds from germinating until after they are dispersed and water is available
* makes seeds less susceptible to damage from freezing
* cell structures survive through several molecular mechanisms, including preservative concentrated sugars which maintain the integrity of the plasma membrane and proteins

Fruit development

* ovary walls thicken to form the pericarp
* formed from the mother plant’s tissues, not the embryo

Pericarp

* part of the fruit that surrounds and protects the seed(s)
* may be dry or fleshy

Simple fruits

* develop from a single flower containing a single carpel or several fused together
* most common type of fruit
* ex: cherries

Aggregate fruit

* develop from a single flower which contains multiple separate carpels
* ex: blackberry

Multiple fruits

* develop from many flowers
* ex: pineapple

Accessory fruits

* develop from other modified floral tissues rather than the ovary
* ex: strawberries (the red part) is derived from the receptacle, while each individual seed is its own fruit

Fruit functions

* aid in seed dispersal—important to the fitness of the young sporophyte, particularly for long-lived species which would otherwise have to compete with the parent for light, water, and nutrients
* protects seeds from physical damage and seed predators

Mechanisms for seed dispersal (dry fruits)

* simply falling to ground
* wind, propulsion, animals
* often have structural elements that aid in dispersal (such as maple “helicopters”)

Mechanisms for seed dispersal (fleshy fruits)

* animals are most common; coevolved mutualistic relationships, similar to pollinators

Mechanisms for discouraging seed predators

* lacing fruits and seeds with poisonous or harmful /painful compounds such as capsaicin, which the seed dispersers lack the receptors for

Seed dormancy

* usually a feature of seeds from species that inhabit seasonal environments with extended cold/dry periods
* adaptation that allows seeds to remain viable until conditions improve
* may be maintained for centuries and still remain viable

ABA (seed dormancy)

* triggers the accumulation of storage compounds, desiccation tolerance, and prevention of germination


* mutants which cannot make/respond to ABA exhibit viviparity

Viviparity

* property in which seeds germinate on the parent plant as soon as they are mature

Water uptake (seed germination)

* first event in germination; 3 phases
* rapid influx of water, dramatically increased oxygen consumption and protein synthesis but no new mRNA transcription (driven by mRNA stored in the seed before maturation)
* extended period where water uptake slows/stops, mRNA transcription and translation, multiplication of mitochondria
* resumption of water uptake as growth begins, allowing cells to develop enough turgor pressure to burst from the seed coat

Radicles (embryonic root)

* emerges first
* develops into mature root system
* key event—seedling must have a source of water in order to grow

Emerging shoot

* eudicots: hook-shaped to protect the apical meristem from damage as the shoot moves through the soil
* monocots: protected by the coleoptile
* usually emerges shortly after the radicle appears

Vegetative development

* produces the non-reproductive portions of the plant body (leaves, roots, stem)

Reproductive development

* produces reproductive structures

Embryogenesis

* developmental process by which a single-celled zygote becomes a multicellular embryo

Angiosperm embryogenesis

* zygote —> apical + basal cell; establishment of asymmetrical apical-basal axis
* apical cell —> globular stage embryo; basal cell + suspensor; radial axis (plant body interior/exterior axis) becomes apparent
* globular stage embryo —> heart stage embryo (protoderm, ground meristem, procambium progenitors)
* root and shoot system precursors form from embryonic tissues

Apical cell

* gives rise to almost the entire embryo

Suspensor

* provides route for nutrient transfer from the parent plant to the developing embryo

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Cotyledons

* often absorb nutrients from the endosperm and supply them to the rest of the embryo
* embryonic leaf

Hypocotyl

* “under-cotyledons”
* embryonic stem

Radicle

* embryonic root

Meristem formation

* SAM and RAM are specified as cotyledons, hypocotyl, and radicle take shape
* rate of cell division in meristems dictated by cell-cell signals produced in response to environmental cues
* daughter cells of meristem grow in specific directions and initially differentiate into epidermal, ground, and vascular tissue (produced and arranged along the radial axis)

Arabidopsis

* model plant (like the Drosophila of plants)

Apical mutants

* mutants lacking cotyledons

Basal mutants

* mutants lacking hypocotyls and radicles

Central mutants

* mutants lacking hypocotyls

*MONOPTEROS*

* gene responsible for basal mutants
* activated in response to auxin
* codes for transcription factor that activates genes responsible for root structures

Auxin

* cell-cell signaling molecule produced in SAM and transported toward basal parts, resulting in auxin gradient along the apical-basal axis that provides positional information (similar to bicoid in Drosophila)
* responsible for plant root gravitropism

Bicoid

* chemical deposited in embryonic Drosophila that signals for development of anterior structures (head, etc)

*PHANTASTICA (PHAN)*

* gene with role in specifying the adaxial-abaxial axis
* mutations result in funky looking leaves

Leaf axes

* mediolateral
* proximal-distal
* adaxial-abaxial