Chapter 31 (part 2) : Reproduction of flowering plants

flowers

A flower is a reproductive shoot Develop from a floral bud containing shoot apical meristem that produces reproductive organs (instead of leaves) -Sepals protect unopened, developing flower bud -Petals attract pollinators -Stamens produce pollen -Carpels produce ovules and receive pollen via pollination

Endosperm nourishes the embryo

Endosperm is rich in starch, proteins, lipids, vitamins and minerals supplied by the parent sporophyte Supplies nutritional needs of the developing embryo and often the young seedling Monocots retain most endosperm in the mature seed -Starchy seeds such as corn, wheat, rice Eudicot seeds consume most of the endosperm and store food inside two cotyledons(embryonic leaves) -Includes protein rich seeds such as beans, peas, peanuts

seedling growth

Eudicots -Hypocotyl hook first penetrates the soil crust, protecting the epicotyl shoot tips and delicate SAM -Growth of the hypocotyl pulls the shoot tips and (usually) the cotyledons from the soil Monocots -Protective coleoptile sheath penetrates the soil before the shoot and first foliage leaves emerge

Production of Male Gametophytes

Stamens produce male gametophytes -Vascular tissue in filaments delivers nutrients from parental sporophyte to anthers -Each anther contains 4 microsporangia, where haploid microspores are produced by meiosis -Microspores develop into small immature male gametophytes called pollen grains

Control of pollen germination

Stigma determines whether or not pollen grains germinate and pollen tubes grow Pollen from foreign species is prevented from germinating Some plant species allow self-pollination (self-fertilization); others only allow cross-pollination Self-incompatibility (SI): rejection of pollen that is too genetically similar to the parent plant -Involves interaction between pollen and pistil proteins -May prevent pollen tube formation

Embryogenesis and seed development

After fertilization, zygote divides into two cells: -Small terminal cell will become the embryo -Large basal cell will become the suspensor Early embryo (proembryo) depends on food supplied directly by parent plant through the suspensor Older embryo relies on nutrients from the endosperm Cotyledons and meristems form, but the embryo dehydrates (mature seeds are only 5-15% water) and becomes dormant Integuments of the ovule develop into a tough seed coat made of sclereid cells -Serves as water and oxygen barrier -Also shields embryo from UV radiation and microbial attack

Seed germination

After seed dispersal (via fruits), favorable conditions can initiate seed germination Germination begins with imbibition - absorption of water to rehydrate and activate the embryo Seed swells, generating pressure that ruptures seed coat Developmental genes are induced resulting in synthesis of enzymes and hormones necessary for growth and development of the seedling plant Cell division occurs in the root apical meristem (RAM) of the radicle, which emerges from the seed and grows downward Shoot apical meristem (SAM) also begins to undergo cell division and the shoot breaks through the soil surface

Alternation of generations

All plants cycle through 2 multicellular life stages Termed alternation of generations Diploid (2n), the spore-producing sporophyte -The sporophyte produces haploid spores by meiosis Haploid, gamete-producing gametophyte -Gametophyte produces gametes by mitosis -Gametes fertilize to produce the next sporophyte

Duration of the plant life cycle

Annuals complete the life cycle in one season and die after producing seeds (e.g. beans, corn, wheat, rice) Biennials grow during the first year and flower (reproduce) during the second year (e.g. beets, carrots, parsley) -May not flower until after a cold treatment (i.e. winter) Perennials live for many years and may produce seeds each year after maturity -Many perennials produce wood through secondary growth -Shoots of herbaceous (non-woody) perennials may die or enter dormancy in winter, but roots remain alive and rhizomes or tubers/bulbs generate new shoots each year (e.g. grasses)

Fruit development

As embryos and seeds develop, the ovary surrounding the seeds develops into a fruit Ovary wall develops into a pericarp (fruit wall) Function of fruit is to disperse seeds Diverse fruit types foster different modes of dispersal (e.g. wind, water, animals)

Pollen grains

At dispersal, pollen grain is typically a 2-celled male gametophyte contained within the microspore cell wall Larger tube cell stores proteins and lipids for later stages of male gametophyte development -Forms pollen tube if pollination to compatible stigma occurs Smaller generative cell divides to produce two sperm cells, typically after pollination Possess a tough pollen wall made of highly resistant biopolymer called sporopollenin Additional pigmented pollen coat encloses pollen wall -Includes lipids and proteins that aid attachment to stigma

Production of the female gametophyte

Carpels produce and nurture female gametophytes -Veins of vascular tissue deliver nutrients from the parent sporophyte to ovules within an ovary -Ovule consists of a megasporangium and enclosing integuments -Megasporangium produces haploid megasporesby meiosis -One surviving megaspore within the ovule generates the female gametophyte (embryo sac) *Typically consists of 7 cells: egg cell, 2 supportive synergids, 3 antipodal cells, large central cell with two polar nuclei

Structure of mature embryo in a seed

Eudicots possess two cotyledons; monocots have one Embryonic root is called a radicle -Develops into main taproot after eudicots germinate -Possesses a root apical meristem Shoot apical meristem is at the base of the cotyledons Epicotyl: embryonic stem above the cotyledons Hypocotyl: embryonic stem below the cotyledons

Double fertilization

Fertilization occurs within the female gametophyte (embryo sac) One sperm cell fuses with the egg to form the diploid zygote Other sperm fuses with the two nuclei of the central cell to form a triploid (3n) cell that develops into the endosperm Endosperm will serve as nutritive tissue as the zygote develops into an embryo (young sporophyte) Fertilized ovule will develop into a seed

Pollen tube growth

Pollen tube grows through the style by the process of tip growth controlled by the tube cell -New cytoplasm and cell wall material are added to the elongating pollen tube cell by Golgi vesicles -Plugs of the polysaccharide callose seal off older parts of the pollen tube *Concentrates cytoplasm near the tip and increases turgor pressure for continued tip growth Mature pollen tube delivers sperm to the female gametophyte (embryo sac) in the ovule

pollination and pollen germination

Pollination: delivery of a pollen grain to the stigma of a pistil by wind or an animal pollinator If compatible, pollen grain absorbs water from the stigma, rehydrates and male gametophyte matures -Generative cell in the pollen grain divides by mitosis to produce two sperm cells -Tube cell grows a pollen tube through the style to the ovary and into the micropyle of an ovule -Sperm are delivered to embryo sac for fertilization

Evolutionary shifts in plant life cycle

Primitive land plants (such as mosses) -Gametophyte is dominant -Sporophyte is small and dependent on the gametophyte Major trend in plant evolution -Larger, more complex sporophyte -Smaller, less complex gametophyte Flowering plants (angiosperms) -Sporophyte is large, complex and independent -Gametophyte reduced to a few cells within flowers; produce gametes but dependent on the sporophyte