Chapter 39 Reproduction in Flowering Plants

Biology 102

Tri-County Technical College

Pendleton, SC

Angiosperm Life Cycle

Pollen from anther lands on sticky stigma of carpel’s tip If capable, can self-pollinate but not the rule

Pollen grain germinates & grows pollen tube down through styleTube reaches ovary, grows through micropyle and discharges 2 sperm cells into embryo sacDouble fertilization occurs as 1 sperm unites with egg (2N zygote) and other sperm fuses with 2 nuclei in embryo sac’s central cell (3N endosperm)

ALC, cont.

After double fertilization, ovule matures into a seedEmbryo is 2N, has rudimentary root and 1 or 2 cotyledons (seed leaves)Endosperm is 3N, divides repeatedly forming endosperm rich in starch/other food reservesSeed coat is 2N; seed germinates into new sporophyte generation and will produce spores that germinate into gametophyte generation (mega and microgametophyte)

Angiosperm Life Cycle Visual

Spore Production Review

Spores are haploid reproductive structures

Sporophyte (always 2N [diploid]) produces spores

Spores produced by megasporangium and microsporangium

Diploid to haploid only way to do that is by meiosis

Gamete Production Review

Gametophyte (always N [haploid]) produces gametes (sperm/egg)

Female gametophyte (megagametophyte) called embryo sac develops in megasporangia Will eventually produce an egg & other cells

Male gametophyte (microgametophyte) called pollen grain develops in microsporangia

Gametophyte (N0 produces gametes (N) takes mitosis to do that

Pollen Formation

Pollen grain is immature male gametophyte that develops within anther of stamens of angiospermsIn sporangial chamber of anther, diploid microsporocytes under meiosis to form 4 haploid microsporesHaploid microspore nucleus undergoes mitosis to produce generative cell and tube cellWall of microspore thickens and sculptured into species-specific patternThese 2 cells and thickened wall are pollen grain or immature male gametophyte

Pollen/Microgametophyte

Pollen grain (microgametophyte/male gametophyte) develop in microsporangia

Can be viewed as synonymous, but botanist would object

Mature Microgametophyte info

Pollen grain becomes mature male gametophyte when generative cell (nuclues) divides to form 2 sperm cells

Both sperm have haploid nuclei

Tube cell (with its nucleus) responsible for pollen tube growth

Embryo Sac

Ovule is structure that forms within chambers of plant ovary and contains the female gametophyte called embryo sacMegasporocyte in sporangium of each ovule grows & undergoes meiosis to form 4 haploid nuclei Only one typically survives

Surviving megaspore grows and its nucleus undergoes 3 mitotic divisions forming one large cell with 8 haploid nuclei

Embryo Sac, cont.

Membranes partition this into multicellular embryo sac

Within the embryo sac one egg cell located at one end, flanked by 2 other cells called synergids

At opposite end are 3 antipodal cells

Other 2 nuclei (polar nuclei) share cytoplasm of large central cell

At end containing egg is the micropyle (opening through integuments surrounding embryo sac)

Embryo Sac Visual

Double Fertilization

Double fertilization is union of 2 sperm cells with two cells of embryo sac

Pollen grain with tube enclosing 2 sperm = mature male gametophyte

Directed by chemical attractant (usually calcium) tip of pollen tube enters through micropyle and discharges 2 sperm nuclei into embryo sac

Double Fertilization, cont.

One sperm unites with egg to form zygote

Other sperm combines with two polar nuclei to form 3N nucleus in large central cell of embryo sack

This central cell with give rise to endosperm which is a food storing tissue

Double Fertilization Visual

Pass the ripened ovary, please!!

After fertilization, ovary wall of flowering plant, together with its seeds, develop into a fruit

May consist of only mature ovary and its seeds or may include other parts of flower or structures closely related to it

Fruit protects the seeds and aids in their dispersal by wind or animals

Fruit develops from ovary of flower while seeds are developing from the ovules

Ripened Ovary, cont.

True fruit is a ripened ovaryPollination triggers hormonal changes that cause ovary to growWall of ovary thickens to become pericarpTransformation of flower into fruit parallels seed developmentIn most plants, fruit does NOT develop without fertilization of the ovulesParthenocarpic plants are an exception

Fruits, III

In some angiosperms, other floral parts contribute to formation of fruitSimple fruit derived from single ovary (cherries are fleshy; soybeans are dry)Aggregate fruit derived from single flower with several separate carpels (strawberries)Multiple fruit derived from an inflorescence or separate tightly clustered flowers (pineapple/figs)Ovule develops into seed containing embryo and food supplyProtective seed coat formed from integuments of ovule

Monocots versus [Eu]DicotsMonocot seed has only 1 cotyledon whereas dicot seed has two cotyledonsSome dicots have fleshy cotyledons because they absorbed food from endospermOthers (castor bean) retain food supply as endosperm and have very thin cotyledons Cotyledons will absorb food from endosperm

and transfer to embryo when seed geminates

Mono vs. Di, cont.

Seed of monocot has only ONE cotyledonMembers of grass family (including wheat and corn) have specialized type of cotyledon called scutellum (small shield) Scutellum very thin with large surface area pressed

against endosperm for absorbing nutrients during germination

Embryo of grass seed enclosed by sheath consisting of a coleorhiza (covers root) and a coleoptile (covers embryonic shoot)

Mono vs. Di Visual

Sowing seeds…so to speak

Fruits may be fleshy and/or dry and either edible or inedible

Dry, winged fruits (ash, elm, maple) blown by wind

Water disperses some fruits (coconuts/water lilies)

Some “hitch” ride on animals (stick-tights)

Some travel through birds and animals (enough said)

Some “carried” by animals

Photoperiodism

Photoperiod defined as regulation of processes (flowering, etc.) by the changing length of day (or night)Numerous genes participate in process Floral meristem identity genes initiate cascade of

further gene expression Pattern formation genes determine spatial organization

of the whorls of organs Organ identity genes that work in concert to specify the

successive whorls Homeotic genes—their products are transcription factors that

mediate expression of still further genes

Photoperiod, cont.

Only requires ONE leaf for plant to detect photoperiod and for floral buds to developAll leaves removed = no photoperiod detection occursBelieved unidentified hormone produced in leavesmoves to budOr may change in relative [ ]s of 2 or more hormonesThis hormone (or mixture) appears to be same in both long-day and short-day plants

Photoperiod, cont.

Combination of environmental cues (photoperiod) and internal signals (hormones) induces transition of bud’s meristem from vegetal state into flowering state

**Plant hormone not yet isolated but inferred to cause conversion of a vegetative shoot apex to a flower is called FLORIGEN

Will it come back next year?

Annual plant completes entire life cycle (seed to flower) in less than one year

Biennial plant grows for all or part of one year, lives on into a second year during which it flowers, forms seed(s), and dies

Perennial plant lives for a few to many years during which both growth and flowering occur

Day or Night?

Short-day plants require light period shorter than a critical length—generally flower in late summer, fall, and winter Chrysanthemums, poinsettias, some soybeans

Long-day plants flower only when light period is greater than certain number of hours, generally late spring and summer Spinach, radish, lettuce, iris, many cereals

Day or Night, cont.

Day-neutral plants are unaffected by photoperiod and flower when reach certain stage of maturity Tomatoes, rice, dandelions

Discovered in 1940s that NIGHT LENGTH and not day length controlled flowering and other responses to photoperiod

If daytime period broken by brief exposure to darkness, there is NO effect on flowering

Day or Night, cont.

If nighttime period interrupted by short exposure to light, plants DO not flowerShort-day plants flower if night is longer than critical lengthLong-day plants need a night shorter than a critical length**Key concept is it is the AMOUNT and timing of darkness that plants are actually using as photoperiod stimulii

Photoperiod Visual

Photoperiod Visual II

Phytochrome and Florigen

Phytochrome is regulatory plant pigment existing in either active or inactive formDifferent wavelengths of light can drive it from one form to the otherFlorigen name given suspected “flowering hormone”Photoperiodic induction of leaf induces it to start and continue production of florigenTransported to other parts of plant switching target parts to reproductive parts

And the Beat Goes On…

Circadian rhythm is physiological cycle with frequency of about 24 hours

Organism sheltered from environmental cues, rhythm may deviate from 24 hrs (free-running periods) and can vary from 21-27 hrs

Clock may take days to reset once cues change

“Period” is length of one cycle; “Amplitude” is magnitude of change over course of a cycle

Rhythm, cont.

Period is remarkably insensitive to temperature

Periods are highly persistent—continue in environment in which there is NO alternation of light or dark

Periods can be “entrained” (within limits) by light-dark cycles that differ from 24 hrs

Brief exposure to light can shift rhythm causing a phase shift

Some Key Concepts

Sexual reproduction involves some type of gametes (sperm/egg; +/-) How those gametes gotten together is real hoot

Asexual reproduction does NOT involve production of gametes“Production of offspring from single parent; occurs without genetic recombination, resulting in a clone

Concepts, cont.

Vegetative reproduction occurs when meristemic tissues composed of dividing, undifferentiated cells can sustain or renew growth indefinitely Parenchyma cells can also divide and

differentiate into various types of specialized cells

Two major natural mechanisms of vegetative reproduction

Concepts III

Fragmentation is separation of parent plant into parts that reform whole plant Most common type of vegetative reproduction

Apomixis is production of seeds without meiosis and fertilizationDiploid cell in ovule gives rise to embryoOvules mature into seeds which are dispersedBest example is dandelion

Just the facts…

Sexual reproduction benefits include generation of variation (asset when environment changes)Produces seeds for dispersal to new locations and which can wait until environment becomes favorableAsexual: in stable environment, can clone many copies in short period of timeProgeny are mature fragments of parent plantNot as fragile as seedlings produced sexually

Tissue Culture

Many plants cells “totipotent”—possess all genetic info and other capabilities necessary to form new individual

Cultures of undifferentiated tissues can give rise to entire plants

Even have way to “de-differentiate” tissues (Fig 16.3, p. 297)

Can use small pieces of tissue from parent plant Orchids, rhodendendrons

Culture, cont.

Culturing tiny bits of apical meristem can produce plants free of viruses

Such tiny bits LACK vascular tissue and viruses tend NOT to enter that tissue