Plant Reproduction and Development Chapter 44 Plant Reproduction and Development

How do plants reproduce? Asexually Existing plant uses mitosis – identical Lilac bushes that sprout new trunks from the root Strawberries and runners Tulips and other bulbs grow new, smaller bulbs Sexually Fusion of gametes from parents

Alternation of generations Plant sexual life cycles alternate between two multicellular stages, haploid and diploid

Sporophyte – multicellular diploid Sexual Life Cycle Sporophyte – multicellular diploid Garden plants, produce flowers Produces specialized reproductive cells that undergo meiosis to form haploid spores Spores undergo mitosis to form multicellular haploid gametophyte Angiosperms and Gymnosperms produce separate male and female gametophyte stages

Sexual Life Cycle of a Flowering Plant Diploid mother cells develop in anthers (male) or ovaries (female) Meiosis produces haploid spores Mitosis of the spores  male & female gametophytes  sperm & egg Pollen carries sperm to flower, sperm travel in tube to female gametophyte Fertilization  diploid zygote Zygote develops into embryo, seedling, mature sporophyte

Alternation of Generations mother cell spores flower MEIOTIC CELL DIVISION stigma male gametophyte (pollen grain) anther In the flower, diploid mother cells develop in the reproductive structures: anthers (male) and ovaries (female) 1 Meiotic cell division of mother cells in the sporophyte produces haploid spores 2 Mitotic cell division of the spores forms male gametophytes (pollen), which produce sperm, and female gametophytes, which produce eggs 3 ovule sperm ovary Pollen carries the sperm to the female reproductive structure of a flower; sperm travel within a pollen tube to the female gametophyte 4 A sperm fertilizes an egg within the female gametophyte, producing a diploid zygote 5 mother cell pollen tube MEIOTIC CELL DIVISION ovule mature sporophyte sperm nuclei spores The zygote develops into an embryo, a seedling, and eventually, a new mature sporophyte 6 female gametophyte egg female gametophyte FERTILIZATION seedling seed haploid (n) diploid (2n) seed fruit embryo

Sexual Life Cycle Varies between Plants Size, complexity and lifespan of sporophyte and gametophyte varies Mosses, liverworts – gametophyte is independent Resulting sporophyte grows on gametophyte Ferns – sperm fertilize eggs in independent gametophyte, zygote begins growing on gametophyte but sporophyte develops its own roots and leaves – becomes dominant stage

Angiosperms and Gymnosperms Differ from mosses, liverworts, ferns Diploid sporophyte is the dominant stage In angiosperms and gymnosperms, sperm is transported within pollen grain. In mosses, liverwort and fern all require water for fertilization (sperm swim to eggs) Gametophytes are very, very small

Flower – reproductive structure of angiosperm Flower Structure Flower – reproductive structure of angiosperm Complete flower – has 4 sets of modified leaves Sepals, petals, stamens, carpels Petunia, rose, lily

Complete Flower Structure Sepal – at base of flower In monocots, resemble petals In dicots – green and leafy Surround and protect flower bud Petals – brightly colored, advertise for pollinators Stamens – attached above petals Filament with anther, pollen Carpel – vase shaped, sticky stigma on elongated style, bulbous ovary at base of carpel – one or more ovules where female gametophyte develops Fertilized ovule becomes seed and dev. into fruit (encloses)

A Complete Flower anther petal stamen filament stigma style carpel sepal ovary ovules (a) A representative dicot flower

Incomplete Flower Structure Lack one or more of 4 floral components Grass (lack petals, sepals) Also described as imperfect Produce separate male and female flowers, often on a single plant (zucchini) American holly, female produces red berries

Zucchini Flowers – male and female

Animation: Pollen Development

Pollen is the Male Gametophyte Develop within anthers of the sporophyte Microspore mother cells develop within pollen sacs of the anther Meiosis produces 4 haploid microspores Each produces an immature male gametophyte (pollen grain)by mitosis, contains the generative cell Tube cell + generative cell in the pollen cell The generative cell undergoes mitosis to form 2 sperm cells.

Male Gametophyte Development pollen sacs microspore mother cell anther Microspore mother cells develop within the pollen sacs of the anther of a flower 1 MEIOTIC CELL DIVISION sporophyte microspores 2 tube cell nucleus Meiotic cell division produces four haploid microspores mature pollen grain Immature pollen grain tube cell cytoplasm sperm cells stigma generative cell The generative cell produces two sperm cells by mitotic cell division; the male gametophyte is now mature 4 Each microspore produces an immature male gametophyte (a pollen grain) by mitotic cell division 3 tube cell nucleus haploid (n) diploid (2n)

Tough, waterproof outercoat Characteristic of the plant species Pollen Tough, waterproof outercoat Characteristic of the plant species Used to identify climate in fossils

Wind-Pollinated Flowers Anther, pollen

Species vary – one to several dozen ovules Female Gametophyte Forms in ovule Species vary – one to several dozen ovules Megapore mother cell develops within ovule Meiosis produces 4 haploid megaspores, 3 degenerate Remaining megaspore form 8 nuclei by mitosis (3X mitosis) Plasma membranes form, 7 cells – 3 at one end (1 N each), one is the egg

Female Gametophyte Development ovule megaspore mother cell A megaspore mother cell develops within each ovule of the ovaries of a flower 1 ovary integuments MEIOTIC CELL DIVISION Cytoplasmic division produces the seven cells of the mature female gametophyte 4 Meiotic cell division produces four haploid megaspores; three degenerate 2 megaspores central cell with two nuclei female gametophyte egg cell The single remaining megaspore forms eight nuclei by mitosis 3 haploid (n) diploid (2n)

Animation: Ovule Development

Pollination and Fertilization Pollen grain lands on stigma Absorbs water, breaks out of coat and elongates through stigma Pollen tube reaches ovule Double fertilization – both sperm fuse with cells of the female gametophyte One sperm fertilizes egg  zygote One sperm fertilizes central cell, mitosis produces endosperm

Pollination and Fertilization of a Flower pollen grain A pollen tube grows down through the style of the carpel to the ovary; the tube cell nucleus travels at the tip of the tube, and the two sperm follow close behind 2 tube cell nucleus sperm pollen tube Pollination occurs when a pollen grain lands on the stigma of a carpel 1 sperm tube cell nucleus Double fertilization: 3 One sperm fuses with the central cell ovule ovary central cell One sperm fuses with the egg cell egg

Animation: Pollination and Fertilization

Fruit and Seed Development Female gametophyte and integuments become seeds Ovary becomes fruit Petals, pollen, stamens dry up and fall off

Development of Fruit and Seeds in a Pepper ripening sepal ovary wall “flesh” of pepper ovary pepper fruit petal ovule seed pepper flower pepper fruits

Three processes transform ovule into seed Seed Development Three processes transform ovule into seed Integuments become seed coat Triploid central cell divides to form endosperm Zygote develops into the embryo As seed matures, embryo differentiates into shoot and root Shoot includes 1 or 2 cotyledons – absorb food from endosperm Monocot – most of endosperm stays in seed until germination Dicot – cotyledons absorb most of the endosperm, so the mature seed is full of embryo

Monocot Dicot Seed Structures Shoot Coleoptile – sheath that surrounds embryonic leaves Dicot Hypocotyl Epicotyl

Seed Development integuments (diploid) seed coat central cell (triploid) endosperm zygote (diploid) embryo fertilized ovule seed (a) Early development of the seed seed coat embryonic root endosperm cotyledon embryonic leaves shoot coleoptile hypocotyl shoot embryonic leaves seed coat embryonic root cotyledons (b) Corn seed (monocot) (c) Bean seed (dicot)

Animation: Embryo and Endosperm Development

Germination – sprouting of seed Embryo grows and breaks out of seed Forms seedling Warmth and moisture are necessary

Some seeds have a period of dormancy Resist adverse environmental conditions Dormancy solves 2 problems Prevents seeds from germinating within moist fruit Environmental conditions optimal for germination may not coincide with conditions that will allow seedling to survive and mature Seeds mature in fall – in temperate climate, it isn’t a good time to germinate In moist, tropical regions dormancy is less common

Additional Requirements for Germination Necessary to break dormancy Drying – often dispersed by fruit eating animals, excreted and dry our Cold – prolonged sub freezing temp. – ensures that seeds released in temperate weather do not germinate Seed coat disruption – weathered or partially digested before germination can occur Desert plants have seeds that are water soluble

Cotyledons Nourish the Developing Plant

Germination Embryo absorbs water, seed coat bursts Root emerges first and grows, absorbing water and minerals Shoot cells elongate and push upward Monocots - energy comes from endosperm, digested by cotyledons and transferred to embryo Dicots – cotyledons have already absorbed endosperm so they transfer energy to embryo

Seeds are buried in soil and must be protected Germination, part 2 Seeds are buried in soil and must be protected Root tip protected by root cap Monocot – coleoptile encloses shoot tip to protect Dicot – shoot forms a hook, as grows clears a path for downward pointing apical meristem Cotyledons are carried out of the soil, become green and photosynthetic, transfer stored and new food to shoot True leaves take over photosynthesis, cotelydons die back

Seed Germination true leaves coleoptile root (a) Corn (monocot) true cotyledon epicotyl seed coat hypocotyl hook cotyledons withered cotyledons hypocotyl root (b) Bean (dicot)

Plants and their Pollinators Coevolution – each as acted as an agent of natural selection on the other Some flowers provide food Beetles, moths, butterflies, hummingbirds Animals distribute pollen Flower colors have coevolved to match the color vision of the animal Bees see UV light so flowers are white, blue, yellow, orange Marking s that point to the center of the flower Structural adaptations - nectar containing tubes, stamens, smell, etc.

UV Patterns Guide Bees to Nectar far- red near UV red orange yellow green blue violet human bee 700 600 500 400 wavelength (nm) (a ) A comparison of color vision in humans and bees human vision bee vision (b ) Flower color patterns seen by humans and bees

“Pollinating” a Pollinator

Vertebrate Pollinators Hummingbirds need a lot of energy so the flowers they pollinate produce more nectar than flowers that are pollinated by insects.

Mating Decoys Particularly orchids Mimic female wasps, bees or flies in smell and shape. Males attempt to copulate but only pick up pollen packet which transfers to the next flower

Nurseries for Pollinators Some insects pollinate the flower, then lay their eggs in the flower’s ovary Milkweed and milkweed bugs Yucca and yucca moth Visit – collect – visit and drill hole, lay eggs – pollinate stigma with pollen Neither can reproduce without the other

Fruit helps disperse seeds Disperse seeds far away so there is no competition Adult plants can withstand more damage than seedlings Species will be more successful if they disperse their seeds a distance Many different types of dispersal Seed dispersion methods

Water-Dispersed Fruit

Wind-Dispersed Fruits

Clingy Fruits

Colored fruit attracts animals Blackberries, raspberries, strawberries, tomatoes, peppers - small seeds that animals swallow Eventually excreted unharmed Some seed coats must be scraped or weakened by an animal’s digestive tract before germination Transported away from its parent plant and ends up with is own fertilizer! Seed dispersal video