Reproduction of Flowering Plants

Chapter 27 Reproduction of Flowering Plants Key Concepts 27.1 Most Angiosperms Reproduce Sexually 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State 27.3 Angiosperms Can Reproduce Asexually

Chapter 27 Opening Question How did an understanding of angiosperm reproduction allow floriculturists to develop a commercially successful poinsettia?

Concept 27.1 Most Angiosperms Reproduce Sexually Most angiosperms reproduce sexually this strategy results in the genetic diversity that is the raw material for evolution.

Concept 27.1 Most Angiosperms Reproduce Sexually Differences between sexual reproduction in angiosperms and in vertebrate animals: Meiosis in plants produces spores, after which mitosis produces gametes. Most plants have alternation of generations. In plants, cells that will form gametes are determined in the adult organism.

Concept 27.1 Most Angiosperms Reproduce Sexually Male and female gametophytes are contained in flowers. A complete flower consists of four concentric groups of organs arising from modified leaves:

Concept 27.1 Most Angiosperms Reproduce Sexually Carpels female sex organs that contain the developing female gametophytes Stamens male sex organs that contain the developing male gametophytes Perfect flowers have both carpels and stamens Imperfect flowers have only male or only female organs

Figure 27.1 Perfect and Imperfect Flowers (Part 1)

Concept 27.1 Most Angiosperms Reproduce Sexually Imperfect flowers: Monoecious male and female flowers on the same plant Dioecious individual plants have only male or only female flowers

Concept 27.1 Most Angiosperms Reproduce Sexually Angiosperm gametophytes are microscopic. Female (megagametophyte), or embryo sac arises from a megaspore. Consists of 7 cells: 1 egg cell 2 synergids (attract pollen tube and receive sperm) 3 antipodal cells degenerate 1 central cell with 2 polar nuclei

Figure 27.2 Sexual Reproduction in Angiosperms

Concept 27.1 Most Angiosperms Reproduce Sexually Male (microgametophytes), or pollen grains, arise from microspores. Consist of 2 cells: Generative cell divides by mitosis to form two sperm cells that participate in fertilization. Tube cell forms pollen tube that delivers the sperm to embryo sac.

Concept 27.1 Most Angiosperms Reproduce Sexually Transfer of pollen from plant to plant: Wind-pollinated flowers have sticky or featherlike stigmas; produce a great number of pollen grains Animal pollination increases the probability that pollen will get to a female gametophyte of the same species.

Concept 27.1 Most Angiosperms Reproduce Sexually Some plants self-pollinate (e.g., Mendel s garden peas) Selfing leads to homozygosity, which can reduce reproductive fitness of offspring (inbreeding depression). Most species have evolved mechanisms to prevent self-pollination.

Concept 27.1 Most Angiosperms Reproduce Sexually Dioecious species: selfing is not possible. Monoecious species: physical separation of male and female flowers, or maturation at different times, prevent selfing. Some species are self-incompatible: pollen from the same plant is rejected. Controlled by a cluster of linked genes called the S locus.

Figure 27.3 Self-incompatibility

Concept 27.1 Most Angiosperms Reproduce Sexually When pollen lands on an appropriate stigma, germination begins with uptake of water. The pollen tube grows through the style to reach the ovule. Pollen tube growth may be guided by a speciesspecific chemical signal produced by the synergids.

Concept 27.1 Most Angiosperms Reproduce Sexually The generative cell divides once to form two haploid sperm cells. Double fertilization: One sperm cell fuses with the egg cell, to form the diploid zygote. The other sperm cell fuses with the two polar nuclei to form a triploid nucleus. This nucleus divides by mitosis to form the endosperm, which contains food for the developing embryo.

Figure 27.4 Double Fertilization

Concept 27.1 Most Angiosperms Reproduce Sexually Fertilization initiates growth and development of the embryo, endosperm, integuments, and carpel. Integuments (tissue layers surrounding megasporangium) develop into the seed coat. Carpel becomes the wall of the fruit that encloses the seed.

Concept 27.1 Most Angiosperms Reproduce Sexually The ovary and the seeds it contains develop into a fruit after fertilization. Functions of fruits: Protect seed from damage by animals and infection by microbial pathogens. Aid in seed dispersal. Fruits may contain other flower parts as well.

Figure 27.5 Angiosperm Fruits (Part 1)

Concept 27.1 Most Angiosperms Reproduce Sexually Diversity of fruit forms reflect dispersal strategies. Some fruits are carried by wind:

Concept 27.1 Most Angiosperms Reproduce Sexually Some fruits attach themselves to animals:

Concept 27.1 Most Angiosperms Reproduce Sexually Some fruits disperse by water: coconuts can float for thousands of miles. Some seeds are swallowed when animals eat the fruits, such as berries. The seeds travel through the animal s digestive tract and are deposited some distance from the parent plant.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Flowering represents a reallocation of energy from vegetative growth to reproductive growth. Flowering may be triggered by environmental cues or as part of a predetermined developmental program.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Annuals complete their lives within a year (many crop plants). Biennials take two years; vegetative growth only in 1 st year, reproductive growth in 2 nd year. Perennials live three or more years many wildflowers, trees, shrubs and typically flower every year.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Shoot apical meristems continually produce leaves, axillary buds, and stem (indeterminate growth). A shoot apical meristem becomes an inflorescence meristem when it produces floral parts. A meristem that produces a single flower is a floral meristem; results in determinate growth (growth of limited extent).

Figure 27.6 The Transition to Flowering (Part 1)

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Genes that determine the transition to floral meristems have been studied in Arabidopsis. Meristem identity genes LEAFY and APETALA1 initiate a cascade of gene expression. Floral organ identity genes: homeotic genes; products are transcription factors that determine whether cells in the floral meristem will be sepals, petals, stamens, or carpels.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State External cues that initiate gene expression for flowering: 1. Photoperiod (day length) Some species flower only when days reach a specific length. Short-day plants (SDPs) flower only when the day is shorter than a critical maximum. Long-day plants (LDPs) flower only when the day is longer than a critical minimum.

Figure 27.7 Photoperiod and Flowering

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Photoperiodic control of flowering synchronizes flowering of plants of the same species in a local population. This promotes cross-pollination and successful reproduction. Floriculturists can vary light exposures in greenhouses to produce flowers at any time of year.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Length of night is actually the critical factor that induces flowering. Length of dark period is critical, even if amount of daylight varies between dark periods. The inductive dark period can be interrupted by red light, but the effect is reversed by far-red light, indicating that phytochrome is the photoreceptor.

Figure 27.8 Night Length and Flowering

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Plants sense night length by measuring the ratio of P fr to P r. Day more red light than far-red; by end of day most phytochrome is P fr. At night P fr is gradually converted back to P r. The longer the night, the more P r there is at dawn. A SDP flowers when ratio of P fr to P r is low at the end of the night; a LDP flowers when this ratio is high.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Phytochrome is located in the leaf. The signal for flowering must be a diffusible chemical that travels from the leaf to the shoot apical meristem. The diffusible chemical is the protein florigen.

Figure 27.9 The Flowering Signal Moves from Leaf to Bud (Part 1)

Figure 27.9 The Flowering Signal Moves from Leaf to Bud (Part 2)

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Florigen (FT) is made in phloem companion cells and travels in the sieve tube elements. It goes to the shoot apical meristem and combines with another protein to stimulate transcription of genes that initiate flowering.

Figure 27.10 Molecular Biology of Flowering (Part 1)

Figure 27.10 Molecular Biology of Flowering (Part 2)

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State The genes involved in flowering: FT (FLOWERING LOCUS T) codes for florigen. CO (CONSTANS) codes for a transcription factor that activates synthesis of FT; expressed in phloem companion cells. FD (FLOWERING LOCUS D) codes for a transcription factor that binds to FT in the shoot apical meristem. The complex activates promoters for meristem identity genes, such as APETALA1.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State External cues that initiate gene expression for flowering: 2. Temperature Some plants flower after a period of cold temperatures (vernalization). Cold temperatures inhibit synthesis of FLC protein, a transcription factor that inhibits expression of FT and FD.

Figure 27.11 Vernalization

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Gibberellins are also involved in flowering. Application of gibberellins to Arabidopsis buds results in activation of the meristem identity gene LEAFY, which in turn promotes the transition to flowering.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Some plants do not need environmental cues for flowering. Example: In some tobacco strains, the terminal bud flowers when the stem has grown 4 phytomers long. The position of the bud determines transition to flowering.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Position may be determined by a concentration gradient of some substance along the apical basal axis of the plant. Example: a diffusible inhibitor of flowering, produced in the roots, whose concentration diminishes with plant height. Evidence suggests that the inhibitor decreases the amount of FLC, allowing the FT FD pathway to proceed.

Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State A positional gradient that acts on FLC is similar to other mechanisms that converge on LEAFY and APETALA1:

Concept 27.3 Angiosperms Can Reproduce Asexually Asexual reproduction (vegetative reproduction) results in offspring that are genetically identical to the parent (clones). Disadvantage: doesn t generate genetic diversity among offspring

Concept 27.3 Angiosperms Can Reproduce Asexually Advantages: Parent can pass on allele combinations that function well, which might otherwise be separated by sexual recombination. Avoids cost of producing flowers. Avoids potentially unreliable processes of crosspollination and seed germination.

Concept 27.3 Angiosperms Can Reproduce Asexually Asexual reproduction often occurs by modification of vegetative organs: Strawberries produce horizontal stems (stolons or runners) from which new plants can grow. Bamboo has underground stems (rhizomes) that also produce new plants. Potato tubers are fleshy underground stems; plants grow from the eyes.

Concept 27.3 Angiosperms Can Reproduce Asexually Garlic bulbs are modified stems and can produce new plants. Kalanchoe produces new plants at the edges of its leaves.

Figure 27.12 Vegetative Reproduction (Part 1)

Concept 27.3 Angiosperms Can Reproduce Asexually Plants that reproduce vegetatively often live in unstable environments (e.g., eroding hillsides) places where germination is unreliable. Beach grasses and other plants with stolons or rhizomes are common on sand dunes and help stabilize the shifting sands.

Concept 27.3 Angiosperms Can Reproduce Asexually Apomixis asexual production of seeds (dandelions, blackberries, some citrus, etc.) Two mechanisms: Megasporocyte does not undergo meiosis, resulting in a diploid egg cell that becomes an embryo and seed. Diploid cells from the integument form a diploid embryo sac, which becomes an embryo and seed. Apomixis results in clones.

Concept 27.3 Angiosperms Can Reproduce Asexually Some crops such as corn are grown as hybrids because the progeny are superior to either parent (hybrid vigor). The hybrids are sterile, and populations of the parent strains must be maintained and crossed every year. An intensive search is on for apomixis genes that could be introduced into crops and allow them to be propagated indefinitely.

Figure 27.13 The Advantage of Asexual Reproduction by Apomixis

Concept 27.3 Angiosperms Can Reproduce Asexually Making stem cuttings and allowing them to root in soil or water is a very old method of reproducing plants vegetatively. Rooting can be encouraged by treating the cuttings with auxin.

Concept 27.3 Angiosperms Can Reproduce Asexually Grafting attaching a bud or piece of stem from one plant to a root-bearing stem of another plant. Used for woody plants. The stock is the root-bearing part; the part grafted on is the scion. The vascular cambia grow together to form a continuous cambium. Most fruit trees and wine grapes are grafted.

Figure 27.14 Grafting

Concept 27.3 Angiosperms Can Reproduce Asexually Meristem culture pieces of shoot apical meristem are cultured on growth media. The plantlets are then planted in the field. Used for strawberries and potatoes to produce virus-free plants. Used in forestry to produce uniform seedlings.

Answer to Opening Question Poinsettias are short-day plants. They are grown in greenhouses where photoperiod is carefully regulated. Wild relatives grow very tall; a shorter, branching variety was propagated asexually by grafting. Short, compact growth was found to be caused by a bacterium, which probably acts by changing cytokinin levels in the plants.

Figure 27.15 A Wild Relative of Poinsettia