Chapter 30 Reproduction and Domestication of Flowering Plants

Overview: Flowers of Deceit Insects help angiosperms to reproduce sexually with distant members of their own species For example, male Campsoscolia wasps mistake Ophrys flowers for females and attempt to mate with them The flower is pollinated in the process Unusually, the flower does not produce nectar and the male receives no benefit © 2011 Pearson Education, Inc. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 38.1 Figure 38.1 Why is this wasp trying to mate with this flower?

Mutualistic symbioses are common between plants and other species Many angiosperms lure insects with nectar; both plant and pollinator benefit Mutualistic symbioses are common between plants and other species Angiosperms can reproduce sexually and asexually Angiosperms are the most important group of plants in terrestrial ecosystems and in agriculture © 2011 Pearson Education, Inc. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Flowers, double fertilization, and fruits are unique features of the angiosperm life cycle Plant lifecycles are characterized by the alternation between a multicellular haploid (n) generation and a multicellular diploid (2n) generation Diploid sporophytes (2n) produce spores (n) by meiosis; these grow into haploid gametophytes (n) Gametophytes produce haploid gametes (n) by mitosis; fertilization of gametes produces a sporophyte © 2011 Pearson Education, Inc.

In angiosperms, the sporophyte is the dominant generation, the large plant that we see The gametophytes are reduced in size and depend on the sporophyte for nutrients The angiosperm life cycle is characterized by “three Fs”: flowers, double fertilization, and fruits For the Discovery Video Plant Pollination, go to Animation and Video Files. © 2011 Pearson Education, Inc.

Germinated pollen grain (n) (male gametophyte) Stamen Stigma Carpel Figure 38.2 Anther Germinated pollen grain (n) (male gametophyte) Stamen Stigma Carpel Anther Style Ovary Filament Ovary Pollen tube Ovule Embryo sac (n) (female gametophyte) Sepal FERTILIZATION Petal Egg (n) Sperm (n) Receptacle Mature sporophyte plant (2n) Zygote (2n) (a) Structure of an idealized flower Key Haploid (n) Seed Germinating seed Figure 38.2 An overview of angiosperm reproduction. Diploid (2n) Seed Embryo (2n) (sporophyte) Simplified angiosperm life cycle (b) Simple fruit

Structure of an idealized flower Figure 38.2a Stamen Stigma Carpel Anther Style Filament Ovary Sepal Petal Figure 38.2 An overview of angiosperm reproduction. Receptacle (a) Structure of an idealized flower

Germinated pollen grain (n) (male gametophyte) Figure 38.2b Anther Germinated pollen grain (n) (male gametophyte) Ovary Pollen tube Ovule Embryo sac (n) (female gametophyte) FERTILIZATION Egg (n) Sperm (n) Zygote (2n) Mature sporophyte plant (2n) Key Figure 38.2 An overview of angiosperm reproduction. Seed Haploid (n) Germinating seed Diploid (2n) Seed Embryo (2n) (sporophyte) (b) Simplified angiosperm life cycle Simple fruit

Flower Structure and Function Flowers are the reproductive shoots of the angiosperm sporophyte; they attach to a part of the stem called the receptacle Flowers consist of four floral organs: sepals, petals, stamens, and carpels Stamens and carpels are reproductive organs; sepals and petals are sterile © 2011 Pearson Education, Inc.

A carpel has a long style with a stigma on which pollen may land A stamen consists of a filament topped by an anther with pollen sacs that produce pollen A carpel has a long style with a stigma on which pollen may land At the base of the style is an ovary containing one or more ovules A single carpel or group of fused carpels is called a pistil © 2011 Pearson Education, Inc.

Complete flowers contain all four floral organs Incomplete flowers lack one or more floral organs, for example stamens or carpels Clusters of flowers are called inflorescences © 2011 Pearson Education, Inc.

Development of Male Gametophytes in Pollen Grains Pollen develops from microspores within the microsporangia, or pollen sacs, of anthers Each microspore undergoes mitosis to produce two cells: the generative cell and the tube cell A pollen grain consists of the two-celled male gametophyte and the spore wall © 2011 Pearson Education, Inc.

If pollination succeeds, a pollen grain produces a pollen tube that grows down into the ovary and discharges two sperm cells near the embryo sac © 2011 Pearson Education, Inc.

Female gametophyte (embryo sac) Figure 38.3 Development of a male gametophyte (in pollen grain) (a) (b) Development of a female gametophyte (embryo sac) Microsporangium (pollen sac) Megasporangium Microsporocyte Ovule Megasporocyte MEIOSIS Integuments Microspores (4) Micropyle Surviving megaspore Each of 4 microspores MITOSIS Ovule Antipodal cells (3) Male gametophyte (in pollen grain) Generative cell (will form 2 sperm) Figure 38.3 The development of male and female gametophytes in angiosperms. Polar nuclei (2) Female gametophyte (embryo sac) Egg (1) Nucleus of tube cell Integuments Synergids (2) 20 m Ragweed pollen grain (colorized SEM) Key to labels Embryo sac Haploid (n) 75 m 100 m (LM) Diploid (2n) (LM)

Development of a male gametophyte (in pollen grain) (a) Figure 38.3a Development of a male gametophyte (in pollen grain) (a) Microsporangium (pollen sac) Microsporocyte MEIOSIS Microspores (4) Each of 4 microspores MITOSIS Male gametophyte (in pollen grain) Generative cell (will form 2 sperm) Figure 38.3 The development of male and female gametophytes in angiosperms. Nucleus of tube cell 20 m Key to labels Ragweed pollen grain (colorized SEM) Haploid (n) 75 m (LM) Diploid (2n)

Development of Female Gametophytes (Embryo Sacs) The embryo sac, or female gametophyte, develops within the ovule Within an ovule, two integuments surround a megasporangium One cell in the megasporangium undergoes meiosis, producing four megaspores, only one of which survives The megaspore divides, producing a large cell with eight nuclei © 2011 Pearson Education, Inc.

This cell is partitioned into a multicellular female gametophyte, the embryo sac © 2011 Pearson Education, Inc.

Female gametophyte (embryo sac) Figure 38.3b (b) Development of a female gametophyte (embryo sac) Megasporangium Ovule Megasporocyte MEIOSIS Integuments Micropyle Surviving megaspore MITOSIS Ovule Antipodal cells (3) Figure 38.3 The development of male and female gametophytes in angiosperms. Polar nuclei (2) Female gametophyte (embryo sac) Egg (1) Integuments Synergids (2) Key to labels Embryo sac Haploid (n) 100 m Diploid (2n) (LM)

Pollination In angiosperms, pollination is the transfer of pollen from an anther to a stigma Pollination can be by wind, water, or animals Wind-pollinated species (e.g., grasses and many trees) release large amounts of pollen © 2011 Pearson Education, Inc.

Abiotic Pollination by Wind Pollination by Bees Figure 38.4a Abiotic Pollination by Wind Pollination by Bees Common dandelion under normal light Hazel staminate flowers (stamens only) Figure 38.4 Exploring: Flower Pollination Hazel carpellate flower (carpels only) Common dandelion under ultraviolet light

Pollination by Moths and Butterflies Pollination by Flies Figure 38.4b Pollination by Moths and Butterflies Pollination by Flies Pollination by Bats Anther Moth Fly egg Stigma Blowfly on carrion flower Long-nosed bat feeding on cactus flower at night Moth on yucca flower Pollination by Birds Figure 38.4 Exploring: Flower Pollination Hummingbird drinking nectar of columbine flower

Coevolution of Flower and Pollinator Coevolution is the evolution of interacting species in response to changes in each other Many flowering plants have coevolved with specific pollinators The shapes and sizes of flowers often correspond to the pollen transporting parts of their animal pollinators For example, Darwin correctly predicted a moth with a 28 cm long tongue based on the morphology of a particular flower © 2011 Pearson Education, Inc.

Figure 38.5 Figure 38.5 Coevolution of a flower and an insect pollinator.

Double Fertilization After landing on a receptive stigma, a pollen grain produces a pollen tube that extends between the cells of the style toward the ovary Double fertilization results from the discharge of two sperm from the pollen tube into the embryo sac One sperm fertilizes the egg, and the other combines with the polar nuclei, giving rise to the triploid food-storing endosperm (3n) © 2011 Pearson Education, Inc.

1 Pollen grain Stigma Pollen tube 2 sperm Style Ovary Ovule Figure 38.6-1 1 Pollen grain Stigma Pollen tube 2 sperm Style Ovary Ovule Polar nuclei Figure 38.6 Growth of the pollen tube and double fertilization. Egg Micropyle

1 2 Pollen grain Stigma Pollen tube Ovule Polar nuclei 2 sperm Style Figure 38.6-2 1 2 Pollen grain Stigma Pollen tube Ovule Polar nuclei 2 sperm Style Egg Ovary Ovule Synergid Polar nuclei Figure 38.6 Growth of the pollen tube and double fertilization. 2 sperm Egg Micropyle

Endosperm nucleus (3n) (2 polar nuclei plus sperm) Figure 38.6-3 1 2 3 Pollen grain Stigma Endosperm nucleus (3n) (2 polar nuclei plus sperm) Pollen tube Ovule Polar nuclei 2 sperm Style Egg Ovary Zygote (2n) Ovule Synergid Polar nuclei Figure 38.6 Growth of the pollen tube and double fertilization. 2 sperm Egg Micropyle

Seed Development, Form, and Function After double fertilization, each ovule develops into a seed The ovary develops into a fruit enclosing the seed(s) © 2011 Pearson Education, Inc.

Endosperm Development Endosperm development usually precedes embryo development In most monocots and some eudicots, endosperm stores nutrients that can be used by the seedling In other eudicots, the food reserves of the endosperm are exported to the cotyledons © 2011 Pearson Education, Inc.

Embryo Development The first mitotic division of the zygote splits the fertilized egg into a basal cell and a terminal cell The basal cell produces a multicellular suspensor, which anchors the embryo to the parent plant The terminal cell gives rise to most of the embryo The cotyledons form and the embryo elongates © 2011 Pearson Education, Inc.

Ovule Proembryo Endosperm nucleus Suspensor Integuments Cotyledons Figure 38.7 Ovule Proembryo Endosperm nucleus Suspensor Integuments Cotyledons Zygote Basal cell Shoot apex Root apex Seed coat Suspensor Terminal cell Figure 38.7 The development of a eudicot plant embryo. Basal cell Endosperm Zygote

Ovule Endosperm nucleus Integuments Zygote Terminal cell Basal cell Figure 38.7a Ovule Endosperm nucleus Integuments Zygote Figure 38.7 The development of a eudicot plant embryo. Terminal cell Basal cell Zygote

Proembryo Suspensor Cotyledons Basal cell Shoot apex Root apex Figure 38.7b Proembryo Suspensor Cotyledons Basal cell Shoot apex Root apex Seed coat Suspensor Figure 38.7 The development of a eudicot plant embryo. Endosperm

Structure of the Mature Seed The embryo and its food supply are enclosed by a hard, protective seed coat The seed enters a state of dormancy A mature seed is only about 5–15% water © 2011 Pearson Education, Inc.

In some eudicots, such as the common garden bean, the embryo consists of the embryonic axis attached to two thick cotyledons (seed leaves) Below the cotyledons the embryonic axis is called the hypocotyl and terminates in the radicle (embryonic root); above the cotyledons it is called the epicotyl The plumule comprises the epicotyl, young leaves, and shoot apical meristem © 2011 Pearson Education, Inc.

(a) Common garden bean, a eudicot with thick cotyledons Figure 38.8 Seed coat Epicotyl Hypocotyl Radicle Cotyledons (a) Common garden bean, a eudicot with thick cotyledons Seed coat Endosperm Cotyledons Epicotyl Hypocotyl Radicle (b) Castor bean, a eudicot with thin cotyledons Figure 38.8 Seed structure. Scutellum (cotyledon) Pericarp fused with seed coat Endosperm Coleoptile Epicotyl Hypocotyl Coleorhiza Radicle (c) Maize, a monocot

(a) Common garden bean, a eudicot with thick cotyledons Figure 38.8a Seed coat Epicotyl Hypocotyl Radicle Cotyledons Figure 38.8 Seed structure. (a) Common garden bean, a eudicot with thick cotyledons

The seeds of some eudicots, such as castor beans, have thin cotyledons © 2011 Pearson Education, Inc.

(b) Castor bean, a eudicot with thin cotyledons Figure 38.8b Seed coat Endosperm Cotyledons Epicotyl Hypocotyl Radicle Figure 38.8 Seed structure. (b) Castor bean, a eudicot with thin cotyledons

A monocot embryo has one cotyledon Grasses, such as maize and wheat, have a special cotyledon called a scutellum Two sheathes enclose the embryo of a grass seed: a coleoptile covering the young shoot and a coleorhiza covering the young root © 2011 Pearson Education, Inc.

Scutellum (cotyledon) Pericarp fused with seed coat Figure 38.8c Scutellum (cotyledon) Pericarp fused with seed coat Endosperm Coleoptile Epicotyl Hypocotyl Coleorhiza Radicle Figure 38.8 Seed structure. (c) Maize, a monocot

Seed Dormancy: An Adaptation for Tough Times Seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling The breaking of seed dormancy often requires environmental cues, such as temperature or lighting changes © 2011 Pearson Education, Inc.

Seed Germination and Seedling Development Germination depends on imbibition, the uptake of water due to low water potential of the dry seed The radicle (embryonic root) emerges first Next, the shoot tip breaks through the soil surface © 2011 Pearson Education, Inc.

In many eudicots, a hook forms in the hypocotyl, and growth pushes the hook above ground Light causes the hook to straighten and pull the cotyledons and shoot tip up © 2011 Pearson Education, Inc.

Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Figure 38.9 Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Hypocotyl Hypocotyl Radicle Seed coat (a) Common garden bean Foliage leaves Figure 38.9 Two common types of seed germination. Coleoptile Coleoptile Radicle (b) Maize

Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Figure 38.9a Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Hypocotyl Hypocotyl Figure 38.9 Two common types of seed germination. Radicle Seed coat (a) Common garden bean

In maize and other grasses, which are monocots, the coleoptile pushes up through the soil © 2011 Pearson Education, Inc.

Foliage leaves Coleoptile Coleoptile Radicle (b) Maize Figure 38.9b Figure 38.9 Two common types of seed germination. Radicle (b) Maize

Fruit Form and Function A fruit develops from the ovary It protects the enclosed seeds and aids in seed dispersal by wind or animals A fruit may be classified as dry, if the ovary dries out at maturity, or fleshy, if the ovary becomes thick, soft, and sweet at maturity © 2011 Pearson Education, Inc.

Fruits are also classified by their development Simple, a single or several fused carpels Aggregate, a single flower with multiple separate carpels Multiple, a group of flowers called an inflorescence © 2011 Pearson Education, Inc.

Pineapple inflorescence Figure 38.10 Stigma Style Carpels Stamen Flower Petal Ovary Stamen Stamen Sepal Stigma Ovary (in receptacle) Ovule Ovule Pea flower Raspberry flower Pineapple inflorescence Apple flower Each segment develops from the carpel of one flower Remains of stamens and styles Carpel (fruitlet) Stigma Sepals Seed Ovary Figure 38.10 Developmental origin of fruits. Stamen Seed Receptacle Pea fruit Raspberry fruit Pineapple fruit Apple fruit (a) Simple fruit (b) Aggregate fruit (c) Multiple fruit (d) Accessory fruit

Carpels Stamen Ovary Stamen Stigma Ovule Pea flower Raspberry flower Figure 38.10a Carpels Stamen Ovary Stamen Stigma Ovule Pea flower Raspberry flower Carpel (fruitlet) Stigma Seed Ovary Figure 38.10 Developmental origin of fruits. Stamen Pea fruit Raspberry fruit (a) Simple fruit (b) Aggregate fruit

Pineapple inflorescence Figure 38.10b Stigma Style Flower Petal Stamen Sepal Ovary (in receptacle) Ovule Pineapple inflorescence Apple flower Remains of stamens and styles Each segment develops from the carpel of one flower Sepals Figure 38.10 Developmental origin of fruits. Seed Receptacle Pineapple fruit Apple fruit (c) Multiple fruit (d) Accessory fruit

An accessory fruit contains other floral parts in addition to ovaries © 2011 Pearson Education, Inc.

Fruit dispersal mechanisms include Water Wind Animals © 2011 Pearson Education, Inc.

Dispersal by Wind Dispersal by Water Dandelion fruit Tumbleweed Figure 38.11a Dispersal by Wind Dandelion fruit Tumbleweed Dandelion “seeds” (actually one-seeded fruits) Winged seed of the tropical Asian climbing gourd Alsomitra macrocarpa Winged fruit of a maple Dispersal by Water Figure 38.11 Exploring: Fruit and Seed Dispersal Coconut seed embryo, endosperm, and endocarp inside buoyant husk

Dispersal by Animals Fruit of puncture vine (Tribulus terrestris) Figure 38.11b Dispersal by Animals Fruit of puncture vine (Tribulus terrestris) Squirrel hoarding seeds or fruits underground Ant carrying seed with nutritious “food body” to its nest Figure 38.11 Exploring: Fruit and Seed Dispersal Seeds dispersed in black bear feces

Flowering plants reproduce sexually, asexually, or both Many angiosperm species reproduce both asexually and sexually Sexual reproduction results in offspring that are genetically different from their parents Asexual reproduction results in a clone of genetically identical organisms © 2011 Pearson Education, Inc.

Mechanisms of Asexual Reproduction Fragmentation, separation of a parent plant into parts that develop into whole plants, is a very common type of asexual reproduction In some species, a parent plant’s root system gives rise to adventitious shoots that become separate shoot systems © 2011 Pearson Education, Inc.

Figure 38.12 Figure 38.12 Asexual reproduction in aspen trees.

Apomixis is the asexual production of seeds from a diploid cell © 2011 Pearson Education, Inc.

Advantages and Disadvantages of Asexual Versus Sexual Reproduction Asexual reproduction is also called vegetative reproduction Asexual reproduction can be beneficial to a successful plant in a stable environment However, a clone of plants is vulnerable to local extinction if there is an environmental change © 2011 Pearson Education, Inc.

However, only a fraction of seedlings survive Sexual reproduction generates genetic variation that makes evolutionary adaptation possible However, only a fraction of seedlings survive Some flowers can self-fertilize to ensure that every ovule will develop into a seed Many species have evolved mechanisms to prevent selfing © 2011 Pearson Education, Inc.

Mechanisms That Prevent Self-Fertilization Many angiosperms have mechanisms that make it difficult or impossible for a flower to self-fertilize Dioecious species have staminate and carpellate flowers on separate plants © 2011 Pearson Education, Inc.

Staminate flowers (left) and carpellate flowers (right) Figure 38.13 Staminate flowers (left) and carpellate flowers (right) of a dioecious species (a) Figure 38.13 Some floral adaptations that prevent self-fertilization. Stamens Styles Styles Stamens Thrum flower Pin flower (b) Thrum and pin flowers

Others have stamens and carpels that mature at different times or are arranged to prevent selfing © 2011 Pearson Education, Inc.

The most common is self-incompatibility, a plant’s ability to reject its own pollen Researchers are unraveling the molecular mechanisms involved in self-incompatibility Some plants reject pollen that has an S-gene matching an allele in the stigma cells Recognition of self pollen triggers a signal transduction pathway leading to a block in growth of a pollen tube © 2011 Pearson Education, Inc.

Vegetative Propagation and Agriculture Humans have devised methods for asexual propagation of angiosperms Most methods are based on the ability of plants to form adventitious roots or shoots © 2011 Pearson Education, Inc.

Clones from Cuttings Many kinds of plants are asexually reproduced from plant fragments called cuttings A callus is a mass of dividing undifferentiated cells that forms where a stem is cut and produces adventitious roots © 2011 Pearson Education, Inc. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Grafting A twig or bud can be grafted onto a plant of a closely related species or variety The stock provides the root system The scion is grafted onto the stock © 2011 Pearson Education, Inc.

Test-Tube Cloning and Related Techniques Plant biologists have adopted in vitro methods to create and clone novel plant varieties A callus of undifferentiated cells can sprout shoots and roots in response to plant hormones © 2011 Pearson Education, Inc.

(a) (b) (c) Developing root Figure 38.14 Figure 38.14 Cloning a garlic plant. (a) (b) (c) Developing root

Transgenic plants are genetically modified (GM) to express a gene from another organism Protoplast fusion is used to create hybrid plants by fusing protoplasts, plant cells with their cell walls removed © 2011 Pearson Education, Inc.

Figure 38.15 Figure 38.15 Protoplasts. 50 m

Humans modify crops by breeding and genetic engineering Humans have intervened in the reproduction and genetic makeup of plants for thousands of years Hybridization is common in nature and has been used by breeders to introduce new genes Maize, a product of artificial selection, is a staple in many developing countries © 2011 Pearson Education, Inc.

Figure 38.16 Figure 38.16 Maize: a product of artificial selection.

Plant Breeding Mutations can arise spontaneously or can be induced by breeders Plants with beneficial mutations are used in breeding experiments Desirable traits can be introduced from different species or genera The grain triticale is derived from a successful cross between wheat and rye © 2011 Pearson Education, Inc.

Plant Biotechnology and Genetic Engineering Plant biotechnology has two meanings In a general sense, it refers to innovations in the use of plants to make useful products In a specific sense, it refers to use of GM organisms in agriculture and industry Modern plant biotechnology is not limited to transfer of genes between closely related species or varieties of the same species © 2011 Pearson Education, Inc.

Reducing World Hunger and Malnutrition Genetically modified plants may increase the quality and quantity of food worldwide Transgenic crops have been developed that Produce proteins to defend them against insect pests Tolerate herbicides Resist specific diseases © 2011 Pearson Education, Inc.

Nutritional quality of plants is being improved For example, “Golden Rice” is a transgenic variety being developed to address vitamin A deficiencies among the world’s poor © 2011 Pearson Education, Inc.

Cassava roots harvested in Thailand Figure 38.17 Figure 38.17 Impact: Fighting World Hunger with Transgenic Cassava Cassava roots harvested in Thailand

Reducing Fossil Fuel Dependency Biofuels are made by the fermentation and distillation of plant materials such as cellulose Biofuels can be produced by rapidly growing crops such as switchgrass and poplar Biofuels would reduce the net emission of CO2, a greenhouse gas The environmental implications of biofuels are controversial © 2011 Pearson Education, Inc.

The Debate over Plant Biotechnology Some biologists are concerned about risks of releasing GM organisms (GMOs) into the environment © 2011 Pearson Education, Inc.

Issues of Human Health One concern is that genetic engineering may transfer allergens from a gene source to a plant used for food Some GMOs have health benefits For example, maize that produces the Bt toxin has 90% less of a cancer-causing toxin than non-Bt corn Bt maize has less insect damage and lower infection by Fusarium fungus that produces the cancer-causing toxin © 2011 Pearson Education, Inc.

GMO opponents advocate for clear labeling of all GMO foods © 2011 Pearson Education, Inc.

Possible Effects on Nontarget Organisms Many ecologists are concerned that the growing of GM crops might have unforeseen effects on nontarget organisms © 2011 Pearson Education, Inc.

Addressing the Problem of Transgene Escape Perhaps the most serious concern is the possibility of introduced genes escaping into related weeds through crop-to-weed hybridization This could result in “superweeds” that would be resistant to many herbicides © 2011 Pearson Education, Inc.

Efforts are underway to prevent this by introducing Male sterility Apomixis Transgenes into chloroplast DNA (not transferred by pollen) Strict self-pollination © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.