Plant Reproduction and Biotechnology Chapter 38 Plant Reproduction and Biotechnology

Pollination – The Key to Angiosperm Success Pollination enables gametes to come together within a flower In angiosperms, the dominant sporophyte Produces spores that develop within flowers into male gametophytes (pollen grains) Produces female gametophytes (embryo sacs)

An Overview of Angiosperm Reproduction Figure 38.2a, b Anther at tip of stamen Filament Anther Stamen Pollen tube Germinated pollen grain (n) (male gametophyte) on stigma of carpel Ovary (base of carpel) Ovule Embryo sac (n) (female gametophyte) FERTILIZATION Egg (n) Sperm (n) Petal Receptacle Sepal Style Ovary Key Haploid (n) Diploid (2n) (a) An idealized flower. (b) Simplified angiosperm life cycle. See Figure 30.10 for a more detailed version of the life cycle, including meiosis. Mature sporophyte plant (2n) with flowers Seed (develops from ovule) Zygote (2n) Embryo (2n) (sporophyte) Simple fruit (develops from ovary) Germinating seed Carpel Stigma Fertilization in a flower begins with pollination when one pollen grain containing three haploid nuclei, one tube nucleus, and two sperm nuclei lands on the sticky stigma of the flower. The pollen grain absorbs moisture and sprouts, producing a pollen tube that burrows down the style into the ovary. The two sperm nuclei travel down the pollen tube into the ovary. Once inside the ovary, the two remaining sperm nuclei enter the ovule through the micropyle. One sperm nucleus fertilizes the egg and becomes the embryo (2n). The other sperm nucleus fertilizes the two polar bodies and becomes the triploid (3n) endosperm (food for growing embryo). THIS PROCESS IS KNOWN AS DOUBLE FERTILIZATION because two fertilizations occur. After fertilization, the ovule becomes the seed and the ripened ovary becomes the fruit.

Flower Structure Flowers are the reproductive shoots of the angiosperm sporophyte – they attract pollinators! The are composed of four floral organs: sepals, petals, stamens, and carpels that are attached to the stem at the receptacle. Stamens and carpels are the male and female reproductive organs Sepals and petals are nonreproductive organs

Gametophyte Development and Pollination In angiosperms Pollination is the transfer of pollen from an anther to a stigma If pollination is successful, a pollen grain produces a structure called a pollen tube, which grows down into the ovary and discharges sperm near the embryo sac

Pollen Pollen develops from microspores within the sporangia of anthers 3 A pollen grain becomes a mature male gametophyte when its generative nucleus divides and forms two sperm. This usually occurs after a pollen grain lands on the stigma of a carpel and the pollen tube begins to grow. (See Figure 38.2b.) Development of a male gametophyte (pollen grain) (a) 2 Each microsporo- cyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain. Pollen sac (microsporangium) Micro- sporocyte spores (4) Each of 4 microspores Generative cell (will form 2 sperm) Male Gametophyte Nucleus of tube cell Each one of the microsporangia contains diploid microsporocytes (microspore mother cells). 1 75 m 20 m Ragweed pollen grain Figure 38.4a MEIOSIS MITOSIS KEY to labels Haploid (2n) Diploid (2n)

Embryo sacs develop from megaspores within ovules Key to labels MITOSIS MEIOSIS Ovule Integuments Embryo sac Mega- sporangium sporocyte Micropyle Surviving megaspore Antipodel Cells (3) Polar Nuclei (2) Egg (1) Synergids (2) Development of a female gametophyte (embryo sac) (b) Within the ovule’s megasporangium is a large diploid cell called the megasporocyte (megaspore mother cell). 1 Three mitotic divisions of the megaspore form the embryo sac, a multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective tissue). 3 Female gametophyte Diploid (2n) Haploid (2n) Figure 38.4b 100 m The megasporocyte divides by meiosis and gives rise to four haploid cells, but in most species only one of these survives as the megaspore. 2

Double Fertilization After landing on a receptive stigma A pollen grain germinates and produces a pollen tube that extends down between the cells of the style toward the ovary The pollen tube then discharges two sperm into the embryo sac (double fertilization) One sperm fertilizes the egg The other sperm combines with the polar nuclei, giving rise to the food-storing endosperm After fertilization, ovules develop into seeds and ovaries into fruits

Growth of the Pollen Tube and Double Fertilization Stigma Polar nuclei Egg Pollen grain Pollen tube 2 sperm Style Ovary Ovule (containing female gametophyte, or embryo sac) Micropyle Ovule Polar nuclei Two sperm about to be discharged Endosperm nucleus (3n) (2 polar nuclei plus sperm) Zygote (2n) (egg plus sperm) Figure 38.6 If a pollen grain germinates, a pollen tube grows down the style toward the ovary. 1 The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule. 2 One sperm fertilizes the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sac’s large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm. 3

After double fertilization From Ovule to Seed After double fertilization Each ovule develops into a seed The ovary develops into a fruit enclosing the seed(s) As the embryo develops from the zygote, the seed stockpiles proteins, oils, and starch (which is why seeds are such major nutritional sinks).

Mechanisms That Prevent Self-Fertilization Many angiosperms have mechanisms that make it difficult or impossible for a flower to fertilize itself Stigma Stigma Anther with pollen Pin flower Thrum flower Figure 38.5

Self Incompatibility Prevents Self-Fertilization Dioecious - in plant biology, having the male and female reproductive parts on different individuals of the same species, for example: staminate flowers (lacking carpels) and carpellate (lacking stamens) The most common anti-selfing mechanism in flowering plants is known as self-incompatibility, the ability of a plant to reject its own pollen If a pollen grain from an anther happens to land on a stigma of a flower on the same plant, a biochemical block prevents the pollen from completing its development and fertilizing the egg. Researchers are unraveling the molecular mechanisms that are involved in self-incompatibility The plant’s response in analogous to our immune response – except is rejects self and accepts non-self

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

Structure of the Mature Seed The embryo and its food supply are enclosed by a hard, protective seed coat Endosperm- In angiosperms, a nutrient-rich tissue formed by the union of a sperm with two polar nuclei during double fertilization. The endosperm provides nourishment to the developing embryo in angiosperm seeds Seed coat- A tough outer covering of a seed, formed from the outer coat of an ovule. In a flowering plant, the seed coat encloses and protects the embryo and endosperm Hypocotyl- In an angiosperm embryo, the embryonic axis below the point of attachment of the cotyledon and above the radicle. Radicle- embryonic root of a plant Epicotyl- In an angiosperm embryo, the embryonic axis above the point of attachment of the cotyledons and below the first pair of miniature leaves

Cotyledons A cotyledon "seed leaf“ is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants (angiosperms). Species with one cotyledon are called "monocots“. Plants with two embryonic leaves are termed "dicots“. In the case of dicot seedlings whose cotyledons are photosynthetic, the cotyledons are functionally similar to leaves. However, true leaves and cotyledons are developmentally distinct. Cotyledons are formed during embryogenesis, along with the root and shoot meristems, and are therefore present in the seed prior to germination. True leaves, however, are formed post-embryonically (i.e. after germination) from the shoot apical meristem, which is responsible for generating subsequent aerial portions of the plant.

Dicots In a common garden bean (dicot) the embryo consists of the hypocotyl, radicle, and thick cotyledons Figure 38.8a (a) Common garden bean, a eudicot with thick cotyledons. The fleshy cotyledons store food absorbed from the endosperm before the seed germinates. Seed coat Radicle Epicotyl Hypocotyl Cotyledons

Monocots The embryo of a monocot has a single cotyledon, a coleoptile, and a coleorhiza Figure 38.8c (c) Maize, a monocot. Like all monocots, maize has only one cotyledon. Maize and other grasses have a large cotyledon called a scutellum. The rudimentary shoot is sheathed in a structure called the coleoptile, and the coleorhiza covers the young root. Scutellum (cotyledon) Coleoptile Coleorhiza Pericarp fused with seed coat Endosperm Epicotyl Hypocotyl Radicle

Dormancy Seed Germination As a seed matures it dehydrates and enters a phase referred to as dormancy Seed dormancy (adaptation for tough times) 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 cues An example of this is after extreme heats of forest fires or the extreme colds of winter Dormant seeds can be viable and capable of germinating can vary from a few days to more than a few decades

Germination of seeds depends on the physical process called imbibition From Seed to Seedling Germination of seeds depends on the physical process called imbibition The uptake of water due to low water potential of the dry seed

From Seed to Seedling - Germination The radicle is the first organ to emerge from the germinating seed In many dicots a hook forms in the hypocotyl, and growth pushes the hook above ground Figure 38.10a Foliage leaves Cotyledon Hypocotyl Radicle Epicotyl Seed coat Common garden bean. In common garden beans, straightening of a hook in the hypocotyl pulls the cotyledons from the soil. (a)

Germination in Monocots Use a different method for breaking ground when they germinate The coleoptile Pushes upward through the soil and into the air Figure 38.10b Foliage leaves Coleoptile Radicle Maize. In maize and other grasses, the shoot grows straight up through the tube of the coleoptile. (b)

From Ovary to Fruit A fruit develops from the ovary and protects the enclosed seeds. It also aids in the dispersal of seeds by wind or animals. Fruits exist in different types: Simple fruit- A fruit derived from a single carpel or several fused carpels Aggregate fruit- A fruit derived from a single flower that has more than one carpel Multiple fruit- A fruit derived from an inflorescence Accessory fruit- A fruit, or assemblage of fruits, in which the fleshy parts are derived largely or entirely from tissues other than the ovary Fruits usually ripen at about the same time that their seeds finish development

Pineapple inflorescence Types of Fruit Fruits are classified into several types depending on their developmental origin Carpels Flower Ovary Stamen Stigma Stamen Ovule Pea flower Raspberry flower Pineapple inflorescence Carpel (fruitlet) Each segment develops from the carpel of one flower Stigma Seed Ovary Stamen Pea fruit Raspberry fruit Pineapple fruit Simple fruit. A simple fruit develops from a single carpel (or several fused carpels) of one flower (examples: pea, lemon, peanut). (a) Aggregate fruit. An aggregate fruit develops from many separate carpels of one flower (examples: raspberry, blackberry, strawberry). (b) Multiple fruit. A multiple fruit develops from many carpels of many flowers (examples: pineapple, fig). (c) Figure 38.9a–c

Asexual Reproduction in Plants Many flowering plants clone themselves by asexual reproduction Many angiosperm species reproduce both asexually and sexually Sexual reproduction generates the genetic variation that makes evolutionary adaptation possible Asexual reproduction in plants is called vegetative reproduction

Mechanisms of Asexual Reproduction Fragmentation Is the separation of a parent plant into parts that develop into whole plants Is one of the most common modes of asexual reproduction Apomixis Is the ability of some plant species to reproduce asexually without fertilization by a male gamete

Asexual Propagation – Clonal Species In some species the root system of a single parent gives rise to many adventitious shoots that become separate shoot systems This is known as vegetative reproduction – the collection of individuals are clones Figure 38.11

Advantages & Disadvantages With asexual reproduction there is no need for a pollinator Asexual reproduction also allows a plant to pass on its entire genetic legacy unchanged by another parents genetic information In the wild, only a small fraction of seeds (produced through sexual reproduction) can survive long enough to become parent plants themselves But - sexual reproduction provides variation in offspring which can be advantageous in unstable environmental conditions Sexual reproduction can allow for continuation of a species due to the great amount of variation that occurs in the offspring

Vegetative Propagation and Agriculture Humans have devised various methods for asexual propagation of angiosperms Many kinds of plants Are asexually reproduced from plant fragments called cuttings In grafting, a modification of vegetative reproduction from cuttings a twig or bud from one plant can be grafted onto a plant of a closely related species or a different variety of the same species

Compare/Contrast Table Comparing Plant Propagation Methods Method Procedure Cuttings A length of stem that includes lateral buds is cut from the parent plant and partially buried in soil or rooting mixture to take root. A piece of stem is cut from the parent plant and attached to another plant. A piece of lateral bud is cut from the parent plant and attached to another plant. Grafting Budding