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Chapter 38

BIOL 1030 Chapter 38: Chapter 38 Angiosperm Reproduction and Biotechnology

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University of Manitoba
Biological Sciences
BIOL 1030
Scott Kevin

Chapter 38 Angiosperm Reproduction and Biotechnology Lecture Outline Overview: To Seed or Not to Seed • Sexual reproduction is not the sole means by which flowering plants reproduce. • Many species can also reproduce asexually, creating offspring that are genetically identical to them. • The propagation of flowering plants by sexual and asexual reproduction forms the basis of agriculture. • For 10,000 years, plant breeders have altered the traits of a few hundred angiosperm species by artificial selection, transforming them into today’s crops. Concept 38.1 Pollination enables gametes to come together within a flower Sporophyte and gametophyte generations alternate in the life cycles of plants. • The life cycles of angiosperms and other plants are characterized by an alternation of generations, in which haploid (n) and diploid (2n) generations take turns producing each other. • The diploid plant, the sporophyte, produces haploid spores by meiosis. • These spores divide by mitosis, giving rise to multicellular male and female haploid plants—the gametophytes. • The gametophytes produce gametes—sperm and eggs. • Fertilization results in diploid zygotes, which divide by mitosis to form new sporophytes. • In angiosperms, the sporophyte is the dominant generation, the conspicuous plant we see. • Over the course of seed plant evolution, gametophytes became reduced in size and dependent on their sporophyte parents. • Angiosperm gametophytes are the most reduced of all plants, consisting of only a few cells. • In angiosperms, the sporophyte produces a unique reproductive structure, the flower. • Male and female gametophytes develop within the anthers and ovules, respectively, of a sporophyte flower. • Pollination by wind, water, or animals brings a male gametophyte (pollen grain) to a female gametophyte contained in an ovule embedded in the ovary of a flower. • Union of gametes (fertilization) takes place within the ovary. • Ovules develop into seeds, while the ovary itself develops into the fruit around the seed. Flowers are specialized shoots bearing the reproductive organs of the angiosperm sporophyte. • Flowers, the reproductive shoots of the angiosperm sporophyte, are typically composed of four whorls of highly modified leaves called floral organs, which are separated by very short internodes. • Unlike the indeterminate growth of vegetative shoots, flowers are determinate shoots in that they cease growing once the flower and fruit are formed. • The four kinds of floral organs are the sepals, petals, stamens, and carpels. • Their site of attachment to the stem is the receptacle. • Sepals and petals are sterile. • Sepals, which enclose and protect the floral bud before it opens, are usually green and more leaflike in appearance than the other floral organs. • In many angiosperms, the petals are brightly colored and advertise the flower to insects and other pollinators. • Stamens and carpels are the male and female reproductive organs, respectively. • A stamen consists of a stalk (the filament) and a terminal anther containing chambers called pollen sacs. • The pollen sacs produce pollen. • A carpel has an ovary at the base and a slender neck, the style. • At the top of the style is a sticky structure called the stigma that serves as a landing platform for pollen. • Within the ovary are one or more ovules. • Some flowers have a single carpel. • In others, several carpels are fused into a single structure, producing an ovary with two or more chambers, each containing one or more ovules. • The anthers and the ovules bear sporangia, where spores are produced by meiosis and where gametophytes later develop. • The male gametophytes are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers. • The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries. • Pollination is the transfer of pollen from an anther to a stigma. • It begins the process by which the male and female gametophytes are brought together so their gametes can unite. • Pollination occurs when pollen released from anthers is carried by wind, water, or animals to land on a stigma. • Each pollen grain produces a pollen tube, which grows down into the ovary via the style and discharges sperm into the embryo sac, fertilizing the egg. • The zygote gives rise to an embryo. • The ovule develops into a seed, and the entire ovary develops into a fruit containing one or more seeds. • Fruits carried by wind, water, or animals disperse seeds away from the source plant where the seed germinates. • Numerous floral variations have evolved during the 130 million years of angiosperm history. • Plant biologists distinguish between complete flowers, those having all four organs, and incomplete flowers, those lacking one or more of the four floral parts. • A bisexual flower is equipped with both stamens and carpels. • All complete and many incomplete flowers are bisexual. • A unisexual flower is missing either stamens (therefore, a carpellate flower) or carpels (therefore, a staminate flower). • A monoecious plant has staminate and carpellate flowers at separate locations on the same individual plant. • For example, maize and other corn varieties have ears derived from clusters of carpellate flowers, while the tassels consist of staminate flowers. • A dioecious species has staminate flowers and carpellate flowers on separate plants. • For example, date palms have carpellate individuals that produce dates and staminate individuals that produce pollen. • In addition to these differences based on the presence of floral organs, flowers vary in size, shape, and color. • Much of this diversity represents adaptations of flowers to different animal pollinators. • The presence of animals in the environment has been a key factor in angiosperm evolution. Male and female gametophytes develop within anthers and ovaries, respectively; pollination brings them together. • The male gametophyte begins its development within the sporangia (pollen sacs) of the anther. • Within the sporangia are microsporocytes, each of which will form four haploid microspores through meiosis. • Each microspore can give rise to a haploid male gametophyte. • A microspore divides once by mitosis and produces a generative cell and a tube cell. • The generative cell will eventually form sperm. • During maturation of the male gametophyte, the generative cell passes into the tube cell. • The tube cell, enclosing the generative cell, produces the pollen tube, which delivers sperm to the egg. • This is a pollen grain, an immature male gametophyte. • This two-celled structure is encased in a thick, ornate, distinctive, and resistant wall. • A pollen grain becomes a mature gametophyte when the generative cell divides by mitosis to form two sperm cells. • In most species, this occurs after the pollen grain lands on the stigma of the carpel and the pollen tube begins to form. • The pollen tube grows through the long style of the carpel and into the ovary, where it releases the sperm cells in the vicinity of the embryo sac. • Ovules, each containing a single sporangium, form within the chambers of the ovary. • One cell in the sporangium of each ovule, the megasporocyte, grows and then goes through meiosis, producing four haploid megaspores. • In many angiosperms, only one megaspore survives. • This megaspore divides by mitosis three times without cytokinesis, forming in one cell with eight haploid nuclei. • Membranes partition this mass into a multicellular female gametophyte—the embryo sac. • Three cells sit at one end of the embryo sac: two synergid cells flanking the egg cell. • The synergids function in the attraction and guidance of the pollen tube. • At the other end of the egg sac are three antipodal cells of unknown function. • The other two nuclei, the polar nuclei, share the cytoplasm of the large central cell of the embryo sac. • The ovule now consists of the embryo sac and the surrounding integuments, layers of protective tissue from the sporophyte that will eventually develop into the seed coat. • Pollination, which brings male and female gametophytes together, is the first step in the chain of events that leads to fertilization. • Some plants, such as grasses and many trees, release large quantities of pollen on the wind to compensate for the randomness of this dispersal mechanism. • At certain times of the year, the air is loaded with pollen, as anyone plagued by pollen allergies can attest. • Some aquatic plants rely on water to disperse pollen. • Most angiosperms interact with insects or other animals that transfer pollen directly between flowers. Plants have various mechanisms that prevent self-fertilization. • Some flowers self-fertilize or “self,” but most angiosperms have mechanisms that make this difficult or impossible. • The various barriers that prevent self-fertilization contribute to genetic variety by ensuring that sperm and eggs come from different parents. • Dioecious plants cannot self-fertilize because they are unisexual. • In plants with bisexual flowers, a variety of mechanisms may prevent self-fertilization. • For example, in some species stamens and carpels mature at different times. • Alternatively, they may be arranged in such a way that it is mechanically unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower. • The most common anti-selfing mechanism is self- incompatibility, the ability of a plant to reject its own pollen and that of closely related individuals. • 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 an egg. • The self-incompatibility systems in plant are analogous to the immune response of animals. • Both are based on the ability of organisms to distinguish “self” from “nonself.” • The key difference is that the animal immune system rejects nonself, but self-incompatibility in plants is a rejection of self. • Recognition of “self” pollen is based on genes for self-incompatibility, called S-genes, with dozens of different alleles in a population. • If a pollen grain and the carpel’s stigma have matching alleles at the S-locus, then the pollen grain fails to initiate or complete the formation of a pollen tube. • Because the pollen grain is haploid, it will be recognized as “self” if its one S-allele matches either of the two S-alleles of the diploid stigma. • Although self-incompatibility genes are all referred to as S-loci, such genes have evolved independently in various plant families. • As a consequence, self-recognition blocks pollen tube growth by different molecular mechanisms. • In some cases, the block occurs in the pollen grain itself, called gametophytic self-incompatibility. • In some species, self-recognition leads to enzymatic destruction of RNA within the rudimentary pollen tube. • RNases are present in the style of the carpel, and they can enter the pollen tube and attack its RNA only if the pollen is of a “self” type. • In other cases, the block is a response by the cells of the carpel’s stigma, called sporophytic self-incompatibility. • In some species, self-recognition activates a signal transduction pathway in epidermal cells that prevents germination of the pollen grain. • Germination may be prevented when cells of the stigma take up additional water, preventing the stigma from hydrating the relatively dry pollen. • Basic research on self-incompatibility may lead to agricultural applications. • Many agricultural plants are self-compatible. • Plant breeders sometimes hybridize different varieties of a crop plant to combine the best traits of the varieties and counter the loss of vigor that can result from excessive inbreeding. • To maximize hybrid seed production, breeders currently prevent self-fertilization by laboriously removing anthers from the parent plants that provide the seeds or by developing male sterile plants. • Eventually, it may be possible to impose self-incompatibility on species that are normally self-compatible. Concept 38.2 After fertilization, ovules develop into seeds and ovaries into fruits Double fertilization gives rise to the zygote and endosperm. • After landing on a receptive stigma, the pollen grain absorbs moisture and germinates, producing a pollen tube that extends down the style toward the ovary. • The nucleus of the generative cell divides by mitosis to produce two sperm, the male gametes. • The germinated pollen grain contains the mature male gametophyte. • Directed by a chemical attractant, possibly calcium, the tip of the pollen tube enters the ovary, probes through the micropyle (a gap in the integuments of the ovule), and discharges two sperm within the embryo sac. • Both sperm fuse with nuclei in the embryo sac. • One sperm fertilizes the egg to form the zygote. • The other sperm combines with the two polar nuclei to form a triploid nucleus in the central cell. • This large cell will give rise to the endosperm, a food-storing tissue of the seed. • The union of two sperm cells with different nuclei of the embryo sac is termed double fertilization. • Double fertilization ensures that the endosperm will develop only in ovules where the egg has been fertilized. • This prevents angiosperms from squandering nutrients. • Normally nonreproductive tissues surrounding the embryo have prevented researchers from visualizing fertilization in plants, but recently, scientists have been able to isolate sperm cells and eggs and observe fertilization in vitro. • The first cellular event after gamete fusion is an increase in cytoplasmic Ca2+ levels, which also occurs during animal gamete fusion. • In another similarity to animals, plants establish a block to polyspermy, the fertilization of an egg by more than one sperm cell. • In plants, this may be through deposition of cell wall material that mechanically impedes sperm. • In maize, this barrier is established within 45 seconds after the initial sperm fusion with the egg. The ovule develops into a seed containing an embryo and a supply of nutrients. • After double fertilization, the ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s). • As the embryo develops, the seed stockpiles proteins, oils, and starch. • Initially, these nutrients are stored in the endosperm. • Later in seed development in many species, the storage function is taken over by the swelling storage leaves (cotyledons) of the embryo itself. • Endosperm development usually precedes embryo development. • After double fertilization, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” having a milky consistency. • It becomes multicellular when cytokinesis partitions the cytoplasm between nuclei. • Cell walls form, and the endosperm becomes solid. • Coconut “milk” is an example of liquid endosperm and coconut “meat” is an example of solid endosperm. • The endosperm is rich in nutrients, which it provides to the developing embryo. • In most monocots and some dicots, the endosperm also stores nutrients that can be used by the seedling after germination. • In many dicots, the food reserves of the endosperm are completely exported to the cotyledons before the seed completes its development, and consequently the mature seed lacks endosperm. • The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell and a terminal cell. • The terminal cell gives rise to most of the embryo. • The basal cell continues to divide transversely, producing a thread of cells, the suspensor, which anchors the embryo to its parent. • The suspensor functions in the transfer of nutrients to the embryo from the parent. • The terminal cell divides several times and forms a spherical proembryo attached to the suspensor. • Cotyledons begin to form as bumps on the proembryo. • A eudicot, with its two cotyledons, is heart-shaped at this stage. • Only one cotyledon develops in monocots. • After the cotyledons appear, the embryo elongates. • Cradled between cotyledons is the embryonic shoot apex with the apical meristem of the embryonic shoot. • At the opposite end of the embryo axis is the apex of the embryonic root, also with a meristem. • After the seed germinates, the apical meristems at the tips of the shoot and root sustain primary growth as long as the plant lives. • During the last stages of maturation, a seed dehydrates until its water content is only about 5–15% of its weight. • The embryo stops growing and becomes dormant until the seed germinates. • The embryo and its food supply are enclosed by a protective seed coat formed by the integuments of the ovule. • In the seed of a common bean, the embryo consists of an elongate structure, the embryonic axis, attached to fleshy cotyledons. • Below the point at which the fleshy cotyledons are attached, the embryonic axis is called the hypocotyl; above it is the epicotyl. • At the tip of the epicotyl is the plumule, consisting of the shoot tip with a pair of miniature leaves. • The hypocotyl terminates in the radicle, or embryonic root. • While the cotyledons of the common bean supply food to the developing embryo, the seeds of some dicots, such as castor beans, retain their food supply in the endosperm and have cotyledons that are very thin. • The cotyledons will absorb nutrients from the endosperm and transfer them to the embryo when the seed germinates. • The embryo of a monocot has a single cotyledon. • Members of the grass family, including maize and wheat, have a specialized cotyledon called a scutellum. • The scutellum is very thin, with a large surface area pressed against the endosperm, from which the scutellum absorbs nutrients during germination. • The embryo of a grass seed is enclosed by two sheathes, a coleorhiza, which covers the young root, and a coleoptile, which covers the young shoot. The ovary develops into a fruit adapted for seed dispersal. • As the seeds are developing from ovules, the ovary of the flower is developing into a fruit, which protects the enclosed seeds and aids in their dispersal by wind or animals. • Fertilization triggers hormonal changes that cause the ovary to begin its transformation into a fruit. • If a flower has not been pollinated, fruit usually does not develop, and the entire flower withers and falls away. • The wall of the ovary becomes the pericarp, the thickened wall of the fruit, while other parts of the flower wither and are shed. • In some angiosperms, other floral parts contribute to the fruit. • In apples, the fleshy part of the fruit is derived mainly from the swollen receptacle, while the core of the apple fruit develops from the ovary. • Fruits are classified into several types, depending on their developmental origin. • A typical fruit is derived from a single carpel or several fused carpels and is called a simple fruit. • Some simpler fruits are fleshy, like a peach, while others are dry, like a pea pod. • An aggregate fruit results from a single flower that has more than one carpel, each forming a small fruit. • The fruitlets are clustered together on a single receptacle, like a raspberry. • A multiple fruit develops from an inflorescence, a group of flowers tightly clustered together. • When the walls of the ovaries thicken, they fuse together and form one fruit, as in a pineapple. • The fruit usually ripens about the same time as its seeds are completing their development. • For a dry fruit such as a soybean pod, ripening is a little more than senescence of the fruit tissues, which allows the fruit to open and release the seeds. • The ripening of fleshy fruits is more elaborate, its step
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