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Home , Antheridium, Archegonium, Flowering plant, Gametophyte, Plant reproduction, Seta

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Plant

Contributed by: Scott D. Russell Publication : 2014

The formation of a new that is either an exact copy or recombination of the genetic makeup of its parents. There are three types of considered here: (1) , in which a vegetative forms a clone of the parent; (2) , in which reproductive components undergo a nonsexual form of production of offspring without genetic rearrangement, also known as ; and (3) , in which (reduction ) leads to formation of male and that combine through syngamy (union of gametes) to produce offspring. See also: PLANT; PLANT .

Vegetative reproduction

Unlike , may be readily stimulated to produce identical copies of themselves through . In animals, only a few cells, which are regarded as stem cells, are capable of generating lineages, organs, or new . In contrast, plants generate or produce stem cells from many plant cells of the , stem, or that are not part of an obvious generative lineage—a characteristic that has been known as totipotency, or the general ability of a single cell to regenerate a whole new plant. This ability to establish new plants from one or more cells is the foundation of plant biotechnology. In biotechnology, a single cell may be used to regenerate new organisms that may or may not genetically differ from the original . If it is identical to the parent, it is a clone; however, if this plant has been altered through , it is known as a genetically modified organism (GMO). The characteristic of modifying the genetics of an organism is known as genetic engineering. See also: BIOTECHNOLOGY; CLONING; GENETIC ENGINEERING.

Regeneration of new plants from a single cell requires stringent conditions to induce a single cell, or undifferentiated cell mass (known as a callus ), to establish , stems, and organs. Specific chemical and hormonal stimulation is required to form organs, typically roots first, then photosynthetic stems and , and ultimately flowers and autonomous plants. See also: TISSUE CULTURE.

Plants often produce organs for vegetative propagation, known as propagules, which can form unlimited numbers of identical offspring. In , commonly known as maternity plant, small plantlets form in notches on the leaves, complete with roots, and each is capable of forming a new, genetically identical plant. Although such prolific propagules are rare, many vegetative parts of the plant are capable of continuing growth when separated from the plant, and some organs can establish clonal offspring through formation of vegetative organs. These stem organs often arise from horizontal or (runners), or erect , and also serve as storage organs, such as enlarged . Leafy storage stems, known as , are similar to in AccessScience from McGraw-Hill Education Page 2 of 10 www.accessscience.com

WIDTH:CFig. 1 Vegetative reproduction through storage organs: , tuberosum, ; , alternifolius, umbrella plant; runner, × ananassa, ; , cepa,;, sp. (After A. D. Bell and A. Bryan, Plant Form: An Illustrated Guide to , Timber Press, Portland, OR, 2008)

providing a renewal (Fig. 1). Roots and leaves are particularly known to proliferate through fragmentation in some plants. Invasive plants usually show a special propensity for proliferation through vegetative reproduction. See also: .

Asexual reproduction

The sexual organs of plants normally produce the next generation through genetic recombination, but under certain circumstances may hijack the processes of embryogenesis to produce clonal that is identical to a parent. A common example is dandelion, which is triploid (having three complete sets in a nucleus) and produces only clonal seed. In dandelion, the three sets of homologous cannot pair, causing conventional meiosis to fail, yet they retain the capacity to form an . Thus, asexual reproduction AccessScience from McGraw-Hill Education Page 3 of 10 www.accessscience.com

uses structures typical of sexual reproduction to form and disperse clones. The particulars of this process vary with .

Deviation from sexual pathways has been documented at many stages in reproduction. In some species, meiosis or syngamy may be omitted or adjacent cells may outcompete sexual for the ability to form . Plants omitting meiosis and syngamy result in clonal seed—a process known as apomixis. Since such clonal may display many of the reproductive advantages of seeds, including dormancy and storage reserves, the production of apomictic offspring of crop plants has been a subject of considerable commercial interest. In plants such as , in which increases of seed productivity depend on hybrid vigor, purebred parental stocks need to be crossed each time that seed is produced. However, if it were possible to produce apomictic seeds of maize, it would not be necessary to cross plants to obtain high-producing crops. Instead, clonal seed would be used to retain hybrid vigor without crossing and this would match the yield of the original seed. Although a promising idea, the introduction of apomixis in crop plants has been more difficult than anticipated. See also: AGRICULTURAL SCIENCE (PLANT).

Sexual reproduction

In contrast to the aforementioned clonal reproduction, sexual reproduction generates genetically different products through crossing-over, recombination, and an assortment of chromosomes during meiosis, which results in that are rarely identical in seed plants. The fusion or syngamy of the male and female gametes during sexual reproduction combines the genetic material of the new offspring into a typically genetically distinct organism. See also: CHROMOSOME; CROSSING-OVER (GENETICS); ; RECOMBINATION (GENETICS).

Although all plants produce embryos and are thus , not all plants are seed plants. Some plants are nonvascular, including , liverworts, and , which constitute the . The remaining vascular plants alive today include various free-sporing vascular plants, consisting of , lycopods, and horsetails, and seed-producing plants, including and angiosperms. Despite a wealth of diversity in histories, the offspring of the embryos do not directly form gametes. Animals are quite distinct from plants as they give rise directly to gametes without any further steps. Plants, by contrast, undergo an obligate alternation of generations between -producing and -producing . See also: BRYOPHYTA; EMBRYOBIONTA.

Sporophytes produce spores through meiosis. During meiosis, paternally derived and maternally derived chromosomes pair with their counterpart chromosomes, resulting in closely aligned, homologous chromosomes. When a copy of each of the maternal and paternal chromosomes is present in the cells, the organism is called a 2n or diploid individual. Sporocytes are parental cells that subsequently form the spores (typically four for each sporocyte) through the process of meiosis. In meiosis (or reduction division), the homologous chromosomes segregate during the first half of meiosis (meiosis I), which halves the former sporophyte number of chromosomes, and only one of each of the parental chromosomes is transmitted into the intermediate dyad AccessScience from McGraw-Hill Education Page 4 of 10 www.accessscience.com

cell. During the second half of meiosis (meiosis II), each chromosome separates at its connecting centromere into chromatids, thus forming four haploid (1n) spores for each precursor sporocyte. During meiosis I, crossing-over and independent assortment of chromosomes result in genetic products that are unique, recombinant forms of the parents’ genetic material. See also: MEIOSIS.

Nonvascular and nonseed plants are called free-sporing plants because their spores are released into the environment and establish the next generation after leaving the , in which they were formed. All spores have a highly resistant that is composed of sporopollenin, a chemical product that is so resistant to breakdown that spores form a vital link in the record of plants. In some cases, only spores or and no megafossils have been recovered for some species. Spore walls may be preserved for up to hundreds of millions of in the geological record. Normally, spores germinate and then divide to form the next generation of plants, which is known as the . The gametophyte is a 1n (haploid) plant that divides mitotically and forms the tissues and organs that produce the gametes. See also: .

Similar to animals, the female gamete of plants is known as an cell and the male gamete is a cell. Since gametophytes form small haploid plants through mitotic division, their gametes are identical, which is in marked contrast to animals. In plants, genetic variability is generated through spore production; in animals, genetic variability is generated through gamete production. Higher plants, however, produce fewer gametes, and the male gametophytes in seed plants produce only two.

The fusion of a 1n sperm cell with a 1n , known as fertilization or syngamy, results in the establishment of a2n . This zygote forms an embryo, which in plants establishes the next sporophyte generation. Subsequent to zygote formation, all of its derivatives divide mitotically. With the continued multiplication and differentiation of its derivatives, an embryo is formed, representing the new offspring, a sporophyte, which completes the life cycle.

In plants lacking traditional vascular tissues, such as mosses, liverworts, and hornworts, the persistent and thus dominant generation is the gametophyte, which directly forms the gametes mitotically. On the other hand, in the vascular plants, the persistent and dominant generation is the sporophyte. In vascular and nonvascular plants, the other generation tends to be transient and reduced in size and complexity—in the nonvascular plants, the sporophyte is reduced largely to a sporangium; in vascular plants, the gametophyte is reduced largely to produce gametes.

The familiar form of nonvascular plants is the green, photosynthetic gametophyte generation, which when reproductively active gives rise to gamete-producing organs, forming either sperm cells (in an ) or an egg cell (in an ) [Fig. 2]. A major outbreeding strategy is the “splash cup” mechanism, in which water may splash sperm cells up to a meter or more away from their origin to the archegonium of another plant. Although sperm cells are flagellated and motile, they can only swim and receive female erotactins (chemical sexual attractants) from the archegonium if they are in the same water droplet; they swim relatively short AccessScience from McGraw-Hill Education Page 5 of 10 www.accessscience.com

WIDTH:CFig. 2 In nonvascular plants such as mosses, a diploid forms on the top of the sporophyte (1) in which meiosis occurs, giving rise to haploid spores (2). These spores germinate, producing gametophytes that form male gametes (3) or female gametes (4), corresponding to swimming sperm cells and egg cells, respectively. During syngamy, gametes fuse, producing a diploid zygote (5) and sporophyte (6). Capsule formation completes the life cycle. (After M. Hoefnagels, Biology: Concepts and Investigations, McGraw-Hill, New York, 2009)

distances given their short life span and limited food stores. Upon sperm fusion with the egg cell, a small zygote forms that develops subsequently into an embryo within the archegonium. Upon maturation, the embryo remains connected with the gametophyte through a foot, transmitting food through the into a single spore-producing capsule or sporangium. Thus, the sporophyte generation in mosses, liverworts, and hornworts is essentially a parasite on the gametophyte. With the completion of meiosis in the capsule, spores mature and are released, completing the life cycle of the sporophyte.

In vascular plants, however, a green, free-living sporophyte is the familiar form of the plant, and the gametophytes are transitory. Free-sporing vascular plants alive today include ferns, lycopods, and horsetails. In AccessScience from McGraw-Hill Education Page 6 of 10 www.accessscience.com

these plants, spores arise from numerous sporangia located in groups of sori (ferns) or small cones (lycopods and horsetails) and are freely disseminated and not nutritionally dependent on the sporophyte (Fig. 3). The spores of these plants are small and form gametophytes that are at most a few millimeters in size before forming the gamete-producing archegonia and antheridia, which generate the egg and sperm cells, respectively. Their gametophytes are free living, but might be overlooked without a hand or mistaken for a nonvascular plant, as they lack . The classical gametophyte is heart-shaped with a notch and is just a single cell in thickness, except on a central archegonial pad. Gametophytes appear to be variably sensitive to dehydration, as there are some ferns that compete well even in semiarid conditions. Although some gametophytes produce both male and female gametes on a single gametophyte, smaller gametophytes may produce only sperm cells. Free-sporing vascular plants also benefit from splash mechanisms to disseminate their sperm cells because the ability of motile sperm cells to travel long distances is limited. The sperm travels to the egg cell, which is retained within the archegonium, and fuses with the egg cell during syngamy to form a zygote. The subsequent young embryo initially obtains nutrition from the gametophyte, but quickly breaks through the wall of the archegonium and becomes an independent plant with the establishment of the young sporophyte. Despite the possibility of multiple sporophytes arising from a gametophyte, typically only one matures. See also: LYCOPHYTA; POLYPODIOPHYTA; SPHENOPHYTA.

In seed plants, the spores consist of two with two distinct sizes—the smaller corresponds to a male spore (), which forms the male gametophyte (pollen), whereas the larger corresponds to a female spore (), which forms the female gametophyte (Fig. 4). In flowering plants (angiosperms) and the most advanced gymnosperms, neither an antheridium nor archegonium is formed—the number of cells is too small—although the precursor may be viewed as an antheridial or archegonial initial. The are freely disseminated into the environment, whereas the megaspore is retained throughout development, within an immature seed, known as an . The ovule is protected by an integument, leaving a small hole, known as a micropyle, through which the male gametophyte grows to deposit the sperm cells. and ,which are regarded as less highly derived among the gymnosperms, produce large multiflagellated sperm cells, but and gnetophytes produce nonmotile sperm cells that are conveyed by the into the ovule. The female gametophyte may contain more than 100,000 cells and contains archegonia in most gymnosperms. In conifers, multiple seasons may be required for maturation, as buds may develop, fertilize, and form embryos during different years. See also: PINOPHYTA; POLLEN; ; SEED.

In flowering plants, the pollen is reduced to only three cells—a tube cell for guidance and delivery, and two sperm cells that migrate into the tube (Fig. 4). Pollen may be disseminated with either two cells (bicellular pollen) or three cells if the two sperm cells have formed precociously before dissemination (tricellular pollen). Although most of the pollen is covered with sporopollenin, typically a small weakened area or aperture may be present, through which the pollen tube germinates and elongates by tip growth. Pollen germinates on and penetrates the and style, ultimately arriving in the , which contains the . AccessScience from McGraw-Hill Education Page 7 of 10 www.accessscience.com

WIDTH:CFig. 3 Life cycle of a typical fern, representing free-sporing vascular plants. Sporangia on the underside of (1) produce spores via meiosis (2) that form haploid gametophytes (3). The gametophyte produces egg cells and swimming sperm cells (4), which undergo syngamy to produce a zygote (5) and ultimately the mature sporophyte, completing the life cycle. (After M. Hoefnagels, Biology: Concepts and Investigations, McGraw-Hill, New York, 2009)

In flowering plants, the female gametophyte (or embryo sac) consists of a highly reduced complement of cells, typically composed of eight nuclei and seven cells. An egg cell flanked by two small cells known as synergids occupies the micropylar pole, where the pollen tube enters; a two-nucleate central cell occupies the center; and three antipodals occupy the opposite, chalazal pole. Tubes are attracted and guided to the synergids via chemical signals that control where tubes enter and discharge their contents. One of the sperm fuses with the egg cell to form the embryo and young , whereas the other sperm fuses with the central cell to form a nutritive , which serves as a food source. Fusion with both sperm cells is known as and is characteristic of angiosperms. See also: FERTILIZATION (PLANT); MAGNOLIOPHYTA. AccessScience from McGraw-Hill Education Page 8 of 10 www.accessscience.com

WIDTH:DFig. 4 Life cycle of an angiosperm shows parallel meiosis occurring in the male (top) and female (bottom). Meiosis forms the male microspores and female . Upon pollination, a pollen tube germinates on the stigma, grows down the style, and enters a synergid, where it deposits two sperm cells. During double fertilization, one sperm cell fuses with the egg cell to form the zygote (and young embryo), whereas the other fuses with the central cell with its polar nuclei to form the nutritive endosperm. (After M. Hoefnagels, Biology: Concepts and Investigations, McGraw-Hill, New York, 2009)

Modern eight-nucleate and seven-celled embryo sacs are believed to have evolved from simple, four-nucleate embryo sacs. The four-nucleate embryo sac consisted of one functional module, which doubled to form the eight-nucleate form. The egg, flanked by two synergids and one polar nucleus, corresponds to one module, whereas the antipodals and the other polar nucleus form a second module. Orientation of the modern embryo sac, in turn, is controlled by an concentration gradient. Auxin is at its highest concentration near the micropylar pole, establishing the egg and synergids. However, when this gradient is experimentally reversed, the polarity of the embryo sac is correspondingly reversed. Molecular factors controlling gamete cell identity and number are just beginning to be characterized. See also: AUXIN.

Floral induction

Flowering in angiosperms is controlled by a number of critical factors, including plant maturity, nutritional status, vernalization, , and hormonal signaling. The onset of floral competence is early in annuals and weedy species, whereas perennials such as and may require up to a decade before they produce their first flowers. This shift to sexual maturity often is correlated with the transition from juvenile to adult growth form for that species. With regard to nutritional status, scarcity of nearly any essential mineral element may delay flowering, but phosphorous needs are especially increased during flowering and seed production. Plants in temperate climates frequently require vernalization, which is exposure of the plant to adequate chill AccessScience from McGraw-Hill Education Page 9 of 10 www.accessscience.com

hours to permit flowering; this is particularly true for crops. Plants also display photoperiodism in measuring day length. Short-day (SD) plants require long nights and therefore bloom in the late summer or autumn, whereas long-day (LD) plants require short nights and flower at the start of the summer, when daylight is longest. Hormonal signaling is also present in many plants. Gibberellin is known to trigger flowering in some LD plants. A particularly challenging task in some species is inducing flowers from callus in tissue culture, but this has been achieved by balancing contributions of both cytokinin and auxin. Induction of flowering thus appears to depend on multiple factors. See also: PHOTOPERIODISM; PLANT HORMONES; PLANT JUVENILITY; VERNALIZATION.

When plants are stimulated to flower, they release a long-distance signal known as florigen, a compound produced in the leaf and transported by to the apex, where it induces flowering. This hormone-like compound was first demonstrated in the 1930s through experiments, which indicated that the flowering signal was both transmissible and universal. The best modern candidate for the long-sought florigen is a compound containing the FLOWERING LOCUS T (FT) protein, which is transmitted through the phloem. When the photoperiodic CONSTANS induces the formation of FT in the leaf, and then FT protein travels to the shoot apex, where it combines with FD (another protein), the process of flowering begins, with the shoot apex induced to become a reproductive apex. Florigen is integrated with other floral signals through the interaction of phytochromes and other light-sensitive plant pigments that detect day length with the CONSTANS protein. Such pigments appear to be involved with posttranscriptional regulation of K CONSTANS and thereby modulate the requirements to construct florigen. See also: FLORIGEN; PHYTOCHROME.

Flowering also can be stimulated by temperature, which can be a potent inducer of flowering that operates independently of CONSTANS. A quantitative trait that modulates thermal sensitivity known as FLOWERING LOCUS M can trigger flowering as well. In determining their flowering status, plants sense specific metabolic and timing cues that relate to successful reproduction. Since the most favorable conditions for flowering and seed set may coincide with specific environmental cues, plants that are well adapted to detect these indicators can optimize their reproductive investment. This is an example of a feed-forward system of selection capable of fine-tuning the flowering characteristics of myriad plants. See also: . Scott D. Russell

Keywords vegetative reproduction; asexual reproduction; sexual reproduction; floral induction; meiosis; fertilization; spore; gametophyte

Bibliography

A. D. Bell and A. Bryan, Plant Form: An Illustrated Guide to Flowering , Timber Press, Portland, OR, 2008 AccessScience from McGraw-Hill Education Page 10 of 10 www.accessscience.com

P. H. Raven, R. F. Evert, and S. E. Eichhorn, Biology of Plants, 7th ed., W. H. Freeman, New York, 2005

S. D. Russell and T. Dresselhaus (eds.), Special Issue: Sexual Plant Reproduction, vol. 21, pp. 1–88, Springer, Berlin∕Heidelberg, 2008

V. Sundaresan and M. Alandete-Saez, Pattern formation in miniature: The female gametophyte of flowering plants, Development, 137:179–189, 2010 DOI: http://doi.org/10.1242/dev.030346

Additional Readings

A. Gaston et al., PFRU, a single dominant locus regulates the balance between sexual and asexual plant reproduction in cultivated strawberry, J. Exp. Bot., 64(7):1837–1848, 2013 DOI: http://doi.org/10.1093/jxb/ert047

R. Jones et al., The Molecular Life of Plants, Wiley-Blackwell, Chichester, West Sussex, UK, 2013

E. P. Solomon, L. R. Berg, and D. W. Martin, Biology, 9th ed., Brooks∕Cole, Belmont, CA, 2011

H. Wollmann and F. Berger, Epigenetic reprogramming during plant reproduction and seed development, Curr. Opin. Plant Biol., 15(1):63–69, 2012 DOI: http://doi.org/10.1016/j.pbi.2011.10.001

K. E. Zinn, M. Tunc-Ozdemir, and J. F. Harper, Temperature stress and plant sexual reproduction: Uncovering the weakest links, J. Exp. Bot., 61(7):1959–1968, 2010 DOI: http://doi.org/10.1093/jxb/erq053

Artificial Vegetative Propagation

Biological Diversity: Nonvascular Plants and Nonseed Vascular Plants

Biological Diversity: Seed Plants

Flowering Plant Reproduction: Fertilization and

Flowering Plant Reproduction: Flower Structure

International Association of Sexual Plant Reproduction Research (IASPRR)

Natural Vegetative Propagation