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Plant reproduction

Contributed by: Scott D. Russell

Publication year: 2014

The formation of a new plant that is either an exact copy or recombination of the genetic makeup of

its parents. There are three types of plant reproduction considered here: (1) vegetative reproduction, in which

a vegetative organ forms a clone of the parent; (2) asexual reproduction, in which reproductive components

undergo a nonsexual form of production of offspring without genetic rearrangement, also known as apomixis;

and (3) sexual reproduction, in which meiosis (reduction division) leads to formation of male and female gametes

that combine through syngamy (union of gametes) to produce offspring. See also: PLANT; PLANT PHYSIOLOGY.

Vegetative reproduction

Unlike animals, plants may be readily stimulated to produce identical copies of themselves through cloning. In

animals, only a few cells, which are regarded as stem cells, are capable of generating cell lineages, organs, or new

organisms. In contrast, plants generate or produce stem cells from many plant cells of the root, stem, or leaf 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 organism. If it is identical to the parent, it is a

clone; however, if this plant has been altered through molecular biology, 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 tissue), to establish roots, stems, and organs. Specific chemical and

hormonal stimulation is required to form organs, typically roots first, then photosynthetic stems and leaves, 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 Bryophyllum, 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 rhizomes or stolons (runners), or erect corms, and also

serve as storage organs, such as enlarged tubers. Leafy storage stems, known as bulbs, are similar to buds in

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WIDTH:CFig. 1 Vegetative reproduction through storage organs: tuber, Solanum tuberosum, potato; rhizome, Cyperusalternifolius, umbrella plant; runner, Fragaria × ananassa, strawberry; bulb, Allium cepa, onion; corm, Gladiolussp. (After A. D. Bell and A. Bryan, Plant Form: An Illustrated Guide to Flowering Plant Morphology, Timber Press,Portland, OR, 2008)

providing a renewal bud (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: PLANT PROPAGATION.

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 seed that is identical to a

parent. A common example is dandelion, which is triploid (having three complete chromosome sets in a

nucleus) and produces only clonal seed. In dandelion, the three sets of homologous chromosomes cannot pair,

causing conventional meiosis to fail, yet they retain the capacity to form an embryo. Thus, asexual reproduction



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

with species.

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 embryos for the ability to form seedlings.

Plants omitting meiosis and syngamy result in clonal seed—a process known as apomixis. Since such clonal seeds

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 maize, 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 spores 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); GAMETOGENESIS; RECOMBINATION (GENETICS).

Although all plants produce embryos and are thus embryophytes, not all plants are seed plants. Some plants are

nonvascular, including mosses, liverworts, and hornworts, which constitute the bryophytes. The remaining

vascular plants alive today include various free-sporing vascular plants, consisting of ferns, lycopods, and

horsetails, and seed-producing plants, including gymnosperms and angiosperms. Despite a wealth of diversity in

life 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 spore-producing sporophytes and gamete-producing gametophytes. 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 sporophyte 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



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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 sporangium, in which they were formed. All

spores have a highly resistant cell wall that is composed of sporopollenin, a chemical product that is so resistant

to breakdown that spores form a vital link in the fossil record of plants. In some cases, only spores or pollen and

no megafossils have been recovered for some species. Spore walls may be preserved for up to hundreds of

millions of years in the geological record. Normally, spores germinate and then divide to form the next

generation of plants, which is known as the gametophyte. The gametophyte is a 1n (haploid) plant that divides

mitotically and forms the tissues and organs that produce the gametes. See also: MITOSIS.

Similar to animals, the female gamete of plants is known as an egg cell and the male gamete is a sperm 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 egg cell, known as fertilization or syngamy, results in the establishment of

a 2n zygote. 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 antheridium) or an

egg cell (in an archegonium) [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



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WIDTH:CFig. 2 In nonvascular plants such as mosses, a diploid capsule forms on the top of the sporophyte (1) in whichmeiosis occurs, giving rise to haploid spores (2). These spores germinate, producing gametophytes that formmale 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 completesthe 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 seta 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



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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 lens or mistaken for a nonvascular plant, as

they lack vascular tissue. The classical fern 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 sexes with two distinct sizes—the smaller corresponds to a male spore

(microspore), which forms the male gametophyte (pollen), whereas the larger corresponds to a female spore

(megaspore), 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 microspores are freely

disseminated into the environment, whereas the megaspore is retained throughout development, within an

immature seed, known as an ovule. 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. Cycads and Ginkgo, which

are regarded as less highly derived among the gymnosperms, produce large multiflagellated sperm cells, but

conifers and gnetophytes produce nonmotile sperm cells that are conveyed by the pollen tube 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; POLLINATION; 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 stigma and style, ultimately arriving in the ovary, which contains the ovules.



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WIDTH:CFig. 3 Life cycle of a typical fern, representing free-sporing vascular plants. Sporangia on the underside offronds (1) produce spores via meiosis (2) that form haploid gametophytes (3). The gametophyte produces eggcells and swimming sperm cells (4), which undergo syngamy to produce a zygote (5) and ultimately the maturesporophyte, 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 seedling, whereas the other sperm fuses with the central cell to form a nutritive

endosperm, which serves as a food source. Fusion with both sperm cells is known as double fertilization and is

characteristic of angiosperms. See also: FERTILIZATION (PLANT); MAGNOLIOPHYTA.



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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 megaspores. Upon pollination, a pollen tube germinates on thestigma, grows down the style, and enters a synergid, where it deposits two sperm cells. During doublefertilization, one sperm cell fuses with the egg cell to form the zygote (and young embryo), whereas the otherfuses 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 auxin 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, photoperiodism, and hormonal signaling. The onset of floral competence is early in annuals and

weedy species, whereas perennials such as trees and shrubs 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



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hours to permit flowering; this is particularly true for tree fruit 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 phloem to the shoot apex, where it induces flowering. This hormone-like

compound was first demonstrated in the 1930s through grafting 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: FLOWER.

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 Plant Morphology, Timber Press,

Portland, OR, 2008



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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 Fruits

Flowering Plant Reproduction: Flower Structure

International Association of Sexual Plant Reproduction Research (IASPRR)

Natural Vegetative Propagation


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