Plant Reproduction

Abstract

Reproduction is required for the generation of new individuals. In land plants, reproduction can be asexual, where the offspring are genetically identical to the parent, or sexual, creating genetic variation. The life cycle of plants involves two alternating generations; a sporophyte that makes spores and a gametophyte that produces male and female gametes. During land plant evolution, this life cycle has altered from the gametophyte generation being the larger dominant generation to the sporophyte being the dominant plant we are familiar with. The structures that make spores and gametes, and the way reproductive cells are exchanged and dispersed, have also altered greatly as plants became better adapted to living in drier environments. Reproduction in the flowering plants has been extensively studied due to their economic importance, and we are beginning to uncover intriguing details of the molecular events that control critical aspects of the process in model and crop plants.

Key Concepts

  • Asexual reproduction produces genetically identical offspring and is beneficial under good conditions.
  • Plants alternate between a diploid sporeā€producing generation and a haploid gameteā€producing generation.
  • In plants, spores are produced by meiosis and can be either released into the environment (bryophytes and nonseed plants) or develop within maternal tissues (seed plants).
  • Distinct male and female gametes are produced in plants by mitotic divisions of haploid cells.
  • Pollen is an evolutionary advance as it enables the dispersal and the delivery of the male gametes to the female gametes in the absence of free water.
  • Seeds aid in the dispersal of offspring by providing a protective coat and a nutrition source to support seedlings during germination.
  • The time of flowering is controlled through both internal signals and environmental cues, which control the production of a flower promoting protein.
  • The development of the male and female gametophytes in flowering plants requires precise regulation to ensure the correct cell types are generated in the correct locations.
  • Pollination is the process by which a pollen grain lands on a receptive female and grows a pollen tube through the female tissue towards an ovule.
  • Seed development in flowering plants requires two fertilisation events (double fertilisation) to produce the embryo and an endosperm needed for seed development.

Keywords: gametophyte; sporophyte; asexual reproduction; sexual reproduction; gamete; spore; seed; flower; pollen; embryo sac

Figure 1. Examples of reproductive structures in a range of plant taxa. (a) A leaf of the ‘mother of thousands’ plant (Bryophyllum daigremontianum) with plantlets along the margins. Reproduced from CrazyD licensed under CC‐BY 2.5. via Wikimedia Commons. (b) Gemmae in a gemmae cup on a liverwort thallus. Reproduced from Holger Casselmann licensed under CC‐BY 2.5. via Wikimedia Commons. (c) Leafy green moss gametophytes with smaller brown sporophytes growing from the gametophytes. Reproduced from Manfred Morgner licensed under CC‐BY 2.5. via Wikimedia Commons. (d) Fern sporophyll with sori on the underside. Reproduced from Petritap licensed under CC‐BY 2.5. via Wikimedia Commons. (e) Pine cone of Pinus radiata. Reproduced from minicooper93402 licensed under CC‐BY 2.5. via Wikimedia Commons. (f) Flowers of Arabidopsis thaliana. Reproduced from Marie‐Lan Nguyen licensed under CC‐BY 2.5. via Wikimedia Commons.
Figure 2. The plant life cycle alternation of generations. The plant life cycle is divided into the diploid sporophyte (yellow) and haploid gametophyte (green) generations. The mature diploid sporophyte produces specialised structures called sporangia in which haploid spores are produced by meiosis. The spores divide mitotically to produce the gametophyte. The gametophyte produces distinct male and female gametes, the egg and the sperm, respectively. In some plants, this occurs in specialised structures called archegonia and antheridia. The egg and sperm fuse to produce a diploid zygote that develops into the sporophyte generation. In different taxa, either the sporophyte or the gametophyte can reproduce asexually (green arrows). Red arrows indicate the steps where the chromosome number changes.
Figure 3. A typical flower. A typical flower is composed of four whorls of organs, labelled on the left. The complex of transcription factors required for the development of each whorl according to the ABCE model is also shown. The reproductive organs form in whorls 3 and 4 and their component parts are indicated on the right. The stamen (whorl 3) consists of the anther and filament. The carpel (whorl 4) consists of the stigma and style supporting pollen germination and pollen‐tube growth, the transmitting tract for pollen‐tube growth and guidance and the ovary, containing numerous ovules.
Figure 4. Development of the male and female reproductive cells in A. thaliana. (a) Pollen development. Pollen development is divided into microsporogenesis and microgametogenesis. Microsporogenesis involves the meiotic division of a microsporocyte to form microspores. During microgametogenesis, the microspore undergoes an asymmetric division to produce a vegetative cell and a generative cell (or male germ cell). The smaller generative cell is engulfed within the cytoplasm of the vegetative cell, where it divides to produce two sperm cells. A thick cell wall consisting of an inner intine and an outer exine is deposited during pollen development. (b) Development of the embryo sac. During megasporogenesis, a megasporocyte undergoes a meiotic division to produce four megaspores. Only one megaspore, the functional megaspore, survives to progress through megagametogenesis. In megagametogenesis, the megaspore undergoes a mitotic division without cell division and one nucleus migrates to each pole, then two further mitotic divisions occur. Upon cellularisation, three antipodal cells form at one end and two synergids and an egg cell form at the other end. One nucleus from each pole (known as the polar nuclei) migrates to the centre and is incorporated into the central cell. The mature embryo sac consists of eight nuclei and seven cells.
Figure 5. Pollination and fertilisation. (a) Pollination. During pollination, pollen grains adhere to the stigma where they become hydrated. The pollen grains then germinate and the pollen tube grows through the style and into the transmitting tract. Upon receiving the appropriate signals, a single pollen tube will leave the transmitting tract and enter an ovule. (b) Fertilisation. After entering the receptive synergid in the ovule, the pollen‐tube tip bursts releasing the sperm cells (orange) and the synergid degrades (fainter). One sperm cell will fuse the egg cell and the second sperm cell will fuse with central cell in a process called double fertilisation (indicated by arrows).
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Further Reading

Evert RF and Eichhorn SE (2013) Raven Biology of Plants, 8th edn. London: W. H. Freeman. Palgrave. Macmillan.

Ma H and Sundaresan V (2010) Development of flowering plant gametophytes. Current Topics in Developmental Biology 91: 379–412.

Suwabe K, Suzuki G and Watanabe M (2010) Achievement of genetics in plant reproduction research: the past decade for the coming decade. Genes and Genetic Systems 85: 297–310.

Taiz L, Zeiger E, Møller IM and Murphy A (2015) Plant Physiology and Development, 6th edn. Sunderland: Sinauer Associates Inc.

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How to Cite close
Brownfield, Lynette, and Twell, David(Jan 2016) Plant Reproduction. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002046.pub2]