Plant Reproduction

Learning Objectives

  • Compare and contrast the life cycles of angiosperms (flowering plants), gymnosperms (conifers), non-seed vascular plants (ferns), and nonvascular plants (mosses)
  • Describe the structures and functions of the flower, seed, and fruit in the angiosperm life cycle
  • Explain the process, locations, and significance of angiosperm gametogenesis and fertilization, including double fertilization
  • Explain the process and significance of seed maturation, dormancy, and germination
  • Predict mechanisms of pollination based on flower characteristics and dispersal based on fruit characteristics

Sexual reproduction in plants: Alternation of Generations

The text below is adapted from OpenStax Biology 32.1

Plants have two distinct multicellular stages in their life cycles, a phenomenon called alternation of generations (in contrast to the haplontic and diplontic life cycles). These two stages are the multicellular, haploid gametophyte and the multicellular diploid sporophyte.  This is very different from most types of animal reproduction where there is only one multicellular stage: a diploid organism which produces single-celled haploid gametes.

Before we revisit this life cycle, a reminder of some terms:

  • Gamete: a mature haploid male or female germ cell that is able to unite with another of the opposite sex in sexual reproduction to form a zygote
  • Spore: a minute, typically one-celled, reproductive unit capable of giving rise to a new individual without sexual fusion

Gametes are always haploid, and spores are usually haploid (spores are always haploid in the plant alternations of generations life cycle).

In the alternation of generations life cycle, illustrated below, there is a mature multicellular haploid stage and a mature mulitcellular diploid stage. The multicelluar haploid stage (the gametophyte) produces gametes via mitosis which fuse to form a diploid zygote. The zygote develops into a mature multicellular diploid individual (the sporophyte), which produces haploid spores via meiosis. The haploid spores then develop into a mature multicellular haploid individual.

Note the multicellular stages are named for what they produce, not what they come from.  The gametophyte makes gametes, and the sporophyte makes spores.

Alternation of Generations. Image credit: Menchi, Wikimedia Commons. https://en.wikipedia.org/wiki/File:Sporic_meiosis.png

Though all plants display an alternation of generations life cycle, there are significant variations in different lineages of plants, consistent with their evolutionary history:

  • In seedless non-vascular plants, or bryophytes (mosses), the haploid gametophyte is larger than the sporophyte (the plant structure that you see is the gametophyte); this is a gametophyte-dominated life cycle. The sporophyte is attached to and dependent on the gametophyte. (By “dominated” we mean “the stage of the plant you can see by eye.”)
  • In seedless vascular plants (ferns), the sporophyte is larger than the gametophyte (the plant structure that you see is the sporophyte), but the gametophyte is free-living and independent from the diploid sporophyte.
  • The life cycle of angiosperms (flowering plants) and gymnosperms (conifers) is dominated by the sporophyte stage (the plant structure that you see is the sporophyte), with the gametophyte remaining attached to and dependent on the sporophyte (reverse of bryophytes).
  • Though they both have sporophyte-dominated life cycles, angiosperms and gymnosperms differ in that angiosperms have flowers, fruit-covered seeds, and double fertilization, while gymnosperms do not have flowers, have “naked” seeds, and do not have double fertilization (more on this later).

The video below describes reproduction in gametophyte-dominant nonvascular plants (eg, mosses):

The video below describes reproduction in sporophyte-dominant vascular plants (eg, gymnosperms and angiosperms):

 

Reproduction in angiosperms

The information below is adapted from OpenStax Biology 32.1

We’ll look more closely at reproduction in angiosperms, which are unique among plants for three defining features: they have flowers, they have fruit-covered seeds, and they reproduce via a process called double fertilization.

  • Flowers are adaptations to attract pollinators
  • Fruits are adaptations to facilitate seed dispersal
  • Double fertilization is an adaptation to invest resources for nourishment of the developing embryo, in a unique way compared to other plants

All the information below is specific to angiosperms, unless otherwise noted.

Flower Structure

A typical flower has four “layers,” illustrated and described below from external to internal structures:

  • The outermost layer consists of sepals, green, leafy structures which protect the developing flower bud before it opens.
  • The next layer is comprised of petals, modified leaves which are usually brightly colored, which help attract pollinators.
  • The third layer contains the male reproductive structures, the stamens. Stamens are composed of anthers and filaments. Anthers contain the microsporangia, the structures that produce the microspores, which go on to develop into male gametophytes. Filaments are structures that support the anthers.
  • The innermost layer contains one or more female reproductive structures, the carpel. Each carpel contains a stigma, style, and ovary. The ovaries contain the megasporangia, the structures that produce the megaspores, which go on to develop into female gametophytes. The stigma is the location where pollen (the male gametophyte) is deposited by wind or by pollinators. The style is a structure that connects the stigma to the ovary.

The parts of the flower include the sepal, petals, stamens, and carpels. Image credit: OpenStax Biology, modification of work by Mariana Ruiz Villareal

The Pollen Grain: the Male Gametophyte

Pollen is the male gametophyte in angiosperms and gymnosperms.  Pollen is often described in everyday language as plant sperm, but this is not the case! As the male gametophyte, pollen is a multicellular, haploid stage that produces the sperm.

Pollen development occurs in a structure called the  microsporangium (micro = small), located within the anthers. The microsporangia (plural of microsporangium) are pollen sacs in which the microspores develop into pollen grains.

Shown is (a) a cross section of an anther at two developmental stages. The immature anther (top) contains four microsporangia, or pollen sacs. Each microsporangium contains hundreds of microspore mother cells that will each give rise to four pollen grains. The tapetum supports the development and maturation of the pollen grains. Upon maturation of the pollen (bottom), the pollen sac walls split open and the pollen grains (male gametophytes) are released. (b) In these scanning electron micrographs, pollen sacs are ready to burst, releasing their grains. Image credit: OpenStax Biology; credit b: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

As a spore, the microspore is haploid, but it is derived from a diploid cell. Within the microsporangium, the diploid microspore mother cell divides by meiosis to give rise to four microspores, each of which will ultimately form a pollen grain, illustrated below. This process is similar to production of gametes in animals (note that haploid gametes in plants are produced by mitosis from a haploid gametophyte). Upon maturity, the microsporangia burst, releasing the pollen grains from the anther where they have the opportunity to be transported to stigmas by wind, water, or an animal pollinator.

Mature pollen grains contain two cells: a generative cell and a pollen tube cell (see, I told you pollen is multicellular!). The generative cell is contained within the larger pollen tube cell. When the pollen grain reaches a stigma, it undergoes a process called germination (which is not the same as seed germination). During pollen germination, the tube cell forms a pollen tube through the style to the bottom of the ovary, the generative cell migrates through it to enter the ovary for fertilization. During its transit inside the pollen tube, the generative cell divides to form two male gametes (sperm cells). Both sperm cells are required for successful fertilization in angiosperms.

Pollen develops from the microspore mother cells. The mature pollen grain is composed of two cells: the pollen tube cell and the generative cell, which is inside the tube cell. The pollen grain has two coverings: an inner layer (intine) and an outer layer (exine). The inset scanning electron micrograph shows Arabidopsis lyrata pollen grains. (Image credit: OpenStax Biology pollen micrograph: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Due to its protective covering that prevents desiccation (drying out) of the sperm, pollen is an important adaptation in facilitating colonization of land by plants. Pollen allows angiosperms and gymnosperms to reproduce away from water, unlike mosses and ferns which require water for sperm to swim to the female gametophyte.

The Embryo Sac: The Female Gametophyte

While the details may vary between species, the general development of the female gametophyte, or embryo sac, has two distinct phases. First, a single cell in the diploid megasporangium (mega = large), located within the ovules, undergoes meiosis to produce four megaspores. Only one megaspore survives, again similar to gamete production in animals.

In the second phase of female gametophyte development, the surviving haploid megaspore undergoes mitosis without complete cell division to produce an eight-nucleate, seven-cell female gametophyte, the embryo sac, illustrated below. Two of the nuclei (the polar nuclei) move to the center of the embryo sac and fuse together, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm, which will ultimately provide nourishment for the developing embryo (analogous to yolk in animal eggs). Three nuclei position themselves on the end of the embryo sac opposite the micropyle (the site where sperm enter the embryo sac) and develop into the antipodal cells, which later degenerate to provide nourishment to the embryo sac. The nucleus closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells. The synergids help guide the pollen tube for successful fertilization. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.

A structure called the integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat after fertilization and protect the entire seed. Just like the evolution of pollen, the evolution of the seed was an important adaptation allowing plants to colonize land away from water due to the protection of the embryo within the plant. (Thus the seed is analogous to the amniotic egg in animal reproduction.) The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization. The ovule wall will become part of the fruit.

As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments (dark green) and has an opening called a micropyle. Inside the embryo sac are three antipodal cells, two synergids, a central cell, and the egg cell. Image credit: OpenStax Biology.

Double Fertilization

The text below was adapted from Openstax Biology 32.2

The phenomenon of double fertilization, or two fertilization events, is unique to angiosperms and does not occur in any other type of plant or other organism.

As described above, after pollen is deposited on the stigma, it germinates and grows through the style to reach the ovule. The pollen tube cell grows to form the pollen tube, guided to the micropyle by chemical signals from the synergid cells. The generative cell travels through the tube to the egg and divides mitotically to form two sperm cells. One sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm, which serves as a source of nutrition for the developing embryo. Together, these two fertilization events in angiosperms are known as double fertilization, illustrated below. After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed, and the ovary become the fruit, usually surrounding the seed.

In angiosperms, one sperm fertilizes the egg to form the 2n zygote, and the other sperm fertilizes the central cell to form the triploid (3n) endosperm. This is called a double fertilization. Image credit: OpenStax Biology

After fertilization, the zygote enters a temporary period of development (shown below). It first divides to form two cells: the upper cell, or apical cell, and the lower cell, or basal cell. The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The suspensor does not become part of the future plant, but instead provides a route for nutrition to be transported from the mother plant to the growing embryo. In this way the suspensor is a type of “extra-embryonic” tissue and is analogous to the umbilical cord in placental mammals. The apical cell also divides, giving rise to the proembryo (the actual embryo that will develop into a plant). The endosperm accumulates starches, lipids, and proteins, and then nourishes the developing cotyledons (embryonic leaves). The cotyledons will serve as an energy store for later embryo development. The seed then loses up to 95% of its water and embryonic development is suspended: the seed enters a period of dormancy for dispersal, and growth is resumed only when the seed germinates. Once development is reactivated, the developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.

Shown are the stages of embryo development in the ovule of a shepherd’s purse (Capsella bursa). After fertilization, the zygote divides to form an upper terminal cell and a lower basal cell. (a) In the first stage of development, the terminal cell divides, forming a globular pro-embryo. The basal cell also divides, giving rise to the suspensor. (b) In the second stage, the developing embryo has a heart shape due to the presence of cotyledons. (c) In the third stage, the growing embryo runs out of room and starts to bend. (d) Eventually, it completely fills the seed. (credit: OpenStax Biology, modification of work by Robert R. Wise; scale-bar data from Matt Russell)

The image below puts each of these steps in context with each other:

Image credit: LadyofHats Mariana Ruiz – based on Judd, Walter S. , Campbell, Christopher S. , Kellog, Elizabeth A. andStevens, Peter F. 1999. Plant Systematics: A PhylogeneticApproach.Sinauer Associates Inc.ISBN 0-878934049., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1671506

This video gives a simplified (but very engaging) overview of double fertilization, as well as reviewing flower structure:

Avoiding self-pollination

In angiosperms, pollination is the transfer of pollen from an anther to a stigma. Many plants can both self-pollinate and cross-pollinate. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual.

Individuals who are well-adapted to current conditions may not be well adapted if and when conditions change; therefore, genetic diversity is beneficial in changing environmental or stress conditions (this is the main advantage of sexual reproduction, after all!).   Although self-pollination less energetically demanding since it does not require production of nectar or extra pollen as food for pollinators, self-pollination leads to less genetic diversity in the population since genetic material from the same plant is used to form gametes, and eventually, the zygote. In contrast, cross-pollination (or out-crossing) leads to greater genetic diversity because the microgametophyte and megagametophyte are derived from different plants.

Because cross-pollination allows for more genetic diversity, evolution has selected for many ways to avoid self-pollination in different species:

  • The pollen and the ovary mature at different times
  • The flowers have physical features that prevent self-pollination, such differences in anther and stigma length
  • Male and female flowers located on different parts of the plant
  • The male and female flowers are located on different plants, such that each plant makes only male or only female gametophytes
  • “Incompatibility genes” prevent pollen from germinating into the stigma (illustrated below)

Self-incompatibility genes determine whether pollen can germinate, preventing fertilization by pollen with the same genotype.

Incompatibility genes are one of the more complex ways that plants prevent self-pollination. Self-incompatibility is controlled by a gene called the S (sterility) locus. If the pollen and the stigma have the same version (allele) of the gene, then then stigma sends signals that prevent the pollen from germinating.

Pollination Syndromes

It may sound like a disease, but pollination “syndrome” just means the way a particular plant species is pollinated. The majority of pollinators are animals, including insects (like bees, flies, and butterflies), bats, or birds. Some plant species are pollinated by abiotic agents, such as wind and water. Plants that are pollinated by animals must either produce nectar to attract and feed the animals, or extra pollen that is eaten by the animals. Plants that are pollinated by wind or water must produce massive quantities of pollen since the probability of the pollen landing on a stigma of the right species is low (wind and water pollination is analogous to broadcast spawning). The mechanism of pollination and the features of the flower are tightly linked:

  • Colored, highly scented flowers tend to be pollinated by beesbutterflies, wasps, or flies. These insects are active during the day, and are able to detect bright colors and have a strong sense of smell. Different smells attract different pollinators, with sweet smells attracting bees and butterflies, and rotting smells attracting flies. Many insect-pollinated flowers have additional color patterns in the UV range, which insects are capable of seeing while humans cannot.
  • White or pale-colored, highly scented flowers tend to be pollinated by moths and bats which are active at night. The light coloring makes them easier to see at night, and they tend to smell musky or fruity. Flowers pollinated by bats are larger than those pollinated by moths.
  • Brightly colored, odorless flowers tend to be pollinated by birds, which do not have a strong sense of smell. The flowers tend to have a curved, tubular shape to accommodate the bird’s beak.
  • Small green, petal-less flowers tend to be pollinated by wind. Wind-pollinated flowers do not produce nectar, but must produce excessive quantities of pollen. Gymnosperms such as pines, which do not have flowers, are also pollinated by wind.
  • Some aquatic plants are pollinated by water; the pollen floats and the water carries it to another flower.

Some examples of different pollination syndromes are shown below:

Left: Insects, such as bees, are important agents of pollination. Middle: Hummingbirds have adaptations that allow them to reach the nectar of certain tubular flowers. Right: A person knocks pollen from a pine tree. (credit: OpenStax Biology, left: modification of work by Jon Sullivan, Lori Branham)

And this video briefly describes the different pollination syndromes listed above:

Seed Dormancy and Germination

As described above, a seed enters a period of temporary development after fertilization; in most species, the seed then enters a period of stasis (inactivity), called dormancy. Dormancy is triggered by loss of up to 95% of the seed’s water content, which dehydrates the seed, causes extremely low metabolic activity, and “concentrates” the seed’s sugars to protect the cells from freezing during winter months. Dormancy can last months, years, or even centuries in some cases.

Once conditions are appropriate for seedling growth, the seed will then germinate or re-initiate development. The signal to initiate seed germination is indicator that conditions are favorable for growth and, depending on the species, can include:

  • water, indicating the start of the rainy season and rehyrdating the seed
  • specific wavelengths of light, indicating favorable sunlight conditions necessary for photosynthesis and the seed is not buried too far under the soil
  • a sustained period of cold, indicating that the seed does not germinate until the cold season is over
  • fire, typical of forest trees, indicating reduced competition from existing tall tree
  • scarification, or chemical treatment with acids, indicating that the seed has passed through the digestive tract of an animal

Fruit and Seed Dispersal

After fertilization, the ovary of the flower develops into the fruit. While we tend to think of fruits as being sweet, biologically a fruit is any structure that develops from an ovary after fertilization. The biological purpose of the fruit is seed dispersal, allowing the seed to be spread far from the mother plant, so they may find favorable and less competitive conditions in which to germinate and grow.

Some fruit have built-in mechanisms so they can disperse by themselves, whereas others require the help of agents like wind, water, and animals. As with pollination syndromes and flower structure, you can often predict a fruit’s dispersal mechanism based on structure, composition, and size:

  • Propulsion-dispersed fruits, such as violets, actually “explode” out of the plant.
  • Wind-dispersed fruits, such as dandelions, are lightweight and may have wing or parachute-like appendages that allow them to be carried by the wind.
  • Water-dispersed fruits, such as coconuts, are light or buoyant, giving them the ability to float.
  • Animal-dispersed fruits may be either have tiny “hooks” all over them so that they attach to passing animals and later fall off in a new location (like sandburs), or very sweet or fatty so that they will be eaten and deposited in a new location in the feces (like blackberries). Fruits dispersed by birds tend to be brightly colored as birds have a highly developed since of sight; fruits dispersed by mammals tend to be highly scented as mammals have a highly developed sense of smell.

Fruits and seeds are dispersed by various means. (a) Dandelion seeds are dispersed by wind, the (b) coconut seed is dispersed by water, and the (c) acorn is dispersed by animals that cache and then forget it. (credit OpenStax Biology a: modification of work by “Rosendahl”/Flickr; credit b: modification of work by Shine Oa; credit c: modification of work by Paolo Neo)