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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:
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. Though all plants display an alternation of generations life cycle, there are significant variations in different lineages of plants, consistent with their evolutionary history:
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 angiospermsThe 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.
All the information below is specific to angiosperms, unless otherwise noted. A typical flower has four “layers,” illustrated and described below from external to internal structures:
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. 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. 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. 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. Double FertilizationThe 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. 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. The image below puts each of these steps in context with each other: 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:
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 SyndromesIt 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:
Some examples of different pollination syndromes are shown below: And this video briefly describes the different pollination syndromes listed above: 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:
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:
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