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AccessScience from McGraw-Hill Education Page 1 of 10 www.accessscience.com 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 AccessScience from McGraw-Hill Education Page 2 of 10 www.accessscience.com WIDTH:CFig. 1 Vegetative reproduction through storage organs: tuber, Solanum tuberosum, potato; rhizome, Cyperus alternifolius, umbrella plant; runner, Fragaria × ananassa, strawberry; bulb, Allium cepa,onion;corm,Gladiolus sp. (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 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 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 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 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
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