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Chapter SummaryThe reproductive organ in plants is the flower; thus when a shoot apical meristem shifts from vegetative, or non-reproductive, development to reproductive development, there is a corresponding shift from leaf production to flower production in the shoot apical meristem. A meristem that has shifted from making leaves to making flowers is called an inflorescence meristem [fig. 10.1]. The reduced leaves that are made during flowering are called bracts and are morphologically different from leaves. The axillary meristems that arise during flowering are called floral meristems and are located at the axil junction of the bracts and the stem.Flowering is triggered by a combination of internal and environmental cues: daylength [fig. 10.2], signaling from the leaves [fig. 10.3], and temperature. Some arctic plants require cold temperatures before flowering, called vernalization, and this is independent of daylength. During flower production, the inflorescence meristem initiates the formation of bracts and floral meristems. Then the floral meristems initiate the formation of floral organs to form flowers. Genes involved in flowering effect the morphology of flowers, and some of these mutants are important for agriculture, such as the cal mutation in cauliflower [fig. 10.4]. Other genes involved in flowering effect the initiation of flowering by effecting the transition from inflorescence meristem to floral meristem, such as the leafy gene [fig. 10.5]. Most flowering plants produce flowers with a whorled phyllotaxy, usually four distinct whorls of floral organs with radial symmetry [fig. 10.6]. The flower is thus patterned in concentric circles of whorls numbered whorl 1 (W1) at the periphery through whorl 4 (W4) at the center. Whorl 1 (W1) will make the sepals of the flower, which are photosynthetic leaflike structures located at the base of the flower that protect the flower bud. Whorl 2 (W2) will make the petals of the flower, and will attract pollinators to the flower. Whorl 3 (W3) will form the stamens, which are the male parts of the flower, and will make the male gametes and store them in the anthers. The innermost whorl 4 (W4) will form the carpels, which are the female parts of the flower and will produce an ovary and eggs. The patterning of the flower and the differentiation of the distinct whorls are controlled by different genes, which have been identified by studying mutations that alter whorl identity [summarized in table 10.1]. Distinct expression domains for these different genes in the floral meristem can be detected before the initiation and development of floral primordium [fig. 10.7]. When considering plant reproduction, it is important to understand that during the plant life cycle there is an alternation of generations between the diploid stage (2n), called the sporophyte, and the haploid stage (n), called the gametophyte [fig. 10.8]. The sporophyte makes the spores of the plant, and the gametophyte makes the male and female gametes of the plant. When we consider a typical flowering plant, we are observing the sporophyte portion of the life cycle, which begins with seed germination. The sporophyte represents the period between fertilization and meiosis, which produces the spores. The gametophyte generation represents the period during which the spores form the male and female gametes within the flower; the gametophyte phase ends with fertilization. Male spores, or microspores, will form pollen grains, which are the mature male gametophyte, and they are made inside the male reproductive organ, the stamen. Female spores, or megaspores, will form the embryo sac, which is the mature female gametophyte, and it is located inside the female reproductive organ, the pistil. The pistil is a very large organ located in the center of the flower and consists of the ovary topped by the style and the stigma, where pollination will occur. The ovary encloses one or several ovules where the megaspores and embryo sacs are formed and which are attached to the ovary by a stalk [fig. 10.10]. The embryo sac is a complex structure, containing different cell types: an egg cell, two synergid cells, three antipodals, and a binucleate central cell. The embryo sac is polarized and has a micropylar end, where the egg cell is located, and a chalazal end, where the antipodals are located. During pollination, the pollen grain lands on the female stigma located at the tip of the pistil [fig. 10.9], which will only allow pollen from the same species to germinate. The pollen grain will send out a long pollen tube that will extend through the style, allowing passage of the sperm cells to the embryo sac for fertilization to occur. During fertilization, one sperm cell will fertilize the egg cell to form the diploid embryo, and the other sperm cell will unite with the two central cells to form the triploid endosperm, which will provide nutrients for the developing embryo. The products of fertilization are packaged into a seed, which is formed by the surrounding ovules within the pistil of the flower and represents the final stage of ovule development. Fertilization will also induce formation of the seed coat and fruit development in fruit bearing plants. In most plants, the seed desiccates and imposes a period of dormancy on embryogenesis. During this time the cotyledons, or "seed leaves," serve as food storage units and absorptive organs in monocots (having one cotyledon), dicots (having two coyledons), and gymnosperms (having many cotyledons). The early cell divisions of a plant embryo separate the embryo proper from the suspensor cell and establish cells that will become the protoderm [fig. 10.11]. In the earliest stages the future shoot apical meristem (which comes from apical cells of the embryo proper) and root meristem (which comes from the uppermost cell of the suspensor, called the hypophysis) can be identified. Development proceeds through many stages (globular, heart, torpedo, and curled) until the cotyledons are fully formed. Plant embryos show domains of gene expression early in development during the globular stage and suggest that developmental compartments form early and may provide insight into how the shoot apical meristem is formed during early embryogenesis [fig. 10.14]. It is important to remember that plants can undergo sexual and asexual reproduction, and embryos can be formed either sexually or asexually. The process by which a plant produces seed asexually, in the absence of fertilization, is called apomixy [fig. 10.12]. Apomictic development can be the result of either an unreduced female megaspore, called diplospory, or a cell from the nucellus, called apospory. Plant embryos can also arise from somatic tissue when exposed to the proper hormones [fig. 10.13], and these plants do not form seed and do not become dormant.
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