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Biology LibreTexts

30.5B: Genetic Control of Flowers

  • Page ID
    13758
  • A variety of genes control flower development, which involves sexual maturation and growth of reproductive organs as shown by the ABC model.

    Learning Objectives

    • Diagram the ABC model of flower development and identify the genes that control that development

    Key Points

    • Flower development describes the process by which angiosperms (flowering plants) produce a pattern of gene expression in meristems that leads to the appearance of a flower; the biological function of a flower is to aid in reproduction.
    • In order for flowering to occur, three developments must take place: (1) the plant must reach sexual maturity, (2) the apical meristem must transform from a vegetative meristem to a floral meristem, and (3) the plant must grow individual flower organs.
    • These developments are initiated using the transmission of a complex signal known as florigen, which involves a variety of genes, including CONSTANS, FLOWERING LOCUS C and FLOWERING LOCUS T.
    • The last development (the growth of the flower’s individual organs) has been modeled using the ABC model of flower development.
    • Class A genes affect sepals and petals, class B genes affect petals and stamens, class C genes affect stamens and carpels.

    Key Terms

    • sepal: a part of an angiosperm, and one of the component parts of the calyx; collectively the sepals are called the calyx (plural calyces), the outermost whorl of parts that form a flower
    • stamen: in flowering plants, the structure in a flower that produces pollen, typically consisting of an anther and a filament
    • verticil: a whorl; a group of similar parts such as leaves radiating from a shared axis
    • biennial: a plant that requires two years to complete its life cycle
    • whorl: a circle of three or more leaves, flowers, or other organs, about the same part or joint of a stem
    • apical meristem: the tissue in most plants containing undifferentiated cells (meristematic cells), found in zones of the plant where growth can take place at the tip of a root or shoot.
    • angiosperm: a plant whose ovules are enclosed in an ovary
    • perennial: a plant that is active throughout the year or survives for more than two growing seasons
    • primordium: an aggregation of cells that is the first stage in the development of an organ

    Genetic Control of Flowers

    Flower development is the process by which angiosperms produce a pattern of gene expression in meristems that leads to the appearance of a flower. A flower (also referred to as a bloom or blossom) is the reproductive structure found in flowering plants. There are three physiological developments that must occur in order for reproduction to take place:

    image

    Anatomy of a flower: Mature flowers aid in reproduction for the plant. In order to achieve reproduction, the plant must become sexually mature, the apical meristem must become a floral meristem, and the flower must develop its individual reproductive organs.

    1. the plant must pass from sexual immaturity into a sexually mature state
    2. the apical meristem must transform from a vegetative meristem into a floral meristem or inflorescence
    3. the flowers individual organs must grow (modeled using the ABC model)

    Flower Development

    A flower develops on a modified shoot or axis from a determinate apical meristem (determinate meaning the axis grows to a set size). The transition to flowering is one of the major phase changes that a plant makes during its life cycle. The transition must take place at a time that is favorable for fertilization and the formation of seeds, hence ensuring maximal reproductive success. In order to flower at an appropriate time, a plant can interpret important endogenous and environmental cues such as changes in levels of plant hormones and seasonable temperature and photoperiod changes. Many perennial and most biennial plants require vernalization to flower.

    Genetic Control of Flower Development

    When plants recognize an opportunity to flower, signals are transmitted through florigen, which involves a variety of genes, including CONSTANS, FLOWERING LOCUS C and FLOWERING LOCUS T. Florigen is produced in the leaves in reproductively favorable conditions and acts in buds and growing tips to induce a number of different physiological and morphological changes.

    From a genetic perspective, two phenotypic changes that control vegetative and floral growth are programmed in the plant. The first genetic change involves the switch from the vegetative to the floral state. If this genetic change is not functioning properly, then flowering will not occur. The second genetic event follows the commitment of the plant to form flowers. The sequential development of plant organs suggests that a genetic mechanism exists in which a series of genes are sequentially turned on and off. This switching is necessary for each whorl to obtain its final unique identity.

    ABC Model of Flower Development

    In the simple ABC model of floral development, three gene activities (termed A, B, and C-functions) interact to determine the developmental identities of the organ primordia (singular: primordium) within the floral meristem. The ABC model of flower development was first developed to describe the collection of genetic mechanisms that establish floral organ identity in the Rosids and the Asterids; both species have four verticils (sepals, petals, stamens and carpels), which are defined by the differential expression of a number of homeotic genes present in each verticil.

    In the first floral whorl only A-genes are expressed, leading to the formation of sepals. In the second whorl both A- and B-genes are expressed, leading to the formation of petals. In the third whorl, B and C genes interact to form stamens and in the center of the flower C-genes alone give rise to carpels. For example, when there is a loss of B-gene function, mutant flowers are produced with sepals in the first whorl as usual, but also in the second whorl instead of the normal petal formation. In the third whorl the lack of B function but presence of C-function mimics the fourth whorl, leading to the formation of carpels also in the third whorl.

    image

    ABC model of flower development: Class A genes (blue) affect sepals and petals, class B genes (yellow) affect petals and stamens, class C genes (red) affect stamens and carpels.

    Most genes central in this model belong to the MADS-box genes and are transcription factors that regulate the expression of the genes specific for each floral organ.

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