Biol 118 1st Edition Lecture 22Outline of Last Lecture I. IntroductionII. FertilizationIII. Cleavage IV. GastrulationV. OrganogenesisVI. Cell DifferentiationOutline of Current Lecture I. IntroductionII. Life Cycle of a Flowering PlantIII. EmbryogenesisIV. What Genes & Proteins Set Up Body AxesV. Growth & DevelopmentVI. What Genes & Proteins Determine Leaf Shape?VII. Reproductive DevelopmentVIII. ABC ModelCurrent LectureIntroduction- Plants grow and develop throughout liveso Most plant cells retain ability to de-differentiate - Arabidopsis thaliana is model organismThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Relatively easy to growo Produce large numbers of offspring o Complete entire life cycle in 6 weeksLife Cycle of a Flowering Plant- Life cycle begins with gametogenesis: gamete formation- Fertilization: occurs when sperm and egg combine in a womb-like ovule inside the protective female reproductive structure of a flower- Development continue inside ovule with embryogenesiso Many plants, ends with the maturation of ovule into a seed Contains dormant embryo & supply of nutrients; surrounded by protective coat- When conditions are favorable, seed undergoes germination: forms seedling- Seedling undergoes organogenesis & becomes an adult plant with vegetative (non-reproductive) organso 3 vegetative organs: leaves, roots & stems Later cells in stem are converted to reproductive structures, producing flowersEmbryogenesis- Takes place inside ovule as the seed matureso Produces a tiny, simplified plant- Zygote divides asymmetrically producing large basal cell& a small apical cello Basal (bottom): gives rise to suspensor: anchors the embryo as it developso Apical (top): gives rise to mature embryoo Help establish the apical-basal axis (top and bottom) of plant- Radial axis (inside & outside) is established when embryo is in its globular state- Initial leaves (cotyledons) are connected to root by stem-like hypocotylo Make up the shoot, which becomes the aboveground portion of the plant bodyo Root forms belowground portion- Shoot apical meristem (SAM) & root apical meristem (RAM) form nexto Meristem: consists of undifferentiated cells that divide repeatedly & some daughter cells become specialized cells o SAM: Exists at the tips of shootso RAM: Found at the tips of the roots- Plant growth and development take place without cell migration- Plant embryonic structures take shape because cell divisions occur in precise orientations- Along with 2 body axes produced, also produces 3 embryonic tissueso Epidermis: Outer protective coveringo Ground tissue: Mass of cells that may later differentiate into specialized cells for photosynthesis, food storage & other functiono Vascular tissue: In center of plant; differentiate into specialized cells that transport food & water between root and shootWhat Genes & Proteins Set Up Body Axes- Monopteros: gene that is critical in setting up the apical-basal axiso Codes for monopteros protein, a transcription factor- Auxin: cell-to-cell signal moleculeo Produced in the shoot apical meristem- transported toward basal parts of the embryoo Concentration of auxin along apical-basal axis forms concentration gradient that provides positional informationo Part of a regulatory cascade that triggers monopteros production & other regulatory transcription factors specific to cells in the developing hypocotyl and roots, setting up the apical-basal axisGrowth & Development- Plants must adjust to changing environmental conditions through continuous growth & development of roots, stems, & leaveso Possible because of the meristems that are located at the tips of shoots & roots- Further plant body development is driven by the meristemso Allow the plant to grow in any direction both above and below the ground o Rate and direction of cell growth are dictated by cell-cell signals produced as a result of environmental cuesWhat Genes & Proteins Determine Leaf Shape?- Initiation of leaf developments depends on the concentration of auxin in parts of the SAM- Three leaf axes form:o Proximal-distal (end-tip)o Lateral (left-right)o Upper-lower (adaxial-abaxial)- Gene called phantastica is critical in setting up the upper-lower axis of leaves o Protein product is regulatory transcription factor that triggers expression of genes that cause cells to form the upper surface of leaves & suppresses transcription of genes required for forming the lower leaf surfaceo Changes in phan expression may underlie some evolutionary changes in leaf shapeReproductive Development- Unlike animals, plants do not have germ cells that are set aside early in development - Floral meristem: modified shoot apical meristem that produces flowers containing reproductive organso Produces 4 whorls of organs- modified leaves Sepals- Found on outside of the flower & provide it with protection Petals- Inside sepal- Enclose the reproductive organs - Colored to attract pollinators Stamens- Inside petals- Produce pollen Carpels- In the middle- Contain egg-producing ovules- Several types of mutant flowering plants are homeotic mutants in which one kind of floral organ is replaced by anothero 3 types of mutants 1: only carpels & stamens 2: Only sepals & carpels 3: only petals & sepalsABC Model- ABC Model: hypothesis for genetic control of flower development- Three basic ideas that underlie the modelo Each of the 3 genes involved is expressed in 2 adjacent whorlso A total of four different combinations of gene products can occuro Each of the 4 combinations of gene products triggers the development of a different floral organ- Hypotheses regarding the proteins governing flower developmento A protein: causes cell to form sepalo A + B: forms petalso B + C: specify stamenso C: designates cells as precursors of carpelso A & C cancel each other out- MADS Box: DNA binding domain contained in DNA sequences of floral organ identity geneso Part of the regulatory cascade that controls floral organ identity genes- Floral genes encode regulatory transcription factors that bind to enhancers/other regulatory sequenceso Trigger the expression of genes required for sepal, petal, carpel & stamen
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