Biol 118 1st Edition Lecture 22 Outline of Last Lecture I Introduction II Fertilization III Cleavage IV Gastrulation V Organogenesis VI Cell Differentiation Outline of Current Lecture I Introduction II Life Cycle of a Flowering Plant III Embryogenesis IV What Genes Proteins Set Up Body Axes V Growth Development VI What Genes Proteins Determine Leaf Shape VII Reproductive Development VIII ABC Model Current Lecture Introduction Plants grow and develop throughout lives o Most plant cells retain ability to de differentiate Arabidopsis thaliana is model organism These 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 grow o Produce large numbers of offspring o Complete entire life cycle in 6 weeks Life 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 embryogenesis o 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 nonreproductive organs o 3 vegetative organs leaves roots stems Later cells in stem are converted to reproductive structures producing flowers Embryogenesis Takes place inside ovule as the seed matures o Produces a tiny simplified plant Zygote divides asymmetrically producing large basal cell a small apical cell o Basal bottom gives rise to suspensor anchors the embryo as it develops o Apical top gives rise to mature embryo o 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 hypocotyl o Make up the shoot which becomes the aboveground portion of the plant body o Root forms belowground portion Shoot apical meristem SAM root apical meristem RAM form next o Meristem consists of undifferentiated cells that divide repeatedly some daughter cells become specialized cells o SAM Exists at the tips of shoots o 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 tissues o Epidermis Outer protective covering o Ground tissue Mass of cells that may later differentiate into specialized cells for photosynthesis food storage other function o Vascular tissue In center of plant differentiate into specialized cells that transport food water between root and shoot What Genes Proteins Set Up Body Axes Monopteros gene that is critical in setting up the apical basal axis o Codes for monopteros protein a transcription factor Auxin cell to cell signal molecule o Produced in the shoot apical meristem transported toward basal parts of the embryo o Concentration of auxin along apical basal axis forms concentration gradient that provides positional information o 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 axis Growth Development Plants must adjust to changing environmental conditions through continuous growth development of roots stems leaves o Possible because of the meristems that are located at the tips of shoots roots Further plant body development is driven by the meristems o 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 cues What 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 surface o Changes in phan expression may underlie some evolutionary changes in leaf shape Reproductive 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 organs o 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 another o 3 types of mutants 1 only carpels stamens 2 Only sepals carpels 3 only petals sepals ABC Model ABC Model hypothesis for genetic control of flower development Three basic ideas that underlie the model o Each of the 3 genes involved is expressed in 2 adjacent whorls o A total of four different combinations of gene products can occur o Each of the 4 combinations of gene products triggers the development of a different floral organ Hypotheses regarding the proteins governing flower development o A protein causes cell to form sepal o A B forms petals o B C specify stamens o C designates cells as precursors of carpels o A C cancel each other out MADS Box DNA binding domain contained in DNA sequences of floral organ identity genes o Part of the regulatory cascade that controls floral organ identity genes Floral genes encode regulatory transcription factors that bind to enhancers other regulatory sequences o Trigger the expression of genes required for sepal petal carpel stamen formation
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