BIO 311D 1st Edition Lecture 31Outline of Last Lecture I. Flowers, double fertilization, and fruits are unique features of the angiosperm life cycleII. Flower Structure and FunctionIII. Development of Male Gametophytes in Pollen GrainsIV. Development of Female Gametophytes (Embryo Sacs)V. PollinationVI. Coevolution of Flower and PollinatorVII. Double FertilizationVIII. Seed Development, Form, and FunctionIX. Endosperm DevelopmentX. Structure of the Mature SeedOutline of Current Lecture I. Signal transduction pathways link signal reception to responseII. ReceptionIII. TransductionIV. ResponseV. Post-Translational Modification of Preexisting ProteinsVI. Transcriptional RegulationVII. Plant hormones help coordinate growth, development, and responses to stimuliVIII. The Discovery of Plant HormonesIX. AuxinX. Stem ElongationCurrent LectureSignal transduction pathways link signal reception to response• A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots• These are morphological adaptations for growing in darkness, collectively called etiolation• After exposure to light, a potato undergoes changes called de-etiolation, in which shootsand roots grow normally• A potato’s response to light is an example of cell-signal processing• The stages are reception, transduction, and responseReception• Internal and external signals are detected by receptors, proteins that change in responseto specific stimuli• In de-etiolation, the receptor is a phytochrome capable of detecting lightTransduction• Second messengers transfer and amplify signals from receptors to proteins that cause responses• Two types of second messengers play an important role in de-etiolation: Ca2+ ions and cyclic GMP (cGMP)• The phytochrome receptor responds to light by– Opening Ca2+ channels, which increases Ca2+ levels in the cytosol– Activating an enzyme that produces cGMPResponse• A signal transduction pathway leads to regulation of one or more cellular activities• In most cases, these responses to stimulation involve increased activity of enzymes• This can occur by transcriptional regulation or post-translational modificationPost-Translational Modification of Preexisting Proteins• Post-translational modification involves modification of existing proteins in the signal response• Modification often involves the phosphorylation of specific amino acids• The second messengers cGMP and Ca2+ activate protein kinases directlyTranscriptional Regulation• Specific transcription factors bind directly to specific regions of DNA and control transcription of genes• Some transcription factors are activators that increase the transcription of specific genes• Other transcription factors are repressors that decrease the transcription of specific genesDe-Etiolation (“Greening”) Proteins• De-etiolation activates enzymes that– Function in photosynthesis directly– Supply the chemical precursors for chlorophyll production– Affect the levels of plant hormones that regulate growthPlant hormones help coordinate growth, development, and responses to stimuli• Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plantThe Discovery of Plant Hormones• Any response resulting in curvature of organs toward or away from a stimulus is called a tropism• In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light• They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present• They postulated that a signal was transmitted from the tip to the elongating regionThe Discovery of Plant Hormones• In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance• In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experimentsA Survey of Plant Hormones• Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ• In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cellsAuxin• The term auxin refers to any chemical that promotes elongation of coleoptiles• Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA• Auxin is produced in shoot tips and is transported down the stem• Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cellThe Role of Auxin in Cell Elongation• According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane• The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric• With the cellulose loosened, the cell can elongateThe Role of Auxin in Cell Elongation• Auxin also alters gene expression and stimulates a sustained growth responseAuxin’s Role in Plant Development• Polar transport of auxin plays a role in pattern formation of the developing plant• Reduced auxin flow from the shoot of a branch stimulates growth in lower branches• Auxin transport plays a role in phyllotaxy, the arrangement of leaves on the stem• Polar transport of auxin from leaf margins directs leaf venation pattern• The activity of the vascular cambium is under control of auxin transportPractical Uses for Auxins• The auxin indolbutyric acid (IBA) stimulates adventitious roots and is used in vegetative propagation of plants by cuttings• An overdose of synthetic auxins can kill plants– For example 2,4-D is used as an herbicide on eudicots• Cytokinins are so named because they stimulate cytokinesis (cell division)Control of Cell Division and Differentiation• Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits• Cytokinins work together with auxin to control cell division and differentiationControl of Apical Dominance• Cytokinins, auxin, and strigolactone interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds• If the terminal bud is removed, plants become bushierAnti-Aging Effects• Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues• Gibberellins