BIO 311D 1st Edition Lecture 31 Outline of Last Lecture I Flowers double fertilization and fruits are unique features of the angiosperm life cycle II Flower Structure and Function III Development of Male Gametophytes in Pollen Grains IV Development of Female Gametophytes Embryo Sacs V Pollination VI Coevolution of Flower and Pollinator VII Double Fertilization VIII Seed Development Form and Function IX Endosperm Development X Structure of the Mature Seed Outline of Current Lecture I Signal transduction pathways link signal reception to response II Reception III Transduction IV Response V Post Translational Modification of Preexisting Proteins VI Transcriptional Regulation VII Plant hormones help coordinate growth development and responses to stimuli VIII The Discovery of Plant Hormones IX Auxin X Stem Elongation Current Lecture Signal 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 shoots and roots grow normally A potato s response to light is an example of cell signal processing The stages are reception transduction and response Reception Internal and external signals are detected by receptors proteins that change in response to specific stimuli In de etiolation the receptor is a phytochrome capable of detecting light Transduction 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 Ca 2 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 cGMP Response 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 modification Post 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 directly Transcriptional 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 genes De 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 growth Plant 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 plant The 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 region The 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 experiments A 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 cells Auxin 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 cell The 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 elongate The Role of Auxin in Cell Elongation Auxin also alters gene expression and stimulates a sustained growth response Auxin 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 transport Practical 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 differentiation Control 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 bushier Anti 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 have a variety of effects such as stem elongation fruit growth and seed germination Stem Elongation Gibberellins are produced in young roots and leaves Gibberellins stimulate growth of leaves and stems In stems they stimulate