Chapter 39 Plant Response to Internal and External Signals Concept 39.1: 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 Figure 39.2 (a) Before exposure to light
(b) After a weeks exposure to natural daylight A potatos response to light is an example of cell-signal processing The stages are reception, transduction, and response
Figure 39.3 CELL WALL 1 Reception CYTOPLASM 2 Transduction
3 Response Relay proteins and second messengers Receptor Hormone or environmental stimulus
Plasma membrane Activation of cellular responses 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: 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 cGMP 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 plants 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
Figure 39.5 RESULTS Shaded side Control
Light Illuminated side Boysen-Jensen Light Darwin and Darwin
Light Gelatin (permeable) Tip removed Opaque
cap Transparent cap Opaque shield over curvature
Mica (impermeable) 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
walls fabric With the cellulose loosened, the cell can elongate Auxins 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 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 buds ability to suppress development of axillary buds If the terminal bud is removed, plants become bushier Gibberellins 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 cell elongation and cell division
Figure 39.10 (b) Grapes from control vine (left) and gibberellin-treated vine (right) (a) Rosette form (left) and gibberellin-induced bolting (right)
Brassinosteroids Brassinosteroids are chemically similar to the sex hormones of animals They induce cell elongation and division in stem segments They slow leaf abscission and promote xylem differentiation
Abscisic Acid Abscisic acid (ABA) slows growth Two of the many effects of ABA Seed dormancy Drought tolerance Seed Dormancy Seed dormancy ensures that the seed will germinate only in optimal conditions
In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold Precocious (early) germination can be caused by inactive or low levels of ABA Drought Tolerance ABA is the primary internal signal that enables plants to withstand drought
ABA accumulation causes stomata to close rapidly Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection The effects of ethylene include response to mechanical stress, senescence, leaf abscission,
and fruit ripening Senescence Senescence is the programmed death of cells or organs A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants
Fruit Ripening A burst of ethylene production in a fruit triggers the ripening process Ethylene triggers ripening, and ripening triggers release of more ethylene Fruit producers can control ripening by picking green fruit and controlling ethylene levels Photoperiodism and Responses to Seasons
Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year Photoperiodism is a physiological response to photoperiod Photoperiodism and Control of Flowering Some processes, including flowering in many species, require a certain photoperiod
Plants that flower when a light period is shorter than a critical length are called short-day plants Plants that flower when a light period is longer than a certain number of hours are called longday plants Flowering in day-neutral plants is controlled by plant maturity, not photoperiod Gravity Response to gravity is known as gravitropism
Roots show positive gravitropism; shoots show negative gravitropism Figure 39.UN03
