Plant Responses to Internal and External Signals Chapter 39 Plant Responses to Internal and External Signals

Overview: Stimuli and a Stationary Life Linnaeus noted that flowers of different species opened at different times of day and could be used as a horologium florae, or floral clock Plants, being rooted to the ground, must respond to environmental changes that come their way For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light © 2011 Pearson Education, Inc.

Figure 39.1 Figure 39.1 Can flowers tell you the time of day?

(1) CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Figure 39.3 (1) CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Relay proteins and Activation of cellular responses second messengers Receptor Figure 39.3 Review of a general model for signal transduction pathways. Hormone or environmental stimulus Plasma membrane

Glucose 1-phosphate (108 molecules) Figure 11.16 Reception Binding of epinephrine to G protein-coupled receptor (1 molecule) Transduction (41) Inactive G protein Active G protein (102 molecules) Inactive adenylyl cyclase Active adenylyl cyclase (102) ATP Cyclic AMP (104) Inactive protein kinase A Active protein kinase A (104) Figure 11.16 Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine. Inactive phosphorylase kinase Active phosphorylase kinase (105) Inactive glycogen phosphorylase Active glycogen phosphorylase (106) Response Glycogen Glucose 1-phosphate (108 molecules)

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 (2-3) © 2011 Pearson Education, Inc.

(a) Before exposure to light (b) Figure 39.2 Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes. (a) Before exposure to light (b) After a week’s exposure to natural daylight

Activation of cellular responses second messengers Figure 39.3 A potato’s response to light is an example of cell-signal processing The stages are reception, transduction, and response CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Relay proteins and Activation of cellular responses second messengers Receptor Figure 39.3 Review of a general model for signal transduction pathways. Hormone or environmental stimulus Plasma membrane

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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

De-etiolation (greening) response proteins Figure 39.4-3 (4-6) 1 Reception 2 Transduction 3 Response Transcription factor 1 CYTOPLASM NUCLEUS Plasma membrane cGMP Protein kinase 1 P Second messenger Transcription factor 2 Phytochrome P Cell wall Protein kinase 2 Transcription Light Translation Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response. Ca2 channel De-etiolation (greening) response proteins Ca2

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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Activated relay molecule Figure 11.10 Signaling molecule Receptor Activated relay molecule Inactive protein kinase 1 Active protein kinase 1 Inactive protein kinase 2 ATP Phosphorylation cascade ADP Active protein kinase 2 P PP P i Figure 11.10 A phosphorylation cascade. Inactive protein kinase 3 ATP ADP P Active protein kinase 3 PP P i Inactive protein ATP ADP P Active protein Cellular response PP P i

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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli Hormones were first discovered and animals and defined with the following criteria: 1. signal produced that acts elsewhere in the body 2. Binds to a specific receptor and triggers responses 3. Travels via circulatory system Plant hormones (aka plant growth regulators) are chemical signals that modify or control one or more specific physiological processes within a plant (7-8) © 2011 Pearson Education, Inc.

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 (9) © 2011 Pearson Education, Inc.

(10-11) RESULTS Shaded side Control Light Illuminated side Figure 39.5 RESULTS (10-11) Shaded side Control Light Illuminated side Boysen-Jensen Light Darwin and Darwin Figure 39.5 Inquiry: What part of a grass coleoptile senses light, and how is the signal transmitted? Light Gelatin (permeable) Mica (impermeable) Tip removed Opaque cap Trans- parent cap Opaque shield over curvature

Figure 39.6 Control RESULTS Excised tip on agar cube Growth-promoting chemical diffuses into agar cube Control (agar cube lacking chemical) Offset cubes Went Experiment He gave this substance the name auxin (greek for increase). Its structure was later found to be IAA (indoleacetic acid) (12-13) Figure 39.6 Inquiry: Does asymmetrical distribution of a growth-promoting chemical cause a coleoptile to grow toward the light?

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 © 2011 Pearson Education, Inc.

Auxin Roles Stimulates stem elongation (low concentrations) Promotes formation of lateral and adventitius roots Regulates development of fruit Enhances apical dominance Fuctions in phototropism and gravitropism Promotes vascular differentiation Retards leaf abscission (14) © 2011 Pearson Education, Inc.

Practical Uses for Auxins The auxin indolbutyric acid (IBA) stimulates adventitious roots and is used in vegetative propagation of plants by cuttings Monocots have inactivating enzymes, dicots do not An overdose of synthetic auxins can kill plants For example 2,4-D is used as an herbicide on eudicots (15) © 2011 Pearson Education, Inc.

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 in shoots and roots. (16-17partial next two on following slides) © 2011 Pearson Education, Inc.

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 Cytokinin promotes lateral bud growth. © 2011 Pearson Education, Inc.

“Stump” after removal of apical bud Figure 39.9 Lateral branches “Stump” after removal of apical bud (b) Apical bud removed Figure 39.9 Apical dominance. Axillary buds (a) Apical bud intact (not shown in photo) (c) Auxin added to decapitated stem

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 © 2011 Pearson Education, Inc.

Gibberellins Gibberellins have a variety of effects, such as 1) stem elongation, 2) fruit growth, 3) pollen development and 4)seed germination (18) © 2011 Pearson Education, Inc.

Gibberellins are produced in young roots and leaves 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 © 2011 Pearson Education, Inc.

Grapes from control vine (left) and gibberellin-treated vine (right) Figure 39.10 (b) Grapes from control vine (left) and gibberellin-treated vine (right) Figure 39.10 Effects of gibberellins on stem elongation and fruit growth. (a) Rosette form (left) and gibberellin-induced bolting (right)

Abscisic Acid Abscisic acid (ABA) slows growth Unlike previous discussed hormones it inhibits growth When first discovered it was thought to play a primary role in leaf abscission (no longer thought) Two of the many effects of ABA Seed dormancy Drought tolerance (Closes stomata) Inhibited growth (19-20) © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection Auxin can also stimulate ethylene production. The effects of ethylene include response to mechanical stress, senescence, leaf abscission, and fruit ripening (21-22) © 2011 Pearson Education, Inc.

The Triple Response to Mechanical Stress Ethylene induces the triple response, which allows a growing shoot to avoid obstacles The triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth © 2011 Pearson Education, Inc.

Ethylene concentration (parts per million) Figure 39.13 Figure 39.13 The ethylene-induced triple response. 0.00 0.10 0.20 0.40 0.80 Ethylene concentration (parts per million)

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 © 2011 Pearson Education, Inc.

Leaf Abscission A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls © 2011 Pearson Education, Inc.

Table 39.1 Table 39.1 Overview of Plant Hormones

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 © 2011 Pearson Education, Inc.

Concept 39.3: Responses to light are critical for plant success Light cues many key events in plant growth and development Effects of light on plant morphology are called photomorphogenesis Plants detect not only presence of light but also its direction, intensity, and wavelength (color) A graph called an action spectrum depicts relative response of a process to different wavelengths © 2011 Pearson Education, Inc.

Phytochromes absorb red wavelengths of light. Different plant responses can be mediated by the same or different photoreceptors There are two major classes of light receptors: blue-light photoreceptors and phytochromes Phytochromes absorb red wavelengths of light. Red light (660nm) increased germination, while far-red light (730nm) inhibited germination The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome (24-25 & 27) © 2011 Pearson Education, Inc.

RESULTS Red Dark Red Far-red Dark Dark (control) Red Far-red Red Dark Figure 39.17 RESULTS Red Dark Red Far-red Dark Dark (control) Figure 39.17 Inquiry: How does the order of red and far-red illumination affect seed germination? Red Far-red Red Dark Red Far-red Red Far-red

Blue-Light Photoreceptors Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism (26) © 2011 Pearson Education, Inc.

(28-30) Cross out 31 Pr Pfr Red light Figure 39.19 Pr Pfr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Figure 39.19 Phytochrome: a molecular switching mechanism. Slow conversion in darkness (some plants) Enzymatic destruction (28-30) Cross out 31

Biological Clocks and Circadian Rhythms Many plant processes oscillate during the day Many legumes lower their leaves in the evening and raise them in the morning, even when kept under constant light or dark conditions © 2011 Pearson Education, Inc.

Figure 39.20 Figure 39.20 Sleep movements of a bean plant (Phaseolus vulgaris). Noon Midnight

Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock” Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle The clock may depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes (32 Skip examples) © 2011 Pearson Education, Inc.

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 (33) © 2011 Pearson Education, Inc.

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 long-day plants Flowering in day-neutral plants is controlled by plant maturity, not photoperiod (34) © 2011 Pearson Education, Inc.

Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness © 2011 Pearson Education, Inc.

(35) 24 hours (a) Short day (long-night) plant Light Flash of light Figure 39.21 24 hours (a) Short day (long-night) plant Light Flash of light Darkness Critical dark period (b) Long-day (short-night) plant Figure 39.21 Photoperiodic control of flowering. (35) Flash of light

Red light can interrupt the nighttime portion of the photoperiod A flash of red light followed by a flash of far-red light does not disrupt night length Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light © 2011 Pearson Education, Inc.

Short-day (long-night) plant Long-day (short-night) plant Figure 39.22 24 hours R R FR Figure 39.22 Reversible effects of red and far-red light on photoperiodic response. R FR R R FR R FR Short-day (long-night) plant Long-day (short-night) plant Critical dark period

Long-day plant grafted to short-day plant Figure 39.23 24 hours 24 hours 24 hours Graft Figure 39.23 Experimental evidence for a flowering hormone. Short-day plant Long-day plant grafted to short-day plant Long-day plant

A Flowering Hormone? Photoperiod is detected by leaves, which cue buds to develop as flowers The flowering signal is called florigen Florigen may be a macromolecule governed by the FLOWERING LOCUS T (FT) gene (36) © 2011 Pearson Education, Inc.

Figure 39.UN03 Figure 39.UN03 Summary figure, Concept 39.2