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Plant Responses to Internal & External Signals

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Presentation on theme: "Plant Responses to Internal & External Signals"— Presentation transcript:

1 Plant Responses to Internal & External Signals
CAMPBELL & Reece Chapter 39

2 Stimuli & a Stationary Life
observation about plants: 1 part of plant sends signals to another sense gravity & direction of light responds to environmental stimuli & internal signals

3 Stimuli & the Stationary Life
animals have behavioral responses to stimuli plants generally respond to environmental cues by adjusting its patterns of growth & development as result: plants of same species vary in body form much more than animals

4 all organisms have ability to receive specific environmental & internal signals & respond to them in ways that enhance survival & reproductive success. Plant cells have cellular receptors used to detect important changes in their environment Change in daylight hours Insect eating their leaves

5 Signal Transduction Pathways
@ cellular level plants & all other eukaryotes are surprisingly similar in signaling mechanisms in order to respond to any stimuli cell must have receptor molecule that is sensitive to & affected by specific stimuli

6 Etiolation / De-etiolation
morphological adaptations for growing in the dark: plant in dark allocates as much nrg as possible to elongation of stems  break ground b/4 exhausts nutrients in tubers After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.

7 Signal Transduction Step 1: Reception proteins that change shape in response to specific stimuli usually a weak signal binds to a receptor causing it to undergo a conformational change

8 Signal Transduction amplification of message thru 2nd messengers
Step 2: Transduction amplification of message thru 2nd messengers 2nd messengers: small molecules or ions transfer signal from receptor to other proteins that carry out the response

9 Signal Transduction Step 3: Response usually involves increasing activity of an enzyme by: post-translational modification of pre-existing proteins transcriptional regulation

10 Signal Transduction in Plants

11 Post-Translational Modification of Pre-Existing Proteins
mostly involves phosphorylation of specific a.a.  alters protein’s hydrophobicity & activity cAMP & Ca++ activate protein kinases which then phosphorylates another protein   phosphorylation of a transcription factor Protein phosphatases dephosphorylate specific protein = off switch for activated proteins

12 Transcriptional Regulation
changing transcription factors  turns genes on (or off) involves transcription factors or repressors probably mechanism used for developmental changes

13 De-Etiolation (“Greening”) Proteins
proteins involved in making chlorophyll precursors or certain plant hormones are either synthesized or activated


15 Plant Hormones hormone (Gr): to excite chemical messengers produced in 1 part of organism & transported to other parts bind to specific receptors in target cells trigger responses some scientists consider plant hormones as plant growth regulators to describe organic cpds (natural or synthetic) that modify or control 1 or more specific physiological processes in a plant

16 Plant Hormones control every aspect of plant growth & development to some degree

17 Phototropism Tropism: any growth response that results in plant organs curving toward or away from stimuli Phototropism: growth towards light (+ phototropism) or away from light (- phototropism)

18 Phototropism

19 Phototropism & Auxin

20 Auxin

21 Auxin produced in shoot apical meristems & young leaves
high levels found in developing fruits & seeds Functions: stimulates cell elongation promotes formation of lateral & adventitious roots regulates development of fruit enhances apical dominance functions in phototropism & gravitropism promotes vascular differentiation


23 Abscisic Acid (ABA) can be made in all plant cells, found in all plant tissues Functions: inhibits growth promotes stomata closure during drought stress promotes seed dormancy, leaf senescence promotes desiccation tolerance

24 ABA

25 Cytokinins mostly made in roots  transported up Functions:
regulate cell division in shoots & roots modify apical dominance promotes lateral bud growth promotes movement of nutrients to sink tissues stimulates seed germination delays leaf senescence


27 Gibberellins found in meristems of apical buds & roots
young leaves, & developing seeds are primary sites of production Functions: stimulate stem elongation, pollen development, pollen tube growth, fruit growth, & seed development & germination regulate sex determination & transition from juvenile to adult phases

28 Gibberellins

29 Brassinosteroids found in all plant tissues, several types, act locally Functions: in shoots promote cell division promote root growth when low concentrations, when high retard growth promote xylem differentiation inhibit phloem differentiation promote seed germination & pollen tube formation

30 Brassinosteroids

31 Strigolactones carotenoid-derived made in roots in response to low phosphate levels or high auxin flow from shoots Functions: promote seed germination control of apical dominance controls mycorrhizal fungi attraction to root

32 Strigolactones

33 Ethylene gas, produced by most parts of plant
amt increases as plant ages or during ripening of fruit or if plant wounded or stressed Functions: promotes ripening of most fruits enhances rate of senescence promotes root & root hair formation promotes flowering in pineapple family

34 Hormones that Affect Seed Germination


36 Plant Responses to Light
Photomorphogenesis: the effects of light on plant morphology effects of light on plants includes: photosynthesis triggers key events in plant growth & development allows plants to measure the passage of days & seasons

37 Plant Responses to Light
plants detect the presence or absence intensity direction wavelength (color) of light

38 Action Spectrum depicts the relative effectiveness of different wavelengths of radiation in driving a particular process useful in studying any process that depends on light (phototropism included) by comparing action spectra with absorption spectra of pigments  close correspondence for a given pigment suggests the pigment is the photoreceptor mediating the response

39 2 Major Classes of Light Receptors
action spectra reveal that red & blue light most important colors in regulating a plant’s photomorphogenesis Major Classes of light receptors: Blue-light photoreceptors Phytochromes


41 Blue-Light Photoreceptors
Functions: phototropism opening of stomata slowing of hypocotyl elongation that occurs when seedling breaks ground

42 Blue-Light Photoreceptors
plants use 3 or more photoreceptors to detect blue light Cryptochrome similar to DNA repair enzymes inhibits stem elongation Phototropin protein kinase mediates phototrophic curvature Zeaxanthin with #2  stoma opening

43 Phytochromes as Photoreceptors
act like molecular “on/off” switches red light turns them on far-red light turns them off regulate: shade avoidance germination of many seeds

44 Photoreversible States of Phytochrome

45 Phytochromes & Shade Avoidance
provides plant with information about quality of light during daylight hrs amt of red & far-red light ~= plants use ratio of the 2 to determine quality of light ex: tree under canopy getting more far-red than red light  uses its resources to grow taller/ tree getting mostly red light will use resources to grow bushier

46 Circadian Rhythms cycles with ~ 24 frequencies not affected by any known environmental variables

47 Photoperiodism regulates time of flowering in many species: Short-Day Plants require a night longer than some critical value to flower ex: mums, poinsiettias, some varieties of tobacco, soy beans Long-Day Plants need night length shorter than some critical value to flower ex: spinach, radish, lettuce, irises, many cereals

48 Photoperiodism flower when reach certain maturity
3. Day-Neutral Plants unaffected by photoperiod flower when reach certain maturity ex: tomatoes, rice, dandelions

49 Photoperiodism some plants require 1 single exposure to photoperiod required to flower others need several successive days of required times & others only respond to photoperiod if previously exposed to some environmental stimulus (period of cold or warm weather) Vernalization: period of cold b/4 flowering

50 Vernalization

51 Florigen flowering signal, probably a protein, made in leaves under certain conditions travels  shoot apical meristems inducing them to switch from vegetative  reproductive growth


53 Gravitropism bending of an organ in response to gravity
roots show + gravitropism shoots show – gravitropism stratoliths (starch-filled plastids) enable roots to detect gravity

54 Thigmotropism measuring leaf length affects its future growth
growth response to touch changes in plant form due to mechanical pertubation plants very sensitive to mechanical stress ex: measuring leaf length affects its future growth

55 Thigmotropism resemble nervous system action potentials
rapid leaf movements involve transmission of electrical impulses called action potentials resemble nervous system action potentials but thousands times slower

56 Environmental Stresses
Major Response DROUGHT ABA production, reducing water loss by closing stomata FLOODING Formation of air tubes that help roots survive O2 deprivation SALT Avoiding osmotic water loss by producing solutes high concentrations HEAT Synthesis of heat-shock proteins , which reduce protein high temperatures COLD Adjusting membrane fluidity, avoiding osmotic water loss, producing antifreeze proteins


58 Responses to Attacks by Herbivores
plants release chemicals that are distasteful or toxic ex: canavanine an unusual a.a., similar to arginine insect that eats plant incorporates this a.a. in place of arginine which adversely affects protein shape  alters functions  insect dies jackbean

59 Defenses Against Herbivores
some plants able to attract predatory animals that help defend plant against herbivores Ex: parasitoid wasps inject their eggs into caterpillars eating plant eggs hatch inside & larvae eat their way out

60 Defenses Against Herbivores
chemicals released in response to herbivores can also function as early warning system for nearby plants of same species in response neighboring plants release biochemical responses making them less vulnerable to attack ex: strawberries, lima beans


62 Defenses Against Pathogens
Hypersensitive Response: seals off infection & destroys both pathogen & infected host cells in region is localized & specific infected area release antimicrobial molecules use methlysalicyclic acid as signaling chemical to rest of plant rest of plant then activates systemic acquired resisitance


64 System Acquired Resistance
nonspecific protection against diverse pathogens


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