Chapter 39: Plant responses to internal

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Chapter 39 Plant responses to internal
external signals

• Plants sessile
• must respond to environmental changes/cues by
  adjusting patterns of growth development
• Signal transduction pathways in plants
• (ex. De- etiolation/greening)
• Environmental signal? conformational change in
  protein
• Ex. Phytochrome (photoreceptor) in cytoplasm
  detects light
• Second messengers amplify signal
• Ex. Phytochrome activation? increases levels of
  cGMP Ca2? changes in ion channels? activating
  protein kinases?activate other enzymes
• Regulation of one or more cell activities
• usually by activating enzymes or stimulating gene
  transcription
• Ex. Secondary messengers activate transcription
  factors? stimulate photosynthesis enzymes,
  chlorophyll production, or affect hormones that
  regulate growth

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Plant hormones their responses

• Hormoneschemical signals that coordinate
  different parts of an organism
• Types
• Auxin (AA)
• Cytokinins
• Gibberellins
• Brassinosteroids
• Abscisic acid (ABA)
• Ethylene

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Auxin (AA)/Indoleacetic acid (IAA)

• Functions
• Stimulate elongation of cells in young shoot
• Stimulates proton pumps? action potential?
  activates expansin enzymes?allow cell membrane to
  expand?more H2O enters
• Increases gene expression of growth proteins
• Induces cell division in vascular cambium
  differentiation of secondary xylem
• Regulates fruit development
• Produced in
• Embryo of seed
• Apical meristems

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Cytokinins

• Functions
• Stimulates cytokinesis
• Influences differentiation
• Stimulates germination
• keeps plant green longer
• Works in conjunction with auxin
• Most common cytokininzeatin
• Produced in
• Actively growing tissues, particularly roots,
  embryos, fruit

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Gibberellins

• Functions
• Elongate stems (stimulates expansin enzymes)
• Stimulate leaf growth
• Stimulate fruit growth
• Stimulate seed germination (break dormancy)
• Produced in
• Roots
• Young leaves
• Embryos of seed

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Brassinosteroids

• Functions
• Induce cell elongation division in stem
  segments seedlings
• Slow leaf abscission
• Promote xylem differentiation
• Inhibit root growth
• Chemically similar to cholesterol animal sex
  hormones
• Produced in
• Seeds
• Fruit
• Shoots
• Leaves
• Floral buds

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Abscisic Acid (ABA)

• Functions
• Inhibits growth
• Closes stomata during H2O stress (alters Ca2
  levels which effects K)
• Promotes seed dormancy
• Produced in
• Leaves
• Stems
• Roots
• Green fruit
• Slows growth
• Inactivated by large amounts of H2O, light or
  prolonged cold exposure
• ABA to giberellin ratio often determines whether
  seed germinates or remains dormant

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Ethylene

• Functions
• Promote fruit ripening (attracts animals to
  spread seeds)
• Causes enzymes to break down cell walls to soften
  fruit
• Causes enzymes to convert starches to sugars for
  taste
• Activates scent color to display ripeness
• May promote or inhibit development of roots,
  leaves, flowers (dependant on species)
• Opposes auxin affects
• Activates enzymes to break down chemical
  components of cells leading to leaf abscission or
  plant death after flowering
• positive feedback mechanism
• Produced in
• Tissues or ripening fruit
• Nodes of stems
• Aging leaves flowers

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Photomorphogenesis

• effects of light on plant morphology
• Light effects plant growth development
• Plants detect light presence, direction,
  intensity, wavelength (color)
• Blue-light photoreceptors
• Initiates responses in plants including
• Phototropism (phototropin blue pigment)
• Light induced stomata opening (carotenoid based
  zaxanthin)
• Light induced retardation of hypocotyl elongation
  upon emerging from soil (cryptochrome pigment)
• Phytochromes photoreceptors
• Photoreceptor switches between Pr Pfr forms
• When Pr active- growth inhibited Pfr active-
  growth stimulated
• Responses include
• Stimulating seed germination
• Aid in tracking seasons
• Shade avoidance

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Biological clocks circadian rhythms

• Physiological processes of plant including
  production of photosynthesis enzymes, stomata
  opening/closing, raising/lowering of leaves
  follow a 24 hour pattern even under controlled
  conditions
• Circadian rhythms
• Cycle in 24 hour frequency
• Not necessarily connected to an environmental
  variable
• Without environmental signal rhythm occurs in a
  21-27 hour frequency
• Light places rhythms on a 24 hour cycle
• In plants a result of blue light photoreceptors
  phytochrome
• Light causes phytochrome to switch between Pr
  Pfr
• Allow plants to adjust to seasonal differences in
  day night

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Photoperiodism

• Physiological response of plant to photoperiod
  (relative length of day or night)
• Seasonal responses of plant
• Short day plants (long night)
• Flower only if light period is shorter than
  critical length
• Flower in late summer, fall, or winter
• Long day plants (short night)
• Flower only if light period is longer than
  critical length
• Flower in late spring or summer
• Day neutral plants
• Flowering unaffected by photoperiod
• Plants really respond to night length not day
  length as names suggest
• Leaves detect photoperiod
• signal buds to develop flowers through signal
  florigen
• Florigen activates organ identity genes

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Other environmental influences on plant growth
abiotic

• Gravity
• Shoot grows upward (negative gravitropism)
• Roots grow downward (positive gravitropism)
• Statoliths (starch dense plastids) settle at
  lower portions of cell triggering redistribution
  of Ca2 thereby affecting auxin
• Help determine up vs. down in plant
• Mechanical stimuli
• Thigmomorphogenesis- plant changes from due to
  mechanical perturbances
• Viny plants grasp grow around supports it
  encounters to stabalize itself
• Touch sensitive plants can send action potentials
  similar to animal nerve impulses to neighboring
  organs

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Other environmental influences on plant growth
abiotic

• Drought
• Low H2O in leaf- guard cells lose turgor- stomata
  close
• H2O deficit increases abscisic acid in leaf to
  keep stomata closed
• Cell expansion inhibited by lack of H2O young
  leaves do not grow- decreases surface area for
  H2O loss
• Conserves H2O but minimizes photosynthesis
  thereby decreasing crop yield
• Flooding
• Too much H2O suffocates roots- no O2 for
  respiration
• Excess H2O- increased ethylene root cortex
  undergoes apoptosis creating air tubes for O2 to
  reach submerged roots
• Salt stress
• Too much salt lowers H2O potential of soil
  causing roots to lose H2O to soil
• Plant responds by producing organic compound
  solutes that keep Y of cells lower than soil

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Other environmental influences on plant growth
abiotic

• Heat stress
• High heat denatures enzymes of plant, damaging
  plant metabolism
• Transpiration lowers leaf temps through
  evaporative cooling
• Heat shock proteins produced which protect
  enzymes from denaturing
• Cold stress
• Cold causes problems with membrane fluidity
  transport
• Plants increase unsaturated fatty acids
  membrane solutes to prevent dehydration keep
  membrane fluid

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Other environmental influences on plant growth
biotic

• Defenses against herbivores
• Thorns
• Chemical defenses
• Distasteful or toxic compound production
• Chemicals that attract defensive predators
• Defenses against pathogens
• First line epidermis periderm
• Second line chemicals released to kill pathogen
  prevent its spread from infection site
• R genes produce proteins that bind to pathogen
  protein
• Elicitors produced
• Oligosaccharins- released from damaged cell wall
  to stimulate production of antimicrobial
  phytoalexins
• PR proteins activated that directly attack
  pathogen
• Stimulate strengthening of cell wall to prevent
  spread