Regulation of Plant Growth

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Regulation of Plant Growth
37
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• Features that maximize plants ability to obtain
  resources for growth and reproduction
• Meristems allow growth throughout the plants
  life
• Post-embryonic organ formation new organs can
  develop throughout life
• Differential growth they can grow organs most
  needed, e.g., more leaves

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• Plants must monitor their environment and
  redirect growth as appropriate
• A plants environment is never completely stable
• light changes day to night, and with seasons
• neighbor plants compete for light, nutrients,
  etc.

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09/10/winding_road.jpg
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• Signals (environmental cues, photoreceptors, and
  hormones) affect three fundamental processes
• Cell division
• Cell expansion
• Cell differentiation

http//aggie-horticulture.tamu.edu/faculty/davies/
students/ngo
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• Plant development is regulated in complex ways.
• Four factors regulate growth
• Presence of environmental cues
• Receptors, e.g. photoreceptors, to sense
  environmental cues
• Hormones mediate effects of cues
• The plants genome

www.ryanphotographic.com/images/Scenes/
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• Seeds are dormant cells do not divide, expand,
  or differentiate
• As seed begins to germinate, it takes up
  (imbibes) water
• Growing embryo obtains chemical building blocks
  by digesting food stored in seed
• Germination is completed when radicle (embryonic
  root) emerges
• Now called a seedling

http//imagessvt.free.fr/physioV/germination
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• If seedling germinates underground, it must
  elongate rapidly, and cope with darkness for a
  time
• Series of photoreceptors directs this stage of
  development
• Early seedling development varies in monocots and
  eudicots

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• Seed dormancy may last weeks, months, or years.
• Mechanisms that maintain dormancy include
• Exclusion of water or oxygen by impermeable seed
  coat
• Mechanical restraint of embryo by tough seed coat
• Chemical inhibition of embryo development

Iris seeds
www.aphotoflora.com
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• Seed dormancy must be broken for germination to
  begin
• Seed coats may be abraded by physical processes,
  or chemically in the digestive tract of an animal
• Soil microorganisms or freeze-thaw cycles may
  soften seed coats
• Fire ends dormancy for many seeds by melting
  waterproof wax in seed, or by cracking the seed
  coat
• Leaching of chemical inhibitors by soaking in
  water can also end dormancy

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• Advantages of seed dormancy
• Survival through unfavorable conditions
• Prevent germination while still attached to
  parent plant
• Seeds that must be scorched by fire avoid
  competition by germinating only in fire-scarred
  areas
• Long-distance dispersal of seeds

www.biol.canterbury.ac.nz/mistletoes/images
Mistletoe seedling
Jack pine seedling sprouting following a fire in
Wisconsin
http//nature.org/initiatives/fire/work
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• Dormancy of some seeds is broken by exposure to
  light
• Germinate at or near soil surface
• Tiny with little food reserves and would not
  survive if they germinated deep in the ground
• Large seeds with large food reserves, germinate
  only when buried deeply, and in darkness (light
  inhibited)

Photo 37.19 Corn, squash, and Arabadopsis (small
brown) seeds.
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• Process of germination
• Imbibition, or uptake of water, is first step
• Seeds water potential is very negative ? water
  will enter if seed coat is permeable
• Expanding seeds exert tremendous force
• Enzymes activated with hydration
• RNA and proteins are synthesized and respiration
  increases
• Initial growth is by expansion of pre-formed
  cells, not cell division

Comparison of non-imbibed and imbibed (swollen)
pea seeds
www.cropsci.uiuc.edu/classes/cpsc112/images/SeedsG
erm
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• During early stages of plant development, plants
  respond to internal and external cues
• Responses are initiated and maintained by two
  types of regulators
• Hormones
• Photoreceptors

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• Hormones
• Regulatory chemicals that act at low
  concentrations at sites distant from where they
  were produced
• Each plant hormone is produced in many cells, and
  has multiple roles interactions can be complex

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• Photoreceptors involved in many developmental
  processes
• They are pigments (molecules that absorb light)
  associated with proteins
• Light acts directly on photoreceptors ?
• regulate processes of development

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if
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• Plants use signal transduction pathways series
  of biochemical reactions by which a cell responds
  to a stimulus
• Protein kinase cascades amplify responses to
  signals as in other organisms ? regulates genes
  expression

http//www.bio.miami.edu/dana/pix/de-etiolation_pa
thway.jpg
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• Plants genome ultimately determines the limits
  of plant development
• The genome encodes plants master plan, but its
  interpretation depends on environmental
  conditions

Environmental effects on plant growth can be
tested in the lab using genetically identical
plants to sort out genomic vs. environmental
causation
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ml/homepa1.jpg
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• Much recent progress in understanding plant
  growth and development has come from studies of
  Arabidopsis thaliana
• Used as model organism it is small, matures
  quickly, its genome is small and has been fully
  sequenced
• Mutants provide insights into mechanisms of
  hormones and receptors

http//aggie-horticulture.tamu.edu/faculty/davies/
students/ngo
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• One technique for identifying genes involved in a
  plant signal transduction pathway is called a
  genetic screen
• Mutants are created by insertion of transposons
  or point mutations by a chemical mutagen, usually
  ethyl methane sulfonate
• A large number of mutated plants are then
  screened for a specific phenotype, usually
  something easy to see or measure
• Once mutant plants have been selected, their
  genotypes and phenotypes are compared to those of
  wild-type plants

http//www.cepceb.ucr.edu/images/members/raikhel/F
ig9_031504.gif
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Figure 37.3 A Genetic Screen
Test tube has mutagen
Exposed seeds are then grown and exposed
to ethylene, one grows taller (shows that it has
a gene that has mutated to make it resistant to
methylene
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Gibberellins

• In Asia, foolish seedling disease in rice
  causes plants to grow rapidly ? tall and spindly,
  and dies before producing seeds
• It is caused by an ascomycete fungus Gibberella
  fujikuroi
• The fungus releases a molecule that stimulates
  plant growth (first isolated in 1925)

Asci of Gibberella fujikuroi
G. fujikuroi on maize
www.rbgsyd.gov.au/__data/page/2288/
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Gibberellins

• The action of gibberellin was studied in dwarf
  strains of corn and tomatoes.
• Gibberellin applied to seedlings of the dwarf
  strains caused them to grow as tall as wild type
  plants.
• Wild-type plants were shown to have much more
  gibberellin than dwarf strains.

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Gibberellins

• Gibberellins are a class of plant hormone that
  stimulate stem elongation.
• They belong to a family of common plant
  metabolites called diterpenoids.
• They have multiple roles in regulating plant
  growth, as shown by experiments in which
  gibberellins are blocked at various stages of
  plant development.

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Gibberellins

• Gibberellins regulate fruit growth.
• Seedless grape varieties have smaller fruit than
  seeded varieties.
• Experimental removal of seeds resulted in small
  fruits, suggesting seeds were the source of a
  growth regulator.
• Spraying young seedless grapes with gibberellins
  caused them to grow as large as seeded varieties.

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Gibberellins

• In germinating cereal seeds, gibberellins diffuse
  through the endosperm to surrounding tissue
  called the aleurone layer underneath the seed
  coat
• Gibberellins trigger a cascade in this layer,
  causing it to secrete enzymes to digest the
  endosperm.

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Gibberellins

• In the beer brewing industry, gibberellins are
  used to enhance malting (germination) of
  barley.
• Breakdown of the endosperm produces sugar that is
  fermented to alcohol.

http//4e.plantphys.net/images/ch20/wt2002c_s.jpg
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Gibberellins

• Inhibitors of gibberellin synthesis cause
  reduction in stem elongation in wild-type plants.
• These inhibitors are used in greenhouses to
  prevent plants from becoming tall and spindly.
• Also used to prevent bolting (producing a tall
  stem that flowers) in plants such as cabbage.

Bolting
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Auxin

• Auxins are a group of plant hormones
• Most important is indoleacetic acid (IAA)
• Discovery of auxin traced to Charles Darwin and
  his son Francis, who were studying plant
  movements
• Phototropism is growth of plant organs towards
  light (or away from light, as roots do)

Photo 37.9 Phototropism Plants grow toward
light.
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Auxin

• Darwins worked with canary grass
• Young grass seedlings have a coleoptile a
  sheath that protects it as it pushes through soil
• Coleoptiles are phototropic
• If coleoptile tip was covered, there was no
  phototropic response. A signal travels from tip
  to growing region

Light Source
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Auxin

• In 1920s, Fritz Went removed coleoptile tips and
  placed cut surfaces on agar
• When agar was then placed on cut plants, they
  showed phototropic response
• A hormone had diffused into agar blockit was IAA

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Auxin

• Lateral distribution of auxin causes plant
  movements
• Carrier proteins move to one side of cell rather
  than to the base
• When light strikes coleoptile on one side, auxin
  moves to other side, and elongation increases on
  that side.
• Coleoptile bends toward light (phototropism)

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Auxin

• If shoot is tipped over, even in dark, auxin will
  move to lower side
• Cell growth results in bending of shoot so that
  it grows up gravitropism.
• Upward gravitropic response of shoots is negative
  gravitropism downward response of roots is
  positive gravitropism

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Auxin

• How does a plant cell sense light and gravity?
• Phototropismmembrane receptor (phototropin)
  absorbs blue light
• When activated, a signal transduction pathway
  results in redistribution of auxin transport
  carriers
• Gravitropism
• some plant cells have large plastids called
  amyloplasts that store starch
• These plastids tend to settle on downward side of
  a cell in response to gravity
• This may disturb ER membranes and trigger auxin
  transport

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Auxin

• Abscission detachment of old leaves from stem
• Auxin inhibits abscission, which results from
  breakdown of cells in abscission zone of petiole
• Timing of leaf fall is determined in part by
  decrease in movement of auxin from blade through
  petiole

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Auxin

• Fruit development normally depends on
  fertilization of the egg
• If unfertilized ovaries are treated with auxin or
  gibberellins, fruit will form parthenocarpy
• Some plants spontaneously form parthenocarpic
  fruits (e.g., grapes, bananas, some cucumbers).

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Auxin

• Auxin is essential for plant survival
• No mutants without auxin have ever been found.
• Some synthetic auxins are used as herbicides
• 2,4-D is lethal to eudicots at concentrations
  harmless to monocots
• Eudicots cant break down the 2,4-D, and grow
  themselves to death.
• 2,4-D is a selective herbicide that can be used
  on lawns and cereal crops to kill eudicot weeds

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What Are the Effects of Cytokinins, Ethylene, and
Brassinosteroids?

• Plant cells such as parenchyma cells can be grown
  in a medium containing sugars and salts
• The cells will divide continuously until they run
  out of nutrients.
• Early work on cell culturing showed that coconut
  milk was the best growth supplement. A molecule
  in the milk likely stimulated cell division.

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• Several experiments identified adenine
  derivatives called cytokinins as the factor that
  stimulates cell division
• Over 150 different cytokinins have been isolated

http//4e.plantphys.net/images/ch21/wt2102a_s.png
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• Cytokinins have many effects
• With auxin, they stimulate rapid cell division in
  tissue cultures
• Cause light-requiring seeds to germinate in
  darkness
• In cell cultures, high cytokinin-to-auxin ratio
  promotes formation of shoots a low ratio
  promotes formation of roots

http//www2.ulg.ac.be/cedevit/image/hormones/utili
s-horm_e.gif
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• Inhibit stem elongation but cause lateral
  swelling of stems and roots
• Stimulate axillary buds to grow.
  Auxin-to-cytokinin ratio controls extent of
  branching
• Delay senescence of leaves

http//www2.ulg.ac.be/cedevit/image/hormones/utili
s-horm_e.gif
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• Ethylene gas is produced by all parts of a plant
• promotes senescence
• promotes leaf abscission
• Balance of ethylene and auxin control leaf
  abscission
• Speeds ripening of fruit
• Ripening fruit loses chlorophyll and break down
  cell walls
• once ripening begins, more and more ethylene is
  produced

Ripening apple gives off ethylene gas, which then
causes leaf abscission in holly
www.cropsci.uiuc.edu/classes/cpsc112/images/PGR
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• Commercial fruit growers use ethylene gas to
  speed up fruit ripening
• Ripening can be delayed by using scrubbers to
  remove ethylene gas from storage chambers
• Cut flowers are sometimes put into silver
  thiosulfate solution to inhibit ethylene
  (probably by combining with ethylene receptors)

Effect of using ethylene on green tomatoes (on
right)
www.cropsci.uiuc.edu/classes/cpsc112/images/PGR
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• Plant steroid hormones were not discovered until
  the 1970s.
• Brassinosteroids were first isolated from mustard
  family plants
• Stimulated cell elongation, pollen tube
  elongation, and vascular tissue differentiation
• But inhibited root elongation.
• Mutant plants that dont make brassinosteroids or
  have defects in signal transduction pathway are
  usually dwarf, infertile, and slow to develop.
• These effects can be reversed by adding small
  amounts of brassinosteroi