Transcript Life: The Science of Biology, 8e

37
Features that maximize plants’
ability to obtain resources for
growth and reproduction:
• Meristems allow growth
throughout the plant’s life
• Post-embryonic organ
formation — new organs can
develop throughout life
• Differential growth — they
can grow organs most
needed, e.g., more leaves
http://www.ncsec.org/team8/fp.gif
Plants must monitor their
environment and redirect
growth as appropriate
 A plant’s environment is
never completely stable
 light changes day to
night, and with seasons
 neighbor plants compete
for light, nutrients, etc.
http://www.howplantswork.net/wpcontent/uploads/2009/10/winding_road.jpg
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
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 plant’s genome
www.ryanphotographic.com/images/Scenes/
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
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
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
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
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
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.
Process of germination
 Imbibition, or uptake of water, is first step
 Seed’s 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 nonimbibed and imbibed
(swollen) pea seeds
www.cropsci.uiuc.edu/classes/cpsc112/images/SeedsGerm
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
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
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
http://www.scielo.br/img/fbpe/gmb/v24n1-4/9424f1.gif
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/deetiolation_pathway.jpg
Plant’s genome ultimately determines the limits of
plant development
 The genome encodes plant’s “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
http://www.odec.ca/projects/2005/ster5b0/public_html/homepa1.jpg
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, it’s 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
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/Fig9_031504.gif
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
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/
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.
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.
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.
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.
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
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
Auxins are a group of plant hormones

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
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.

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
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 block…it was
IAA
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)
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
How does a plant cell sense light and gravity?
 Phototropism—membrane 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
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
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).
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 can’t 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
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.
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
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/utilis-horm_e.gif
Inhibit stem
elongation but cause
lateral swelling of
stems and roots
 Stimulate axillary buds
to grow. Auxin-tocytokinin ratio
controls extent of
branching
 Delay senescence of
leaves

http://www2.ulg.ac.be/cedevit/image/hormones/utilis-horm_e.gif
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
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 don’t 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