37. features that maximize plants’ ability to obtain resources for growth and reproduction:...
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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/wp-content/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
www.aphotoflora.com
Iris seeds
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
http://nature.org/initiatives/fire/work
Jack pine seedling sprouting following a fire in Wisconsin
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
www.cropsci.uiuc.edu/classes/cpsc112/images/SeedsGerm
Comparison of non-imbibed and imbibed (swollen) pea seeds
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/de-etiolation_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
http://www.odec.ca/projects/2005/ster5b0/public_html/homepa1.jpg
Environmental effects on plant growth can be tested in the lab using genetically identical plants to sort out genomic vs. environmental causation
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 thengrown and exposed toethylene, one grows taller(shows that it has a genethat has mutated to make itresistant 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)
www.rbgsyd.gov.au/__data/page/2288/
Asci of Gibberella fujikuroi
G. fujikuroi on maize
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 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
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-to-cytokinin ratio controls extent of branching
Delay senescence of leaves
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
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)
www.cropsci.uiuc.edu/classes/cpsc112/images/PGR
Effect of using ethylene on green
tomatoes (on right)
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
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