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/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