regulation of plant growth

what is differentiation

the process by which cells become different from one another

how do plant cells increase in size

by taking in what in what is called the stress-relaxation cycle

how does the stress-relaxation cycle work

the cycle begins when turgor pressure acts on the cell wall. This physical stress alters the structure of the material that makes up the cell wall. Some changes are reversible, some arent and will become permanently stretched out. Then the wall relaxes, turgor pressure falls and the cycle repeats.

what can change the rate of cell wall expansion

by changing the chemical composition of the wall. Some plant hormones increase the rate of cell expansion by making the cell wall looser.

what dictates the direction of maximal cell expanion

it usually can be predicted by the arrangement of cortical microtubules (cytoskeletal proteins that are anchored to the cell membrane)

what is a dormant seed

when the development of a seed stops even when external conditions are adequate for development

how do seeds maintain dormancy

• exclusion of water or oxygen from the embryo

• mechanical restraint of the embryo by a tough seed coat

• chemical inhibition of germination by growth regulators

• photodormancy: some seeds need a period of light or dark before they can germinate

• thermodormancy: some seeds need high or low termperature to germinate

why do seeds become dormant

dormancy ensures that the seed will germinate at a time suitable for the plant to complete its life cycle

• some seeds need the cold of winter to germinate in the spring

when do seeds germinate

seeds begin to germinate or sprout when dormancy is broken and environmental conditions are satisfactory.

how does germination start

imbibition. The seed takes in water because of the polar macromolecules, like cellulose in the seed. Seeds also take in water because of their very negative water protential

process of germination

starch, proteins, and lipids are hydrolyzed to provide energy and building blocks for the growing embryo. Reserves are stored in cotyledons. Germination is complete when radicle emerges from seed coat.

what is a cotyledon

the first leaf or leaves of the embryo

what is a radicle

embryonic root

what do photoreceptors allow to happen

when seeds germinate underground they need to know which way is up. Photoreceptors that sense light and gravity direct this stage of development

what is the coleoptile

in monocots its a sheath of cells which protects the shoot as it grows and pushes its way through the soil

what are photoreceptors

they detect changes in the quality and direction of light as well as the timing of light availability.

what are tropisms

a directional response to a stimulus

what is phototropism

the growth or movement of a plant towards light

what is gravitropism

the growth of a plant towards gravity

what is thigmotropism

the growth of a plant in response to touch

what are plant hormones

chemical signals in very low concentrations who play multiple regulatory roles at target sites often distinct from where they are produced.

effects of gibberellins on plant growth and development

• stem elongation

• fruit growth

• mobilization of seed reserves

where is auxin produced

in the shoot apex and diffused down the shoot in a polar fashion

main roles of auxin

plant growth and development

what does the polar transport of auxin depend on

1. diffusion across a cell membrane

2. membrnae protein asymmetry

3. proton pumping/chemiosmosis

4. ionization of a weak acid

explain how the polar transport of auxin depends on the diffusion across a cell membrane

polar molecules diffuse across cell membranes less readily than nonpolar molecules

explain how the polar transport of auxin depends on membrane protein asymmetry

some proteins that transport auxin are located in specific portions of the cell membrane. In the stem, most of the transport proteins that allow auxin to move out of the cell are located at the bottom end of the cell

explain how the polar transport of auxin depends on proton pumping/ chemiosmosis

a proton pump moves protons from the cytoplasm to the cell wall, thereby increasing the intracellular pH and decreasing the pH in the cell wall.

Proton pumping also sets up an electrochemical gradient, which provides potential energy to drive auxin transport

explain how the polar transport of auxin depends on ionization of a weak acid

the main form of auxin, indole-3-acetic acid, is a weak acid. When the pH is low, this reaction is driven to the right, and HA (non-ionized auxin) is the predominant form. When the pH is higher, there is more A- (ionized auxin)

how are polar auxin transporters distributed

they are distributed such that the hormone predominately moves down the axis of the shoot. However, lateral redistribution of auxin is responsible for directional plant growth

what is step 1 of polar auxin transport

auxin enters the cell by passive diffusion as an unchanged acid or through auxin import proteins, symported with proton.

what is step 2 of polar auxin transport

proton pumps in the cell membrane maintain the cell wall at an acidic pH and set up a chemiosmotic gradient to drive the transport of auxin

what is step 3 of polar auxin transport

auxin- predominates in the cytosol, which has a neutral pH

what is step 4 of polar auxin transport

auxin- exits the cell via auxin anion efflux(move out of cell) carriers that are concentrated at the bottom end of each cell

what is step 5 of polar auxin transport

in the cell wall, the lower pH causes auxin- to become auxin, which can diffuse into the next cell or enter through an auxin-proton symporter

why do plants bend toward the light

the lateral movement of auxins toward the shaded side within the tip, this will stimulate cell elongation on the shaded side as the auxin travels down, causing the coleoptile to grow toward the light

auxin’s role in leaf abscission

leafs normally detach in a process called abscission, but auxin prevents this.

auxin’s role in root initiation

serves as the primary chemical signal for root initiation. They dictate where and when new roots form

auxin’s role in apical dominance

auxin helps maintain apical dominance

what is apical dominance

its where apical (terminal) buds inhibit the growth of axillary buds, resulting in the growth of a single main stem with minimal branching

auxin’s role in fruit development

fruit development normally needs prior fertilization, but auxins causes formation without fertiliation

auxin’s role in cell expansion

auxin regulates the cell wall expansion

what is cell expansion driven by

the uptake of water, which enters the cytoplasm of the cell and accumulates in its central vacuole.

explain the acid growth hypothesis

auxin increases the pumping of protons from the cytoplasm into the cell wall, this lowers the pH of the wall and activates the enzyme expansins which loosen the cell wall, making it easier to stretch as the cell expands.

what are the two aspects of light

1. its quality, the wavelengths of light that can be absorbed by molecules in the plant

2. its quantity, the intensity and duration of light exposure

which color is most effective in inducing the coleoptiles to curve

blue light

What is phototropin

a blue-light receptor protein which absorbs blue light

what is cryptochromes

it is a class of blue-light receptors which absorb blue and ultraviolet light

list some of the red light induced changes

the apical hook straightens, the rudimentary leaves unfold, and chlorophyll is made

key feature of red light responses

they are reversible by far-red light (710 - 740nm)

what are phytochromes

red light receptors

how do phytochromes switch between states

the default state, which absorbs red light, is Pr. When Pr absorbs a photon of red light it is converted into Pfr

What is Pfr

the active form of phytochrome - the form that triggers important biological processes in various plants, like chlorophyll synthesis

what does the chromophore of Pr do

the chromophore of Pr preferentially absorbs red light; when it does so, it changes conformation and the phytochrome is converted to the Pfr form.

what part of phytochrome absorbs red and far-red light

its a covalently attached pigment called a chromophore

what happens to the chromophore of Pfr when it absorbs far-red light

when the chromophore of Pfr absorbs far-red light, hte phytochrome is converted back to the Pr form

what is dark reversion

Pfr form can slowly revert to the Pr form. This even occurs gradually and doesn’t not require light

what happens to plants growing in the shade

in the shade of other plants, the ratio of red to far-red light is low (0.13:1) which stimulates cell elongation and the plants grow upward toward the sunlight

what is etiolation

To outcompete neighbors for sunlight, the plant directs energy into rapid stem elongation and upward growth rather than branching, resulting in tall, thin plants (etiolation

how does the conversion from Pr form to Pfr form actually happen

when Pr absorbs red light, the chromophore changes shape, causing change in the conformation of the protein itself, from Pr to Pfr. This results in the movement of Pfr from cytoplasm to nucleus, once there it can stimulate genes that were repressed and repress genes that were formerly stimulated.