Chapter 35, pages 750-756
I. Plant Movements and Growth Responses
A. Tropisms
1. Definition: Plant growth in response to an external stimulus.
2. Phototropism
a. Plant growth in response to the stimulus of light
b.
Etiolated stems - stems grown in the dark that are yellowish
in color and have
long
internodes.
3. Gravitropism - plant growth in response to gravity
4. Thigmotropism - plant growth in response to a mechanical stimulus (touch, etc.)
1. Movements of plants in
response to changes in
turgor
pressure in the cells of the
plant
2. Leaves fold up when you touch a sensitive plant, Mimosa pudica
a. An external stimulus triggers the response
b. Loss of turgor pressure causes the leaves to fold quickly
1. Definition: A biological clock that approximates a 24-hour cycle
2. Phytochrome
a. Probably the photoreceptor chemical involved in resetting the biological clock
b. Plants appear to "anticipate" sunrise and sunset
3.
Nyctinastic movements are sleep movements of the plant
directed by the
biological
clock
II. Hormonal Regulation of Plant Growth
A. Introduction
1. Plants are totipotent
a.
Each plant cell contains all the genetic information to form a complete plant
from
that cell
b. Thus cells need external signals to coordinate growth and development
c. Plant hormones do this coordination
2. How hormones work:
a. Act in low concentrations
b. Function as internal signals
c. How plant hormones initiate their effects is not fully understood
d. Effects of hormones overlap
e. Plant hormones coordinate:
(1) Cell division - new cell production as a result of mitosis
(2) Cell growth - increases in cell size and organ size
(3) Differentiation - specialization in structure and function of the cell
3. Five classes of hormones:
a. Auxins
b. Gibberellins
c. Cytokinins
d. Ethylene
B. Auxins
1. First detected in the
coleoptiles
of
grass
because auxin stimulated elongation of the
grass coleoptiles
a. Only naturally occurring auxin (IAA)
b. Synthesized from the amino acid tryptophan
c. A number of synthetic auxins have been produced
3. Synthesized in the apical meristem of stems
a. Transported polarly
b. Transported in a basipetal (downward) direction [acropetal direction is up]
c. About 10 mm/hr transportation rate (too fast for diffusion)
d. Takes energy to transport auxin
4. Function of auxins
a. Causes cell elongation or growth
(1) Does this by breaking the cross bonds of the cellulose polymers
so that the
cell can expand and then reforms them again
(2) The acid-growth hypothesis suggests that auxin causes hydrogen
ions to be
pumped into the cell wall, activating enzymes
which break the bonds,
making the wall flexible for expansion
(3) Turgor pressure causes the cell wall to expand
b. Phototropism
(1) Sunlight causes
differential flow of auxin down the stem. More auxin flows
down the dark side of the stem than the on the
light side.
(2) Thus more elongation of the cells occurs on the dark side
of the stem and
causes the stem to bend toward the light
(3) IAA (natural auxin only) is destroyed by sunlight
(1) Auxin inhibits the development of lateral buds
(2) Does this by inhibiting the formation of a
vascular
connection between the
lateral bud and the stem
(3) Removing the
apical
bud removes the source of
auxin
and the
lateral
buds
begin to grow
d. Fruit development
(1) IAA produced by seeds in the fruit stimulates the pericarp to develop
(2) IAA applied to flowers can cause fruit development without fertilization
(1)
NAA
(naphthaleneacetic acid) is used to stimulate root development on
cuttings
for
asexual propagation
(2) 2,4-D
(a) A selective herbicide
(b) Kills dicots but not monocots
C. Cytokinins
1. Discovery
a.
1940's and 1950's researchers were trying to find substances that would
induce
plant cells to divide in
tissue
culture
b.
Found that coconut water and
autoclaved
herring sperm DNA would do the
trick
c.
In 1956 the active substance was isolated from herring sperm and was called
cytokinin (because it induced cell division or
cytokinesis)
d. In 1963 the first natural cytokinin was isolated from corn and named zeatin
2. Chemistry
a. Cytokinins are similar to purine and adenine
b. They are very closely related to nucleic acids (particularly t-RNA)
3. Produced in actively growing tissues, particularly roots, embryos, and fruits
4. Transported in the xylem
5. Functions of cytokinin
a. Promotes cell division (mitosis) and differentiation in only some plants
b. Required ingredient in plant tissue culture media for inducing mitosis
c.
In tissue culture cytokinins work with
auxin
to induce
organogenesis
from
callus
(1) High cytokinins to low auxin induces shoot formation in tobacco
(2) Low cytokinins to high auxin induces root formation in tobacco
d. Auxin and cytokinin interact in apical dominance
(1) Auxin
inhibits
lateral
buds from growing while cytokinin stimulates lateral
buds to grow
(2) Reversed in roots: auxin stimulates branch roots and cytokinin inhibits it
e. Delays senescence (aging) of plant cells
(1) Inhibits senescence in plant parts that are cut (like cut flowers)
(2) Promotes maintenance of normal levels of
protein
and
nucleic
acids in cut
plant parts
(3) Stimulates RNA and protein synthesis
(4) Cytokinin sprays, in spray cans, are used to keep cut flowers fresh
(5) Cytokinins can increase shelf-life of fruits and vegetables
(this, to my
knowledge, has not been approved yet by
the FDA)
PUC Home Page | Gilbert Muth Home Page | Botany Syllabus Home Page