Transcript Plant Hormones

Figure 39.0 A grass seedling growing toward a candle’s light
Chapter 39:
Plant Hormones
Review for test
Test tomorrow all multiple choice
Hormones
Chemical signals
Produced in one part of plant and effect
other parts
Trigger responses in target cells and
tissues
Small amounts produce substantial
changes
General plant hormones
Auxins
Gibberellins
Cytokinins
Abscisic acid
Ethylene
Table 39.1 An Overview of Plant Hormones
Study
This!!
Hormonal Action
Hormone (red) binds
to receptor (receptor)
which activate a 2nd
messenger (blue).
This triggers
1.Proton pumps
2.Production of golgi
vesicles
3.Transcription and
translation
H+
Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis
Acid Growth Hypothesis
1. Auxin, produced by apical mersitems in
buds, trigger H+ pumps
2. H+ pumps move ions into cell wall matrix
3. Acidity in wall increases and breaks bonds
in the cellulose fibers
4. Cell wall weakens and water enters
5. Cell elongates
Auxin & Cytokinins
Auxin:
Control of Apical Dominance
Effect on Stems:
Auxin produced by terminal bud inhibits
growth of axillary buds. Plant grows taller
Cytokinins produced by roots stimulate
axillary bud growth
Lower buds grow faster than those close to
terminal
If terminal bud is cut, (i.e. auxin is removed)
and axillary buds grow and plant gets fuller
Auxin:
Control of Apical Dominance
Effect on Roots:
Auxin stimulates root branching
High concentrations of auxin INHIBIT root
growth
Cytokinins inhibit root branching
Apical Dominance
Function of cytokinins
Promotes cell division.
Morphogenesis.
Lateral bud development.
Delay of senescence.
Made in root, travel in xylem
Gibberellin
Discovered in association with Foolish
disease of rice (Gibberella fujikuroi)
uninfected
infected
Effects of Gibberellins
Make in roots and young leaves
General cell elongation in leaves and
stems (not roots).
Found in embryo of seeds and leads to
the breaking of dormancy.
Promotion of flowering.
Commercially used for spraying on
Thompson’s seedless grapes
Figure 39.11 The effect of gibberellin treatment on seedless grapes
Abscisic acid
Functions of abscisic acid
General growth inhibitor.
Prevents seeds from germinating (if
ABA is removed from seeds,
germinations begins!)
Causes closing of stomata
“Winterization”effect on plants
Produced in response to stress.
Ethylene
H
H
\
/
C = C
/
\
H
H
Functions of ethylene
Gaseous
Causes fruit to ripening.
Senescence and abscission.
Interference with auxin transport.
Initiation of stem elongation and bud
development.
Tropism
Tropisms
Phototropism:
shoots or stems bends towards light
enhancing photosynthesis
Gravitropism:
Plants uses statoliths to tell which way is
“up” or “down”
Roots display positive gravitropism (grows
down towards the earth) and the shoot
displays negative gravitropism (grows up
out of the earth)
Growth movement
Phototropism
Light straight above
plants
Auxin amounts
equal on both sides
Cell elongation
equal on both sides
Stems grow straight
up!
Phototropism
Light at an angle
Auxin accumulates
(moves) to the
shaded side
Phototropism
High auxin
concentration promotes
cell growth on shaded
side
Stem grows towards
light source
Positive phototropism
Gravitropism
Shoot:
Grows away from pull
of gravity
Negative gravitropism
Root:
Grows toward pull of
gravity
Positive gravitropism
If plant falls over…
In roots:
Statoliths (starch
grains) settle to the
bottom of the cells
Triggers the
movement of Ca2+
Causes lateral
movement of auxin
Roots and stems respond…
…the effects of auxin on roots and
shoots are opposing!!!!
 IN ROOTS:
 1. Low auxin, cells elongate
 2. High auxin, inhibitory
response, no elongation
 Roots curve down
 IN SHOOTS
 3. Low auxin inhibits
elongation
 4. High auxin, stimulates cell
elongation
 Shoots curve up