Download Mineral Nutrition in Plants II

Plant Responses to Signals
A weed is a plant that has mastered every survival
skill except for learning how to grow in rows.
- anonymous
External Signals
• External signals are used by plant cells
to alter their physiology, morphology
and development,
– physical environment,
– chemical environment,
– biological environment,
• sometimes other plants,
• Plants receive signals at the cell level,
and have no well defined sensory
organs,
• Except for gravity, all other signals are
constantly variable.
Internal Signals
•
Signals can be processed by
growing and non-growing cells,
•
Signals are transduced into
biologically meaningful results
through numerous and coordinated pathways,
– changes in ion flux,
– regulation of metabolic
pathways,
– regulation of gene expression,
– changes in the cytoskeleton.
Signal Transduction
models
Signal
Signal
Signal
Signals
Signal
Response
Response
Response
Linear Model
Network Model
Reception
• Cell surface receptors,
– hydrophilic molecules such as
peptides and carbohydrates
don’t readily cross the
membrane,
• are perceived on the cell
surface,
• Amphiphilic and hydrophobic
molecules may pass through the
membrane to receptors,
– steroid hormones for example,
• Light may be perceived at
the cell surface, or in the
cell.
Plant Receptors
i.e. light responses
• Where does one look?
– no clearly differentiated
organs (i.e., eyes, ears, etc.),
– sensitive tissues, however,
no clearly differentiated
cells,
– lots of responses.
•
•
•
•
•
•
•
•
Germination
(+/-)
Stem length
(-)
Leaf expansion
(+)
Flowering
(+/-)
Phototropism
(+/-)
Stomatal opening
(+)
Chloroplast development (+)
Pigment synthesis
(+)
De-Etiolation (italics)
De-etiolation
greening
•
Etiolated
De-etiolated
Etiolated growth habit,
– long stem,
– unexpanded closed leaves,
• etioplasts vs chloroplasts,
• lack of chlorophyll,
– apical hook,
– short root,
•
Photomorphogenesis,
–
–
–
–
Inhibited stem growth,
Expanded leaves,
Pigmentation.
Root development.
Nature Reviews Molecular Cell Biology 3; 85-93
Phytochrome
plant photoreceptor
• 1920’s,
– researchers observed
chlorophyll deficient mutants
(albinos) that underwent deetiolation when given
physiologically active light,
• 1950’s,
– phytochrome discovered,
• Molecular switch, signal
transducer.
N
Phytochrome
or
chromophore
• Proteins with tetra-pyrrole chromophores,
and transmission kinase domains,
kinase domains
• Phytochrome gene family contains at least
five members,
C
• Gene family members serve different
functions.
The Experiments
Phytochrome Signal
Transduction
hypothesis
Phytochrome is a greening receptor,
cGMP is a second messenger,
Signal
red light
Response
Calcium is a second messenger.
See Fig. 39.2 for general signal transduction.
greening
phytochrome (Pfr)
…enhances the
expression of MYB,
Phytochrome Signal
Transduction
for real
•
Active phytochrome (Pfr) is
transported to the nucleus,
… binds and inactivates transcriptional
repressor (PIF3),
…MYB transcription factor is
expressed, in turn activates
CCA1 transcription,
red light activates
phytocrome, active
phytochrome (Pfr) is
transported to the nucleus,
… CCA1 enhances the expression of
CAB (chlorophyll A/B) proteins.
CCA1 expression, in
turn, enhances greening
proteins.
...thousands of
genes,
phytochromes
…hundreds of
transcription
factors,
...response.
Nature Reviews Molecular Cell Biology 3; 85-93
Plant Responses to Signals II
Phytohormones
No one can look at the plants growing on a bank or on the borders of a thick wood, and doubt that
the young stems and leaves place themselves so that the leaves may be well illuminated...they are
extremely heliotropic; and this probably serves...as a guide (for) the buried seeds through fissures
in the ground or through overlying masses of vegetation, into the light and air.
- Charles Darwin
“The Power of Movement in Plants” (1880)
Charles Darwin was a Plant Physiologist,
Phototropism,
Introduction to the plant hormone Auxin.
Phytohormones
…a plant product that is able to stimulate
physiological responses at very low
concentrations,
– either in the tissue in which it is
synthesised,
– or in other regions of the plant to which
it is transported,
...do not operate in isolation from one
another, but often act in co-ordination
to produce subtle responses,
…affect gene expression, enzyme activity
and membrane function.
Phototropism
Tropism: a growth response in
plants that results in curvature
toward, or away from a stimulus.
Charles Darwin and his son Francis localized the location of perception
for blue light phototropism.
Positive Phototropism: growth toward a light
stimulus
Peter Boysen-Jensen demonstrated that a diffusable substance was
involved .
IAA
Natural auxins...
Peter Went demonstrated that the diffusible substance resulted in cell expansion.
Went isolated the active compound.
Bioassays
auxin
Oat Coleoptile (2 cm)
•
Oat Coleoptile (> 2 cm)
Bioassay: identification (or quantitation) of a biologically active substance by
measuring the effect the substance has on living material.
Auxin Transport
polar
Synthesized in the SAM
Auxin moves basipetally (from apex to base).
Auxin
…induces apical dominance,
SAM intact
SAM removed
“Leader”
basipetal movement of
auxin inhibits axillary buds,
axillary buds are released.
Apical Dominance
SAM intact
SAM removed
basipetal movement of
axillary buds are released,
auxin inhibits axillary buds,
Auxins
…induce vascular differentiation
hydathode
…xylem differentiation occurs
around the wound,
…xylem differentiates between hydathodes
and leaf vasculature
...following the path of auxin
diffusion.
...following the path of basipetal auxin
transport.
Auxins
…induces lateral and adventitious root formation,
Honeysuckle cut stems
Rooting compounds.
[ auxin ]
Auxins
…promotes fruit development,
seeds removed
seed
Normal
…embryo produces auxin that
stimulates fruit development.
seeds removed
+
auxin
Strawberry
See Fig. 39.11
Auxin
…promotes cell expansion,
Auxin activates the H+-ATPase.
Acidification of the cell wall activates expansin (cell wall proteins).
See Fig. 39.11a
Acid Growth Hypothesis
ABP + Auxin
AUXIN BINDING PROTEIN
Time course of action.
…activates the pump.
Mode of action
(hypothesis)
Auxin Binding Protein
auxin receptor?
over-express
ABP1 gene
add auxin
• It’s a protein that is found to bind
auxin, (Auxin Binding Protein,
ABP) has been proposed as a
candidate receptor,
•
A common test for receptors, is to overexpress (express more than the normal
amount of protein), and test to see if the
signal is amplified,
•
ABP passes this test.
•
However, it’s not the Auxin Receptor
Auxin Binding Protein
dead
auxin receptor?
wild type
abp mutant
ABP controls cell division and elongation in embryogenesis.
http://genesdev.cshlp.org/content/15/7/902
TIR1
To know:
TIR1 represents a new
signal transduction
paradigm.
auxin receptor
U
IV
III
U
E2
I
II
IAA
TIR1
IV
U
IV
IAA
III
IAA
III
…affects transcriptional control
RBX
ASK
CSN
CUL1
TIR1, F-box portion of
SCFTIR1 ubiquitin ligase
complex, represents
an auxin receptor
(Dharmasiri et al. (2005) Nature 435: 441-445;
Kepinski and Leyser (2005) Nature 435: 446-451)
AuxRE
GENE “X”
(Harper
al. (2002)
(2000)Plant
PlantMol.
CellBiol.
12: 49:
757-770;
Liscum
& et
Reed
387-400;
Esmon et al.
Int. J.Plant
Dev. Cell
Biol.16:
49: 379-393)
665-674
Tatematsu
et (2005)
al. (2004)
Auxins
…promote gene expression,
5 - 50 minutes
Genes
Products
Concept Map
Receptor
Discovery
Auxin
Function(s)
Transport