Download •Deposition of unique membrane/cell wall material at rhizoid end

Fucus life
cycle
F-actin patch defines position of future rhizoid
•Originally at sperm entry site
•Light/gravity/temp/current cause reassembly of actin patch
(e.g. opposite light, towards gravity)
Figure 4-1
Embryo develops apical/basal polarity
Figure 2-1
•Deposition of
unique
membrane/cell
wall material at
rhizoid end
•Ca 2+ channels
asymmetrically
positioned
•Asymmetric localization of Ca2+ pumps results in a gradient
of Ca2 + ions
Belanger and
Quatrano, 2000
Figure 2-1
1
•Both thallus and
rhizoid walls
have fate
inducing
components
•Directed fusion
of vesicles
deposits
compounds into
cell wall - these
fix cell fate
•Following polarization, axis becomes fixed
• cytoskelton is organized so that vesicles fuse at
rhizoid end
•If stop secretion using brefaldinA, no axis fixation
Belanger and Quatrano, 2000
Figure 4.3 Berger et al., 1994
Marker AtLTP1 is
restricted to the shoot
epidermis - indicates
apical region of embryo
Arabidopsis zygotes also undergo an initial asymmetric cell
division to generate the upper embryo and lower suspensor
•Similar to Fucus, although different evolutionary origins
•Angiosperm embryos are contained within maternal tissue,
thus polarity may depend on maternal polarity
In gnom mutants, first
asymmetric division does
not occur, subsequent
divisions are not normal
•Secretion defect improper cell wall
targeting?
Top cell of
suspensor =
hypophysis - QC
and root cap
columella
Former micropyle
Figure 4-4
But AtlTP1 expression
may be normal, indicting
proper Apical/basal
polarity
is independent of cell
division
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Figure 4-5
Polar auxin
transport controls
many aspects of
post-embryonic
development
•Transported
basipetally, in
shoot vascular
parencyma
•Efflux carriers
(PIN) at base of
cells
•Redirection of
auxin at root tip
results in “reverse
fountain”
Directional flow of auxin in globular and beyond is due to basipetal
placement of PIN proteins (immunolocalization) early in
embryonic development (mid-globular stage)
Steinmann et al., 1999, Figure 4.6
a -c) DR5::GFP activity is
highest in apical cell and
derivatives
d-e) activity is highest in
hypophysis
PIN1
in wild
type
DR5::DTA (diptheria toxin)
made conditional by Gal4
transactivation
•Early activation causes death of
apical cell (g), later causes death
of basal cells (h)
PIN1
in
gnom
•In embryos mutant for gnom PIN1 is not localized
to the basal membrane (but gnom defects are ealier)
Steinmann et al., 1999
immunolocalization of IAA
•I - pre-globular in apical
•J - globular in basal
Friml et al., 2003
3
a) Wild type
a-d) PIN1 localization 1-16 cells - no polarity,
32 cells -basal
Friml et al., 2003
f) - PIN3::GUS
f inset) PIN3 in situ
g) - PIN4
i-o) PIN7 localization
i) PIN7::GUS
j) in situ - basal cells
3k-o immunolocaliation
To 32 cell stage - In
basal cells on side
facing apical
At 32 cells - reverses
b-e) pin7 mutant
Many pin7 mutant embryos show defects: b, c) filamentous,
d) apical/basal boundary not clearly defined, e) apical
defects
Friml et al., 2003
-similar to gnom
gnom
But, pin7 embryos seem to recover at 32 cell stage
Friml et al., 2003
Friml et al., 2003
pin1 pin3 pin4 pin7 quadruple mutants show severe
apical/basal defects, do not recover at 32-cell stage, and
end up looking like gnom
•Functional redundancy amongst PIN genes
•gnom defects are explained by improper
localization of ALL PIN proteins
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Model of Experiments That Have Examined the Mechanisms of the Control of PIN1 Protein Localization
CytochalasinInhibits actin
untreated
BFA inhibits
vesicle
cycling
Friml et al., 2003
a) After asymmetric division, auxin accumulates in apical cell
through apically localized PIN7-dependent transport
b) Auxin is produced in apical region, and PIN1 localization to
basal membranes causes basipetal transport, PIN 7 localization
reverses (PIN7 function is now redundant)
Muday, G. K., et al. Plant Cell
2002;14:293-299
PIN is
internalized
Therefore
less on PM
wt
Copy right ©2002 American
Society of Plant Biologists
35S::PID
gnom
Friml et al., 2003
•Functional redundancy amongst PIN genes
•gnom defects are explained by improper
localization of ALL PIN proteins
•GNOM = ARF-GEF, functionally homologous to
yeast protein, required for vesicle cycling
wt
RPS5A::PI
D
DR5::GUS
in 35S::PID
DR5::GUS
RPS5A::PI
D
RPS5A::PI
D
Overexpression
of PID results in
•reversal in PIN1 polarity (basal (low) to apical (high))
•Lack of basal specific auxin maximum
•Defects in basal structure development
•Defects in basal structures, embryo symmetry
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PID
The Mammalian GLUT4 Asymmetric
Vesicular Targeting Mechanism as a Model
for the Localization of the Auxin Efflux
Carrier
GNOM - to
plasma
membrane
allows
vesicle
trafficking
High PID apical
Low PID - basal
exocytosis
GNOM - allows vesicle cycling - if inhibit, PIN is
internalized
PID - directs vesicle cycling - if increase, PIN is apical
instead of basal
auxin - inhibits endocytosis, which results in an
accumulation of PIN at the PM
auxin causes inhibition of
endocytosis, which results
in an accumulation of PIN
at the PM
endocytosis
Muday, G. K., et al. Plant Cell
2002;14:293-299
A Model for Auxin Response through the TIR1 Auxin Receptor Pathway
But what causes the auxin response?
BDL
AXR6
MP
Woodward, A. W., et al. Plant Cell 2005;17:2425-2429
Copy right ©2005 American Society of Plant Biologists
6
A Model for Auxin Response through the TIR1 Auxin Receptor Pathway
•monopteros (mp)
embryos show
abnormal division
patterns and never
form a root
•Plants can be made in
culture - these have
reduced levels of
auxin transport
•MP is expressed in
subepidermis, then is
restricted to
procambium
BDL
AXR6
MP
Woodward, A. W., et al. Plant Cell 2005;17:2425-2429
Figure 4.7
Copy right ©2005 American Society of Plant Biologists
No degradation
(no activator ARF)
mp
(Hardtke and Berleth, 1998)
axr6
(Hobbie et al., 2000)
(undegradable
Repressor Aux/IAA)
bdl
(Hamann et al., 1999)
Auxin response is necessary to pattern the
basal parts of the plant
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Auxin is directed
to root by PIN4,
where it induces
BDL degradation
and activation of
auxin responsive
genes via MP
WOX2
expressio
n
WOX9
expression
Weijers and Jurgens, 2005
Vogler and Kuhlemeier, 2003
WOX2 expressed only in apical cell, WOX9 only in basal cell
WOX = WUSCHEL-related homeobox
Later WOX9 expression depends on MP and BDL
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