Shoot meristem self-organization
and identity
Vegetative shoot apical meristem (SAM)
Wild-type shoot apex
shootmeristemless apex
(from Long et al., 1996, Nature 379: 66-69)
Gene expression in embryonic SAM
WUSCHEL (WUS)
SHOOTMERISTEMLESS (STM)
(from Laux et al., 1998, Cell 95: 805-815)
Wild type
wuschel mutant
(from Laux et al., 1998, Cell 95: 805-815)
Gene expression in embryo SAM
WUSCHEL (WUS)
SHOOTMERISTEMLESS (STM)
(from Laux et al., 1998, Cell 95: 805-815)
WUS expression in adult shoot meristems
(from Laux et al., 1998, Cell 95: 805-815)
clavata mutants have enlarged meristems
Wild type
clavata3 mutant
clv1 and clv3 mutants look similar
(from J. C. Fletcher et al., Science 283, 1911 -1914 (1999))
Expression of CLV1 in Arabidopsis inflorescence shoot meristem
Clark et al., Cell 89: 575-585 (1997)
CLV3 (A-D) and CLV1 (E) expression
(from J. C. Fletcher et al., Science 283, 1911 -1914 (1999))
CLV3 expression in a clv1 mutant meristem
(from J. C. Fletcher et al., Science 283, 1911 -1914 (1999))
Model for CLV3 peptide signaling
Model for shoot apical meristem (SAM) maintenance
Support for model:
In clv mutant, WUS expression domain expands.
clv wus double mutants resemble wus single mutants.
If you overexpress WUS, CLV genes are activated and new meristems form.
Similar regulation in shoot and root meristems
WOX5 is similar in sequence to WUSCHEL.
WOX5 is expressed in the root quiescent center.
Sarkar et al. (2007) Nature 446: 811-814
Similar regulation in shoot and root meristems
Root cap stem cells differentiate in a wox5 mutant.
Sarkar et al. (2007) Nature 446: 811-814
Inflorescence meristem:
Makes flowers instead
of leaves and branches
Stem elongates more
Shoot meristem identities
Sepals, petals, stamens, carpels
Flower meristems
Inflorescence SAM
Induction
of flowering
Vegetative SAM
Leaves,
branches (lateral meristems)
Shoot meristem identities
Sepals, petals, stamens, carpels
Flower meristems (determinate)
Inflorescence SAM (indeterminate)
Induction
of flowering
Leaves,
branches (lateral meristems)
Vegetative SAM (indeterminate)
Vegetative shoot apical meristem (SAM)
Flower
meristems
Shoot
apical
meristem
(SAM)
Flower meristem
Flower
Arabidopsis flower
Stamen
Petal
Sepal
Gynoecium
(fused carpels)
Shoot meristem identities
Sepals, petals, stamens, carpels
Flower meristems (determinate)
Inflorescence SAM (indeterminate)
Induction
of flowering
Leaves,
branches (lateral meristems)
Vegetative SAM (indeterminate)
4 sepals
4 petals
6 stamens
2 carpels
1 2 3 4
Whorls
Arabidopsis mutants with altered flowers
Wild type
apetala1
cauliflower
apetala1
agamous
pistillata
Shoot meristem identities
Sepals, petals, stamens, carpels
Flower meristems
Meristem identity genes (LFY, AP1, CAL)
Inflorescence SAM
Induction
of flowering
Vegetative SAM
Leaves,
branches (lateral meristems)
leafy mutants:
partial conversion of
flowers to lateral
branches and leaves
leafy mutant flowers have no petals or stamens
Wild type
leafy
LEAFY is expressed throughout the flower meristem
(from Weigel et al., 1992, Cell 69: 843-859)
apetala1 mutant flowers lack sepals and petals
and have a new flower where a petal should be
Wild type
apetala1
Flowers are converted into
leafy shoots in leafy apetala1 mutants
Flower meristems behave like inflorescence
meristems in the apetala1 cauliflower double
mutant
ap1 cal
ap1 cal
Fig. 20.41
Why did they call this mutant cauliflower?
ap1 cal
Shoot meristem identities
Sepals, petals, stamens, carpels
Organ identity genes
(AP1, AP2, PI, AP3, AG
SEP1, SEP2, SEP3)
Flower meristems
Inflorescence SAM
Induction
of flowering
Vegetative SAM
Leaves,
branches (lateral meristems)