Shoot meristem self-organization and identity

Vegetative shoot apical meristem (SAM)

(from Long et al., 1996, Nature 379: 66-69)

Wild-type shoot apex shootmeristemless apex

Gene expression in embryonic SAM

WUSCHEL (WUS) SHOOTMERISTEMLESS (STM)

(from Laux et al., 1998, Cell 95: 805-815)

wuschel mutant

(from Laux et al., 1998, Cell 95: 805-815)

Wild type

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)

(from J. C. Fletcher et al., Science 283, 1911 -1914 (1999))

Wild type clavata3 mutant

clv1 and clv3 mutants look similar

clavata mutants have enlarged meristems

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

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.

Model for shoot apical meristem (SAM) maintenance

Sarkar et al. (2007) Nature 446: 811-814

WOX5 is similar in sequence to WUSCHEL.WOX5 is expressed in the root quiescent center.

Similar regulation in shoot and root meristems

Sarkar et al. (2007) Nature 446: 811-814

Root cap stem cells differentiate in a wox5 mutant.

Similar regulation in shoot and root meristems

Inflorescence meristem:

Makes flowers instead of leaves and branches

Stem elongates more

Vegetative SAM

Inflorescence SAM

Flower meristems

Leaves, branches (lateral meristems)

Sepals, petals, stamens, carpels

Induction of flowering

Shoot meristem identities

Vegetative SAM (indeterminate)

Inflorescence SAM (indeterminate)

Flower meristems (determinate)

Leaves, branches (lateral meristems)

Sepals, petals, stamens, carpels

Induction of flowering

Shoot meristem identities

Vegetative shoot apical meristem (SAM)

Flower meristems

Shoot apical meristem (SAM)

Flower meristem Flower

Arabidopsis flower

Sepal

Stamen

Gynoecium(fused carpels)Petal

Vegetative SAM (indeterminate)

Inflorescence SAM (indeterminate)

Flower meristems (determinate)

Leaves, branches (lateral meristems)

Sepals, petals, stamens, carpels

Induction of flowering

Shoot meristem identities

1 2 3 4

4 sepals

4 petals

6 stamens

2 carpels

Whorls

Arabidopsis mutants with altered flowers

apetala1apetala1cauliflower

pistillata

Wild type

agamous

Vegetative SAM

Inflorescence SAM

Flower meristems Meristem identity genes (LFY, AP1, CAL)

Leaves, branches (lateral meristems)

Sepals, petals, stamens, carpels

Induction of flowering

Shoot meristem identities

leafy mutants:partial conversion of flowers to lateral branches and leaves

leafy mutant flowers have no petals or stamens

leafy

Wild type

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 intoleafy shoots in leafy apetala1 mutants

Flower meristems behave like inflorescence meristems in the apetala1 cauliflower double

mutant

ap1 cal

ap1 cal

Fig. 20.41

ap1 cal

Why did they call this mutant cauliflower?

Vegetative SAM

Inflorescence SAM

Flower meristems

Leaves, branches (lateral meristems)

Sepals, petals, stamens, carpels Organ identity genes (AP1, AP2, PI, AP3, AG SEP1, SEP2, SEP3)

Induction of flowering

Shoot meristem identities