Determinate Organs- Leaves

Question 1

What can Arabidopsis mutants help to identify?

Answer 1

Arabidopsis mutants identify genes controlling meristem activity.

Question 2

What do clavata mutants do?

Answer 2

Clavata mutants progressively accumulate more stem cells.
SAM, IM and FM cell number increases in clavata mutants
(because of defect in the transition of cells out of CZ).

Question 3

What do clavata IMs produce?

Answer 3

Produce more floral meristems.

Question 4

What do mutant clavata FMs do?

Answer 4

Produce more floral organs.

Extra carpels – gives larger club-like gynoecium
name clavata is derived from Latin – “clavatus” – shaped like a club

Question 5

What are the known parts of the CLAVATA intracellular signalling pathway?

Answer 5

CLAVATA gene products appear to be part of an intercellular signalling pathway.

CLV1 – receptor like kinase.

CLV2 - receptor-like protein.

CLV3 – small secreted protein.

Question 6

What are wuschel mutants?

Answer 6

Wuschel mutants repeatedly initiate defective shoot meristems –
stop-go development
giving disorganized bunches of leaves (touseled) and very few flowers.

Question 7

What does wuschel do?

Answer 7

WUSCHEL promotes stem cell identity.

Question 8

What do lateral organs derive from?

Answer 8

Lateral organs derive from founder cells.

Question 9

What are founder cells?

Answer 9

Founder cells are:

• specified by their position at the meristem periphery.
• morphologically indistinguishable from surrounding cells at this stage.

Activation of founder cells involves increased cell divisions to create a lateral organ primordium.

Further growth and differentiation produce the leaf.

Question 10

What is phyllotaxy and when is it determined?

Answer 10

The pattern of leaves on the shoot. Defined positions of leaves.

Reflects the pattern of organ initiation at the meristem.

Phyllotaxy is determined very early in leaf development,
at the stage of primordium initiation.

Question 11

What is the most common form of phyllotaxy?

Answer 11

Spiral phyllotaxy.

In most plants (~80%), leaves form in a regular spiral pattern.

Question 12

At what angle do leaves form in spiral phyllotaxy?

Answer 12

In spiral phyllotaxy, leaves form about 137.5°apart.

Initiate at specific angle.
Optimum spacing of leaves to capture optimum amount of light.

The Golden Ratio
137.5/360 = 0.382

Question 13

How are leaf primordia termed?

Answer 13

Leaf primordia are numbered P1, P2 etc from youngest to oldest.

The next leaf to form is called the incipient primordium, I1.

Question 14

How can primordia influence the position of other primordia?

Answer 14

Primordia can influence position of where next ones will form.
Secrete inhibitor.

Leaf primordia are surrounded by zones of inhibition in which new primordia cannot form.

Inhibition decreases as primordia grow away from the meristem periphery.

Question 15

How do the incipient primordia get their positional information?

Answer 15

The apex of the pin1 mutant is bare – it fails to produce lateral organs.
Therefore, polar auxin transport is necessary for primordium formation.

Auxin- hormone involved in almost everything in plants.
Pin mutant not initiating primordia of any kind.
Flow of auxin must be involved in primordium formation.

Question 16

What is required for organogenesis?

Answer 16

Local auxin maximum.

Question 17

Where does auxin accumulate?

How is auxin maxima driven?

Answer 17

Auxin accumulates at I1 position.

Auxin maxima is driven by PIN localistion- Localised maximum of hormone.

Question 18

How does PIN1 polarity affect the loaction of next primordium?

Answer 18

A subsequent reversal in PIN1 polarity changes the position of the auxin maximum, specifying the site of the next primordium.

Question 19

What happens to PIN1 after primordium initiation?

Answer 19

After primordium initiation, PIN1 distribution changes, directing auxin flow into the developing midvein of the leaf.

Require auxin to promote formation.
Not spread of inhibitor, but taking away of activator.

Question 20

How is leaf identity initiated?

Answer 20

Cells in the primordia are functionally distinct from cells in the meristem.

They become determinate – growth stops once the leaf is formed.

Question 21

What promotes differentiation?

Answer 21

Primordium-specific genes.

Question 22

What are the ARP genes and what do they do?

Answer 22

The ARP genes

“ARP” is derived from three homologous genes 
ASYMMETRIC LEAF1 (AS1) - Arabidopsis.
ROUGH SHEATH2 - Maize.
PHANTASTICA (PHAN) 
–Antirrhinum (snapdragon).

ARP genes encode MYB transcription factors.
Expressed in cells of leaf primordia.
Promote growth, differentiation and determinacy.

(ARP- transcription factors.
Highly related proteins.
Only expressed in emerging primordium and promote primordium.)

Question 23

Where is AS1 expressed and what does it do?

Answer 23

ASYMMETRIC LEAF1 (AS1), an ARP, is expressed in cotyledons but not in the meristem.

Prevent differentiation by controlling expression of ARP1.

Question 24

What happens if ARP activity is lost?

Answer 24

Increased leaf indeterminancy and complexity.

Very similar phenotypes to KNOX-OE.

Question 25

What do as1 mutants have?

Answer 25

as1 mutants have lobed leaves and ectopic meristems
on leaf surface.

as1 mutants ectopically
express homeobox transcription factors.

Question 26

How do ARP and KNOX1 genes affect each other?

Answer 26

The activities of ARP and KNOX1 genes are mutually antagonistic.

The two classes of transcription factors are mutually repressive, and help establish a separate identity for the emerging leaf primordium.

Question 27

What are the two types of leaf complexity?

Answer 27

Simple and compound.

KNOX factors are crucial for this difference.

Question 28

What is the state of KNOX1 expression in simple leaves?

Answer 28

In plants with simple leaves, KNOX1 expression remains off in leaf primordia.

Question 29

What is the state of KNOX1 expression in compound leaves?

Answer 29

In plants with compound leaves, KNOX1 expression turns back on in primordia.

Make primordia on the leaf primordia.

Question 30

What are the two types of leaf polarity?

Answer 30

Adaxial (top side)

Abaxial (bottom side)

Question 31

How is leaf poalrity affected by the meristem?

Answer 31

Lateral organs have a polarity with respect to the meristem.

The meristem side is adaxial, and far side is abaxial.

(The meristem side is adaxial, and far side is abaxial.)

Question 32

How are the adaxial and abaxial sides of the leaf different?

Answer 32

adaxial / abaxial sides of the leaf are functionally specialised.

Adaxial- palisade mesophyll – densely packed photosynthetic cells.

Abaxial- spongy mesophyll – loosely packed to facilitate gas exchange.

Question 33

What is the Sussex signal?

Answer 33

Meristem-derived positional signal for ad/abaxial polarity.

If signal blocked, only get one surface- leaf can’t distinguish top and bottom sides.

Question 34

What is the phantastica mutant?

Answer 34

Temperature sensitive transposons.
Phan- snapdragon version of asymmetric leaves.

The phantastica mutant has radially symmetrical leaves.
PHAN encodes an ARP transcription factor.

Mutant phan leaves are abaxialized. PHAN is necessary for adaxial cell fate.
Needs transcription factor to promote differentiation.

Question 35

What are kanadi mutants?

Answer 35

Arabidopsis mutant.

Loss of function kanadi mutants have radial, adaxialized leaves.

A kanadi mutant has radial, adaxialized leaves, showing that KANADI genes promote abaxial fate.

Opposite function as PHAN, yet similar loss of lamina outgrowth.

KANADI genes also encode transcription factors.

Question 36

How do KAN and PHAN control leaf polarity?

Answer 36

Mutual repression of transcription factors controls leaf polarity and lamina outgrowth.

Another example of antagonism between TFs as a developmental circuit guiding morphogenesis.

KAN and PHAN repress each other.