Determinate Organs- Leaves Flashcards
What can Arabidopsis mutants help to identify?
Arabidopsis mutants identify genes controlling meristem activity.
What do clavata mutants do?
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).
What do clavata IMs produce?
Produce more floral meristems.
What do mutant clavata FMs do?
Produce more floral organs.
Extra carpels – gives larger club-like gynoecium
name clavata is derived from Latin – “clavatus” – shaped like a club
What are the known parts of the CLAVATA intracellular signalling pathway?
CLAVATA gene products appear to be part of an intercellular signalling pathway.
CLV1 – receptor like kinase.
CLV2 - receptor-like protein.
CLV3 – small secreted protein.
What are wuschel mutants?
Wuschel mutants repeatedly initiate defective shoot meristems –
stop-go development
giving disorganized bunches of leaves (touseled) and very few flowers.
What does wuschel do?
WUSCHEL promotes stem cell identity.
What do lateral organs derive from?
Lateral organs derive from founder cells.
What are founder cells?
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.
What is phyllotaxy and when is it determined?
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.
What is the most common form of phyllotaxy?
Spiral phyllotaxy.
In most plants (~80%), leaves form in a regular spiral pattern.
At what angle do leaves form in spiral phyllotaxy?
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
How are leaf primordia termed?
Leaf primordia are numbered P1, P2 etc from youngest to oldest.
The next leaf to form is called the incipient primordium, I1.
How can primordia influence the position of other primordia?
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.
How do the incipient primordia get their positional information?
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.
What is required for organogenesis?
Local auxin maximum.
Where does auxin accumulate?
How is auxin maxima driven?
Auxin accumulates at I1 position.
Auxin maxima is driven by PIN localistion- Localised maximum of hormone.
How does PIN1 polarity affect the loaction of next primordium?
A subsequent reversal in PIN1 polarity changes the position of the auxin maximum, specifying the site of the next primordium.
What happens to PIN1 after primordium initiation?
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.
How is leaf identity initiated?
Cells in the primordia are functionally distinct from cells in the meristem.
They become determinate – growth stops once the leaf is formed.
What promotes differentiation?
Primordium-specific genes.
What are the ARP genes and what do they do?
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.)
Where is AS1 expressed and what does it do?
ASYMMETRIC LEAF1 (AS1), an ARP, is expressed in cotyledons but not in the meristem.
Prevent differentiation by controlling expression of ARP1.
What happens if ARP activity is lost?
Increased leaf indeterminancy and complexity.
Very similar phenotypes to KNOX-OE.
