L18 - Introduction to Plant Development and Meristems

Question 1

What are the advantages of studying a small number of model plants?

Answer 1

• Traits like short lifecycle and lots of progeny
• Often beneficial to have many researchers working on one organism (builds a more complete picture and often conserved)

Question 2

Give the main differences between plant and animal development and why this emerges

Answer 2

Plants are generally indeterminate
- Autotrophs, must create large SA

Very plastic development

Plants have no unique parts
- Subject to herbivory so must survive if one part eaten

Cell wall inhibits cells rearranging so development controlled by division + elongation

Question 3

Where are meristems present and what is their function?

Describe the shoot apical meristem in more detail

Answer 3

Present at growing tips of plants
- Shoot apical meristem, root apical meristem, auxillary meristems

Shoot Apical Meristem:
- Dome of cells w/ central zone of slowly dividing stem cells
- One daughter cells remains stem cell in meristem
- Other daughter cell fed into meristem periphery for rapid division and stem/leaf formation

Question 4

Describe the function of the shoot apical meristem maintenance feedback loop and its general mechanism in 5 steps

Answer 4

Maintains meristem identity

1) WUSCHEL TF expressed in middle of SAM.

2) WUS promotes CLV3 peptide

3) CLV3 peptide binds CLV1/2

4) CLV1/2 inhibit WUS, reducing meristem growth

5) Less meristem = less WUS = reverse occurs

Question 5

How does WUS move and what is the evidence for this?

What is the effect of stopping this movement?

How does CLV3 move?

Answer 5

• WUS moves via plasmodesmata between cells
• Evidence as WUS RNA detected in smaller central meristem region than WUS protein
• Blocking movement by inducing extra callose production in WUS domains arrested meristem
• CLV3 seen in extracellular space in cell walls
• Moves apoplastically

Question 6

Aside from the WUS and CLAVATA genes name another gene that is used to maintain the meristem

Answer 6

SHOOT MERISTEMLESS (STM)
- Mobile TF, prevents cells obtaining organ identity
- Broader expression than WUS
- Works with WUS to induce CLV3
- STM mutants don’t maintain meristems, all converted to lateral organs

Question 7

Why is auxin thought to be important in phyllotaxis?

Describe how auxin is transported through plant tissues

Answer 7

Inhibition of organ formation by an organ thought to be through auxin depletion

• Auxin transported through plant tissues by specific influx + efflux proteins
• Can also passively diffuse into cells but not out of cells
• Most commonly imported by PIN transporters + exported by PIN-FORMED transporters

Question 8

Explain Hofmeister’s general model for phyllotaxis

Draw the relevant diagram

Answer 8

• New organs form on active ring in peripheral zone of meristem (central zone excluded)
• New organ inhibits other organs forming near
• Meristem grows, moving organ away
• Zone of inhibition reduced until low enough for new organ formation to initiate

(See diagram on pg 7)

Question 8

What is phyllotaxis?

Describe the four most common patterns observed

Answer 8

Phyllotaxis - Arrangement of repeat plant organs around the stem e.g. leaves

Spiral: Organs at 137.5˚

Decussate: Two opposite leaves then 90˚ shift with two more opposite leaves

Distichuos: Leaves produced one at a time at 180˚ to each other (common in grasses)

Whorled: Many leaves made at once

Question 9

Give two examples of plant phenotypes that show the importance of auxin in organ formation

Answer 9

• PIN mutants has no flowers, only a naked inflorescence (pin like)
• 1-N-Naphthylphthalamic acid (NPA) inhibits PIN transporters, gives “pin” phenotype
• Auxin application to NPA treated or PIN meristems restores organ function (transport of auxin via PIN auxin dependent)

Question 10

Describe the mechanism in which auxin depletion influences phyllotaxis

Answer 10

• PIN proteins polarise toward high auxin, reinforcing auxin maxima
• Eventually, sufficient auxin causes outgrowth
• Auxin maxima causes STM reduction, allowing organ formation
• Surrounding area depleted of auxin, inhibiting new organ formation
• Relative size of meristem compared to primordia gives different patterns

Question 11

Describe how the different parts of a flower develop and how the flower is made determinate

Answer 11

• Leaves modified by A, B, C function TFs to produce different floral organs
• AG gene shuts off WUS in floral meristem after carpel development
• Floral meristem used up, making flower determinate