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Developmental Biology > Lecture 10 - Plant stem cells and meristems > Flashcards

Lecture 10 - Plant stem cells and meristems Flashcards

1
Q

What is the shoot apical meristem and what is its structure?

A
  • the source of all above ground organs
  • key to the continued growth and function of the meristem
  • small population of densely packed stem cells at the tip of the meristem
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2
Q

What do the small population of densely packed stem cells at the tip of the meristem differentiate into?

A

Vascular tissues

Below L3

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3
Q

What are the functions of the shoot apical meristem?

A
  1. Make lateral organs on the flanks

2. Maintain the meristem (through asymmetric cell division)

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4
Q

How do the stem cells of the shoot apical meristem divide?

A

Stem cell divides into a daughter cell that will differentiate into a new type and another that will replace the parent and maintain the stem cell population ,provding a new source of cells for differentiation

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5
Q

How can we find out how meristematic stem cells work?

A

Reveal key genes by mutagenesis screens

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6
Q

What have mutagenesis screens revealed about the phenotype of a wuschel mutant and what was the experiment?

A

Wild type
-produces leaves in bullk then eventually stops producing leaves and produces long shoot before producing leaves again
Wuschel mutant
-‘stop start’ phenotype/development
-‘stop’ = missing a dome with initiation of leaves
-‘start’ = lots of domes
-instead of one cell remaining as a stem cell both cells divide meaning that the sc population is not maintained
-continually produces whirls of leaves then a short shoot region then whirls of leaves
-eventually will flower

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7
Q

What is the molecular mechanism of wuschel?

A
  • encodes a transcription factor
  • expressed in the heart of the SAM in a small number (in situ hybridisation)
  • lack of WUS fails to result in maintainin the SAM
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8
Q

What is the function of the CLAVATA genes?

A

-important meristematic function in development and regulation

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9
Q

What is the CLAVATA mutant phenotyp?

A
  • opposite to wuschel
  • ‘runaway expanding meristem ‘
  • enlarged meristem ‘strap like’
  • stem cell population is enlarged
  • tends to expand laterally
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10
Q

How many CLAVATA genes are there and what are their features?

A

3 CLAVATA genes

  • same mutant phenotype
  • CLV1 and CLV2 encode receptor kinases
  • CLV3 encodes a secreted pre-protein cleaved to give an extracellular peptide
  • overlapping domains of expression
  • CLV genes expressed in a slightly broader region of meristem than WUS
  • lact of the CLV genes results in a runaway expanding meristem
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11
Q

What type of pathways of regulating the meristem size do CLAVATA and WUS mutants show?

A 
CLAVATA 
-restrictive pathway reduced
-promotive pathway increased
-bigger flowers
-more cells
WUS
-promotive pathway reduced
-restrictive pathway increased
-reduction in the number of stemcells

Balance between restrictive and promotive pathway regulated meristem size

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12
Q

How do WUS and CLV interact?

A

SAM maintainence feedback loop
-WUS gene essential for stem cell identity
-domain of WUS kept in check by the CLV signalling pathway
-WUS feeds back to up-regulate CLV3 expression
CLV3 (signalling peptide) binds to CLV1 (receptor) which represses the activity of WUS (transcription factor) which normally promotes the expression of CLV3

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13
Q

What is epistasis?

A

When the effect of one gene depends on the presence of one or more modifier genes
-identify when do a double gene knockout, and the phenotype you see is of one of the genes, that means that that gene is epistatic to the other
LEARN THAT ^ IT IS V. IMPORTANT

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14
Q

How is the wus mutation epistatic to the clv mutation?

A
  • in the double mutant the wus phenotype is all you can see
  • suggest wus is at the end of the pathway
  • fits in with the SAM maintenence feedback loop
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15
Q

What happens to the domain of expression of WUS in a clv mutant phenotype?

A
  • normally expression in a small number of cell in the SAM
  • in clv mutant, WUS expession expands massively beneath the surface (lose the repressive part of the pathway, no CLV3 to repression wus expression), meristam is much larger
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16
Q

What happens to the domain of expression of CLV3 in a wus mutant?

A
  • strong expression in the SAM of the WT
  • reduction of CLV3 expression in the same domain in a wus mutant
  • wus mutant leads to a reduction in CLV3 expression (promotive part of the pathway)
17
Q

What does overexpression of CLV3 result in?

A
  • a phenotype like loss of wus

- ‘stop start’ phenotype

18
Q

What experiment was done to overexpress CLV3?

A
  • take promoter and join to the coding region of the CLV3 gene in bacteria, extra copy of the CLV3 gene
  • insert construct into vector and transform into plants to overexpress CLV3
  • use the 35s promotor
  • from the polyflower mesoic virus
  • expressed strongly in most plant cells
19
Q

Why over express CLV3 not CLV1?

A

if over express the ligand will only make a difference in cells that are expressing the receptor, where CLV3 will endogenously act

20
Q

Where do arabidpsis flowers develop from?

A

Auxillary shoot meristems (floral meristems)

  • initate floral organs (modified leaves - initaite in the same way and meristem functions in the same way)
  • CLAVATA switched off in flowers
  • grow in a determinate way (unlike SAM and leaves - must terminate the wus/clv pathway)
21
Q

What are the floral organs and what is their pattern of development?

A 
Sepal 
Petal 
Stamen 
Carpel
(from the outside in)
22
Q

What are the effects of the WUS and CLV mutations in flowers?

A
  • similar to effects on the SAM
  • clv mutant = more stamen
  • wus mutant =meristem terminates early, not all of the organs are produced
23
Q

How is the meristem function significantly different to that of the SAM?

A
  • the floral meristem gets ‘used up’
  • when making the carpels
  • must mean wus switched off
  • grows in a determinate way
24
Q

What is the phenotype of the agamous mutant?

A

-can’t make carpels
-mutant floral meristems do not terminate
-no stamen, carpels, lots of petals
-should have three petal whirls (petals, stamen, carpel) but carrys on producing petals indefinately
Shows that wus is epistatic to ag

25
Q

What is agamous?

A
  • transcription factor needed for carpel specification
  • shuts off WUS in the floral meristem
  • c function gene
  • determines reproductive organs in the flower
26
Q

How does agamous fit into the pathway of regulation of meristems?

A

CLV3 binds to CLV1
CLV1 and 3 supress Wus
Wus promotes the expression of AG
AG supresses the expression of Wus

AG negatively regulates Wus to result in determinate growth in the floral meristem

27
Q

What is the evidence that wus is epistatic to ag?

A

agamous/wus double mutant -> wus is the only phenotype seen

28
Q

What do clavata and wuschel mutants reveal about the regulation of gene expression in the SAM and FAM?

A

reveal genes that encode different compartments of a signal transduction pathway that regulates gene expression and balances cell proliferation with the maintenance of a population of stem cell

29
Q

How do CLV1, 2, and 3 form a signalling complex?

A

CLV1 and 2 form a receptor (cannot signal alone, CLV1 has kinase signalling ability)
CLV3 is the ligand that binds to the CLV1,/3 receptor complex to result in the supression of wus gene expression

30
Q

What does CLV expression result in?

A

inhibition of meristem size

31
Q

What does wus expression result in?

A

promotion of meristem size

32
Q

What receptors are related to CLV1/2 and function in broadly the same way?

A

CLE ligand and LRR receptor like kinase signalling in the SAM
CLV3 signalling to CLV1/2 and CRN supress the stem cell pool
CLE ligand signalling to BAM1/BAM2 promotes the stem cell pool

33
Q

What signalling is important in root meristems?

A

CLE dependent signalling

-root meristem regulated in a similar way to the shoot meristem

34
Q

What is a special features of fern meristems?

A

-have single apical cell

35
Q

In flowering plants, what pathway regulated the meristem?

A

wus/clv pathway

Developmental Biology (14 decks)

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