lecture 29 Flashcards
1
Q
What is evidence that auxin is a morphogenetic trigger?
A
- mutants lacking basal structures are defective in auxin signalling (mp/bdl mutants)
- auxin accumulates in the basal region of the embryo prior to hypophysis formation
2
Q
What is the difference between a morphogen and a morphogenetic trigger?
A
- morphogen
- cell fate determined by distribution of a morphogen
- diffusion gradient
- different cell fates at different levels
- the morphogen is instructive along the entire field of cells
- morphogenetic trigger
- instructive in only a small group of cells
- e.g. only when in high concentrations
- these cells acquire a different fate
- but the trigger is not instructive in cells with lower concentrations
3
Q
What is the concept of master regulators?
A
- cell division: no distinction between cells in the field
- patterning: specific sets of transcription factors are activated according to positon
e. g. TF1 in position 1 and 2, TF2 in 3 and 4, TF3 in 5 and 6 - differentiation: cell/tissue specific programme of gene expression: e.g. TF1 –> head genes, TF2 –> thorax genes, TF3 –> abdomen genes
- master regulators orchestrate specific patterns of gene expression
- this is true of animals, is it true of plants?
4
Q
What are master regulators of axial cell fate?
A
- Plethora (PLT) genes encode transcription factors that function to promote root formation
- phenocopy of mp mutants - lack of hypohysis
- need to inactivate all members of the family because of redudancy
- to test whether the PLT genes are master regulators of cell fate, generate transgeneic plants in which Plt genes are mis-expressed
- homeosis - conversion of shoot to root
- this evidence suggests that plethora genes are master regulators
- would expect the plethora genes to activated by auxin, and by mp
5
Q
Does PLT act downstream of auxin/Mp?
A
two lines of evidence suggest PLTs are regulated by auxin
- loss of PLT expression is observed in mp mutants
- PTL gene activity is responsive to exogenously applied auxin (IAA/NAA)
- increase in plethora expression after 24 hours
- PLT expression accumulates in the hypophysis precursor cell in wt embryos at globular stage
- slightly more broadly in the adjacent basal embryonic cells
no expression in mp mutants therefore plethora expression is absolutely dependent on monoptrous activity therefore downstream
6
Q
What is the current model for embryonic root patterning?
A
- auxin –> monoptrous –> plethora
- BUT Mp is not actually expressed in hypohpysis precursor cell
- basal embryo cell just above hypophysis precursor
- auxin –> bdl inhibition –> mp upregulates PIN –> PIN genes encode auxin efflux transporters –> movement of auxin in hypophysis precursor cell
- if this model is correct, then mp mutant prevents auxin from accumulating in precursor cell since it doesn’t trigger expression of PIN proteins
- could we then rescue cell adding auxin to hypothesis ? no.
- this model has something missing
BUT
- exogenously applied auxin does not rescue mp mutants
- must be a MP-dependent signal that moves from the basal cell into the hypophysis precursor
- this has possibly been identified
7
Q
What is organogenesis?
A
- leaves (vegetative organs) arise in the periphery of the shoot apical meristem
- arranged in a regular and predictable pattern - phyllotaxy
- most common arrangement is spiral (~137.5º between adjacent organs)
- new leaves tend to arise in the first available space
- i.e. in a region furthest from existing leaves
- greatest space
- suggests that growing leaves are exerting some kind of inhibition on newly arising leaves –> lateral inhibition
- if you surgically remove a leaf, the position of the new leaf shifts, indicating that the removed leaf was exerting some kind of inhibtion
8
Q
How do we know that auxin promotes organ formation?
A
- mutants with defects in auxin production, transport or perception display changes in phyllotaxy e.g. pin-formed1 (pin1) mutants
- tells us that auxin promotes organ formation
- exogenous application of auxin restores organ formation from the pin1 mutant apex
9
Q
What is field theory of phyllotaxy?
A
- organ formation requires auxin accumulation
- developing organs deplte auxin from surrounding regions
- new organs can only form in regions of least depletion
10
Q
What is PIN localisation in the SAM?
A
- PIN1 accumulates in membranes facing future site of organ initiation
- auxin accumulates at future sites of organ formation and drained from surrounding tissue
- net sink
- PIN1 polarity reverses
- auxin flows to new sites of organ formation
- net source
- sequential auxin peaks in the SAM trigger organ formation
while there is a gradient of auxin, it is only very high levels that trigger organ formation
11
Q
Give a summary of plant morphogens
A
- positional information used extensively in plant development
- no evidence for maternally encoded morphogens
- auxin has morphogen-like qualities
- gradient in tissye
- acts directly on cells via activation of ARFs
- determines cell fate according to concentration (female gametophyte)
- auxin frequently behaves as a morphogenetic trigger
- hypophysis formation
- phyllotaxy
12
Q
What is control of post-embryonic cell fate re: the flower?
A
- floral meristems arise from the inforescence meristem
- produce concentric whorls of organs with distinct identities
- determinate: produces limited number of organs
- genetic analysis identified homeotic mutants - where one floral organ is replaced with another
- three classes of mutant (A, B, C) - affects organs in TWO adjacent whorls
wt flower whorl 1 - 4 sepals whorl 2 - 4 petals whorl 3 - 6 stamens whorl 4 - 2 carpels
13
Q
What is a floral meristem?
A
modified axillary bud
as a meristem able to produce organs along its periphery
produces whorls instead of spirals
ceases activity once 4th whorl is produced
14
Q
What is a class A mutant flower?
A
Wh 1: sepals –> carpels
Wh 2: petals –> stamens
Wh 3: stamens
Wh 4: carpels
apetela2 (ap2) flower
apetala1 (ap1) flower
15
Q
What is a class B mutant flower?
A
Wh1: sepals
Wh 2: petals –> sepals
Wh 3: stamens –> carpels
wh 4: carpels
apetala3 (ap3) flower
pistillata (pi) flower
