Plants Flashcards
(26 cards)
How does a flower arise from a shoot apical meristem?
Mofodication from a vegetive meristem to a reproductive apex - usually an inflorescence. This occurs by organogenesis and histodifferentiation.
Flower organs are produced sequentially in Whorls W1 = Sepals W2 = Petals W3 = Stamens W4 = Carpals
What have flower been proposed to be, and how have we studied them?
Goethe proposed flowers are simply modified leaves.
Studying homeotic mutants that alter organ that appears in a specified positions allows us to understand what genetic mechanisms are behind it. An example being monstrous flowers where stamens > petals.
What are the 3 classes of developmental genes?
1 - flowering time genes
2 - meristem identity genes
3 - floral organ identity genes
There are two types of meristem identity genes
- Promoters of flower meristems e.g. Leafy and Apetela 1
- Promoters of inflorescence meristems e.g. Terminal flower 1
Describe the leafy mutation. When is LFY expressed and what is its job?
Are there any homologues in other angiosperms?
Normally inflorescence has lots of branches with flowers interspersed with more inflorescences. LFY = lots of inflorescence’s and no flowers. Any flowers that do form looks very leafy hence name.
LFY is expressed early from the inner 2 whorls and is also an organ identity gene. It is a TF with a HTH domain binding DNA as a dimer.
Normally leafy promotes flower development
Flo = snapdragon lfy homologue and has the same phenotype.
Describe the AP1 mutation. When is AP1 expressed and what is its job?
This mutations also results in incomplete conversion to flowers. Flowers that do form have sepals but no petals. It is also expressed early from the outer 2 whorls and is also an organ identity gene. AP1 encodes a MADS box domain.
How do LFY and AP1 interact?
In LFY mutant AP1 is repressed. So LFY stimulates AP1. AP1 also stimulates LFY so there is a positive feedback loop. Over expression of both = small rapidly flowering plant.
What is cauliflower?
Enhancer of AP1. No phenotpye when mutated on its own but when mutated with AP1 the inflorescences never arrest and it looks like a cauliflower.
Encodes MADS box domain and also stimulates LFY
Describe the TFL1 mutation. When is TFL1 expressed and what is its job?
In this mutant the shoot meristem is converted to a terminating flower. TFL1 is restricted to the IM. Loss of TFL1 could be resulting in floral meristem genes to be expressed in the inflorescence? FISH shows that normally TFL1 limits expression of FM genes in the IM.
Both AP1 and LFY negatively regulates TFL1 and TFL1 negatively regulates both AP1 and LFY. Can be shown by over-expressing AP1 which leads to a TFL1 mutant. If repeat this on a TFL1 mutant background - phenotype is enhanced.
How was the ABC model first created? What is the ABC model
Analysed organ identity genes by chemically inducing mutations and assess the phenotype for homeotic transformations.
ABC model normal flower = W1 = A = Sepals W2 = A + B = Petals W3 = C + B = Stamens W4 = C = Carpals and termination
Describe class A, B and C mutants and give example genes.
Class A = AP2 = W1 > carpals and W2 > stamens
Class B = AP3 and Pi = W2 > sepals and W3 > carpals
Class C = Ag = W3 >petals and W4 > sepals and indeterminate flower.
A and C function don’t overlap but will fill each others places if the other is missing meaning they both have cadasteral functions
What happens in double and triple mutants - does the model have predictive power?
Yes it is predictive
Just A function = all sepals and indeterminate flower
Just C function = all carpals and terminal flower
Just B function = W1 = leaf, W2+W3 = petal/stamen hybrid and W4 = leaf and indeterminate
No function at all = all leaves ad indeterminacy + hairs showing leaf not sepals
Which was the first of the ABC genes to be identified and was it expressed where we expected it to be?
Are there any orthologues in other angiosperms?
Ag first gene identified due to accidental insertion into Ag2 which phenocopied Ag1. FISH shows Ag limited to W3 and W4 as expected.
Using Ag as a probe identified plena - snapdragon orthologue
Are the Class B gene expressed where we expect them to be?
AP3 restricted to W2 and W3 but Pi in W2 W3 and W4. Both must work together in B function as Pi mutant alone - no change in W4.
If over-express both Pi and AP3 in W4 this phenocopies superman mutant = lots of stamens. In sup mutant AP3 spreads into W4 and you get stamens.
Sup does not repress AP3 as it is only expressed at the W3/W4 boundary - FISH. So Sup must repress W3 proliferation into W4, evidence - over expression of sup causes small plants and sup has ear motif commonly seen in repressors of transcription.
More recent evidence suggests sup might actually be in W4 after all…
Sup regulated by leafy, AP3/Pi and Ag.
Are the Class A gene expressed where we expect them to be?
No - AP2 seen in all 4 whorls but as predicted in AP2 mutant Ag does extend. So does AP2 cooperate with something else to inhibit Ag - Lug?
Lug mutants = enhances AP2 phenotpye and alone is weak A class phenotype but Ag extends in single mutant. Lug = co repressor working with AP2 and SEUSS to stop Ag in W1 and W2.
Issue - Lug and SEUSS expressed in all 4 whorls. However, Ap2 protein only seen in W1 and W2. miR172 seen in W3 and W4 and prevents translation of AP2 mRNA. Altering AP2 to be resistant to miR172 phenocopies Ag mutant. Lug and Seu cooperate to regulate miR172 expression.
How conserved are the cadasteral genes in other angiosperms?
Petunia = new super model?
Sup conserved - same gene same function.
miR172 different function in petunia. Instead W2/W3 boundary maintained by miR169. In arabidopsis miR169 has dought stress function.
What type of proteins TF are the meristem identity and floral organ identity genes that we’ve comes across so far?
AP1 and CAL = MADS box
LFY = HTH - unique
AP3, Pi and Ag = MADS box
AP2 = unique
MADS box proteins bind DNA as dimers an CArG box domains, different combinations promote different organ types.
How were the Class E function gene identified?
Reverse genetics - took MADS box sequence and tested genome for more. Found 3 Ag like genes - names sep1-3.
Single and double mutants - no change. Triple mutants = all sepals and a new flower = BC class mutants.
Seps must cooperate with B and C genes to specify organ type. Othologues found in pertunia and called floral binding proteins. - FBP2 and FBP5
Can we genetically create a petal by dipping a leaf?
Using A and B = no
Use A + B + Sep = yes.
Describe the quartet model
MADS box proteins interact as tetramers to determine whorl organ identity. Confirmed the interactions with protein interaction studies. These Quartets repress/activate by bindign to different CArG domains.
Sepals = AP1 AP1 Sep Sep Petals = AP3 Pi AP1 Sep Stamens = AP3 Pi Ag Sep Carpals = Ag Ag Sep Sep
In quadruple mutants = all leaf like - much hairer than sepals.
What are D class genes?
Carpal specification not as simple as just carpals. Also ovules inside. This is regulated by Seedstick and Shatterproof 1 and 2. When all 3 are mutated then ovules become carpal like. Both are MADS box proteins.
Petunia orthologues = fbp7 and fbp11
How are meristem identity and floral organ identity genes linked?
Lfy expressed very early in flower development - much earlier than ABCDE genes. Makes sense becasue lfy required for floral meristem. Lfy directly regulates AP1 AP3 and Pi and Ag genes by binding to them and stimulating them.
What is wuschel?
Mutants have fewer W3 organs and no W4 organs and whole flower is contracted.
Wus required for stem cell activity in floral meristem and only seen in young flowers.
Both Wus and lfy required to switch on Ag. Over expression of wus in any whorl = extra organs. Ag inhibits wus and wus still present in Ag mutants. Ag inhibtion of wus achieved by knuckles - a zfn.
This is why Ag mutants are indeterminate.
In shoot meristem wus maintains stem cell niche through interaction with other genes.
How is this model conserved in other angiosperms?
Generally a conserved model but with some modifications
Examples - lillies have B function in W1 so have extra petals
Lacadonia - inside out flower with stamens in the middle and carpals surrounding this.
List some of the main differences between plant and animal development
Alterations of generations - sporophyte and gametophyte.
Sperm fertilise haploid eggs and one diploid cell - endosperm
Most of plant development is post embryonic whilst most of animal development is embryonic.
Plant zygote undergoes one asymmetric split into apical and basal cell. Apical cell forms the eventual shoot meristem, basal cell forms the root meristem.
Migration very important for animal development whilst in plants it isn’t. Development relies mostly upon patterns and controlled proliferation - stretch and divide.
