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23.2. 2006 Flower development 1 Computational Systems Biology Flower development Teemu Teeri 23.2. 2006.

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Presentation on theme: "23.2. 2006 Flower development 1 Computational Systems Biology Flower development Teemu Teeri 23.2. 2006."— Presentation transcript:

1 Flower development 1 Computational Systems Biology Flower development Teemu Teeri

2 Flower development 2 Flower development in four parts 1. ABC and beyond 2. Induction of flowering 3. Meristems and prepatterns 4. Regulatory networks

3 Flower development 3 Part 1 ABC and beyond Homeotic genes that determine organ identity in flowers

4 Flower development 4 Sepal Petal Carpel Stamen Arabidopsis

5 Flower development 5 Homeotic mutants Wilhelm Johannsen William Bateson Homeosis: Something has been changed into the likeness of something else Bateson 1894

6 Flower development 6 Homeotic mutants

7 Flower development 7 Normal flower A mutant B mutant C mutant Homeotic mutants grow correct organs in wrong places

8 Flower development 8 B AC sepal petal stamen carpel ABC model for organ identity determination in flowers

9 Flower development 9 ABC model for organ identity determination in flowers

10 Flower development 10 B AC sepal petal stamen carpel B C stamen carpel AC sepal carpel B A sepal petal sepal The ABC model explains homeotic mutants in flowers

11 Flower development 11 B AC sepal petal stamen carpel AC sepal carpel B C stamen carpel B A sepal petal sepal Mutant phenotypes in Arabidopsis

12 Flower development 12 A sepal C carpel A - B - B - C - Double mutants in Arabidopsis

13 Flower development 13 A sepal C carpel A - B - B - C - A - B - C - leaf B AC Double mutants in Arabidopsis

14 Flower development 14 ABC genes in Arabidopsis and snapdragon

15 Flower development 15 MADS N C I K I K C N 56 aa, highly conserved, DNA-binding, dimerisation aa, considerable sequence variability 70 aa, moderately conserved, keratin related, protein-protein interactions Poor or no sequence conservation A region present in AG and related MADS proteins MADS domain family of transcription factors

16 Flower development 16 AGAMOUS APETALA3 Expression domains of ABC MADS-box genes correlate with their function

17 Flower development 17 MADS domain proteins bind DNA as dimers g e n e M2 M1 M2 M1 transcription

18 Flower development 18 The two B-function genes form an autoregulatory loop globosa DEF GLO DEF GLO deficiens DEF GLO DEF GLO

19 Flower development 19 A - B - C - leaf B AC Are they sufficient? No, expression of ABC genes in leaves does not convert leaves into flower organs. ABC MADS-box genes are necessary for development of flower organs

20 Flower development 20 sepal petal anther carpel B AC sepal petal anther carpel Among the ABC MADS- box genes, phylogenetic position and genetic function correlate. Phylogeny

21 Flower development 21 Arabidopsis MADS- box genes AGL2, AGL4 and AGL9 group outside of the ABC genes in fylogeny. When mutated, there is no change in flower phenotype.

22 Flower development 22 Wild typeOrgans W1-W4 Triple mutant W1 W2 W3 W4 In a triple mutant for AGL2, AGL4 and AGL9, all organs in the Arabidopsis flower develop into sepals

23 Flower development 23 Wild typeOrgans W1-W4 Triple mutant W1 W2 W3 W4 AGL2, AGL4 and AGL9 were renamed to SEPALLATA1, SEPALLATA2 and SEPALLATA3

24 Flower development 24 A sepal B - C - The SEPALLATA function (SEP1, SEP2 or SEP3) is needed to fulfill both the B function and the C function in Arabidospis. The triple mutant resembles the double mutant where B and C function genes are inactive

25 Flower development 25 Quaternary complexes of MADS domain proteins

26 Flower development 26 The Quartet Model of flower development

27 Flower development 27 A - B - C - leaf B AC Are they sufficient? ABC and SEP MADS-box genes are necessary for development of flower organs

28 Flower development 28 Rosette leavesCotyledons Conversion of Arabidopsis leaves into petals

29 Flower development 29 Scanning electron microscopy is used to define organ identity

30 Flower development 30 Unifying principles of flower development ABC model –Striking in its simplicity –Applicable to a wide range of flowering plants Central role of LEAFY –Necessary and sufficient to specify a meristem as floral Integrator of floral induction pathways Key activator of the ABC genes B AC sepal petal stamen carpel

31 Flower development 31 Part 2 How do we get there? Induction of flowering

32 Flower development 32 Inflorescence meristem Vegetative meristem Flower meristem CO FLC AGL20 AGL24 LFY/FLO wt Meristems and phase transitions

33 Flower development 33 Multiple inductive pathways control the timing of flowering Long-day photoperiod Gibberellins (GA) Vernalization Autonomous pathway

34 Flower development 34 Induction of flowering Multiple cues

35 Flower development 35 Multiple cues are integrated by FLC, SOC1, FT and LFY Induction of flowering Multiple cues

36 Flower development 36 Meristem identity genes Shoot meristem identity genes –TERMINAL FLOWER 1 (TFL1) Floral meristem identity genes –LEAFY (LFY) –APETALA 1 (AP1)

37 Flower development 37 wild type centroradialis mutant Snapdragon TFL1 –> CEN, LFY –> FLO Inflorescence meristem Flower meristem CEN FLO cen FLO

38 Flower development 38 Meristem identity genes Inflorescence meristem Vegetative meristem Flower meristem wt TFL1 LEAFY TFL1 LFY

39 Flower development 39 TFL1 versus LFY and AP1 35S-LFY 35S-AP1 35S-TFL1 LFY AP1 TFL1

40 Flower development 40 Part 3 Meristems and prepatterns How ABC is laid down?

41 Flower development 41 Meristems are stem cells of the plant

42 Flower development 42 Maintenance of the shoot apical meristem SAM WUS CLA3 SAM WUS CLA3 CLAVATA3 expression is dependent on WUSCHEL Stable feedback loop that maintains the size of SAM WUS expression gives the meristem a prepattern

43 Flower development 43 Other prepatterns UFO UNUSUAL FLOWER ORGANS (UFO) patterns all meristems

44 Flower development 44 Other prepatterns LEAFY marks the flower meristem LEAFY Floral SAM Vegetative SAM

45 Flower development 45 WUS induces AG AG represses WUS WUS AG SAM A wus mutant flower: central organs are missing

46 Flower development 46 WUS induces AG AG represses WUS WUS AG SAM A wus mutant flower: central organs are missing + LEAFY Unlike CLAVATA3, AGAMOUS expression is only initially dependent on WUSCHEL

47 Flower development 47 WUS induces AG AG represses WUS AG SAM LEAFY Unlike CLAVATA3, AGAMOUS expression is only initially dependent on WUSCHEL Repression of the SAM organizer terminates the meristem

48 Flower development 48 WUS induces AG AG represses WUS WUS ag SAM + LEAFY Failure in repression of the SAM organizer keeps the meristem proliferating

49 Flower development 49 AP1 is initially expressed throughout the meristem SAM LEAFY APETALA1 is induced by LEAFY AP1

50 Flower development 50 AG represses AP1 AG SAM LEAFY AP1 B AC

51 Flower development 51 B genes use the UFO prepattern UFO + LEAFY AP3 LEAFY and UFO induce AP3 expression in a region where whors 2 and 3 (petals and stamens) will develop

52 Flower development 52 B genes use the UFO prepattern PI is initially induced also in the center of the flower meristem PI AP3 PI AP3 AP3 PI The B gene autoregulatory loop stabilizes B gene expression

53 Flower development 53 B genes use the UFO prepattern PI is initially induced also in the center of the flower meristem PI AP3+PI PI AP3 AP3 PI The B gene autoregulatory loop stabilizes B gene expression

54 Flower development 54 Patterning ABC genes SAM LEAFY AP1 AP3+PI AG B AC sepal petal stamen carpel

55 Flower development 55 A complete picture…

56 Flower development 56 Part 4 Regulatory networks

57 Flower development 57 Regulatory networks Figure 2. Logical Rules for AP1, AP2, FUL, AP3, and PI. The state of each network node (rightmost column in each table) depends on the combination of activity states of its input nodes (all other columns in each table). X represents any possible value. Comparative symbols ( ) are used when the relative values are important to determine the state of activity of the target node. AP1 (A), AP2 (B), FUL (C), AP3 (D), and PI (E).

58 Flower development 58 Regulatory networks Figure 4. Gene Network Architecture for the Arabidopsis Floral Organ Fate Determination.

59 Flower development 59 Regulatory networks The Steady States of the NetworkModel Coincide with Experimental Gene Expression Profiles The network had 139,968 possible initial conditions, and it attained only 10 fixed-point attractors or steady gene expression states (see supplemental data online for complete basins of attraction). These steady gene states (Table 1) predicted by the model coincide with the gene expression profiles that have been documented experimentally in cells of wild-type Arabidopsis inflorescence meristems and floral organ primordia. For example, in the Infl steady states, floral meristem identity genes (LFY, AP1, and AP2) and floral organ identity genes (AP1, AP2, AP3, PI, SEP, and AG) are off, whereas the inflorescence identity genes (EMF1 and TFL1) are on.

60 Flower development 60 Reading Jack, T. 2004: Molecular and genetic mechanisms of floral control. Plant Cell 16, S1-S17. Espinosa-Soto et al. 2004: A gene regulatory network model… Plant Cell 16:


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