Flowering Time

3. Flowering Time FT Flower Meristem Identity APETALA1, LEAFY, (TERMINAL FLOWER) Flower Organ Identity A, B, C, D Fruit Development FRUITFULL, SHATTERPROOF INDEHISCENT

4. stamen carpel sepal petal Arabidopsis flower

6. 10 13 5 8 2 7 3 * 1 4 12 11 6 9

7. LFY and AP1 are expressed in flower meristems

8. The LFY and AP1 genes are expressed in flower meristems. • In LFY/AP1 double mutants, flower meristems are converted into • shoots (leaves and stems). • LFY and AP1 normally act to promote flower meristem identity, • meaning that they tell cells to form a flower rather than a shoot.

9. The TFL gene is expressed in the shoot apical meristem where it acts to repress LFY and AP1 expression. • In TFL mutants, LFY and AP1 are expressed in the shoot apical meristem, converting it into a “terminal flower”. • The TFL gene normally prevents the shoot meristem from • being converted into a terminal flower.

12. stamen petal sepal carpel 1 2 3 4

13. 1 2 3 4 A A

14. 1 2 3 4 B B

15. 1 2 3 4 C C

16. petal petal sepal sepal carpel stamen stamen B B A C C A 1 2 3 4 3 2 1

17. A = sepals A + B = petals B + C = stamens C = carpels ABC Model Bowman and Meyerowitz, 1991

18. gene protein protein structure by Schumacher et al. Nature/vol410/26 April 2001

19. petal stamen sepal carpel sepal leaf The C gene is active in cells that will form stamens and carpels

21. A The “A” gene is active in cells that will form sepals and petals

22. B The “B” gene is active in cells that will form petals and stamens

23. C The “C” gene is active in cells that will form stamens and carpels

24. The activities of the “A” and “C” genes do not overlap

25. In “C” mutant, “A” activity expands throughout the flower

26. In “A” mutant, “C” activity expands throughout the flower

30. normal flower

31. petal petal sepal sepal stamen carpel carpel stamen normal A mutant B B B B A C C A C C C C carpel carpel stamen stamen stamen stamen

32. sepals are replaced by carpels “A” mutant

33. sepal sepal sepal sepal carpel carpel carpel petal petal sepal sepal carpel stamen stamen normal B mutant B B A C C A A C C A

34. petals are replaced by sepals “B” mutant

35. normal C mutant B B A C C A petal petal sepal sepal carpel stamen stamen B B A A A A sepal petal petal petal petal sepal *

36. stamens are replaced by petals “C” mutant

37. carpels are replaced by a new flower “C” mutant

38. normal C mutant B B A C C A petal petal sepal sepal carpel stamen stamen B B A A A A sepal petal petal petal petal sepal *

39. petal stamen sepal carpel sepal leaf The C gene is active in cells that will form stamens and carpels

40. C (AGAMOUS) gene Summary • The C gene is expressed in cells that will later form stamens and carpels. • The C gene is necessary for stamen and carpel development. • The C gene is necessary to prevent the indeterminate growth of the flower meristem.

41. Determinacy vs. Indeterminacy • Wild-type flowers are “determinate” structures meaning that they produce a defined number of organs. • C mutant flowers are “indeterminate” structures in that they continuously produce new flower organs. • The C gene is necessary to prevent the indeterminate growth of the flower meristem.

42. Activation of the C gene (AG) • LFY + WUS bind to the C gene regulatory sequences and activate transcription. • LFY provides the “flower specificity” and WUS provides the “central zone” specificity to ensure C gene activation in the center of flower meristems.

43. Indeterminacy of C mutant flowers • The C (AG) gene negatively regulates WUS expression. • In wild-type flowers, WUS expression disappears soon after the C gene is activated. • In C mutant flowers, WUS expression is never turned off. • In C mutant flowers, WUS activity causes the indeterminate (continuous) growth of the flower meristem. • If mutations in the C gene are combined with mutations in WUS (the double mutant), flowers are determinate. This demonstrates that WUS is responsible for the indeterminate phenotype of C mutant flowers.