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Spring 2008 BIO 350M/388M Molecular mechanisms of flowering

Spring 2008 BIO 350M/388M Molecular mechanisms of flowering Sibum Sung Section of Molecular Cell & Developmental Biology University of Texas at Austin When to flower is a matter of the survival of species Floral transition (Aukerman and Amasino, 1998)

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Spring 2008 BIO 350M/388M Molecular mechanisms of flowering

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  1. Spring 2008 BIO 350M/388M Molecular mechanisms of flowering Sibum Sung Section of Molecular Cell & Developmental Biology University of Texas at Austin

  2. When to flower is a matter of the survival of species

  3. Floral transition (Aukerman and Amasino, 1998) Transition in developmental programs: from vegetative growth to reproduction

  4. Floral Induction

  5. Daylength in flowering

  6. “Maryland Mammoth” cultivar of tobacco requires Short Days for flowering (Allard & Garner, 1918) Green house grown (kept as Long Days) “Maryland Mammoth” plant does not flower

  7. Floral induction of leaves of Short Day plant Perrila crispa “Something” must be produced in leaves and “move” to the meristem Long journey to identify “Florigen” began (Lang and Zeevart)

  8. Concept of Florigen (flowering hormone) A substance made in leaves which induces the shoot to flower, and it is “graft-transmissible” Only problem is “No one could ever isolate or purify such a substance (and many tried!)”. “Florigen” remained a hypothesis for decades until……

  9. Genetics came to the rescue Arabidopsis: Long Day Plant Flowering is induced by Long Day (LD) Certain late flowering mutants are blind to photoperiod i.e) contstans (co), flowering locus t (ft) CO: transcriptional co-activator FT: transcriptional co-activator (?)

  10. Inductive Photoperiod CO FT (and other floral integrators) Floral Transition

  11. How plants (and other organisms) measure the length of day and night ? External coincidence model: (originally proposed by Boenning, 1936) Light has a dual role in this model: - entrains the circadian oscillation of light- and dark-sensitive phases - directly required for the production of the signal.

  12. Photoperiod-dependent activation of FT is explained by the external coincidence model

  13. Photoperiod-dependent activation of FT is explained by the external coincidence model CO mRNA is circadian regulated. CO protein is not stable in dark in Arabidopsis

  14. FT is a floral activator both in LD and SD plants Long Day Plant Short Day Plant (Kobayashi & Weigel, 2007) CO in Long Day plants and similar proteins in Short Day plants are regulated in opposite ways

  15. FT is Florigen! ? (at least a part of it) In Arabidopsis (top), a leaf protein moves from a flowering graft into a nonflowering mutant, causing a stem and blossoms to form (Corbesier et al., 2007). In rice (bottom), the equivalent protein (green) shows up in the shoot apical meristem (Tamaki et al., 2007) Of course, FT protein may move with other substances. Stay tuned, since Florigen has a long history of misleading scientists

  16. Regulation of floral induction Besides the photoperiod-dependent regulation, floral transition is under controls of many other cues. (Kobayashi & Weigel, 2007)

  17. Some plants need winter to flower Vernalization Acquisition of the competence to flower in the spring by exposure to the prolonged cold of winter (Chouard, 1960).

  18. Memory of winter can be mitotically stable From Lang & Melchers memory in Hyocyamus niger

  19. Intron I Vernalization Requirement: Natural Variation (Klaus Napp-Zinn) Suppression of FLC acts via Region A of Intron I Region A pIDA FLC in Col-flc pIDA in Col-flc (pIDA responds to vernalization)

  20. FLC (FLOWERING LOCUS C) is a potent repressor of flowering Inductive Photoperiod CO FT (and other floral integrators) Floral Transition FLC

  21. FLC repression by vernalization is mitotically stable FLC VIN3 VIN3 UBQ

  22. Genetic screening for vernalization insensitive (vin) mutants • VRN2: Homolog of Su(z)12 (Gendall et al., 2001) • VRN1: Myb DNA binding protein (Levy et al., 2002) • VIN3: PHD finger protein (Sung & Amasino, 2004) • LHP1: LIKE-HETEROCHROMATIN PROTEIN (Sung et al., 2006) • VIL1: PHD finger protein (Sung et al., 2006; Greb et al., 2007) • VIN5: Histone arginine methyl transferase (Sung, Schmitz, Amasino, 2008) • VIN7: PAH2 domain protein (Sung, Schmitz, Amasino, unpub) • ; Genes are involved in the regulation of chromatin structure

  23. Nucleosome core particle (Luger et al., 1997)

  24. Modifications of nucleosomes (Turner, 2005) • Charged amino acid residues on the exposed face of histone proteins can be covalently modified

  25. Histone code Active chromatin (ON) Repressed chromatin (OFF) High in: H3K9Me, H3K27Me High in: Acetylation, H3K4Me, H3S10P Modified histone could be recognized by activation/repression complexes and establish stable activation/repression chromatin (Jenuwein and Allis, 2001)

  26. FLC repression by vernalization is mitotically stable FLC VIN3 VIN3 UBQ

  27. Dynamics of FLC chromatin Active FLC Chromatin High in Ac; H3K4Me; H3S10P H2A.Z Histone variant Repressed FLC Chromatin High in H3K9Me; H3K27Me H4R3MeS2; LHP1 VIL1, VIN5 VIN3, VIL1, VIN5 VRN2, VRN1, etc. (from Sung and Amasino, 2005)

  28. Environmentally induced chromatin changes ON; Fall VIL1, VIN5 VIN3, VIL1, VIN5 VRN2, VRN1, etc. WINTER! OFF; Spring (from Sung and Amasino, 2005)

  29. FLC (FLOWERING LOCUS C) is a potent repressor of flowering Inductive Photoperiod CO FT (and other floral integrators) Floral Transition FLC

  30. Stable repression of FLC provides competence to flower upon inductive photoperiod Inductive Photoperiod CO FT (and other floral integrators) Floral Transition FLC

  31. Molecular basis of the vernalization response • FLC is a potent repressor of flowering. • Competence: in Arabidopsis, largely a function of FLC expression level. Vernalization leads to competence via repression of FLC. • Mitotic stability: Vernalization-mediated repression of FLC via histone modifications that are hallmarks of epigenetic silencing

  32. Fall: flowering repressed FLC is highly expressed and thus represses FT activation Short Days prevents CO-FT activation

  33. FLC is repressed and FLC chromatin undergoes changes

  34. Stable repression of FLC by chromatin changes eliminates antagonistic effect on FT activation • Long Days promote CO-FT activation

  35. FLC resetting • During meiosis, FLC is reactivated • (reversing chromatin structure)

  36. Multiple flowering control pathways Many genes have variations even within same species ; provide flexibilities to adapt to local environments

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