Controls on Flowering

1. Controls on Flowering

2. FLOWERING • Light and Flowering • Flowering types: LDP, SDP, DNP • Critical day (night) length • A period of bright light needed prior to darkness, additional amount of light needed to extend day is small • Light break in night removes effect • R/FR shows phytochrome

3. Response to brief period of light in an extended dark period (longer than 24 hr) oscillates between inhibition and promotion of flowering • This shows that the state of phytochrome reacts with rhythm rather than a simple decay in hour-glass fashion.

4. Photophase Skotophase 0 12 24 Hours • Rhythms • All living organisms contain circadian rhythms with a light (photo) phase and a dark (skoto) phase

5. Rhythms • All living organisms contain circadian rhythms with a light (photo) phase and a dark (skoto) phase • If light exceeds photophase it will interfere with skotophase and reset the rhythm

6. Measurement of phytochrome in a SDP shows that it does changes from Pfr to Pr early in the dark period SDP Phytochrome Proportion present as Pfr Light 0 9 Hours of darkness

7. PHY and CRY as floral regulators • phyA promotes flowering. The Arabidopsis phyA mutant flowers later. • phyB plays an inhibitory role in floral initiation. phyB mutant flowered earlier. • Either a mutation or an overexpression of the CRY2 gene resulted in the reduced sensitivity to photoperiods.

8. Cryptochromes and phytochromes are the pigments setting the clock in plants. • Plants with phyA, phyB, and cry2 knocked out are consistent with the hypothesis that these three photopigments combine to mediate photoentrainment

9. How photoreceptors may regulate flowering time in response to different photoperiods. The daylength signal may be transmitted by photoreceptors, through the circadian clock, to affect floral initiation. The circadian clock may regulate (gate) the signal transduction of photoreceptors affecting floral initiation, being permitted at the peak, but denied at the trough of the circadian rhythm.

10. Mutations in genes that affect circadian rhythms affect flower timing. Flower timing mutants confer defects in circadian timing. elf3 (early flowering 3). ELF3 is a putative transcription factor for which both mRNA and protein abundance oscillate. ELF3 interacts with PHYB. Late flowering gi (gigantea). GI mRNA oscillates. Period of leaf movement shortened in gi but period of gene expression rhythm lengthens GI thought to be involved in PHYB signalling. Not a transcriptioin factor. Function unknown.

11. Summary Flowering governed by interaction between the form of phytochrome, cryptochrome, and the state of the endogenous rhythm, itself originally entrained by these pigments through dawn or dusk signals.

13. LD SD SDP SDP SD LD • The Induction Stimulus • Plants very sensitive clock • Number of induction cycles needed varies with species: 1 to approx. 7 • Light perceived by leaves; apex responds • Only 1 leaf needed • Influence transmitted across a graft • Transported in phloem • Flowering hormone = Florigen • Consists of the product of the gene called CONSTANS a protein or mRNA. When expressed in phloem companion cells, plants flower. • Once flowering induced plant remains in induced state; perennials reset by time or winter dormancy

14. Photoreceptors, Rhythms, Florigen & Flowering This is for Arabidopsis, a LDP

15. Juvenility • Some plants will not flower in juvenile phase • Contain phytochrome • Hormonal state appears inappropriate for flowering: • In conifers non-polar GAs increase at maturity • In English ivy GA causes a reversion to the juvenile phase and this is reversed back to the adult form by ABA

16. Vernalization • Low temperature promotion of flowering readiness prior to inducing photoperiods. • Needed by plants that naturally go through a winter prior to flowering. LD Cold

17. Vernalization • Most such plants are LDP biennials (e.g., sugar beet, carrot), though some may be perennials (LDP or SDP) (e.g. chrysanthemum). • Vernalization needs 5-10o for ca 6 weeks. • plus 4-5 days of following intermediate temperature to stabilize the vernalization. • High temperatures prior to stabilization cause devernalization.

18. The longer the exposure to cold the more rapidly floral initiation occurs. Rye Days from planting to flowering 50 10 Days of Cold

19. Plants requiring vernalization. (A) A biennial cabbage (Brassica oleracea) variety growing for five years without cold exposure vs a summer‑annual variety of B. oleracea that flowers rapidly without vernalization. Summer annual and vernalization‑requiring types of henbane (B) and Arabidopsis (C). Plants were grown in LD without vernalization. In both a single‑dominant gene (FLC) is responsible for the vernalization‑requiring habit (right). The annuals (left) have a non-functional flc allele.

20. Gene action in vernalization Flower forming genes

21. A speculative model of an Arabidopsis circadian clock. Light input via phytochromes and cryptochromes is mediated through ELF3 and GI, or through PIF3. PHYA-PIF3 and PHYB-PIF3 interactions are known to occur. PIF3 binds to CCA1 and LHY promoters and possibly to other targets in the clock. (McClung 2001)