Control of Flowering by

Photoperiod Requires a

Transmissible Signal

The spatial separation between the site of perception of the photoperiodic signal (LEAVES) and the site of flowering (SHOOT APICAL MERISTEM) logically requires a transmissible signal that carries the information from the leaf to the shoot apex.

Hypothesis of a Floral Hormone Suggested by Russian plant physiologist

Mikhail Chailakhyan in 1936 FLORIGEN

- hormone that stimulates flowering- term used to describe the hypothesized hormone-like molecules responsible for controlling and/or triggering flowering in plants

Xanthium strumarium (cocklebur)Short Day Plant

Grafting - a horticultural technique whereby tissues from one plant are inserted into those of another so that the two sets of vascular tissues may join together.

Hypothesis of a Floral Hormone In Arabidopsis thaliana, florigen is likely the Flowering Locus T (FT) protein.

In some short day cereals (such as rice) florigen is likely a protein called Hd3a.

Hypothesis of a Floral Hormone Mechanism:

1. Initiation (photoperiod-regulated initiation)- In Arabidopsis thaliana, the signal is

initiated by the production of messenger RNA (mRNA) coding a transcription factor called CONSTANS (CO).

- CO protein promotes transcription of another gene called Flowering Locus T (FT).

Hypothesis of a Floral Hormone Mechanism:

2. Translocation (signal translocation via the phloem)

- The FT protein resulting from the short period of CO transcription factor activity is then transported via the phloem to the shoot apical meristem.

Hypothesis of a Floral Hormone Mechanism:

3. Flowering (induction of flowering at the shoot apical meristem)

- The FT protein interacts with FD protein to activate floral identity genes, thus inducing flowering. - Specifically, arrival of FT at the shoot apical meristem and formation of the FT/FD heterodimer is followed by the increased expression of at least one direct target gene, APETALA 1 (AP1), along with other targets, such as SOC1 and several SPL genes, which are targeted by a microRNA.

FT protein-acts as a long range signal

FD protein-protein mediating signals from the floral pathway integrator FT at the shoot apex

APETALA 1 gene-gene that specifies flower meristem identity and is also required for normal development of sepals and petals

SOC1 gene- triggers LFY gene expression

LFY gene- gene that causes meristems to develop into flowers instead of shoots

FLC- responsible for vernalization- negative regulator of SOC1

Temperature and

Photoperiodism

Low Temperature Requirement of Winter Annuals and Biennial Flowering Plants

• Vernalization= the acquisition of a plant's ability toflower or germinate in the spring by exposure to the prolonged cold of winter

Winter Cereals

• Winter Strains= will not normally flower

during a single growing season; but must be planted in the fall in order to flower and produce a crop the following year

• Spring Strains= will flower and produce

grain in the same year they are planted

Winter Cereals (Experiment)• Spring strain of Rye (Secale cereale var. Petkus)

= Long day plant= under long days, flowers are initiated after 7 leaves have been produced= under short days, flowers will not appear until 22 leaves have been produced

Winter Cereals (Experiment)• Spring strain of Rye (Secale cereale var. Petkus)

= germinated and grown at normal temperature (18°C) is not a LD plant and flowers only after 22 leaves have been produced regardless of photoperiod= if subjected at low temperature (1°C) for several weeks, it will flower early in response to long days just as spring strain does

Hyoscyamus niger (Black Henbane)• Annual strain

= typical Long day plant= grows in a rosette habit under short day, but undergoing extensive stem elongation (BOLTING) and flowering under long days

Hyoscyamus niger (Black Henbane)• Biennial strain

= it has a cold requirement that must be satisfied before it will bolt and flower as a long day plant= optimal temperature is 3-17°C

• Some day neutral plants have a cold requirement if satisfied, flowering will proceed regardless of photoperiod.

Example:Chrysanthemum Campanula medium

Gibberellin as Substitute for Vernalization

• Gibberellins will normally substitute for cold requirement in vernalizable long day plants and for long days in other LD plants.

• If such plants are treated with gibberellins, they produce flowers without subjecting the plants to cold and photoperiodic treatments.

Gibberellin as Substitute for Vernalization

• Example:Henbane is a rosette-leaved long day plant which requires cold treatment for flowering.

- If such untreated plants are sprayed with gibberellins, the plants produce flowers.

Gibberellin as Substitute for Vernalization

• Gibberellins do not usually substitute for short days in Short Day plants.

Exception: Campanula