Week 4 Wednesday: Flowering - Part 1 (9/23)

Floral induction

An environmental stimulus leads to floral initiation

Floral initiation

All of the developments necessary for the irreversible commitment by the meristem to produce an inflorescence

Endogenous

Originating from within

Exogenous

Originating from outside

Short-day plant (SDP)

Flowering under short days

Long-day plant (LDP)

Flowering under long days

Day-neutral plant (DNP)

Flowering independent of day length

Short-long-day plant (SLDP)

Flowering when short days are followed by long days

Long-short-day plant (LSDP)

Flowering when long days are followed by short days

Obligate (qualitative) flowering

Absolute requirement to a particular environmental condition, and will remain vegetative without inductive photoperiod

Facultative (quantitative) flowering

Flowering is promoted but will occur in the absence of the condition. Will eventually flower without the inductive photoperiod

Autonomous flowering plants

In response to endogenous developmental factors, but usually sensitive to irradiance; internal developmental cues lead to floral initiation

What does flowering depend on?

The dark period

Night-break lighting: blue, green, red

Lots of red and blue are required, but only a small amount of green is required to inhibit flowering

Germination under far-red/red light

Germination is inhibited under far-red light, and promoted under red light

Phytochrome forms

Pr (inactive) and Pfr (active)

Red light activates phytochrome (Pfr), while far-red light inactivates it (Pr). The light turns the molecule to its opposite form

Dark reversion

Energentically favorable change of Pfr (active) to Pr (inactive) over time

Hourglass hypothesis

A now-disproved hypothesis where plants track time via a non-oscillating biochemical “countdown” that resets every day, rather than an internal clock

It was disproven by the fact that night breaks could prevent/promote flowering

External coincidence model

Organisms use a daily biological clock to create a photo-inducible phase, and plant responses occur when the sensitive phase (peak) lines up with light or dark

Antiflorigen

A signal protein that suppresses flowering and counteracts the flower-promoting action of florigen

CONSTANS (CO) gene

The plant’s internal 24-hour circadian rhythm makes the CO (CONSTANS) gene to produce CO mRNA in the late afternoon/dusk. In the light, photoreceptors stabilize the CO protein but in the dark it is destroyed

CO gene relation to FTmRNA

FT mRNA (Flowering locus T) is the florigen signal that tells the plant to start flowering. The CO protein must be present to turn on FT expression