However, most of what is known about how plants make flowers comes from research on plants that do rely on environmental guidance for flower initiation.
It’s Time To Flower
The correct timing of flowering is essential to maximize reproductive success in angiosperms.
And many flowering plants rely on the photoperiod (specifically, the relative night length) as an environmental signal to tell seasonal time. (To see how, please see previous posts about How Plants Tell Time and Why Plants Tell Time.)
The latest scientific evidence supports the hypothesis that the flower-inducing hormone florigen is actually a protein called FT. (The name “FT” comes from the finding that this protein is genetically coded for by the gene “Flowering Locus T” in plant molecular biologists’ favorite experimental plant Arabidopsis thaliana.)
Briefly, FT is produced in the leaves and is transported via the phloem to the shoot apical meristem (SAM).
Here FT acts like a molecular “alarm-clock”, evoking a complex genetic scenario, which culminates in flower formation (more on this to come).
But what sets off this “alarm-clock”, i.e. the production of FT in the leaves?
Turns out the story involves red, far-red, and blue light, the length of the night, and the plant’s biological clock. (Please note: Why is night length more important than day length? I don’t have the room to present the evidence here, but for a great summary, please see an animated explanation.)
1. Most of this information is based on genetic research using the plant Arabidopsis thaliana. (Although specific genes and proteins vary, depending on plant species, it appears that the basic story presented below holds for most photoperiodic flowering plants.)
2. Arabidopsis is a so-called “Long-Day” (LD) flowering plant (in reality, a “short-night” plant, but don’t get me started). So, adjustments in the story need to be made for so-called “Short-Day” (SD) plants. (Yes, they really are “long-night” plants.)
3. In Arabidopsis, florigen is likely the FT protein. In some SD cereals (such as rice), florigen is likely a protein called Hd3a, which is very similar to the Arabidopsis FT protein.
A Light-Sensitive, Flowering Alarm Clock
The so-called biological clock in plants is set primarily in the leaves by phytochromes, which are sensitive to red and far-red light.
They get help from blue-light-sensitive cryptochrome.
These photoreceptors interact with “clock-genes” that cause some proteins in plant cells to cycle with a circadian rhythm.
One of these proteins regulates the gene that codes for florigen (FT in Arabidopsis and Hd3a in rice, for instance).
Thus, florigen cycles in the leaves also with a circadian rhythm.
Briefly, in LD (“short-night”) plants florigen apparently peaks not long after sundown, and then slowly degrades during the night. If the nights are too long, the florigen level is below the threshold level to induce flowering at dawn, when the leaves begin to transport material to the SAM via the phloem. (Please note: florigen appears to be synthesized primarily by leaf vein cells adjacent to the phloem.)
Conversely, in SD (“long-night”) plants, the florigen apparently peaks long after sundown. So, if the night is too short, at dawn, the florigen hasn’t exceeded the threshold level to trigger flowering.
A Simplified Model of Florigen Cycling and Transport in a “short-night” (Arabidopsis) versus a “long-night” (rice) plant. Proteins (red) include: FT (Flowering Locus T), Hd3a (Heading date 3a), CO (Constans) , HD1 (Heading date 1), and Ehd1 (Early heading date 1). Proposed florigens in Arabidopsis (FT) and rice (Hd3a) cycle in leaves with circadian rhythm, but with different phases in response to different photoperiods. At dawn, when photosynthesis resumes in leaves, phloem transport from leaves to SAMs also resumes, carrying florigen. If florigen is above a threshold level, then it may trigger the floral transition in the SAM.
1. Greenup, A., et al. (2009) “The molecular biology of seasonal flowering-responses in Arabidopsis and the cereals.” Annals of Botany, Vol. 103, pp. 1165-1172. (Full Text)
2. Mach, J. (2011) “Will the real florigen please stand up? Sorting FT homlogs in maize.” The Plant Cell, Vol. 23, p. 843. (Full Text)
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