By: Wendy Langhans
The lives of so many creatures revolve around a 24 hour cycle – everything from bacteria to plants and animals – including human beings. They even have a name for this cycle: Circadian rhythm. According to one definition, these are biological processes which “repeat once a day” and are endogenously created – which means the “rhythms persist in the absence of external cues”. To visualize how a circadian rhythm works, picture an alarm clock. Once it’s set, it goes off every day at the same time.
But clocks aren’t the only way we measure time. People need to know the time of day and time of year; they need a clock AND a calendar. So do plants. They also need a botanical calendar. In Wisconsin, apple trees blossom in May. If they blossomed too early, say, in sub-zero January, there would be no apples in August and September.
Recently, scientist have discovered how some plants ingeniously use their circadian clock to produce a botanical calendar. Let’s take a look at how this works in Arabidopsis, a plant related to mustard and cabbage and that serves as “a model plant” in biological research.
In Arabidopsis, flowering is induced by a protein known as “FLOWERING LOCUS T”. As explained here, FLOWERING LOCUS T “travels from the leaves to the shoot apex, a part of the plant where cells are undifferentiated, meaning they can either become leaves or flowers. At the shoot apex, this protein starts the molecular changes that send cells on the path to becoming flowers.” OK, so we know there’s a bio-chemical pathway involved. But how does the plant know it’s the right time of year to produce FLOWERING LOCUS T?
Here’s what scientists have recently discovered. Every day, late in the afternoon, Arabidopsis produces a protein known as “FKF1”. This protein is a photoreceptor and is activated by sunlight. When the days are short, there is no late afternoon sunshine, so no activation occurs. But in the spring, when the days are longer, sunlight activates FKF1. Once activated, FKF1 “activates the flowering mechanisms involving FLOWERING LOCUS T” and the plant begins to grow flowers.
To summarize, here’s how a plant uses an alarm clock to produce a botanical calendar:
ALARM CLOCK. Every day, in the late afternoon => FKF1 protein.
BOTANICAL CALENDAR. Late afternoon daylight => activates FKF1 protein => FLOWERING LOCUS T protein => Flowers.
Of course, this explanation leaves one major question about botanical calendars unanswered: if people hang calendars containing photos of flowers, what kind of photos would be in a Flower’s calendar?
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