A tale of two chromophores
We took to the woods.
I said, I have things to do. We won’t be long, she replied.
I had seen a bit of green clubmoss on our last walk, a sure sign of things to come. Clubmosses are tough, determined, and among the first to break winter dormancy and to begin the inexorable journey which leads to reproduction. I said, find me some nice clubmoss. Not just any clubmoss. I wanted to photograph clubmoss that was particularly vigorous. Vigorous clubmoss that was well illuminated. Vigorous clubmoss that was well illuminated, and which had a pleasing bit of snow surrounding it. You’re not the least bit picky, she said. We walked on.
I found some, she called. Found some what, I said. She was pointing to a nice bit of Lycopodium. Beautiful. I lay on the ground. The frame counter skipped from 2250 to 2255. We walked on.
I can’t resist this opportunity to describe how it is that, having been dormant for several months during winter, plants know when winter is over and when to begin to grow again. Wait too long, and the weather may change before you flower and set seed. Wait not long enough, and run the risk of frost. The quick answer to the question, How do plants know, is that they don’t, in fact, know anything at all but they do respond, biochemically, in ways which switch their physiological state from one of deep dormancy to another of intense activity.
Like the lives of most other organisms, those of plants are controlled, in part, by a circadian rhythm, an internal clock, the cadence of which is determined by the rotation of our planet on its axis. Cycles of growth and reproduction are complex and influenced by cycles of day and night, by the movement of our planet about the sun, and by changes in the relative durations of light and dark.
So, back to the question of how is it that plants are able to determine day length. We learned in grade school that plants rely on photosynthesis for nutrition. This would lead one to predict that plants should grow and reproduce when sunshine (and access to nutrient) is plentiful and that they should go dormant when the days are short. Surely. But again, how are plants able to determine the length of a day? The answer is found in pigment.
Although some plant pigments are responsible for making plants colorful, others do more than make these organisms pleasant to look at. A number are sensitive to light and are, at the same time, physiologically active. Light sensitive plant pigments provide a link between environmental cue and physiological response. One in particular, phytochrome, is responsible for the response to day length and belongs to a large class of pigments called chromophores (molecules capable of absorbing light energy). The thing about phytochrome is that the very act of absorbing a photon at a particular wavelength changes its three dimensional structure, and this allows it to then respond to a photon of a different wavelength. The phytochrome which absorbs red light is written Pr and is called the red form, while the form which absorbs in the far-red is written Pfr, and is called the far-red form.
The Pr form of phytochrome is physiologically inactive while its Pfr form is active. When a photon of red light strikes Pr the phytochrome changes shape to form Pfr and, as you might have guessed, when a photon of far-red light strikes Pfr the phytochrome shifts back to the Pr form. These forms are interconvertible and differentially active. One other thing you need to know to make sense of all of this is that, in darkness, Pfr shifts to Pr. What really matters to a plant, in terms of detecting both annual and daily cycles, is the balance between the two forms of phytochrome. Let’s simplify and say that a plant will do its thing if Pfr > Pr and that it will go dormant if the reverse is true, and Pr > Pfr. So, during that time of the year when there is more light (short nights, less Pfr being converted to Pr, there will be more active phytochrome around) the plant will grow and reproduce. Alternately, during that time of year when there is less light (long nights, more Pfr being converted to Pr, more inactive phytochrome around) the plant will enter dormancy. To end this discussion, remember that Pfr is a physiologically responsive protein. It acts as a transcription factor and is therefore responsible for driving cellular mechanisms which result in tissue growth and reproduction.
So now, here is an image of a clubmoss, shown responding to an increase in day light or, more precisely and as you and I now know, a decrease in the length of night.