Winter Plant Adaptation, Part 3: Keeping Time

I sit at my computer this morning grateful for warmth in the midst of this week’s extreme low temperatures. This is a tough week for plants and humans alike, especially those without shelter.

We have been geeking out over the past couple of posts about how leaf buds and woody plants prepare for the cold we are experiencing right now. How do they do it when they cannot run for the hot-house? Organisms, from bacteria to fungi to mammals to humans, possess an internal clock. Biological clocks and circadian rhythms are made up of proteins and gene interactions responding to light. Biological clocks might be useful to an organism needing to anticipate environmental changes such as the the onset of winter. Changing light levels, it turns out, are a far more consistent means of anticipating environmental changes than, say, temperature or relative humidity. It should be stated however, in the absence of environmental cues like day/night cycles, organisms still experience these biological rhythms, so it is ultimately an internal deal.

Plants use two groups of similar proteins to respond to time. Cryptochromes respond to the blue end of the visible light spectrum and are largely responsible for growth responses: things like phototropism, stomata opening and making of chlorophyll. Phytochromes are a second set of light responsive proteins involved in plant circadian rhythms. These proteins respond to the red and infrared end of the visible light spectrum, and they are largely responsible for leaves dropping in the fall, among other photomorphogenic (or light-regulated) behavior.

The phytochrome molecule makes up the core of a plant’s biological clock known as the “oscillator”. Phytochrome exists in nature in two forms whose cell concentrations oscillate back and forth throughout the day and season. The Pr  form absorbs light best at wavelengths around 666 nm. When red light from the sun hits this molecule, it transforms into a slightly different form known as Pfr. Pfr absorbs best in the infrared end of the spectrum, 730 nm to be exact, and varying concentrations are responsible for the varying developmental behavior described above including when a plant is being shaded. When Pfr absorbs infrared light, it changes back to Pr, thus the oscillation. That “simple” chemical clock not only keeps track of the daily time, but the calendar day. The ratio of Pr to Pfr changes depending on the season. The concentrations of Pr are greater than Pfr in the spring due to longer exposure to red light than infrared. The opposite is true of the ratio in the fall.

So, what does this have to do with cold tolerance and acclimation in plants? Cold stress, both short term wild swings (like we experienced this week) and long term seasonal changes, triggers a whole bunch of different gene and chemical activity. How these genes and chemicals all interplay is not completely known. It does appear there are several ways a plant initiates some of the cold stress strategies mentioned in my last post, some independent and some dependent on light exposure. Light seems to play an active role regulating the genes responsible for the dehydration strategy described. A plant utilizing more than one means of responding to cold, dependent and independent of light, makes sense to me since you would want to be able to prepare for winter AND deal with the short-term wild temperature swings occurring throughout the fall, winter and early spring.

Okay, so is not that the coolest thing? Botanical stopwatches! Who knew? Well, you do now, thanks to this blog.

Josh Steffen, Horticulture and Facilities Manager