I’ve been thinking a lot about sleep lately, mostly because I wrote a story on the topic for the March issue of Popular Science. Sleep, of course, is a key part of our lives as humans. We do it every night (or at least we should). We also see other animals sleep, from our pet dogs snoozing at our feet to the wild birds roosting in the trees outside our windows. But what about the life forms that frequent Our Modern Plagues? Do microbes rest? Do insects regularly turn in at some point each day? What about plants, from crops to invasive species?
And if all these organisms do sleep, or exhibit some parallel behavior, are scientists manipulating the trait to our benefit?
To explore these questions, I will post a three-part series over the next several weeks. First up: plants.
So, do plants sleep? I posed the question to several plant experts and the short answer is no, at least not in the literal sense. Plants don’t have central nervous systems that seem to be key in what we think of as sleep in humans. But plants do have circadian rhythms tuned to Earth’s 24-hour light-dark cycle, which they maintain even if they’re kept in light fulltime, just as we do. And that is where things get really interesting.
Cyclical cues
For us, the circadian cycle determines when we should sleep and when we should wake up: sunlight enters our eyes each morning, triggering cells in the brain that control levels of the hormone melatonin, which, in part, controls drowsiness. The more melatonin, the sleepier we are—levels drop in the daytime and rise at night. And while our main sleep clock resides in the brain, we also have clock genes in nearly all of the cell types throughout the body, and vital physiological processes occur as we sleep.
Plants also go through physiological changes during each stage of the day, says Janet Braam, a plant biologist at Rice University. “There are likely diverse and very important advantages of circadian clock function to plants,” she told me by email. For example, “we know that plants use the clock to be able to monitor day length and thus can prepare for seasonal changes (like winter) before the weather actually changes.”
Indeed, plant behavior is tightly controlled by the sun. During the day, plants soak up sunlight during photosynthesis, the process they use to get energy. But when the sun goes down, plants’ opportunity to eat disappears and other physiological processes take over, including energy metabolism and growth.
Plants can anticipate the dawn each day and follow the sun to maximize their photosynthesis potential. The sunflower in the time-lapse below, for example, sways back and forth as the sun rises and falls, and the videos at this great website sLowlife show corn seedlings bowing towards a light bulb and sunflower seedlings that appear to dance as they reach for sunlight.
Braam’s team at Rice has found that the circadian rhythm in certain plants also determines when they launch chemical defenses against predators. A 2012 study on Arabidopsis, a highly studied flowering plant related to cabbage, found that the plant’s circadian cycle helps ward off cabbage looper caterpillars. A set of plants kept on a normal day/night cycle anticipated the time the caterpillars typically eat and gave off a pungent chemical to discourage feeding. Plants forced on a light cycle 12 hours out of phase didn’t do this and were thus chewed up.
And in a study from last June, the researchers showed that cabbage retains its circadian rhythm after harvest, including the cyclical production of a chemical called 4MSO, which may have anti-cancer properties.
Making the most of a situation
From an evolutionary perspective, it makes sense that plants have such a robust chemical life tuned into their environment. Robertson McClung, a plant geneticist at Dartmouth, points out that, just like any other organism, plants need to eat and protect themselves from predators. Yet unlike an animal, plants can’t walk to find food or seek shelter. To survive, they must take advantage of various stages of the day to maximize food intake and growth. They also sense the cyclical activity of insects and other predators to prep chemical defenses, or small changes in temperature or the timing of sunlight to prepare for different seasons.
What does this mean for us? Understanding how to control the nutritional composition of the vegetables in our fridge could improve our diet and health—say, by eating a piece of cabbage when 4MSO is at its peak. Manipulating the chemical signals of plants could also provide an option for pest control, or encourage higher crop yields.
“Everybody working on the plant circadian clock has that as their goal,” explains McClung, referring to the idea that we might exploit plants’ natural rhythms. “There is a sense now, there is emerging data that you may be able to improve crop productivity through the manipulation of the circadian clock.”
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Additional reading:
sLowlife Exhibit – Plants-In-Motion, Indiana University
C. Robertson McClung, “Plant circadian rhythms,” The Plant Cell 18:4 792-803 (2006)
Danielle Goodspeed et al, “Postharvest circadian entrainment enhances crop pest resistance and phytochemical cycling,” Current Biology 23:13 1235-1241 (2013)
Danielle Goodspeed et al, “Arabidopsis___ synchronizes jasmonate-mediated defense with insect circadian behavior,” _PNAS 109:12 4674-4677 (2012)
