Humans have been observing the circadian rhythms of plants for millennia, even if they couldn’t fully explain what they saw. In a new piece for The Conversation, Hua Lu, associate professor of biological sciences at UMBC, and Linda Wiratan ’19, biochemistry and molecular biology, survey current knowledge of plants’ daily rhythms and how recent findings could influence human medical care.
“The internal time-sensing mechanism,” known as the circadian clock, “allows many living organisms to keep track of time and coordinate their behaviors along 24-hour cycles,” write Lu and Wiratan. Important advances in circadian clock research led to the 2017 Nobel Prize in physiology or medicine being awarded for work that explained the molecular basis of circadian rhythms.
Lu and Wiratan, among others, are studying plants’ circadian clocks “for insights into how they affect the health and well-being of all life on Earth,” they write. They point out that bacteria, fungi, insects, plants, and mammals are all affected by circadian rhythms, which are regulated by the circadian clock’s central oscillator, “an elaborate network of genes that turn each other’s activity on and off,” forming “complex feedback loops that accurately calibrate time.”
While the exact genes involved differ across organisms, the basic feedback-loop mechanism is the same. “Such amazing balancing acts reflect organisms’ abilities to anticipate changing environment throughout the day,” Lu and Wiratan write, which includes everything from herbivore activity to light availability.
Researchers have found evidence that improper circadian clock function plays a role in human diseases such as diabetes, obesity, and depression, so a fuller understanding of the clock is important for human health. Plants’ circadian clocks also regulate the function of their immune systems. First-line defenses include ever-present physical features like protective hairs and waxy coatings, but plants also have more sophisticated ways of fending off their foes. Some plant cells can detect the presence of pathogens, triggering a cascade of defense responses such as production of antimicrobial compounds.
“Even in the absence of pathogens, many of these responses show low but rhythmic changes that are influenced by the circadian clock,” Lu and Wiratan say. “When a real attack arrives, the plants’ daily rehearsal of their defense systems ensures a strong and concerted timely defense. Plants with misaligned clocks succumb to the attack.” Their clock allows the plants “to time their defense, which allows them to anticipate likely attacks before they occur and modulate defense responses to real attackers.”
For example, Arabidopsis (a small flowering plant ubiquitous in plant science laboratories) peaks its production of jasmonic acid, a hormone associated with the defense response, at noon, to prepare for the predictable onslaught of cabbage looper caterpillar feeding a few hours later. It’s unknown how in-sync pathogen and plant clocks are, but evidence suggests that some pathogens can manipulate the Arabidopsis clock.
These relationships aren’t limited to plants and their pests. “In humans and mice, some populations of gut microbiota oscillate daily, depending on the host circadian clock,” Lu and Wiratan write, adding that “interestingly, gut microbiota are capable of re-programming the host clock […] Research in this area represents a fascinating and unexplored level of host-invader dynamics.”
All of this research on plants has provided a great deal of knowledge about circadian rhythms “and their role in modulating everything from development to defense,” Lu and Wiratan write. “As researchers continue to untangle more about how these clocks work—including how they influence interactions between hosts and their invading pathogens and pests—new forms of specially-timed precision medicine could be on the horizon.”
The original article, Studying circadian rhythms in plants and their pathogens might lead to precision medicine for people, has been viewed more than 4,750 times and republished in outlets such as Smithsonian Magazine, World Economic Forum, and at Phys.org.
Image: Floral clock in Edinburgh, Ireland. Photo by Patrick Down, used under CC BY-NC 2.0.