How is the sleep-wake cycle maintained in plants? New research about plant proteins may help us to better understand the human circadian rhythm and human disease.
All life forms have biological rhythms. This complex set of internal clocks is regulated mainly by light, or the lack thereof. Human bodies and other diurnal living organisms innately know that light means it’s time for wakefulness, while a lack of light means it is time to sleep. Scientists have studied exactly how light can cause such an impact for centuries, with new discoveries coming out constantly. Surprisingly, it appears that a set of plant proteins called cryptochromes may be responsible for much of the human circadian rhythm.
How is Your Sleep-Wake Cycle Maintained?
In humans, light is perceived by the retinas of our eyes, which converts the light energy to electrical waves passed down nerve fibers to the suprachiasmatic nucleus of the hypothalamus. In turn, this part of the hypothalamus induces other parts of the brain to communicate with the rest of the body about the time of day via hormonal messengers such as melatonin and cortisol. However, plants and bacteria have no brain, yet also respond to light.
How exactly does this occur? An ancient group of proteins known as cryptochromes help simple as well as complex organisms to keep their internal clocks running on time. More interestingly, humans and other animals also have cryptochromes and appear to be affected by them in a variety of ways.
The Role of Cryptochromes
Our circadian rhythms govern not just sleep, but growth, metabolism and many other important tasks. A protein discovered in plants decades ago and found to be present in humans and other animals appears to play an important role. Cryptochromes are a set of proteins present in even the earliest plants that become active in darkness. These proteins are involved in telling animals when to migrate, helping plants know when to grow and even in the growth of human cancer cells. New research has found that they are inhibited by closely associated proteins that sense blue light.
According to the latest studies, plants grow mostly at night because of cues from cryptochromes. When the proteins that inhibit cryptochromes are dysfunctional, the plants grow twice as quickly. This knowledge may be important in the future treatment of cancer, as cryptochromes appear to encourage the growth of cancer cells. It also can be used in agriculture to encourage plants to grow more quickly and to grow to a larger size with less time and fewer resources.
Comparing Apples and Humans?
Many may question how these discoveries about plants can affect human health. However, it is important to remember that everything we know now about chronobiology, or the study of internal clocks, began with studies performed on plants. In the early 18th century scientist Jacques d’Ortous de Mairan questioned whether the opening and closing of mimosa leaves was caused by sunlight. After putting plants indoors with no exposure to light, he noted that they continued to open leaves during the day and close them at night. These plants’ sleep-wake cycle maintained itself without cues such as light and temperature, which led other researchers to look into the matter. This was the first time that scientists realized that, while light plays an important part in plant and animal behavior, we also have internal clocks controlling our activities.
While we understand a great deal about the circadian rhythms of both plants and animals, there is still much to learn. Asking “How is the sleep-wake cycle maintained?” is the first step to solving many puzzles about our health and behavior.
