How Plants Use Photoreceptors to Sense Light

More Than Meets the Eye: Plants' Hidden Ability to Detect Light

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Published in 10-22-2024

By Israel Benítez García . Professor of the Biotechnology Engineering academic program and the Master of Applied Sciences, Polytechnic University of Sinaloa . Mazatlan, Sinaloa, Mexico.

Have you ever wondered how plants, which are motionless and have no eyes, know which way the sun shines? The answer lies in their amazing ability to perceive light. Like us, plants have specialized mechanisms that allow them to detect and respond to different types of electromagnetic radiation. In this article, we will explore how plants "see" the world and how this ability influences their growth and development.

EYE ANTENNAS FOR PERCEIVING LIGHT

"Plants have developed a wide variety of adaptations to optimize light capture. For example, shade plants have larger, thinner leaves to capture the little light available, while sun plants have smaller, thicker leaves to avoid excess radiation. In addition, many plants have developed specialized movements, such as heliotropism, which allows them to orient their leaves toward the most intense light source."

The world is full of signals and stimuli; but each signal is perceived by each animal thanks to its cells having specific receptor antennas, for example, in the human eye, which is the organ that allows us to perceive light, it has cells that contain "photoreceptors", which act as antennas on the surface of the retina. Photoreceptors are specialized rod- and cone-shaped neurons that respond to different wavelengths within the visible light spectrum, converting it into nerve impulses that are directed to the brain for transformation into images. This mechanism is known as phototransduction. The signal emitted by the photoreceptors is carried out by messengers (proteins and ions) within the cell. These participate in biochemical reactions that control the entry and exit of ions such as sodium (Na+) and calcium (Ca+), whose function is to control phototransduction and allow our brain to distinguish colors and shapes. Rod-shaped photoreceptors contain "rhodopsin", a photosensitive protein that allows us to see in low-light conditions and is capable of perceiving blue-green light detected at a wavelength of 500 nanometers (nm), while cone photoreceptors have three proteins called erythropsin perceives red light at 700 nm, "chloropsin" perceives green light at 530 nm and cyanopsin perceives blue light at 430 nm. Thanks to these proteins, the brain's photoreceptors can interpret colors.

VISIBLE LIGHT SPECTRUM

Visible light is the spectrum of electromagnetic radiation that the eyes, through the photoreceptors, perceive within a wavelength of 400 nm to 700 nm; In this electromagnetic range we find blue, green, yellow and red light, that is, at a wavelength of 400 nm, our eyes perceive blue light, at an average wavelength of 580 nm the color yellow and the longer - for example, 700 nm - the red light. This means that the different colors we see at dusk are due to the different wavelengths within the visible light spectrum emitted by the Sun's rays. However, the human eye cannot perceive wavelengths where gamma rays, X-rays and infrared light are found because we do not have photosensitive proteins for these wavelengths. In this sense, could it be that plants can see the light that our eyes do not perceive?

The Plant's Secret Sense: Detecting Sunlight

Photo by Olena Bohovyk

TURNING TOWARDS THE LIGHT

If you look around you, you can see objects with different shapes, colors and textures, or identify the difference between a vase and a television. Plants cannot do this. On the contrary, they are able to see the light of the Sun coming from the east and the light of the Sun setting in the west, as well as see if a day is cloudy or even if you are blocking the light. How do plants know this? Darwin, yes! Charles Darwin, the same one who proposed the theory of evolution, asked himself the same question and together with his son Francis Darwin carried out an experiment in which they showed the effect of light on plants. With their discoveries, the Darwins demonstrated that plants can turn towards the light, this phenomenon is known as phototropism - movement towards the light.

Plants bend towards the light because they perceive the blue light emitted by the sun's rays. Darwin proved this by covering the tip of a plant, thus preventing it from moving towards the light; however, by covering it with a transparent mica, it bent towards the light again. On the other hand, by cutting off the top of the plant, it was unable to bend towards the light. With this simple experiment, Darwin concluded that some factor was transmitted from the tip of the plant to the rest of the stem causing its curvature. This finding does not answer how plants know the direction from which the light comes, but it did show that the structure that perceives light is located at the top of the plant.

How do plants look at the world?

According to Darwin, plants are sensitive to light. How do they manage to perceive light? Is this the enigma you are asking yourself now? The answer lies at the apex of plants - the tips of plants - where proteins similar to those in our eyes are found, and they act in the same way as they perceive light, but the proteins (phytochromes, cryptochromes and phototropins) of their photoreceptors are different.

Photo by Dawid Zawila

Unlike the human eye, which has four photoreceptors, plants have 13 photoreceptors called phytochromes that allow plants to perceive light and differentiate between the colors of the light spectrum. For example, if we put a plant next to a blue light, it will lean towards the beam of light, but if we put it next to a red light, the plant will not lean. This happens because of the type of phytochrome and its chromophore - a protein sensitive to different types of light - responsible for distinguishing between blue light and red light. Red phytochromesI are important for plants because they help them identify when it is daytime - by detecting red light - and nighttime - by perceiving far-off red light.

Thanks to this wonderful interpretation of light through phytochromes, plants can know if you are standing next to them, because when you block the passage of light to the plant, it will detect only the far red light, and if you let the light pass freely, the plants absorb the red light again and they will perceive that there is something next to them. In short, phytochromes act as a sensor, with the red light it turns on and recognizes that it is daytime and with the far red light it turns off and knows that it is nighttime. Plants respond to this situation in a way that you cannot imagine; for example, if you put a large plant next to a small one, the latter will only absorb the far light, therefore, it will not know if it is already dawn, consequently, the plant tends to elongate its stem until it reaches the red light and thus knows when it is day or night. In addition to red phytochromes, plants can see UV light and blue light through four photoreceptors, two called phototropins (important for flowering) and two cryptochromes (important for floral development) as well as photoreceptors with chlorophyll that perceive green light, which is important for carrying out photosynthesis. Therefore, plants see the world depending on its color: blue to grow and develop their flower or fruit, red to distinguish between day and night, green to feed and UV to defend themselves.

FOR REFLECTION

Plants, like us, can perceive colors through specialized structures called phytochromes and, even though plants do not have a brain, they are able to perceive figures and multiple colors, thanks to the fact that their cells can transform the light received into chemical signals to generate a physiological response - germinate, grow, develop flowers and fruits - as well as distinguish between day and night. The next time you're watching the sunset, remember that the plants around you will be enjoying it too.

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López, H. (2015). Electromagnetic radiation waves extraordinary invisible force- Science Unemi, 1(2) 26-29

Darwin, C. (1897). The power of movement in plants. Appleton.