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  • Writer's pictureHirokazu Kobayashi

Plants can distinguish the sun's changing colors!*

Updated: 1 day ago

Hirokazu Kobayashi

CEO, Green Insight Japan, Inc.

Professor Emeritus and Visiting Professor, University of Shizuoka

 

When children draw pictures of the sun, children in Japan and the West often use yellow. This is influenced by the shared and translated picture books in these countries. However, in the West, the sun often has eyes, a nose, and a mouth. In contrast, children in India, the Middle East, and Africa tend to draw the sun in red or orange, likely due to the intense heat they experience from the sun. Sunlight can be decomposed into a light spectrum from blue to red, observed in natural phenomena as a "rainbow." Among these lights, plants do not use green light but rather blue and red light. In the red spectrum, plants can distinguish between light with wavelengths of 680 nm and 700 nm. Blue light is high in energy, whereas red light penetrates deeper into the water. The mechanism by which living organisms convert sunlight into usable energy is called "photosynthesis." Early photosynthetic organisms, existing as bacteria in seawater, likely evolved to utilize red light. On the other hand, the Earth rotates, and during morning and evening, sunlight passes obliquely through the Earth's atmosphere, increasing the distance it travels before reaching the surface. Red light transmits better in the atmosphere; hence, the human eye perceives sunrise and sunset.

 

The history of the discovery of photosynthesis is fascinating. In 1771, the British chemist (clergyman) Joseph Priestley placed a sprig of mint with leaves in water and left it in a sealed glass container exposed to sunlight for several months. Subsequently, candles burned well in that container. After the candle burned out, he placed the mint again in the container and exposed it to sunlight for ten days, and again, the candle burned well. He repeated this experiment ten times, obtaining the same result each time. He divided the air in which the candle had burned into two parts, placing mint in one glass container and leaving the other without mint under the same conditions. The candle burned in the former but not in the latter. Additionally, a mouse remained comfortably in the sealed container, including mint, and was treated with sunlight. This is said to be the discovery of oxygen production through photosynthesis. In the 1950s, American plant physiologist Robert Emerson used diffraction gratings, which had been developed then, to irradiate green algae with various wavelengths of light. When he irradiated with 650 nm and 700 nm light simultaneously as a single light energy source, the efficiency of photosynthesis significantly improved compared to irradiating each wavelength separately. This phenomenon is called the "Emerson Effect," and it led to the discovery of photosystem II (PS II) and photosystem I (PS I), which have absorption characteristics at 680 nm and 700 nm, respectively. Such effects are not observed in primitive photosynthetic bacteria like photosynthetic sulfur bacteria, possessing only PS I. Having two photosystems allows for transporting many protons (H+), producing a large amount of ATP, an energy substance. Additionally, due to PS II, oxygen is made from water, enabling oxygen-dependent organisms like humans to live on Earth.

 

From the above discussion, a potential issue becomes apparent. During sunrise and sunset, the light at 700 nm becomes stronger compared to 680 nm, causing an imbalance in the driving force of PS II and PS I, producing toxic singlet oxygen. To prevent this, plants have acquired a control system that ensures the driving forces of PS II and PS I operate in a one-to-one ratio. This control mechanism, a marvel of nature, was discovered by us using Arabidopsis thaliana, a model plant extensively studied worldwide. Arabidopsis, a member of the mustard family, originates from the Mediterranean. The mechanism involves a molecule called plastoquinone, which senses both driving forces between PS II and PS I, depending on whether it is oxidized or reduced, and regulates the production of protein components of PS I at the genetic expression level. This is not just a fascinating scientific discovery but also a potential solution to the issue of toxic singlet oxygen production. As this is a gene expression control system, it can be used to control the initiation of plant-based biopharmaceutical production. This has been named the "light switch" and is currently utilized in business applications at our company, Green Insight Japan, Inc.




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