UV Signals and Stomata in Plant Photosynthesis
Do you remember Matt Damon planting potatoes in the movie The Martian? Potato seedlings grew under sunlight, which provided food for the Martian and maintained oxygen level in the space module. It is common knowledge that plants turn CO2 into oxygen through photosynthesis, but have you ever considered how does the sunlight make plants to take in CO2 and release oxygen? The answer given by Prof. He Junmin in School of Life Sciences, Shaanxi Normal University is that UV-B radiation and other visible light antagonistically regulate stomatal apertures on plant epidermis to suitable opening levels.
The words ultraviolet light may remind people of ozone hole, and the sunburns or burning sensation on the skin after exposure to direct sunlight. As early as 1980, United Nations Environment Programme predicted that before 2010 animals and plants would be severely affected by increased UV radiation. "But several years have passed and the plants are growing well." He Junmin thought, "Maybe to some extent, the UV light in sunlight is beneficial to plant growth."
But of course this does not mean we can ignore the increasing UV radiation. Scientists have been doing their works on this. As a biologist, He Junmin also hoped to provide some precautions with his plant studies. He wondered if UV radiation had some regulating effects on plants? So since 2000, he has been working on the influence of UV signal on the opening and closure of plant stomata. His research team studied broad beans and arabidopsis and discovered that red and blue lights were important factors in inducing the opening of plant stomata, but too much opening would result in over evaporation, which is harmful to plant growth. Therefore, He Junmin thought there would be light in a certain waveband could make plants release some hormone or signal, which would control stomata opening and affect plant growth.
In the view of a biologist, stoma is an ideal system for a single plant cell to respond to various external stimuli, thus studying the mechanisms of stomatal movement would tell the transmission of signals within plants. Therefore, different plant signal experiments use stomata as a window to observe the internal signaling mechanism inside plants. Although stomata have always been observed and studied, little attention has been paid to the effect and its signal transduction mechanisms of UV-B radiation in sunlight used by photosynthesis on the stomatal movement. He's team studied the mechanism of UV-B radiation regulation of stomatal movement not only to discover the signal transduction pathways of plant cells in response to UV-B radiation, also to understand the plants' self-regulating ability under unfavorable environment such as increased UV-B radiation, which makes it possible for artificially regulating UV-B light to increase plant yield.
Previous studies by He’s team indicate that signaling molecules hydrogen peroxide and nitric oxide have important roles in UV-B-induced plant stomatal closure, but He Junmin wasn't satisfied with this simple explanation. After nearly ten years experiments, they discovered that UV-B receptor UVR8, plant hormone ethylene and heterotrimeric G proteins all have important signaling roles in the UV-B-induced stomatal closure and function upstream of signaling molecules hydrogen peroxide and nitric oxide, which is the first time to place the signaling molecules such as G proteins and ethylene into the UV-B signal transduction pathways in plants.
The discovery of the relationship between UV-B signal transduction and plant hormone as well as G proteins greatly broaden the understanding of plant UV-B signal transduction mechanisms. Currently, He Junmin's team is extending their research to the roles of signaling molecules including ethylene and G proteins in the UV-B radiation-regulated other physiological processes such as gene expression and pollen tube growth, and explores whether these signaling molecules have universal roles in the plant UV-B signal transduction. "If this hypothesis is proved to be true, it will provide new ideas for enhancing plant ability to resist UV-B radiation via transgenic technology, but we need further experimental data to back it up." said He Junmin.