On Aug. 28, 2024, the University of Würzburg announced research using newly generated “optogenetic” tobacco plants to investigated how plants process external signals. Since plants can not relocate when attacked they have developed different strategies that are triggered by certain signals from the environment.

For their study, the research teams worked with tobacco plants that carry ion channels that can be specifically switched on with light. With the help of these light-sensitive proteins, which are obtained from algae and microorganisms, the researchers investigated whether the influx of calcium ions or anion efflux-mediated depolarization of the cell membrane is decisive for the plant’s reaction to a certain stress situation. The research findings were published in the journal Nature.

For their study, the research teams worked with tobacco plants that carry ion channels that can be specifically switched on with light. More than 20 years ago, Peter Hegemann, Georg Nagel and Ernst Bamberg initiated the success of optogenetics, with their discovery and characterization of light-activated ion channels, so-called channelrhodopsins. With the help of these light-sensitive proteins, which are obtained from algae and microorganisms, the JMU researchers were able to experimentally investigate whether the influx of calcium ions or anion efflux-mediated depolarization of the cell membrane is decisive for the plant’s reaction to a certain stress situation. However, the scientists had to do a great deal of preparatory work before they were able to do this.

Point 1: “Like all rhodopsins, including those in our eyes, channelrhodopsins require the small molecule retinal, also known as vitamin A, to absorb light. We humans get retinal mainly from beta-carotene, the provitamin A. However, land plants do not contain retinal, but a lot of beta-carotene,” explains Dr. Shiqiang Gao, co-author of the Nature publication and ‘rhodopsin engineer’ from the Optogenetics lab of the Department of Neurophysiology at JMU.

Point 2: “Most rhodopsins are activated by blue or green light. However, this is always a component of white light,” explains Georg Nagel. As a result, the tobacco plants could not be grown in a greenhouse or under artificial white light, as is usually the case. Only in special growth chambers with red LED light, which can be used photosynthetically, it was possible to avoid unwanted rhodopsin activation. Tests under different growth conditions showed: “Tobacco develops healthily and unchanged under red light compared to greenhouse conditions,” says Dr. Kai Konrad.

Point 3: The expression of chanelrhodopsin in tobacco cells often causes difficulties. In 2021, the Würzburg team of scientists succeeded in expressing the light-activated anion channel GtACR1 in tobacco plant cells. As a result, Georg Nagel’s team was able to develop various channelrhodopsins that were optimized for the permeability of calcium ions. Finally, Dr. Shiqiang Gao and Dr. Shang Yang, both members of Nagel’s group, succeeded in developing a very good calcium-conducting channelrhodopsin XXM 2.0 for targeted expression in tobacco plants.

These newly generated “optogenetic” tobacco plants made it possible to clarify the question of whether calcium influx or membrane depolarization is decisive for the plant’s response to a specific stress situation. “The answer was clear,” says Dr. Kai Konrad, corresponding author. First author Dr. Meiqi Ding from Dr. Konrad’s group explains, “After activation of the anion channel, the leaves wilted and responded with the typical plant response to drought; the plant hormone abscisic acid (ABA) was produced and gene expression was ramped up to protect against desiccation.”