Research Overview: Professor Klaus Apel was a member of the faculty at BTI from 2008 – 2016. He has returned to the Swiss Federal Institute of Technology in Zurich, where he is a professor emeritus. Learn more about his life and work in this profile of Klaus Apel.

How do plants respond to environmental stress?

Plants can endure extreme environmental stress (heat, drought, cold or intense light) through genetically controlled defenses, such as wilting, loss of leaves or stunted growth, but these very defenses can also reduce yields, among other effects. As a result, one effect of global warming could be reduced food production just when the world’s population is burgeoning. Understanding how plants sense and respond to stress at the genetic level is the ultimate objective of Klaus Apel’s laboratory at BTI. His findings could enable scientists to mitigate the negative results of stress, such as yield loss, or fine tune a plant’s ability to survive climate change. It turns out that chloroplasts — the tiny organs that contain chlorophyll and carry out photosynthesis — play an important role in a plant’s ability to sense environmental stress. Conditions such as drought, heat, cold and intense light interfere with the normal photosynthetic process in the chloroplasts, which leads to over-production of sometimes toxic forms of oxygen, called reactive oxygen species (ROS). High levels of ROS were previously considered detrimental to the cell. However, recent work with an Arabidopsis thaliana mutant by Apel and his research group indicates that the release of one type of ROS, called singlet oxygen, in the chloroplast actually triggers a variety of positive stress adaptation responses in the plant. These responses include slowed plant growth, cell death, and the activation of a broad range of defense genes, which normally are turned on only in the presence of pathogens. In further work, Apel’s group proved that certain genetic mutations in Arabidopsis eliminate the plant’s stress responses without interfering with the release of singlet oxygen. It appears these mutations prevent the plant from sensing the presence of singlet oxygen, which in turn prevents symptoms of stress. Apel’s group has identified these mutated genes, which is a first and crucial step toward understanding the genetic basis of the stress response in plants. The results of Apel’s work could lead to plants that cope better with the environmental stress of global warming. Ultimately, such a discovery could help increase food supplies or predict a plant’s susceptibility to environmental changes.

FLU: A negative regulator of chlorophyll biosynthesis

One of the first reactions of plants under stress is the enhanced production of chemically distinct reactive oxygen species (ROS). A major difficulty in elucidating the biological activity of ROS during stress stems from the fact that not only one but several chemically distinct ROS are generated simultaneously, thus making it very difficult to link a particular stress response to a specific ROS. This problem has been alleviated by using the conditional flu mutant of Arabidopsis that allows the production of only singlet oxygen within plastids in a non-invasive, controlled manner. In the dark the flu mutant accumulates protochlorophyllide (Pchlide), a potent photosensitizer that upon illumination generates singlet oxygen. Several singlet oxygen-mediated stress responses have been distinguished during re-illumination of the flu mutant. Furthermore, inactivation of nuclear genes encoding the two closely related plastid proteins Executer1 and Executer2 has been shown to be sufficient to abrogate these stress responses despite the ongoing release of singlet oxygen. By varying the length of the dark period, one can adjust the level of the photosensitizer Pchlide and define conditions that minimize the cytotoxicity of singlet oxygen: either endorse acclimation in flu plants exposed to a very short dark period as one extreme, or promote a genetically controlled cell death response in plants shifted for a longer period in the dark as another extreme. This activity of singlet oxygen assigns a new function to the chloroplast, namely that of a sensor of environmental changes that activates a broad range of stress responses known to be activated also by abiotic and biotic stressors. This work is aimed at dissecting the complexity of singlet oxygen signaling and understanding and eventually also modifying the genetic constraints that determine the adaptability of plants to environmental changes.