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 to induce the production of only one ROS, singlet oxygen, within plastids in a non-invasive, controlled manner (Fig. 1). 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. 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 and 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 to the dark as another extreme (Fig. 2). 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. Our work is aimed at dissecting the complexity of singlet oxygen signalling and understanding and eventually also modifying the genetic constraints that determine the adaptability of plants to environmental changes.
Figure 1 FLU-dependent control of light-dependent Chl biosynthesis. Inactivation of FLU impedes negative feedback control (A) and leads to the overaccumulation of excess Pchlide in etiolated flu seedlings that upon excitation with blue light emits a strong red fluorescence (B, DD). Upon light exposure Pchlide acts as a photosensitizer and triggers the release of 1O2 that results in the rapid bleaching of seedlings (B, DL). Under continuous light Pchlide is immediately reduced via POR to Chlide and does not reach critical levels that may evoke the production of 1O2 (B, LL).
Figure 2 Schematic diagram of 1O2-dependent signal transduction from the plastid to the nucleus that triggers stress responses ranging from acclimation to cell death. The quality of the response can be modulated by shifting flu plants for various lengths of time to the dark, by activating modulators (e.g. PRL1) or by inactivating EX1 and/or EX2.
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Tanz SK, Kilian J, Johnsson C, Apel K, Small I, Harter K, Wanke D, Pogson B, Albrecht V. 2012. The SCO2 protein disulphide isomerase is required for thylakoid biogenesis and interacts with LHCB1 chlorophyll a/b binding proteins which affects chlorophyll biosynthesis in Arabidopsis seedlings. Plant J 69: 743-754
Simkova, K., Kim, C., Gacek, K., Baruah, A., Laloi, C. and Apel, K. 2012. The chloroplast division mutant caa33 of Arabidopsis thaliana reveals the crucial impact of chloroplast homeostasis on stress acclimation and retrograde plastid-to-nucleus signaling. Plant J 69: 701-712
Paddock, T., Lima, D., Mason, M.E., Apel, K. and Armstrong, G.A. 2012. Arabidopsis light-dependent protochlorophyllide oxidoreductase A (PORA) is essential for normal plant growth and development. Plant Mol Biol 78: 447-460
Kim, C., Meskauskiene, R., Zhang, S., Lee, K.P., Lakshmanan Ashok, M., Blajecka, K., Herrfurth, C., Feussner, I. and Apel, K. 2012. Chloroplasts of Arabidopsis are the source and a primary target of a plant-specific programmed cell death signaling pathway. Plant Cell 24: 3026-3039
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Kauss, D., Bischof, S., Steiner, S., Apel, K. and Meskauskiene, R. 2012. FLU, a negative feedback regulator of tetrapyrrole biosynthesis, is physically linked to the final steps of the Mg++-branch of this pathway. FEBS Lett 586: 211-216
Saini, G., Meskauskiene, R., Pijacka, W., Roszak, P., Sjogren, L.L.E., Clarke, A.K., Straus , M. and Apel, K. 2011. 'happy on norflurazon' (hon) mutations implicate perturbance of plastid homeostasis with activating stress acclimatization and changing nuclear gene expression in norflurazon-treated seedlings. Plant J 65: 690-702
Baruah, A., Simkova, K., Apel, K. and Laloi, C. 2009. Arabidopsis mutants reveal multiple singlet oxygen signaling pathways involved in stress response and development. Plant Mol Biol 70: 547-563
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Coll, N.S., Danon, A., Meurer, J., Cho, W.K. and Apel, K. 2009. Characterization of soldat8, a Suppressor of Singlet Oxygen-Induced Cell Death in Arabidopsis Seedlings. Plant Cell Physiol 50: 707-718
Kim, C., Lee, K.P., Baruah, A., Nater, M., Gobel, C., Feussner, I. and Apel, K. 2009. O-1(2)-mediated retrograde signaling during late embryogenesis predetermines plastid differentiation in seedlings by recruiting abscisic acid. Proc Natl Acad Sci U S A 106: 9920-9924
Meskauskiene, R., Wursch, M., Laloi, C., Vidi, P.A., Coll, N.S., Kessler, F., Baruah, A., Kim, C. and Apel, K. 2009. A mutation in the Arabidopsis mTERF-related plastid protein SOLDAT10 activates retrograde signaling and suppresses 1O(2)-induced cell death. Plant J 60: 399-410
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Lee, K.P., Kim, C., Landgraf, F., Apel, K. 2007. EXECUTER1- and EXECUTER2-dependent transfer of stress-related signals from the plastid to the nucleus of Arabidopsis thaliana. Proc Natl Acad Sci U S A 104: 10270-5
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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 mutant by Apel and his research group indicates that the release of one 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 enviromental stress of global warming. Ultimately, such a discovery could help increase food supplies or predict a plant’s susceptibility to environmental changes.