Autoluminescence Imaging: a Non-Invasive Tool for Mapping Oxidative Stress
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Review TRENDS in Plant Science Vol.11 No.10 Autoluminescence imaging: a non-invasive tool for mapping oxidative stress Michel Havaux, Christian Triantaphylide` s and Bernard Genty CEA/Cadarache, DSV, DEVM, Laboratoire d’Ecophysiologie Mole´ culaire des Plantes, UMR 6191 CEA-CNRS-Aix Marseille University, F-13108 Saint-Paul-lez-Durance, France Living organisms spontaneously emit faint light, and includes carotenoids or terpenoids, which can produce this autoluminescence is stimulated in response to many endoperoxides upon oxidation [8,9]. Singlet oxygen is pro- stresses. This phenomenon is attributable to the duced in reactions between two lipid peroxy radicals, and endogenous production of excited states during the light emission peaks in the red (at 634 and 703 nm; oxidative reactions, particularly during peroxidation of Figure 1). By contrast, the bimolecular reaction of alkoxy lipids, which generates light-emitting molecules such as radicals and the cleavage of dioxetanes lead to the forma- triplet carbonyls and singlet oxygen. Using highly sen- tion of excited triplet carbonyls, which in turn decay and sitive cameras, it is now possible to remotely image yield light in the range 350 to 480 nm (blue). Consistently, spontaneous luminescence with a good spatial spectral analyses of the luminescence emission of oxidized resolution, providing a new non-invasive tool for lipids in vitro showed the expected peaks in the blue and mapping oxidative stress and lipid peroxidation in red-near infrared domains [10]. However, in plants, triplet plants. carbonyls can also transfer excitation energy to chlorophyll molecules [11] whose deactivation is associated with The types of luminescence of living organisms photon emission at around 730 nm (far-red light) [12,13]. Plants, animals and bacteria glow continuously. This spon- Formation of activated carbonyls from lipid hydroper- taneous photon emission (SPE) occurs in the ultraviolet oxide decomposition is stimulated by heat [14], and this and visible range of wavelengths with a low intensity of phenomenon is the basis of the luminescence burst À19 À16 À2 10 to 10 Wcm [1]. This is about three orders of observed at 135 8C in plant leaves (Figure 2a) [3,4,15]. magnitude less than the luminescence arising from enzy- The amplitude of the luminescence measured at this tem- matic reactions in fireflies, jellyfishes and some fishes in a perature is two orders of magnitude higher than the room- transitory phenomenon commonly called bioluminescence temperature autoluminescence amplitude. Numerous [2]. It is different from the luminescence of chlorophyll, thermoluminescence studies have validated the use of emitted by photosynthetic organisms following exposure to the luminescence measured at 135 8C as an index of lipid light, which results from recombination of positively peroxidation by correlating the signal amplitude with the charged carriers at the donor side of photosystem II and extent of lipid peroxidation, which can be measured using a electrons on the acceptor side [3,4]. The chlorophyll signal variety of biochemical methods [15–18]. The luminescence decays rapidly in the dark within seconds to minutes. Here, at 135 8C has been used in plants to quantify lipid we provide background information about SPE, with damage after oxidative stress [18–20] or to characterize emphasis on the origin of the signal, and discuss in more the stress resistance of mutants [16,17,21]. Importantly, detail imaging of SPE in plants in comparison with other when the luminescence signal emitted by Arabidopsis available techniques. leaves at 25 8C was plotted against the luminescence emission at 135 8C, a linear regression was obtained Spontaneous photon emission, a signature of biological (Figure 2b), confirming that both signals reflect oxidative oxidation status stress damage and supporting the hypothesis that they SPE, often called biophoton emission, is attributed to the could have a common origin in plants [15]. endogenous production of excited states during oxidative Light produced during lipid peroxidation therefore con- metabolic reactions [5,6] and it is therefore closely related stitutes an internal probe of oxidative stress that has many to the oxidation status of the organism. The major source of potential applications. Lipid peroxidation and oxidative SPE is the slow spontaneous decomposition of lipid per- stress have been measured by room-temperature SPE in oxides and endoperoxides, such as dioxetanes, which gen- a variety of samples, from whole organisms to isolated erates the light-emitting byproducts singlet oxygen and membranes or lipoproteins, and even in human breath excited carbonyl groups [7]. The term ‘lipids’ is used here in (reviewed in Refs [1,15]). In many studies, the intensity a broad sense and is not restricted to fatty acids but also of SPE correlated with other indexes of lipid peroxidative Corresponding author: Havaux, M. ([email protected]). degradation, such as the level of conjugated dienes, the Available online 7 September 2006. malondialdehyde content or the concentration of www.sciencedirect.com 1360-1385/$ – see front matter ß 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2006.08.001 Review TRENDS in Plant Science Vol.11 No.10 481 1 Figure 1. Chemiluminescence associated with lipid peroxidation. The emission of light is related to the production of excited carbonyl compounds and singlet oxygen ( O2) that occurs during lipid peroxide degradation. Self-reaction of lipid peroxy radicals (the Russel reaction) gives a tetroxide intermediate, which decomposes either into a triplet state carbonyl compound (reaction 1a; the triplet state is indicated by a star) or into singlet oxygen (reaction 1b). Self-reaction of alkoxy radicals produces excited carbonyl compounds in a redox reaction (reaction 2). The singlet oxygen that is produced during lipid peroxidation (such as reaction 1b) reacts with unsaturated compounds, leading to the production of dioxetanes, which yield triplet state carbonyl compounds upon decomposition (reaction 3). Upon relaxation, the excited carbonyl compounds emit light in the blue region (380–460 nm; reaction 4a) whereas bimolecular emission of singlet oxygen occurs in the red region (634 and 703 nm; reaction 4b). Abbreviation: hy, energy of a single photon. Adapted from Refs [6,7]. exogenously added lipid hydroperoxides [22–24]. This Imaging of spontaneous photon emission non-invasive technique is useful for real-time monitoring A currently emerging and promising application of SPE is of rapid perturbations of the cellular oxidation state and is the imaging of oxidative stress in intact tissues. Early performed without an exogenous probe. attempts at autoluminescence imaging in the 1990s used Figure 2. Luminescence of Arabidopsis leaves and its variation with temperature. (a) High-temperature luminescence emission from Arabidopsis leaves exposed to photo-oxidative stress. The curves are of wild type (Columbia; 1) and a vte1 npq1 double mutant deficient in the carotenoid zeaxanthin and in vitamin E (2) [17].Plantswere exposed for 3 days to high light stress (1200 mmol mÀ2 sÀ1, 8 h photoperiod) at low temperature (8 8C). The luminescence band peaking at around 135 8Cisattributedtolight- emitting molecules produced during lipid peroxidation [15]. (b) Correlation between the luminescence emission at 25–30 8C (commonly used for imaging) and the amplitude of the luminescence band at 135 8C. The results shown were obtained in a series of experiments on a range of Arabidopsis mutants exposed to various stress conditions. www.sciencedirect.com 482 Review TRENDS in Plant Science Vol.11 No.10 Figure 3. Imaging of the spontaneous luminescence emission from Arabidopsis plants at room temperature (1, double mutant vtc2 npq1 deficient in both ascorbate and zeaxanthin; 2, single mutant vtc2 deficient in ascorbate; 3, wild type) pre-exposed for 4 days to high light stress (4 8C, 1600 mmol mÀ2 sÀ1, 8 h/16 h day/night regime). These mutants are differently sensitive to high light stress [21]. (a) Luminescence, as imaged with a liquid nitrogen-cooled CCD camera (VersArray LN/CCD 1340–1300B, Roper Scientific (as in Ref. [32]), mounted with a back-illuminated CCD from E2V CCD36–40). The emitting sample was imaged on the sensor by a 50-mm focal distance lens with an f-number of 1.2 (F mount Nikkor 50-mm, f:1.2, Nikon) for maximizing light collection. Integration time was 60 min using the full resolution of the CCD (1300 Â 1340 pixels). Plants were adapted to darkness for 2 h before measurements. Randomly distributed white spots are the results of stray radiation in the range of gamma and cosmic rays. The color scale indicates signal intensity from 0 (blue) to 250 (white). (b) Photograph of the corresponding plants. Asterisks indicate the type of leaves used for the malondialdehyde (MDA) analysis in (c) (i.e. larger, mature leaves). (c) MDA level in leaves. MDA levels in mature leaves of the vtc2 npq1 double mutant (1), vtc2 mutant (2) and wild type (3) plants that had been exposed to high light stress for 4 days were determined by HPLC as described in Ref. [17]. Data are mean values of a minimum of three experiments + standard deviation. single photon counting imaging techniques that If leaf autoluminescence does reflect lipid oxidative demonstrated the feasibility of this approach. However, damage, mutants known to be sensitive to oxidative stress most of the images obtained with these systems had low should emit more photons than do wild types. This is spatial resolution. Nevertheless, SPE from the brain of a shown in Figure 3, in which SPE from whole Arabidopsis rat was visualized and the imaged signal correlated with plants was imaged using a liquid nitrogen-cooled CCD cerebral energy metabolism and oxidative stress [25]. SPE camera similar to that described in Ref. [32]. The plants in plant tissues infected with a pathogen, injured mechani- were pre-adapted to darkness for 2 h to allow photosynth- cally or exposed to drought stress were also visualized with esis-related chlorophyll luminescence to decay [13].