Photosynthesis Research (v.114, #2)

Experimental in vivo measurements of light emission in plants: a perspective dedicated to David Walker by Hazem M. Kalaji; Vasilij Goltsev; Karolina Bosa; Suleyman I. Allakhverdiev; Reto J. Strasser; Govindjee (69-96).
This review is dedicated to David Walker (1928–2012), a pioneer in the field of photosynthesis and chlorophyll fluorescence. We begin this review by presenting the history of light emission studies, from the ancient times. Light emission from plants is of several kinds: prompt fluorescence (PF), delayed fluorescence (DF), thermoluminescence, and phosphorescence. In this article, we focus on PF and DF. Chlorophyll a fluorescence measurements have been used for more than 80 years to study photosynthesis, particularly photosystem II (PSII) since 1961. This technique has become a regular trusted probe in agricultural and biological research. Many measured and calculated parameters are good biomarkers or indicators of plant tolerance to different abiotic and biotic stressors. This would never have been possible without the rapid development of new fluorometers. To date, most of these instruments are based mainly on two different operational principles for measuring variable chlorophyll a fluorescence: (1) a PF signal produced following a pulse-amplitude-modulated excitation and (2) a PF signal emitted during a strong continuous actinic excitation. In addition to fluorometers, other instruments have been developed to measure additional signals, such as DF, originating from PSII, and light-induced absorbance changes due to the photooxidation of P700, from PSI, measured as the absorption decrease (photobleaching) at about 705 nm, or increase at 820 nm. In this review, the technical and theoretical basis of newly developed instruments, allowing for simultaneous measurement of the PF and the DF as well as other parameters is discussed. Special emphasis has been given to a description of comparative measurements on PF and DF. However, DF has been discussed in greater details, since it is much less used and less known than PF, but has a great potential to provide useful qualitative new information on the back reactions of PSII electron transfer. A review concerning the history of fluorometers is also presented.
Keywords: Delayed fluorescence; Fluorometers; Photosystem II; Prompt fluorescence

While light is the essential driving force for photosynthetic carbon fixation, high light intensities are toxic to photosynthetic organisms. Prolonged exposure to high light results in damage to the photosynthetic membrane proteins and suboptimal activity, a phenomenon called photoinhibition. The primary target for inactivation is the photosystem II (PSII) reaction center. PSII catalyzes the light-induced oxidation of water at the oxygen-evolving complex. Reactive oxygen species (ROS) are generated under photoinhibitory conditions and induce oxidative post translational modifications of amino acid side chains. Specific modification of tryptophan residues to N-formylkynurenine (NFK) occurs in the CP43 and D1 core polypeptides of PSII. The NFK modification has also been detected in other proteins, such as mitochondrial respiratory enzymes, and is formed by a non-random, ROS-targeted mechanism. NFK has been shown to accumulate in PSII during conditions of high light stress in vitro. This review provides a summary of what is known about the generation and function of NFK in PSII and other proteins. Currently, the role of ROS in photoinhibition is under debate. Furthermore, the triggers for the degradation and accelerated turnover of PSII subunits, which occur under high light, are not yet identified. Owing to its unique optical and Raman signal, NFK provides a new marker to use in the identification of ROS generation sites in PSII and other proteins. Also, the speculative hypothesis that NFK, and other oxidative modifications of tryptophan, play a role in the PSII damage and repair cycle is discussed. NFK may have a similar function during oxidative stress in other biologic systems.
Keywords: Tryptophan; N-formylkynurenine; Photoinhibition; Post translational modification; Proteolysis; Synechocystis PCC. 6803; Reactive oxygen species; High light stress; Photosystem II; Photosynthetic oxygen evolution

The phototransformation pathways of protochlorophyllide forms were studied in 8–14-day-old leaves of dark-germinated wheat (Triticum aestivum L.) using white, 632.8 nm He–Ne laser and 654 nm laser diode light. The photon flux density (PFD) values (0.75–360 μmol photons m−2 s−1), the illumination periods (20 ms–10 s) and the temperature of the leaves (between −60 °C and room temperature) were varied. The 77 K fluorescence spectra of partially phototransformed leaves showed gradual accumulation or even the dominance of the 675 nm emitting chlorophyllide or chlorophyll form at room temperature with 632.8 nm of PFD less than 200 μmol photons m−2 s−1 or with 654 nm of low PFD (7.5 μmol photons m−2 s−1) up to 1 s. Longer wavelength (685 or 690 nm) emitting chlorophyllide forms appeared at illuminations under −25 °C with both laser lights or at room temperature when the PFD values were higher or the illumination period was longer than above. We concluded that the formation of the 675 nm emitting chlorophyllide form does not indicate the direct photoactivity of the 633 nm emitting protochlorophyllide form; it can derive from 644 and 657 nm forms via instantaneous disaggregation of the newly-produced chlorophyllide complexes. The disaggregation is strongly influenced by the molecular environment and the localization of the complex.
Keywords: Chlorophyllide form; Etiolated wheat; Laser illumination; Protochlorophyllide phototransformation; 77 K fluorescence emission

The UV-B photoreceptor UVR8 promotes photosynthetic efficiency in Arabidopsis thaliana exposed to elevated levels of UV-B by Matthew P. Davey; Novita I. Susanti; Jason J. Wargent; Jane E. Findlay; W. Paul Quick; Nigel D. Paul; Gareth I. Jenkins (121-131).
The UV-B photoreceptor UVR8 regulates expression of genes in response to UV-B, some encoding chloroplast proteins, but the importance of UVR8 in maintaining photosynthetic competence is unknown. The maximum quantum yield of PSII (F v/F m) and the operating efficiency of PSII (Φ PSII) were measured in wild-type and uvr8 mutant Arabidopsis thaliana. The importance of specific UVR8-regulated genes in maintaining photosynthetic competence was examined using mutants. Both F v/F m and Φ PSII decreased when plants were exposed to elevated UV-B, in general more so in uvr8 mutant plants than wild-type. UV-B increased the level of psbD-BLRP (blue light responsive promoter) transcripts, encoding the PSII D2 protein. This increase was mediated by the UVR8-regulated chloroplast RNA polymerase sigma factor SIG5, but SIG5 was not required to maintain photosynthetic efficiency at elevated UV-B. Levels of the D1 protein of PSII decreased markedly when plants were exposed to elevated UV-B, but there was no significant difference between wild-type and uvr8 under conditions where the mutant showed increased photoinhibition. The results show that UVR8 promotes photosynthetic efficiency at elevated levels of UV-B. Loss of the DI polypeptide is probably important in causing photoinhibition, but does not entirely explain the reduced photosynthetic efficiency of the uvr8 mutant compared to wild-type.
Keywords: Arabidopsis; ELIP; PSII; SIG5; UV-B; UVR8

All cyanobacteria sequenced to date have multiple psbA genes, encoding the D1 protein. Some of these psbA genes have a series of mutations that would seem to render D1 incapable of binding the Mn4CaO5 metallocluster (Murray, Photosynth Res 110(3):177–184, 2012). Nonetheless, these genes are expressed under specific environmental conditions, such as during N2 fixation in unicellular diazotrophs of the genes Cyanothece. These genes emphasize the clever way that cyanobacteria have learned to deal with a constantly changing environment.
Keywords: Cyanobacteria; Nitrogen fixation; Photosystem II; psbA genes; Gene transcription

We provide a News Report on the 2012 Gordon Research Conference on Photosynthesis held at Davidson College, North Carolina during July 8–13 that focuses on four young investigators who were presented awards during the conference.
Keywords: Brandon Greene; Katrin Linke; Elisabet Romero; Anna Schneider