Prafullachandra Vishnu Sane

A transient exchange of the photosystem II reaction center protein D1:1 with D1:2 during low temperature stress of Synechococcus sp. PCC 7942 in the light lowers the redox potential of QB.

Upon exposure to low temperature under constant light conditions, the cyanobacterium Synechococcus sp. PCC 7942 exchanges the photosystem II reaction center D1 protein form 1 (D1:1) with D1 protein form 2 (D1:2). This exchange is only transient, and after acclimation to low temperature the cells revert back to D1:1, which is the preferred form in acclimated cells (Campbell, D., Zhou, G., Gustafsson, P., Öquist, G., and Clarke, A. K. (1995) EMBO J. 14, 5457–5466). In the present work we use thermoluminescence to study charge recombination events between the acceptor and donor sides of photosystem II in relation to D1 replacement. The data indicate that in cold-stressed cells exhibiting D1:2, the redox potential of QB becomes lower approaching that of QA. This was confirmed by examining theSynechococcus sp. PCC 7942 inactivation mutants R2S2C3 and R2K1, which possess only D1:1 or D1:2, respectively. In contrast, the recombination of Q A − with the S2 and S3 states did not show any change in their redox characteristics upon the shift from D1:1 to D1:2. We suggest that the change in redox properties of QB results in altered charge equilibrium in favor of QA. This would significantly increase the probability of Q A − and P680+ recombination. The resulting non-radiative energy dissipation within the reaction center of PSII may serve as a highly effective protective mechanism against photodamage upon excessive excitation. The proposed reaction center quenching is an important protective mechanism because antenna and zeaxanthin cycle-dependent quenching are not present in cyanobacteria. We suggest that lowering the redox potential of QB by exchanging D1:1 for D1:2 imparts the increased resistance to high excitation pressure induced by exposure to either low temperature or high light.

Changes in the redox potential of primary and secondary electron-accepting quinones in photosystem II confer increased resistance to photoinhibition in low-temperature-acclimated Arabidopsis.

Exposure of control (non-hardened) Arabidopsis leaves for 2 h at high irradiance at 5°C resulted in a 55% decrease in photosystem II (PSII) photochemical efficiency as indicated by Fv/Fm. In contrast, cold-acclimated leaves exposed to the same conditions showed only a 22% decrease in Fv/Fm. Thermoluminescence was used to assess the possible role(s) of PSII recombination events in this differential resistance to photoinhibition. Thermoluminescence measurements of PSII revealed that S2QA- recombination was shifted to higher temperatures, whereas the characteristic temperature of the S2QB- recombination was shifted to lower temperatures in cold-acclimated plants. These shifts in recombination temperatures indicate higher activation energy for the S2QA- redox pair and lower activation energy for the S2QB- redox pair. This results in an increase in the free-energy gap between P680+QA- and P680+Pheo- and a narrowing of the free energy gap between primary and secondary electron-accepting quinones in PSII electron acceptors. We propose that these effects result in an increased population of reduced primary electron-accepting quinone in PSII, facilitating non-radiative P680+QA- radical pair recombination. Enhanced reaction center quenching was confirmed using in vivo chlorophyll fluorescence-quenching analysis. The enhanced dissipation of excess light energy within the reaction center of PSII, in part, accounts for the observed increase in resistance to high-light stress in cold-acclimated Arabidopsis plants.

Photosystem II reaction centre quenching: mechanisms and physiological role

Dissipation of excess absorbed light energy in eukaryotic photoautotrophs through zeaxanthin- and ΔpH-dependent photosystem II antenna quenching is considered the major mechanism for non-photochemical quenching and photoprotection. However, there is mounting evidence of a zeaxanthin-independent pathway for dissipation of excess light energy based within the PSII reaction centre that may also play a significant role in photoprotection. We summarize recent reports which indicate that this enigma can be explained, in part, by the fact that PSII reaction centres can be reversibly interconverted from photochemical energy transducers that convert light into ATP and NADPH to efficient, non-photochemical energy quenchers that protect the photosynthetic apparatus from photodamage. In our opinion, reaction centre quenching complements photoprotection through antenna quenching, and dynamic regulation of photosystem II reaction centre represents a general response to any environmental condition that predisposes the accumulation of reduced QA in the photosystem II reaction centres of prokaryotic and eukaryotic photoautotrophs. Since the evolution of reaction centres preceded the evolution of light harvesting systems, reaction centre quenching may represent the oldest photoprotective mechanism.

Reaction centre quenching of excess light energy and photoprotection of photosystem II

In addition to the energy dissipation of excess light occurring in PSII antenna via the xanthophyll cycle, there is mounting evidence of a zeaxanthin-independent pathway for non-photochemical quenching based within the PSII reaction centre (reaction centre quenching) that may also play a significant role in photoprotection. It has been demonstrated that acclimation of higher plants, green algae and cyanobacteria to low temperature or high light conditions which potentially induce an imbalance between energy supply and energy utilization is accompanied by the development of higher reduction state of QA and higher resistance to photoinhibition (Huner et al., 1998). Although this is a fundamental feature of all photoautotrophs, and the acquisition of increased tolerance to photoinhibition has been ascribed to growth and development under high PSII excitation pressure, the precise mechanism controlling the redox state of QA and its physiological significance in developing higher resistance to photoinhibition has not been fully elucidated. In this review we summarize recent data indicating that the increased resistance to high light in a broad spectrum of photosynthetic organisms acclimated to high excitation pressure conditions is associated with an increase probability for alternative non-radiative P680+QA- radical pair recombination pathway for energy dissipation within the reaction centre of PSII. The various molecular mechanisms that could account for non-photochemical quenching through PSII reaction centre are also discussed.

Acclimation to temperature and irradiance modulates PSII charge recombination.

Acclimation of wild type and the chlorina F2 mutant of barley to either high light or low temperature results in a 2‐ to 3‐fold increase in non‐photochemical quenching which occurred independently of either energy‐dependent quenching (qE), xanthophyll cycle‐mediated antenna quenching or state transitions. Results of in vivo thermoluminescence measurements used to address this conundrum indicated that excitation pressure regulates the temperature gap for S 2 Q B ‐ and S 2 Q A ‐ charge recombinations within photosystem II reaction centers. This is discussed in terms of photoprotection through non‐radiative charge recombination.

Photoprotection of Photosystem II: Reaction Center Quenching Versus Antenna Quenching

Understanding the role of the xanthophyll cycle and elucidating the mechanisms of antenna quenching through the non-photochemical dissipation of excess absorbed energy in the photoprotection of the photochemical apparatus continues to be a major focus of photosynthetic research. In addition to antenna quenching, there is evidence for the non-photochemical dissipation of excess energy through the PS II reaction center. Hence, this photoprotective mechanism is called reaction center quenching. One technique to assess reaction center quenching is photosynthetic thermoluminescence. This technique represents a simple but powerful probe of PS II photochemistry that measures the light emitted due to the reversal of PS II charge separation through the thermally-dependent recombination of the negative charges stabilized on Q− A and Q− B on the acceptor side of PS II with the positive charges accumulated in the S2- and S3-states of the oxygen evolving complex. Changes in the temperature maxima for photosynthetic thermoluminescence may reflect changes in redox potentials of recombining species within PS II reaction centers. Exposure of Synechococcussp. PCC 7942, Pinus sylvestrisL., Arabidopsis thaliana, and Chlamydomonas reinhardtii to either lowtemperatures or to high light induces a significant downshift in the temperature maxima for S2Q− B and S3Q− B recombinations relative to S2Q− A and S3Q− A recombinations. These shifts in recombination temperatures are indicative of lower activation energy for the S2Q− B redox pair recombination and a narrowing of the free energy gap betweenQAandQB electron acceptors. This, in turn, is associated with a decrease in the overall thermoluminescence emission. We propose that environmental factors such as high light and low temperature result in an increased population of reduced QA (Q− A), that is, increased excitation pressure, facilitating non-radiative P680+Q− A radical pair recombination within the PS II reaction center. The underlying molecular mechanisms regulating reaction center quenching appear to be species dependent. We conclude that reaction center quenching and antenna quenching are complementary mechanisms that may function to photoprotect PS II to different extents in vivo depending on the species as well as the environmental conditions to which the organism is exposed.

Excitation pressure regulates the activation energy for recombination events in the photosystem II reaction centres of Chlamydomonas reinhardtii.

Using in vivo thermoluminescence, we examined the effects of growth irradiance and growth temperature on charge recombination events in photosystem II reaction centres of the model green alga Chlamydomonas reinhardtii. We report that growth at increasing irradiance at either 29 or 15 degrees C resulted in comparable downward shifts in the temperature peak maxima (T(M)) for S2QB- charge pair recombination events, with minimal changes in S2QA- recombination events. This indicates that such growth conditions decrease the activation energy required for S2QB- charge pair recombination events with no concomitant change in the activation energy for S2QA- recombination events. This resulted in a decrease in the DeltaT(M) between S2QA- and S2QB- recombination events, which was reversible when shifting cells from low to high irradiance and back to low irradiance at 29 degrees C. We interpret these results to indicate that the redox potential of QB was modulated independently of QA, which consequently narrowed the redox potential gap between QA and QB in photosystem II reaction centres. Since a decrease in the DeltaT(M) between S2QA- and S2QB- recombination events correlated with growth at increasing excitation pressure, we conclude that acclimation to growth under high excitation pressure narrows the redox potential gap between QA and QB in photosystem II reaction centres, enhancing the probability for reaction center quenching in C. reinhardtii. We discuss the molecular basis for the modulation of the redox state of QB, and suggest that the potential for reaction center quenching complements antenna quenching via the xanthophyll cycle in the photoprotection of C. reinhardtii from excess light.

Restricted capacity for PSI-dependent cyclic electron flow in ΔpetE mutant compromises the ability for acclimation to iron stress in Synechococcus sp. PCC 7942 cells☆

Exposure of wild type (WT) and plastocyanin coding petE gene deficient mutant (ΔpetE) of Synechococcus cells to low iron growth conditions was accompanied by similar iron-stress induced blue-shift of the main red Chl a absorption peak and a gradual decrease of the Phc/Chl ratio, although ΔpetE mutant was more sensitive when exposed to iron deficient conditions. Despite comparable iron stress induced phenotypic changes, the inactivation of petE gene expression was accompanied with a significant reduction of the growth rates compared to WT cells. To examine the photosynthetic electron fluxes in vivo, far-red light induced P700 redox state transients at 820nm of WT and ΔpetE mutant cells grown under iron sufficient and iron deficient conditions were compared. The extent of the absorbance change (ΔA(820)/A(820)) used for quantitative estimation of photooxidizable P700(+) indicated a 2-fold lower level of P700(+) in ΔpetE compared to WT cells under control conditions. This was accompanied by a 2-fold slower re-reduction rate of P700(+) in the ΔpetE indicating a lower capacity for cyclic electron flow around PSI in the cells lacking plastocyanin. Thermoluminescence (TL) measurements did not reveal significant differences in PSII photochemistry between control WT and ΔpetE cells. However, exposure to iron stress induced a 4.5 times lower level of P700(+), 2-fold faster re-reduction rate of P700(+) and a temperature shift of the TL peak corresponding to S(2)/S(3)Q(B)(-) charge recombination in WT cells. In contrast, the iron-stressed ΔpetE mutant exhibited only a 40% decrease of P700(+) and no significant temperature shift in S(2)/S(3)Q(B)(-) charge recombination. The role of mobile electron carriers in modulating the photosynthetic electron fluxes and physiological acclimation of cyanobacteria to low iron conditions is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Prasanna K. Mohanty (1934–2013): a great photosynthetiker and a wonderful human being who touched the hearts of many

Prasanna K. Mohanty, a great scientist, a great teacher and above all a great human being, left us more than a year ago (on March 9, 2013). He was a pioneer in the field of photosynthesis research; his contributions are many and wide-ranging. In the words of Jack Myers, he would be a “photosynthetiker” par excellence. He remained deeply engaged with research almost to the end of his life; we believe that generations of researchers still to come will benefit from his thorough and enormous work. We present here his life and some of his contributions to the field of Photosynthesis Research. The response to this tribute was overwhelming and we have included most of the tributes, which we received from all over the world. Prasanna Mohanty was a pioneer in the field of “Light Regulation of Photosynthesis”, a loving and dedicated teacher—unpretentious, idealistic, and an honest human being.

Vidyadhar Govind (Pandit) Tatake (1926–2004): An ingenious instrumentalist, an authority on thermoluminescence, and a lover of classical Indian music

Govind (Pandit) Tatake passed away on September 30, 2004 after a prolonged illness, at Pune, India. His family was with him when he breathed his last. Born on July 24, 1926 in a farmer’s family to Indira and Govind Tatake at Sangli, Maharashtra, India, Pandit had his early education at the prestigious Nutan Marathi Vidyalay in Pune. Graduating from the school he joined the equally illustrious Fergusson College, Pune University, from where he obtained his M. Sc. in Physics in 1953. He was a voracious reader and impressed everyone even in his childhood by his knowledge of several areas including traditional Hindu teaching and philosophy. It was for this reason that he was nicknamed Pandit meaning a learned person. This name stayed with him all his life and his friends always called him by this name. Although he was interested in Physics in general, he had a special liking for electronics and instrumentation. When he studied Physics, electronics was still nascent and hence found only a perfunctory place in the curriculum. So he decided to obtain a postgraduate diploma in Electronics right after his M. Sc. in Physics from University of Pune. At that time there were very few who had specialized training in both of these important disciplines. It was also the time Independent India was anxious to set up world-class institutions for research in leading branches of science. One such area was that of Atomic Energy. The Father of Indian Atomic Energy Establishment, Dr. Homi Bhabha, with the blessings from the then Prime Minister Jawaharlal Nehru, was enlisting intellectuals to undertake research and development in all the areas of Atomic Energy including biological effects and applications of Atomic Energy. A major department of Biology and Agriculture was established, in 1952, in the Atomic Research Establishment. Initially it was located in the premises of the Indian Cancer Research Institute at Parel, Mumbai (then called Bombay), but soon thereafter shifted to the Richardson and Cruddas Vidyadhar Govind (Pandit) Tatake (1926–2004)