Photosynthesis Research (v.124, #2)

Andrew A. Benson, 1917–2015 by Hartmut K. Lichtenthaler; Bob B. Buchanan; Roland Douce; Govindjee (131-135).
On January 16, 2015, Professor Andrew Alm Benson, one of the leading plant biochemists of the twentieth century, died in La Jolla, California, at the age of 97; he was born on September 24, 1917. Benson was known especially for his pioneering studies on photosynthesis (CO2 assimilation, carbon reduction cycle) and plant lipids (phospholipid phosphatidyl glycerol; and the sulfolipid, sulfoquinovosyl diglyceride). A photograph of Benson is shown in Fig. 1. Fig. 1 Photograph of Andrew A. Benson. Source: Annual Review of Plant Biology, Vol. 53, 2002, published with permission
Keywords: The path of carbon in photosynthesis; Calvin–Benson cycle; RuBP; Rubisco; Phosphoglycerate; PGA; Sulfolipid

In the sunlight-fluctuating environment, plants often encounter both light-deficiency and light-excess cases. Therefore, regulation of light harvesting is absolutely essential for photosynthesis in order to maximize light utilization at low light and avoid photodamage of the photosynthetic apparatus at high light. Plants have developed a series of strategies of light-harvesting regulation during evolution. These strategies include rapid responses such as leaf movement and chloroplast movement, state transitions, and reversible dissociation of some light-harvesting complex of the photosystem II (LHCIIs) from PSII core complexes, and slow acclimation strategies such as changes in the protein abundance of light-harvesting antenna and modifications of leaf morphology, structure, and compositions. This review discusses successively these strategies and focuses on the rapid change in antenna size, namely reversible dissociation of some peripheral light-harvesting antennas (LHCIIs) from PSII core complex. It is involved in protective role and species dependence of the dissociation, differences between the dissociation and state transitions, relationship between the dissociation and thylakoid protein phosphorylation, and possible mechanism for thermal dissipation by the dissociated LHCIIs.
Keywords: Chloroplast movement; Leaf movement; LHCII; Photosystem II; qE ; Reversible dissociation

Seasonal changes in the content of dehydrins in mesophyll cells of common pine needles by Natalia Korotaeva; Anatolii Romanenko; Galina Suvorova; Maria V. Ivanova; Lidia Lomovatskaya; Gennadii Borovskii; Victor Voinikov (159-169).
The appearance of dehydrins (DHNs) in cells is required for the development of cold resistance. DHNs are therefore considered specific markers of cold resistance by some authors. DHNs accumulate in plants concomitantly with a reduction of intracellular water content, and presumably protect membranes and proteins from damage caused by moisture loss. DHN content in pine needles increases in spring and autumn when moisture availability and temperatures are most unfavorable. The present work is focused on seasonal changes in DHN content in various mesophyll-cell compartments of pine (Pinus sylvestris L.) needles in association with changes in environmental factors. In spring, the number of thylakoid membranes per granum was lower than in summer and autumn. An increase in needle content of DHNs with approximate masses of 76, 73, 72, 35, and 17 kD in spring and autumn, associated with needle dehydration during this period, is shown here. The largest increase in DHN content was observed in spring, with the highest amount of DHNs presented in chloroplast membrane system including grana thylakoids, stromal thylakoids, and the two chloroplast envelope membranes and in cell walls. In the autumn, most DHNs were localized in chloroplasts and mitochondria.
Keywords: Dehydrins; Pinus sylvestris L.; Seasonal changes; Cell compartments

Carotenoid–chlorophyll coupling and fluorescence quenching in aggregated minor PSII proteins CP24 and CP29 by Christoph-Peter Holleboom; Daniel Alexander Gacek; Pen-Nan Liao; Marco Negretti; Roberta Croce; Peter Jomo Walla (171-180).
It is known that aggregation of isolated light-harvesting complex II (LHCII) in solution results in high fluorescence quenching, reduced chlorophyll fluorescence lifetime, and increased electronic coupling of carotenoid (Car) S1 and chlorophyll (Chl) Qy states, as determined by two-photon studies. It has been suggested that this behavior of aggregated LHCII mimics aspects of non-photochemical quenching processes of higher plants and algae. However, several studies proposed that the minor photosystem II proteins CP24 and CP29 also play a significant role in regulation of photosynthesis. Therefore, we use a simple protocol that allows gradual aggregation also of CP24 and CP29. Similarly, as observed for LHCII, aggregation of CP24 and CP29 also leads to increasing fluorescence quenching and increasing electronic Car S1–Chl Qy coupling. Furthermore, a direct comparison of the three proteins revealed a significant higher electronic coupling in the two minor proteins already in the absence of any aggregation. These differences become even more prominent upon aggregation. A red-shift of the Qy absorption band known from LHCII aggregation was also observed for CP29 but not for CP24. We discuss possible implications of these results for the role of CP24 and CP29 as potential valves for excess excitation energy in the regulation of photosynthetic light harvesting.
Keywords: CP24; CP29; LHCII; Aggregation; Electronic coupling; Fluorescence quenching; Chlorophylls; Carotenoids

High prevalence of diffusive uptake of CO2 by macroalgae in a temperate subtidal ecosystem by Christopher E. Cornwall; Andrew T. Revill; Catriona L. Hurd (181-190).
Productivity of most macroalgae is not currently considered limited by dissolved inorganic carbon (DIC), as the majority of species have CO2-concentrating mechanisms (CCM) allowing the active uptake of DIC. The alternative, diffusive uptake of CO2 (non-CCM), is considered rare (0–9 % of all macroalgal cover in a given ecosystem), and identifying species without CCMs is important in understanding factors controlling inorganic carbon use by eukaryotic algae. CCM activity has higher energetic requirements than diffusive CO2 uptake, therefore when light is low, CCM activity is reduced in favour of diffusive CO2 uptake. We hypothesized that the proportional cover of macroalgae without CCMs (red and green macroalgae) would be low (<10 %) across four sites in Tasmania, southern Australia at two depths (4–5 and 12–14 m); the proportion of species lacking CCMs would increase with decreasing depth; the δ13C values of macroalgae with CCMs would be more depleted with depth. We found the proportion of non-CCM species ranged from 0 to 90 % and included species from all three macroalgal phyla: 81 % of red (59 species), 14 % of brown (three species) and 29 % of green macroalgae (two species). The proportion of non-CCM species increased with depth at three of four sites. 35 % of species tested had significantly depleted δ13C values at deeper depths. Non-CCM macroalgae are more abundant in some temperate reefs than previously thought. If ocean acidification benefits non-CCM species, the ramifications for subtidal macroalgal assemblages could be larger than previously considered.
Keywords: CCMs; Climate change; CO2-concentrating mechanisms; Inorganic carbon acquisition; Irradiance; Seaweed

Violaxanthin de-epoxidase disulphides and their role in activity and thermal stability by Erik Ingmar Hallin; Kuo Guo; Hans-Erik Åkerlund (191-198).
Violaxanthin de-epoxidase (VDE) catalyses the conversion of violaxanthin to zeaxanthin at the lumen side of the thylakoids during exposure to intense light. VDE consists of a cysteine-rich N-terminal domain, a lipocalin-like domain and a negatively charged C-terminal domain. That the cysteines are important for the activity of VDE is well known, but in what way is less understood. In this study, wild-type spinach VDE was expressed in E. coli as inclusion bodies, refolded and purified to give a highly active and homogenous preparation. The metal content (Fe, Cu, Ni, Mn, Co and Zn) was lower than 1 mol% excluding a metal-binding function of the cysteines. To investigate which of the 13 cysteines that could be important for the function of VDE, we constructed mutants where the cysteines were replaced by serines, one by one. For 12 out of 13 mutants the activity dropped by more than 99.9 %. A quantification of free cysteines showed that only the most N-terminal of these cysteines was in reduced form in the native VDE. A disulphide pattern in VDE of C9–C27, C14–C21, C33–C50, C37–C46, C65–C72 and C118–C284 was obtained after digestion of VDE with thermolysin followed by mass spectroscopy analysis of reduced versus non-reduced samples. The residual activity found for the mutants showed a variation that was consistent with the results obtained from mass spectroscopy. Reduction of the disulphides resulted in loss of a rigid structure and a decrease in thermal stability of 15 °C.
Keywords: Violaxanthin de-epoxidase; Cysteine; Mutation; Disulphide bond; Violaxanthin; Zeaxanthin

How will climate change influence grapevine cv. Tempranillo photosynthesis under different soil textures? by Urtzi Leibar; Ana Aizpurua; Olatz Unamunzaga; Inmaculada Pascual; Fermín Morales (199-215).
While photosynthetic responses to elevated CO2, elevated temperature, or water availability have previously been reported for grapevine as responses to single stress factors, reports on the combined effect of multiple stress factors are scarce. In the present work, we evaluated effects of simulated climate change [CC; 700 ppm CO2, 28/18 °C, and 33/53 % relative humidity (RH), day/night] versus current conditions (375 ppm CO2, 24/14 °C, and 45/65 % RH), water availability (well-irrigated vs. water deficit), and different types of soil textures (41, 19, and 8 % of soil clay contents) on grapevine (Vitis vinifera L. cv. Tempranillo) photosynthesis. Plants were grown using the fruit-bearing cutting model. CC increased the photosynthetic activity of grapevine plants grown under well-watered conditions, but such beneficial effects of elevated CO2, elevated temperature, and low RH were abolished by water deficit. Under water-deficit conditions, plants subjected to CC conditions had similar photosynthetic rates as those grown under current conditions, despite their higher sub-stomatal CO2 concentrations. As expected, water deficit reduced photosynthetic activity in association with inducing stomatal closure that prevents water loss. Evidence for photosynthetic downregulation under elevated CO2 was observed, with decreases in photosynthetic capacity and leaf N content and increases in the C/N ratio in plants subjected to CC conditions. Soil texture had no marked effects on photosynthesis and did not modify the photosynthetic response to CC and water-deficit conditions. However, in mature well-irrigated plants grown in the soils with the highest sand content, an important decrease in stomatal conductance was observed as well as a slight decrease in the utilization of absorbed light in photosynthetic electron transport (measured as photochemical quenching), possibly related to a low water-retention capacity of these soils even under well-watered conditions.
Keywords: Climate change; Grapevine; Photosynthesis; Soil texture; Water deficit

Homologs of the Photosystem II (PS II) subunit PsbP are found in plants, algae, and cyanobacteria. In higher plants, PsbP is associated with mature PS II centers, but in cyanobacteria, the homologous CyanoP protein appears sub-stoichiometric to PS II. We have investigated the role of CyanoP by characterizing knockout mutants of the cyanobacterium Synechocystis sp. PCC 6803. Removal of CyanoP resulted in changes to phycobilisome coupling and energy transfer to PS II, but the function of PS II itself remained similar to wild type. We therefore investigated the hypothesis that CyanoP is involved in the biogenesis or repair of PS II by creating a double mutant lacking both CyanoP and the PS II assembly factor Ycf48. This strain exhibited an additive reduction in the amplitude of variable chlorophyll a fluorescence induction relative to either of the single mutants but displayed increased oxygen evolution, slight increases in PS II monomer and dimer levels, and a reduction in accumulation of an early PS II assembly complex containing CP47, compared to the ΔYcf48 strain.
Keywords: Assembly; Biogenesis; CyanoP; Photosystem II; PsbP; Synechocystis sp. PCC 6803; Ycf48

Role of electron transport chain of chloroplasts in oxidative burst of interaction between Erwinia amylovora and host cells by Hamid Abdollahi; Zahra Ghahremani; Kobra Erfaninia; Rahim Mehrabi (231-242).
Erwinia amylovora is a necrogenic bacterium, causing the fire blight disease on many rosaceous plants. Triggering oxidative burst by E. amylovora is a key response by which host plants try to restrain pathogen spread. Electron transport chain (ETC) of chloroplasts is known as an inducible source of reactive oxygen species generation in various stresses. This research was performed to assess the role of this ETC in E. amylovora–host interaction using several inhibitors of this chain in susceptible and resistant apple and pear genotypes. All ETC inhibitors delayed appearance of disease necrosis, but the effects of methyl viologen, glutaraldehyde, and DCMU were more significant. In the absence of inhibitors, resistant genotypes showed an earlier and severe H2O2 generation and early suppression of redox dependent, psbA gene. The effects of inhibitors were corresponding to the redox potential of ETC inhibitory sites. In addition, delayed necrosis appearance was associated with the decreased disease severity and delayed H2O2 generation. These results provide evidences for the involvement of this ETC in host oxidative burst and suggest that chloroplast ETC has significant role in E. amylovora–host interaction.
Keywords: Apple; Pear; Resistance; Reactive oxygen species; Hydrogen peroxide; Electron flux inhibition