Short-term Photoinhibition Research Articles

Permanent Stress Adaptation and Unexpected High Light Tolerance in the Shade-Adapted Chlamydomonas priscui.

The Antarctic photopsychrophile, Chlamydomonas priscui UWO241, is adapted to extreme environmental conditions, including permanent low temperatures, high salt, and shade. During long-term exposure to this extreme habitat, UWO241 appears to have lost several short-term mechanisms in favor of constitutive protection against environmental stress. This study investigated the physiological and growth responses of UWO241 to high-light conditions, evaluating the impacts of long-term acclimation to high light, low temperature, and high salinity on its ability to manage short-term photoinhibition. We found that UWO241 significantly increased its growth rate and photosynthetic activity at growth irradiances far exceeding native light conditions. Furthermore, UWO241 exhibited robust protection against short-term photoinhibition, particularly in photosystem I. Lastly, pre-acclimation to high light or low temperatures, but not high salinity, enhanced photoinhibition tolerance. These findings extend our understanding of stress tolerance in extremophilic algae. In the past 2 decades, climate change-related increasing glacial stream flow has perturbed long-term stable conditions, which has been associated with lake level rise, the thinning of ice covers, and the expansion of ice-free perimeters, leading to perturbations in light and salinity conditions. Our findings have implications for phytoplankton survival and the response to change scenarios in the light-limited environment of Antarctic ice-covered lakes.

Cyclic electron flow (CEF) and ascorbate pathway activity provide constitutive photoprotection for the photopsychrophile, Chlamydomonas sp. UWO 241(renamed Chlamydomonas priscuii).

Under environmental stress, plants and algae employ a variety of strategies to protect the photosynthetic apparatus and maintain photostasis. To date, most studies on stress acclimation have focused on model organisms which possess limited to no tolerance to stressful extremes. We studied the ability of the Antarctic alga Chlamydomonas sp. UWO 241 (UWO 241) to acclimate to low temperature, high salinity or high light. UWO 241 maintained robust growth and photosynthetic activity at levels of temperature (2°C) and salinity (700mM NaCl) which were nonpermissive for a mesophilic sister species, Chlamydomonas raudensis SAG 49.72 (SAG 49.72). Acclimation in the mesophile involved classic mechanisms, including downregulation of light harvesting and shifts in excitation energy between photosystem I and II. In contrast, UWO 241 exhibited high rates of PSI-driven cyclic electron flow (CEF) and a larger capacity for nonphotochemical quenching (NPQ). Furthermore, UWO 241 exhibited constitutively high activity of two key ascorbate cycle enzymes, ascorbate peroxidase and glutathione reductase and maintained a large ascorbate pool. These results matched the ability of the psychrophile to maintain low ROS under short-term photoinhibition conditions. We conclude that tight control over photostasis and ROS levels are essential for photosynthetic life to flourish in a native habitat of permanent photooxidative stress.We propose to rename this organism Chlamydomonas priscuii.

Synechocystis mutants defective in manganese uptake regulatory system, ManSR, are hypersensitive to strong light

High affinity transport of manganese ions (Mn2+) in cyanobacteria is carried by an ABC-type transporter, encoded by the mntCAB operon, which is derepressed by the deficiency of Mn2+. Transcription of this operon is negatively regulated by the two-component system consisting of a sensory histidine kinase ManS and DNA-binding response regulator ManR. In this study, we examined two Synechocystis mutants, defective in ManS and ManR. These mutants were unable to grow on high concentrations of manganese. Furthermore, they were sensitive to high light intensity and unable to recover after short-term photoinhibition. Under standard illumination and Mn2+ concentration, mutant cells revealed the elevated levels of transcripts of genes involved in the formation of Photosystem II (psbA, psbD, psbC, pap-operon). This finding suggests that, in mutant cells, the PSII is sensitive to high concentrations of Mn2+ even at relatively low light intensity.

Cross tolerance to heavy-metal and cold-induced photoinhibiton in leaves of Pisum sativum acclimated to low temperature

Under high light intensity, low temperatures as well as heavy metals induce photoinhibition of PSII and oxidative stress in leaves. Since cold acclimation of leaves ameliorates their capacity of antioxidative defence, cross tolerance between cold-induced and heavy metal-induced photoinhibition was investigated in pea leaves grown at either 22 °C or 6 °C. The experimental conditions were chosen to induce a uniform level of short-term photoinhibition at low temperature or in the presence of CuSO4 or CdCl2 in leaves grown at 22 °C. Under all conditions photoinhibition of PSII was lower in cold-acclimated (6°C-grown) than in non-acclimated (22°C-grown) pea leaves. In darkness PSII was not affected by all treatments. Other parameters like catalase activity, chlorophyll content and metabolite contents were most sensitive to CuSO4, but less affected by CdCl2 and low temperature treatments. Strong oxidation of ascorbate and concomitant loss of catalase activity showed the enhanced oxidative stress in CuSO4-treated leaves. Generally, all measured parameters were less affected in cold-acclimated leaves than in non-acclimated leaves under all experimental conditions. Cold-acclimated pea leaves contained higher levels of ascorbate and particularly of glutathione and a higher capacity to keep the primary electron acceptor of PSII more oxidised. Incubation with heavy metals caused a nearly complete loss of reduced glutathione. It is suggested that reduced glutathione served as a source for phytochelatin synthesis. The extraordinarily high glutathione content in cold-acclimated pea leaves might therefore increase their ability to chelate heavy metals and thus to protect leaves from heavy-metal induced damage.

Mutational and expression analysis of ELIP1 and ELIP2 in Arabidopsis thaliana

Plants exposed to photoinhibitory conditions respond by accumulation of the early light-induced proteins (ELIPs) with a potential photoprotective function. In Arabidopsis thaliana two genes (Elip1 and Elip2) encode for two ELIP proteins: evidence exists that the two genes are differentially regulated but their precise function is unclear. Mutants null for one or the other Elip gene can help in elucidating ELIPs role and here we describe the expression profile of ELIP1 and ELIP2, and the phenotype of such null mutants. Both ELIPs accumulate during greening of etiolated seedlings and in mature plants the transcripts fluctuate diurnally without protein accumulation. Steady-state transcript level of both genes increases in response to high light with transcription of Elip1 much more sensitive than that of Elip2 to increasing irradiation at 22 degrees C. At 4 degrees C instead Elip2 is strongly transcribed even at growing light. Furthermore, only ELIP1 accumulates under high light at 22 degrees C while both proteins accumulate at 4 degrees C. These results indicate the existence of a differential regulation of ELIPs expression in response to light or chilling stress with mechanisms active either at transcriptional and post-transcriptional level. Phenotypically, the mutants behave as the wild type as far as sensitivity to light- or light and cold-induced short-term photoinhibition, while both ELIPs are necessary to ensure a high rate of chlorophyll accumulation during deetiolation in continuous high light.

Projections of the basal retrochiasmatic area: a neural site involved in the photic control of pineal metabolism

It has been shown that the basal retrochiasmatic area (RCHb), situated immediately ventral to the third ventricle behind the suprachiasmatic nucleus and in front of the arcuate nucleus, is implicated in the nocturnal inhibitory process of melatonin production induced by short-term retinal photo-stimulation. In the present study, the projections of the RCHb have been examined using the Phaseolus vulgaris leucoagglutinin (PHA-L) method in the rat. Considering the putative role of the RCHb in contributing to the short-term photo-inhibition of the pineal gland during the night, it is reasonable to suppose that the RCHb may ultimately inhibit the sympathetic outflow of the upper thoracic segments, known to be critically involved in the control of melatonin secretion. Of particular interest, the present anterograde tract-tracing study indicates all possible paths from the RCHb which may conceivably be involved in influencing the sympathetic outflow and, therefore, melatonin production. Thus, apart from a direct projection to the intermediolateral column at thoracic levels of the spinal cord, the RCHb is in a position to control the sympathetic outflow through other potential routes, such as the dorsal parvicellular part of the paraventricular nucleus of the hypothalamus, lateral hypothalamic area, ventromedial nucleus of the hypothalamus, lateral part of the periaqueductal gray and Barrington's nucleus.

Inactivation of the clpP1 gene for the proteolytic subunit of the ATP-dependent Clp protease in the cyanobacterium Synechococcus limits growth and light acclimation.

ClpP functions as the proteolytic subunit of the ATP-dependent Clp protease in eubacteria, mammals and plant chloroplasts. We have cloned a clpP gene, designated clpP1, from the cyanobacterium Synechococcus sp. PCC 7942. The monocistronic 591 bp gene codes for a protein 80% similar to one of four putative ClpP proteins in another cyanobacterium, Synechocystis sp. PCC 6803. The constitutive ClpP1 content in Synechococcus cultures was not inducible by high temperatures, but it did rise fivefold with increasing growth light from 50 to 175 micromol photons m(-2) s(-1). A clpP1 inactivation strain (delta clpP1) exhibited slower growth rates, especially at the higher irradiances, and changes in the proportion of the photosynthetic pigments, chlorophyll a and phycocyanin. Many mutant cells (ca. 35%) were also severely elongated, up to 20 times longer than the wild type. The stress phenotype of delta clpP1 when grown at high light was confirmed by the induction of known stress proteins, such as the heat shock protein GroEL and the alternate form of PSII reaction center D1 protein, D1 form 2. ClpP1 content also rose significantly during short-term photoinhibition, but its loss in delta clpP1 did not exacerbate the extent of inactivation of photosynthesis, nor affect the inducible D1 exchange mechanism, indicating ClpP1 is not directly involved in D1 protein turnover.

Photosynthesis, photoinhibition and low temperature acclimation in cold tolerant plants

Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The photosynthetic response of plants to low temperature is dependent upon time of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperature shiftsinhibits whereas low temperature growthstimulates electron transport capacity and carbon assimilation. However, the photosynthetic response to low temperature is clearly species and cultivar dependent. Winter annuals and algae which actively grow and develop at low temperature and moderate irradiance acquire a resistance to irradiance 5- to 6-fold higher than their growth irradiance. Resistance to short-term photoinhibition (hours) in winter cereals is a reflection of the increased capacity to keep QA oxidized under high light conditions and low temperature. This is due to an increased capacity for photosynthesis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance of cereals. It is proposed that the enhanced photosynthetic capacity reflects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage of carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Long-term exposure (months) of cereals to low temperature photoinhibition indicates that this reduction of photochemical efficiency of PS II represents a stable, long-term down regulation of PS II to match the energy requirements for CO2 fixation. Thus, photoinhibition in vivo should be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which necessarily results in damage or injury to plants. Not all cold tolerant, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering evergreens become dormant during the cold hardening period and generally remain susceptible to photoinhibition. It is concluded that the photosynthetic response to low temperatures and susceptibility to photoinhibition are consequences of the overwintering strategy of the plant species.

Effect of Cold Hardening on Sensitivity of Winter and Spring Wheat Leaves to Short-Term Photoinhibition and Recovery of Photosynthesis

Photoinhibition of photosynthesis and its recovery were studied in wheat (Triticum aestivum L.) leaves grown at nonhardening (20 degrees C) and cold-hardening (5 degrees C) temperatures. Cold-hardened wheat leaves were less susceptible to photoinhibition at 5 degrees C than nonhardened leaves, and the winter cultivars, Kharkov and Monopol, were less susceptible than the spring cultivar, Glenlea. The presence of chloramphenicol, a chloroplastic protein synthesis inhibitor, increased the susceptibility to photoinhibition, but cold-hardened leaves still remained less susceptible to photoinhibition than nonhardened leaves. Recovery at 50 mumol m(-2) s(-1) photosynthetic photon flux density and 20 degrees C was at least biphasic, with a fast and a slow phase in all cultivars. Cold-hardened leaves recovered maximum fluorescence and maximum variable fluorescence in the dark-adapted state during the fast phase at a rate of 42% h(-1) compared with 22% h(-1) for nonhardened leaves. The slow phase occurred at similar rates (2% h(-1)) in cold-hardened and nonhardened leaves. Full recovery required up to 30 h. Fast-recovery phase was not reduced by either lowering the recovery temperature to 5 degrees C or by the presence of chloramphenicol. Slow-recovery phase was inhibited by both treatments. Hence, the fast phase of recovery does not require de novo chloroplast protein synthesis. In addition, only approximately 60% of the photochemical efficiency lost through photoinhibition at 5 degrees C was associated with lost [(14)C]atrazine binding and, hence, with damage to the secondary quinone electron acceptor for photosystem II-binding site. We conclude that the decrease in susceptibility to photoinhibition exhibited following cold hardening of winter and spring cultivars is not due to an increased capacity for repair of photoinhibitory damage at 5 degrees C but reflects intrinsic properties of the cold-hardened photosynthetic apparatus. A model to account for the fast component of recovery is discussed.

Effect of long-term photoinhibition on growth and photosynthesis of cold-hardened spring and winter wheat

The effect of repeated exposure to high light (1200 μmol · m(-2) · s(-1) photosynthetic photon flux density, PPFD) at 5° C was examined in attached leaves of cold-grown spring (cv. Katepwa) and winter (cv. Kharkov) wheat (Triticum aestivum L.) over an eight-week period. Under these conditions, Kharkov winter wheat exhibited a daily reduction of 24% in FV/FM (the ratio of variable to maximal fluorescence in the dark-adapted state), in contrast to 41% for cold-grown Katepwa spring wheat. Both cultivars were able to recover from this daily suppression of FV/FM such that the leaves exhibited an average morning FV/FM of 0.651 ± 0.004. Fluorescence measurements made under steady-state conditions as a function of irradiance from 60 to 2000 μmol · m(-2) · s(-1) indicated that the yield of photosystem II (PSII) electron transport under light-saturating conditions was the same for photoinhibited and control cold-grown plants, regardless of cultivar. Repeated daily exposure to high light at low temperature did not increase resistance to short-term photoinhibition, although zeaxanthin levels increased by three- to fourfold. In addition, both cultivars increased the rate of dry-matter accumulation, relative to control plants maintained at 5° C and 250 μmol · m(-2) · s(-1) PPFD (10% and 28% for Katepwa and Kharkov, respectively), despite exhibiting suppressed fv/fm and reduced photon yields for O2 evolution following daily high-light treatments. Thus, although photosynthetic efficiency is suppressed by a longterm, photoinhibitory treatment, light-saturated rates of photosynthesis are sufficiently high during the high-light treatment to offset any reduction in photochemical efficiency of PSII. We suggest that in these cold-tolerant plants, photoinhibition of PSII may represent a longterm, stable, down-regulation of photochemistry to match the overall photosynthetic demand for ATP and reducing equivalents.

Light and temperature dependent inhibition of photosynthesis in frost-hardened and un-hardened seedlings of pine

Needles of un-hardened and frost-hardended seedlings of Pinus sylvestris and Pinus contorta were exposed to photoinhibitory photon flux densities at temperatures between 0 and 35°C under laboratory conditions. Photoinhibition of photosynthesis was assayed by measuring oxygen evolution under saturating CO2 in a leaf disc oxygen electrode or by recording of photosystem II fluorescence induction kinetics at 77 K. It was demonstrated that frost hardening of pine did not affect the susceptibility of photosynthesis to short time (2 h) photoinhibition at 15°C. The two pine species irrespective of acclimative state were equally sensitive to photoinhibition as assayed by apparent photon yield analyses of photosynthetic oxygen evolution. Plots of the apparent photon yield of oxygen evolution vs. F v /F m revealed a non-linear relationship.In the temperature range of 15-20°C short term photoinhibition caused a loss of F v without effect on F 0 . However, photoinhibition at temperatures lower or higher caused F 0 to increase and decrease, respectively. In fact the decrease of F v v /F upon lowering the temperature was mainly caused by the temperature effect on F 0 . Besides photoinhibition causing the well established quenching of F v by increased radiationless decay somewhere in the reaction center-antenna complex, it is suggested that F 0 generally increases as a result of loss of functional reaction centers causing decreased trapping of excitation energy. However, the high temperature induced quenching of F 0 suggests that the quenching process (or processes) induced under photoinhibitory conditions is temperature dependent; i.e. it increases with the increase of temperature.In pine the photon yield of photosynthesis was much more sensitive to short term photoinhibition than was the rate of light saturated photosynthesis. This difference is explained by photosystem II and electron transport having surplus capacity relative to that of reductive carbon metabolism.

Photosynthetic characteristics of giant kelp (Macrocystis pyrifera) determined in situ

Rates of net photosynthesis and nocturnal respiration by individual blades of the giant kelp Macrocystis pyrifera (L.) C. Agardh in southern California, were determined in situ by measuring oxygen production in polyethylene bags during spring/summer of 1983. Mature blades from different depths in the water column exhibited different photosynthetic characteristics. Blades from the surface canopy (0 to 1 m depth) exhibited higher photosynthetic capacity under saturating irradiance and higher photosynthetic efficiency at low irradiances than blades from 3 to 5 or 7 to 9 m depths. Saturating irradiance was lower for canopy blades than for deeper blades. Canopy blades showed no short-term photoinhibition, but photosynthetic rates of deeper blades were significantly reduced during 1 to 2 h incubations at high irradiances. Results of 1 to 2 wk acclimation experiments indicated that differences between photosynthetic characteristics of blades from different depths were primarily attributable to acclimation light conditions. Vertical displacement of blades within the kelp canopy occurred on a time-scale of 1 min to 1 h. Blades continually moved between the unshaded surface layer and deeper, shaded layers. Vertical movement did not maximize photosynthesis by individual blades; only a small proportion of blades making up a dense surface canopy maintained light-saturated photosynthetic rates during midday incubations. The relatively high photosynthetic rates exhibited by canopy blades over the entire range of light conditions probably resulted from acclimation to intermittent high and low irradiances, a consequence of vertical displacement. Vertical displacement also reduced the afternoon depression in photosynthesis of individual canopy blades. The overall effect of vertical displacement was optimization of total net photosynthesis by the kelp canopy and, therefore, optimization of whole-plant production.