Long-term light adaptation of light-harvesting and energy-transfer processes in the glaucophyte Cyanophora paradoxa under different light conditions.

In response to fluctuation in light intensity and quality, oxygenic photosynthetic organisms modify their light-harvesting and excitation energy-transfer processes to maintain optimal photosynthetic activity. Glaucophytes, which are a group of primary symbiotic algae, possess light-harvesting antennas called phycobilisomes (PBSs) consistent with cyanobacteria and red algae. However, compared with cyanobacteria and red algae, glaucophytes are poorly studied and there are few reports on the regulation of photosynthesis in the group. In this study, we examined the long-term light adaptation of light-harvesting functions in a glaucophyte, Cyanophora paradoxa, grown under different light conditions. Compared with cells grown under white light, the relative number of PBSs to photosystems (PSs) increased in blue-light-grown cells and decreased in green-, yellow-, and red-light-grown cells. Moreover, the PBS number increased with increment in the monochromatic light intensity. More energy was transferred from PBSs to PSII than to PSI under blue light, whereas energy transfer from PBSs to PSII was reduced under green and yellow lights, and energy transfer from the PBSs to both PSs decreased under red light. Decoupling of PBSs was induced by intense green, yellow, and red lights. Energy transfer from PSII to PSI (spillover) was observed, but the contribution of the spillover did not distinctly change depending on the culture light intensity and quality. These results suggest that the glaucophyte C. paradoxa modifies the light-harvesting abilities of both PSs and excitation energy-transfer processes between the light-harvesting antennas and both PSs during long-term light adaption.

Functional characterization of four opsins and two G alpha subtypes co-expressed in the molluscan rhabdomeric photoreceptor.

BACKGROUND: Rhabdomeric photoreceptors of eyes in the terrestrial slug Limax are the typical invertebrate-type but unique in that three visual opsins (Gq-coupled rhodopsin, xenopsin, Opn5A) and one retinochrome, all belonging to different groups, are co-expressed. However, molecular properties including spectral sensitivity and G protein selectivity of any of them are not determined, which prevents us from understanding an advantage of multiplicity of opsin properties in a single rhabdomeric photoreceptor. To gain insight into the functional role of the co-expression of multiple opsin species in a photoreceptor, we investigated the molecular properties of the visual opsins in the present study. RESULTS: First, we found that the fourth member of visual opsins, Opn5B, is also co-expressed in the rhabdomere of the photoreceptor together with previously identified three opsins. The photoreceptors were also demonstrated to express Gq and Go alpha subunits. We then determined the spectral sensitivity of the four visual opsins using biochemical and spectroscopic methods. Gq-coupled rhodopsin and xenopsin exhibit maximum sensitivity at ~ 456 and 475 nm, respectively, and Opn5A and Opn5B exhibit maximum sensitivity at ~ 500 and 470 nm, respectively, with significant UV sensitivity. Notably, in vitro experiments revealed that Go alpha was activated by all four visual opsins, in contrast to the specific activation of Gq alpha by Gq-coupled rhodopsin, suggesting that the eye photoreceptor of Limax uses complex G protein signaling pathways. CONCLUSIONS: The eye photoreceptor in Limax expresses as many as four different visual opsin species belonging to three distinct classes. The combination of opsins with different spectral sensitivities and G protein selectivities may underlie physiological properties of the ocular photoreception, such as a shift in spectral sensitivity between dark- and light-adapted states. This may be allowed by adjustment of the relative contribution of the four opsins without neural networks, enabling a simple strategy for fine-tuning of vision.

Cone-Driven Retinal Responses Are Shaped by Rod But Not Cone HCN1.

Signal integration of converging neural circuits is poorly understood. One example is in the retina where the integration of rod and cone signaling is responsible for the large dynamic range of vision. The relative contribution of rods versus cones is dictated by a complex function involving background light intensity and stimulus temporal frequency. One understudied mechanism involved in coordinating rod and cone signaling onto the shared retinal circuit is the hyperpolarization activated current (Ih) mediated by hyperpolarization-activated cyclic nucleotide-gated 1 (HCN1) channels expressed in rods and cones. Ih opposes membrane hyperpolarization driven by activation of the phototransduction cascade and modulates the strength and kinetics of the photoreceptor voltage response. We examined conditional knock-out (KO) of HCN1 from mouse rods using electroretinography (ERG). In the absence of HCN1, rod responses are prolonged in dim light which altered the response to slow modulation of light intensity both at the level of retinal signaling and behavior. Under brighter intensities, cone-driven signaling was suppressed. To our surprise, conditional KO of HCN1 from mouse cones had no effect on cone-mediated signaling. We propose that Ih is dispensable in cones because of the high level of temporal control of cone phototransduction. Thus, HCN1 is required for cone-driven retinal signaling only indirectly by modulating the voltage response of rods to limit their output.SIGNIFICANCE STATEMENT Hyperpolarization gated hyperpolarization-activated cyclic nucleotide-gated 1 (HCN1) channels carry a feedback current that helps to reset light-activated photoreceptors. Using conditional HCN1 knock-out (KO) mice we show that ablating HCN1 from rods allows rods to signal in bright light when they are normally shut down. Instead of enhancing vision this results in suppressing cone signaling. Conversely, ablating HCN1 from cones was of no consequence. This work provides novel insights into the integration of rod and cone signaling in the retina and challenges our assumptions about the role of HCN1 in cones.

Saturation of visual responses explains size tuning in rat collicular neurons.

The receptive field of many visual neurons is composed of a central responsive area, the classical receptive field, and a non-classical receptive field, also called the "suppressive surround." A visual stimulus placed in the suppressive surround does not induce any response but modulates visual responses to stimuli within the classical receptive field, usually by suppressing them. Therefore, visual responses become smaller when stimuli exceed the classical receptive field size. The stimulus size inducing the maximal response is called the preferred stimulus size. In cortex, there is good correspondence between the sizes of the classical receptive field and the preferred stimulus. In contrast, in the rodent superior colliculus, the preferred size is often several fold smaller than the classical receptive field size. Here, we show that in the rat superior colliculus, the preferred stimulus size changes as a square root of the contrast inverse and the classical receptive field size is independent of contrast. In addition, responses to annulus were largely independent of the inner hole size. To explain these data, three models were tested: the divisive modulation of the gain by the suppressive surround (the "normalization" model), the difference of the Gaussians, and a divisive model that incorporates saturation to light flux. Despite the same number of free parameters, the model incorporating saturation to light performed the best. Thus, our data indicate that in rats, the saturation to light can be a dominant phenomenon even at relatively low illumination levels defining visual responses in the collicular neurons.

Effect of monochromatic light on light adaptation and opsin expression in Ectropis grisescens.

Light has a substantial effect on the behaviour and physiology of nocturnal moths. Ectropis grisescens is a major nocturnal tea pest in China, and light traps are commonly used to control geometrid moths because of their positive phototaxis. However, some moths gather around light traps and enter the light adaptation state, which decreases the efficacy of light traps in controlling this pest. We identified opsin genes and the spectral sensitivities of the photoreceptors of E. grisescens moths. We also determined the effects of several monochromatic lights on opsin gene expression and light adaptation. We detected three types of opsin genes and six spectral sensitive peaks (at 370, 390, 480, 530, 550, and 580 nm). We also observed significant changes in the diurnal rhythm of opsin gene expression under different light conditions. When active males were suddenly exposed to different monochromatic lights, they quickly entered the light adaptation state, and the adaptation time was negatively correlated with the light intensity. Males were most sensitive to 390 nm wavelengths, followed by 544 nm, 457 nm, and 593 nm. Red light (627 nm) did not affect the activity of E. grisescens males but had detectable physiological effects.

Distinct mechanisms of light adaptation of elementary responses in photoreceptors of dipteran flies and American cockroach.

Of many light adaptation mechanisms optimizing photoreceptor functioning in the compound eyes of insects, those modifying the single-photon response, the quantum bump (QB), remain least studied. Here, by recording from photoreceptors of the blow fly Protophormia terraenovae, the hover fly Volucella pellucens, and the cockroach Periplaneta americana, we investigated mechanisms of rapid light adaptation by examining how properties of QBs change after light stimulation and multiquantal impulse responses during repetitive stimulation. In P. terraenovae, light stimulation reduced latencies, characteristic durations, and amplitudes of QBs in an intensity- and duration-dependent manner. In P. americana, only QB amplitudes decreased consistently. In both species, time constants of QB parameters' recovery increased with the strength and duration of stimulation, reaching ∼30 s after bright prolonged 10-s pulses. In the blow fly, changes in QB amplitudes during recovery correlated with changes in half-widths but not latencies, suggesting at least two separate mechanisms of light adaptation: acceleration of QB onset by sensitizing transduction channels and acceleration of transduction channel inactivation causing QB shortening and decrease. In the cockroach, light adaptation reduced QB amplitude by apparently lowering the transduction channel availability. Impulse response data in the blow fly and cockroach were consistent with the inferences from the QB recovery experiments. However, in the hover fly V. pellucens, impulse response latencies and durations decreased simultaneously, whereas amplitudes decreased little, even when bright flashes were applied at high frequencies. These findings indicate the existence of dissimilar mechanisms of light adaptation in the microvilli of different species.NEW & NOTEWORTHY By studying light adaptation of elementary responses in photoreceptors of the blow fly and the cockroach we found three distinct mechanisms. In the blow fly, one mechanism speeds quantum bump onset and another accelerates quantum bump inactivation, decreasing its size. In the cockroach, quantum bump amplitude decreases without changes in kinetics, indicating decreased availability of transduction channels. The findings can be explained by expression of different transduction channels in the flies and cockroaches.

Phosphatic metabolism in dark- and light-adapted rat retinas.

This study defines retinal phosphatic metabolites and their adjustment to illumination in rat retinas under conditions that preserve retinal function. Metabolic data are measured using high-performance liquid chromatography (HPLC) and 31P nuclear magnetic resonance (31P NMR) spectroscopy after 10 min of light exposure in vivo compared with retinas from dark-adapted rats. Multiple high-energy and low-energy phosphatic metabolites of intermediary metabolism were quantified. The concentration of the high-energy phosphate adenosine triphosphate (ATP) remained unchanged from dark- to light-adaptation. Under the same conditions the concentrations of the high-energy phosphates guanosine triphosphate (GTP) and creatine phosphate increased, whereas the inorganic phosphate decreased. Comparing dark-adapted controls with retinas light-adapted either in vitro or in vivo, the evidence is consistent with a light-dependent increase in GTP and a decrease in cyclic guanosine monophosphate. Although cyclic adenosine monophosphate (cAMP) levels were lower in retinas light-adapted in vivo than in the dark-adapted controls, this did not seem to be an effect of light, as cAMP levels decreased similarly after 10 min incubation in dark or light in parallel with recovery of ATP/adenosine diphosphate ratios. This study: (1) reports on retinal metabolic changes with adjustment in illumination, (2) provides baseline measurements of retinal phosphatic metabolites in whole retinas, and (3) reports on the validity of chromatographic and spectroscopic methods used for studying retinal metabolism establishing a high correlation among measurements made using HPLC and 31P NMR.

Effect of Temperature and Light Intensity on the Polar Lipidome of Endophytic Brown Algae Streblonema corymbiferum and Streblonema sp. In Vitro.

The effect of temperature and light intensity on the polar lipidome of endophytic brown algae Streblonema corymbiferum and Streblonema sp. in vitro was investigated. More than 460 molecular species have been identified in four glycoglycerolipids classes, five phosphoglycerolipids classes and one betaine lipid class. The lipids glucuronosyldiacylglycerol and diacylglyceryl-N,N,N-trimethyl-homoserine were found in the algae of the order Ectocarpales for the first time. A decrease in cultivation temperature led to an increase in the unsaturation level in all classes of polar lipids. Thus, at low temperatures, the content of 18:4/18:4 monogalactosyldiacylglycerol (MGDG), 20:5/18:4 digalactosyldiacylglycerol (DGDG), 18:3/16:0 sulfoquinovosyldiacylglycerol (SQDG), 18:3/18:3 and 18:3/18:4 phosphatidylglycerol (PG), 20:4/20:5 and 20:5/20:5 phosphatidylethanolamine (PE), 14:0/20:5, 16:0/20:5 and 20:5/20:5 phosphatidylcholine (PC), 20:5/20:4 phosphatidylhydroxyethylglycine and 18:1/18:2 DGTS increased. At high temperatures, an increase in the content of chloroplast-derived MGDG, DGDG and PG was observed. Both low and high light intensities caused an increase in 20:5/18:3 MGDG and 18:3/16:1 PG. At low light intensity, the content of DGDG with fatty acid (FA) 18:3 increased, and at high light intensity, it was with FA 20:5. The molecular species composition of extraplastid lipids also showed a dependence on light intensity. Thus, the content of PC and PE species with C20-polyunsaturated FA at both sn-positions, 18:1/18:1 DGTS and 16:0/18:1 phosphatidylinositol increased. Low light intensity induced a significant increase in the content of chloroplast-derived 18:1/16:1 phosphatidylethanolamine.

Multimeric and monomeric photosystem II supercomplexes represent structural adaptations to low- and high-light conditions.

An intriguing molecular architecture called the "semi-crystalline photosystem II (PSII) array" has been observed in the thylakoid membranes in vascular plants. It is an array of PSII-light-harvesting complex II (LHCII) supercomplexes that only appears in low light, but its functional role has not been clarified. Here, we identified PSII-LHCII supercomplexes in their monomeric and multimeric forms in low light-acclimated spinach leaves and prepared them using sucrose-density gradient ultracentrifugation in the presence of amphipol A8-35. When the leaves were acclimated to high light, only the monomeric forms were present, suggesting that the multimeric forms represent a structural adaptation to low light and that disaggregation of the PSII-LHCII supercomplex represents an adaptation to high light. Single-particle EM revealed that the multimeric PSII-LHCII supercomplexes are composed of two ("megacomplex") or three ("arraycomplex") units of PSII-LHCII supercomplexes, which likely constitute a fraction of the semi-crystalline PSII array. Further characterization with fluorescence analysis revealed that multimeric forms have a higher light-harvesting capability but a lower thermal dissipation capability than the monomeric form. These findings suggest that the configurational conversion of PSII-LHCII supercomplexes may serve as a structural basis for acclimation of plants to environmental light.

Dopamine D1 and D4 receptors contribute to light adaptation in ON-sustained retinal ganglion cells.

The adaptation of ganglion cells to increasing light levels is a crucial property of the retina. The retina must respond to light intensities that vary by 10-12 orders of magnitude, but the dynamic range of ganglion cell responses covers only ∼3 orders of magnitude. Dopamine is a crucial neuromodulator for light adaptation and activates receptors in the D1 and D2 families. Dopamine type D1 receptors (D1Rs) are expressed on horizontal cells and some bipolar, amacrine, and ganglion cells. In the D2 family, D2Rs are expressed on dopaminergic amacrine cells and D4Rs are primarily expressed on photoreceptors. However, the roles of activating these receptors to modulate the synaptic properties of the inputs to ganglion cells are not yet clear. Here, we used single-cell retinal patch-clamp recordings from the mouse retina to determine how activating D1Rs and D4Rs changed the light-evoked and spontaneous excitatory inputs to ON-sustained (ON-s) ganglion cells. We found that both D1R and D4R activation decrease the light-evoked excitatory inputs to ON-s ganglion cells, but that only the sum of the peak response decrease due to activating the two receptors was similar to the effect of light adaptation to a rod-saturating background. The largest effects on spontaneous excitatory activity of both D1R and D4R agonists was on the frequency of events, suggesting that both D1Rs and D4Rs are acting upstream of the ganglion cells.NEW & NOTEWORTHY Dopamine by bright light conditions allows retinal neurons to reduce sensitivity to adapt to bright light conditions. It is not clear how and why dopamine receptors modulate retinal ganglion cell signaling. We found that both D1 and D4 dopamine receptors in photoreceptors and inner retinal neurons contribute significantly to the reduction in sensitivity of ganglion cells with light adaptation. However, light adaptation also requires dopamine-independent mechanisms that could reflect inherent sensitivity changes in photoreceptors.

Morphological and electrophysiological specializations of photoreceptors in the love spot of hover fly Volucella pellucens.

Studies of functional variability in the compound eyes of flies reveal superior temporal resolution of photoreceptors from the frontal areas that mediate binocular vision, and in males mate recognition and pursuit. However, the mechanisms underlying differences in performance are not known. Here, we investigated properties of hover fly Volucella pellucens photoreceptors from two regions of the retina, the frontal-dorsal "love spot" and the lateral one. Morphologically, the microvilli of the frontal-dorsal photoreceptors were relatively few in number per rhabdomere cross-section, short and narrow. In electrophysiological experiments involving stimulation with prolonged white-noise and natural time intensity series, frontal-dorsal photoreceptors demonstrated comparatively high corner frequencies and information rates. Investigation of possible mechanisms responsible for their superior performance revealed significant differences in the properties of quantum bumps, and, unexpectedly, relatively high absolute sensitivity of the frontal-dorsal photoreceptors. Analysis of light adaptation indicated that photoreceptors from two regions adapt similarly but because frontal-dorsal photoreceptors were depolarized much stronger by the same stimuli than the lateral photoreceptors, they reached a deeper state of adaptation associated with higher corner frequencies of light response. Recordings from the photoreceptor axons were characterized by spike-like events that can significantly expand the frequency response range. Seamless integration of spikes into the graded voltage responses was enabled by light adaptation mechanisms that accelerate kinetics and decrease duration of depolarizing light response transients.

Pseudorandom full-field electroretinograms reflect different light adaptation mechanisms.

PURPOSE: To investigate the magnitude and time course of pseudorandom ffERG during light adaptation. METHODS: Ten healthy subjects (26 ± 10.1 years) underwent 20 min of dark adaptation, and then the ffERG was evoked by pseudorandom flash sequences (4 ms per flash, 3 cd.s/m2) driven by m-sequences (210-1 stimulus steps) using Veris Science software and a Ganzfeld dome over a constant field of light adaptation (30 cd/m2). The base period of the m-sequence was 50 ms. Each stimulation sequence lasting 40 s was repeated at 0, 5, 10, 15 and 20 min of light adaptation. Relative amplitude and latency (corrected by values found at 0 min) of the three components (N1, P1, and N2) of first-order (K1) and first slice of the second-order (K2.1) kernel at 5 time points were evaluated. An exponential model was fitted to the mean amplitude and latency data as a function of the light adaptation duration to estimate the time course (τ) of the light adaptation for each component. Repeated one-way ANOVA followed by Tukey post-test was applied to the amplitude and latency data, considering significant values of p < 0.05. RESULTS: Regarding the K1 ffERG, N1 K1, P1 K1, and N2 K1 presented an amplitude increase as a function of the light adaptation (N1 K1 τ value = 2.66 min ± 4.2; P1 K1 τ value = 2.69 min ± 2.10; and N2 K1 τ value = 3.49 min ± 2.96). P1 K1 and N2 K1 implicit time changed as a function of the light adaptation duration (P1 K1 τ value = 3.61 min ± 5.2; N2 K1 τ value = 3.25 min ± 4.8). N1 K1 had small implicit time changes during the light adaptation. All the K2,1 components also had nonsignificant changes in amplitude and implicit time during the light adaptation. CONCLUSIONS: Pseudorandom ffERGs showed different mechanisms of adaptation to retinal light. Our results suggest that K1 ffERG is generated by retinal mechanisms with intermediate- to long-term light adaptation, while K2.1 ffERG is generated by retinal mechanism with fast light adaptation course.

Threshold versus intensity functions in two-colour automated perimetry.

PURPOSE: Two-colour computerised perimetry is a technique developed for assessing cone- and rod-function at fixed background luminances in retinal disease. However, the state of adaptation during testing is unknown but crucial in the interpretation of results. We therefore aimed to determine the adaptational state of rod- and cone-mechanisms in two-colour perimetry. METHODS: Sensitivity to 480 nm (blue) and 640 nm (red) Goldmann size V targets was determined for 10 normal subjects aged 16 to 46 years at 17 locations in the central 60 degrees of the visual field under scotopic conditions and then from -1.5 log cd m-2 to 2 log cd m-2 (white background) in 0.5 log unit steps. Data were fitted with threshold versus intensity (tvi) functions of the form logT = logT0 + log ((A + A0 )/A0 )n . RESULTS: No clear rod-cone break was observed for 640 nm stimuli. For 480 nm stimuli, transition from rod-detection to cone-detection occurred at mesopic illumination levels, where rod adaptation approached Weber behaviour. Cone detection mechanisms did not display Weber-like adaptation until the background luminance approached 1 log cd.m-2 . Diseases resulting in a "filter effect" - including disorders of the photoreceptors - are therefore predicted to affect sensitivity when rod function is probed with short-wavelength targets under scotopic conditions, but less so under mesopic conditions. Filter effects are similarly anticipated to affect cone function measured using long-wavelength targets under mesopic conditions (e.g., during microperimetry), but less so under photopic conditions. CONCLUSIONS: Asymmetries in adaptation in automated two-colour perimetry are predicted to artefactually favour the detection of losses in rod sensitivity under scotopic conditions and cones under mesopic conditions.

Responses of leaf morphology, NSCs contents and C:N:P stoichiometry of Cunninghamia lanceolata and Schima superba to shading.

BACKGROUND: The non-structural carbohydrates (NSCs), carbon (C), nitrogen (N), and phosphorus (P) are important energy source or nutrients for all plant growth and metabolism. To persist in shaded understory, saplings have to maintain the dynamic balance of carbon and nutrients, such as leaf NSCs, C, N and P. To improve understanding of the nutrient utilization strategies between shade-tolerant and shade-intolerant species, we therefore compared the leaf NSCs, C, N, P in response to shade between seedlings of shade-tolerant Schima superba and shade-intolerant Cunninghamia lanceolate. Shading treatments were created with five levels (0, 40, 60, 85, 95% shading degree) to determine the effect of shade on leaf NSCs contents and C:N:P stoichiometry characteristics. RESULTS: Mean leaf area was significantly larger under 60% shading degree for C. lanceolata while maximum mean leaf area was observed under 85% shading degree for S. superba seedlings, whereas leaf mass per area decreased consistently with increasing shading degree in both species. In general, both species showed decreasing NSC, soluble sugar and starch contents with increasing shading degree. However shade-tolerant S. superba seedlings exhibited higher NSC, soluble sugar and starch content than shade-intolerant C. lanceolate. The soluble sugar/starch ratio of C. lanceolate decreased with increasing shading degree, whereas that of S. superb remained stable. Leaf C:N ratio decreased while N:P ratio increased with increasing shading degree; leaf C:P ratio was highest in 60% shading degree for C. lanceolata and in 40% shading degree for S. superba. CONCLUSION: S. superba is better adapted to low light condition than C. lanceolata through enlarged leaf area and increased carbohydrate reserves that allow the plant to cope with low light stress. From mixed plantation viewpoint, it would be advisable to plant S. superba later once the canopy of C. lanceolata is well developed but allowing enough sunlight.

Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities.

Fucoxanthin-chlorophyll (Chl) a/c-binding proteins (FCPs) are light-harvesting pigment-protein complexes found in diatoms and brown algae. Due to the characteristic pigments, such as fucoxanthin and Chl c, FCPs can capture light energy in blue-to green regions. A pennate diatom Phaeodactylum tricornutum synthesizes a red-shifted form of FCP under weak or red light, extending a light-absorption ability to longer wavelengths. In the present study, we examined changes in light-harvesting and energy-transfer processes of P. tricornutum cells grown under white- and single-colored light-emitting diodes (LEDs). The red-shifted FCP appears in the cells grown under the green, yellow, and red LEDs, and exhibited a fluorescence peak around 714 nm. Additional energy-transfer pathways are established in the red-shifted FCP; two forms (F713 and F718) of low-energy Chl a work as energy traps at 77 K. Averaged fluorescence lifetimes are prolonged in the cells grown under the yellow and red LEDs, whereas they are shortened in the blue-LED-grown cells. Based on these results, we discussed the light-adaptation machinery of P. tricornutum cells involved in the red-shifted FCP.

Adaptation of light-harvesting and energy-transfer processes of a diatom Chaetoceros gracilis to different light qualities.

Diatoms are a major group of microalgae in marine and freshwater environments. To utilize the light energy in blue to green region, diatoms possess unique antenna pigment-protein complexes, fucoxanthin chlorophyll a/c-binding proteins (FCPs). Depending on light qualities and quantities, diatoms form FCPs with different energies: normal-type and red-shifted FCPs. In the present study, we examined changes in light-harvesting and energy-transfer processes of a diatom Chaetoceros gracilis cells grown using white- and single-colored light-emitting diodes (LEDs), by means of time-resolved fluorescence spectroscopy. The blue LED, which is harvested by FCPs, modified energy transfer involving CP47, and suppressed energy transfer to PSI. Under the red-LED conditions, which is absorbed by both FCPs and PSs, energy transfer to PSI was enhanced, and the red-shifted FCP appeared. The red-shifted FCP was also recognized under the green- and yellow-LEDs, suggesting that lack of the shorter-wavelength light induces the red-shifted FCP. Functions of the red-shifted FCPs are discussed.

Regulation of photosystem I-light-harvesting complex I from a red alga Cyanidioschyzon merolae in response to light intensities.

Photosynthetic organisms use different means to regulate their photosynthetic activity in respond to different light conditions under which they grow. In this study, we analyzed changes in the photosystem I (PSI) light-harvesting complex I (LHCI) supercomplex from a red alga Cyanidioschyzon merolae, upon growing under three different light intensities, low light (LL), medium light (ML), and high light (HL). The results showed that the red algal PSI-LHCI is separated into two bands on blue-native PAGE, which are designated PSI-LHCI-A and PSI-LHCI-B, respectively, from cells grown under LL and ML. The former has a higher molecular weight and binds more Lhcr subunits than the latter. They are considered to correspond to the two types of PSI-LHCI identified by cryo-electron microscopic analysis recently, namely, the former with five Lhcrs and the latter with three Lhcrs. The amount of PSI-LHCI-A is higher in the LL-grown cells than that in the ML-grown cells. In the HL-grown cells, PSI-LHCI-A completely disappeared and only PSI-LHCI-B was observed. Furthermore, PSI core complexes without Lhcr attached also appeared in the HL cells. Fluorescence decay kinetics measurement showed that Lhcrs are functionally connected with the PSI core in both PSI-LHCI-A and PSI-LHCI-B obtained from LL and ML cells; however, Lhcrs in the PSI-LHCI-B fraction from the HL cells are not coupled with the PSI core. These results indicate that the red algal PSI not only regulates its antenna size but also adjusts the functional connection of Lhcrs with the PSI core in response to different light intensities.

Early diabetes impairs ON sustained ganglion cell light responses and adaptation without cell death or dopamine insensitivity.

Retinal signaling under dark-adapted conditions is perturbed during early diabetes. Additionally, dopamine, the main neuromodulator of retinal light adaptation, is diminished in diabetic retinas. However, it is not known if this dopamine deficiency changes how the retina responds to increased light or dopamine. Here we determine whether light adaptation is impaired in the diabetic retina, and investigate potential mechanism(s) of impairment. Diabetes was induced in C57BL/6J male mice via 3 intraperitoneal injections of streptozotocin (75 mg/kg) and confirmed by blood glucose levels more than 200 mg/dL. After 6 weeks, whole-cell recordings of light-evoked and spontaneous inhibitory postsynaptic currents (IPSCs) or excitatory postsynaptic currents (EPSCs) were made from rod bipolar cells and ON sustained ganglion cells, respectively. Light responses were recorded before and after D1 receptor (D1R) activation (SKF-38393, 20 µM) or light adaptation (background of 950 photons·µm-2 ·s-1). Retinal whole mounts were stained for either tyrosine hydroxylase and activated caspase-3 or GAD65/67, GlyT1 and RBPMS and imaged. D1R activation and light adaptation both decreased inhibition, but the disinhibition was not different between control and diabetic rod bipolar cells. However, diabetic ganglion cell light-evoked EPSCs were increased in the dark and showed reduced light adaptation. No differences were found in light adaptation of spontaneous EPSC parameters, suggesting upstream changes. No changes in cell density were found for dopaminergic, glycinergic or GABAergic amacrine cells, or ganglion cells. Thus, in early diabetes, ON sustained ganglion cells receive excessive excitation under dark- and light-adapted conditions. Our results show that this is not attributable to loss in number or dopamine sensitivity of inhibitory amacrine cells or loss of dopaminergic amacrine cells.

A method for quantifying pigment position in retinal pigment epithelium.

In wildtype mice, the pigment granules in the retinal pigment epithelium aggregate in the dark towards Bruch's membrane and disperse towards the photoreceptors in the light. We have developed a repeatable method amenable for quantifying pigment position in the RPE from wild type mice by estimating the population density of pigment granules, or pigment density, within 4 µm2 areas in the basal part of cells examined by transmission electron microscopy. To measure pigment position, 2 µm × 2 µm squares were aligned along the apical ends of the basal microvilli. The pigment granules within each 4 µm2 area were counted, and the average pigment density was calculated for each mouse. The average pigment density for light-adapted mice (n = 3 mice) was 1.3 pigment granules/µm2 (± 0.2 pigment granules/µm2). For dark-adapted wildtype mice (n = 3 mice), pigment density was 1.9 pigment granules/µm2 (± 0.3 pigment granules/µm2). Pigment density was statistically significantly different (p < 0.02) between light-adapted and dark-adapted mice, with pigment density higher in the dark-adapted mice. This method was implemented by four observers and their results were compared. No statistically significant differences were found in the measurements acquired by the different observers, illustrating the repeatability of the method.

Light-induced chloroplast movements in Oryza species.

Light-induced chloroplast movements control efficient light utilization in leaves, and thus, are essential for leaf photosynthesis and biomass production under fluctuating light conditions. Chloroplast movements have been intensively analyzed using wild-type and mutant plants of Arabidopsis thaliana. The molecular mechanism and the contribution to biomass production were elucidated. However, the knowledge of chloroplast movements is very scarce in other plant species, especially grass species including crop plants. Because chloroplast movements are efficient strategy to optimize light capture in leaves and thus promote leaf photosynthesis and biomass, analysis of chloroplast movements in crops is required for biomass production. Here, we analyzed chloroplast movements in a wide range of cultivated and wild species of genus Oryza. All examined Oryza species showed the blue-light-induced chloroplast movements. However, O. sativa and its ancestral species O. rufipogon, both of which are AA-genome species and usually grown in open condition where plants are exposed to full sunlight, showed the much weaker chloroplast movements than Oryza species that are usually grown under shade or semi-shade conditions, including O. officinalis, O. eichingeri, and O. granulata. Further detailed analyses of different O. officinalis accessions, including sun, semi-shade, and shade accessions, indicated that the difference in chloroplast movement strength between domesticated rice plants and wild species might result from the difference in habitat, and the shape of mesophyll chlorenchyma cells. The findings of this study provide useful information for optimizing Oryza growth conditions, and lay the groundwork for improving growth and yield in staple food crop Oryza sativa.