Low-light Environments Research Articles

Taxonomy and toxin production of Gambierdiscus carpenteri (Dinophyceae) in a tropical marine ecosystem: The first record from the Philippines

Morphological and phylogenetic analysis showed that the Gambierdiscus isolate from Bolinao, Philippines belongs to the species of G. carpenteri. It was morphologically more similar to the Merimbula strain than the subtropical Florida Keys strain. Growth and toxin production were also investigated at varying levels of temperature, salinity, and irradiance. Gambierdiscus are known to grow favorably in a low light environment. However, this study showed high growth rates of G. carpenteri even at high irradiance levels. Generally, cells produced more toxins at lower treatment levels. Highest cellular toxin content recorded was 7.48 ± 0.49 pg Pbtx eq/cell at culture conditions of 25 °C, 100 μmol photons m−2 s−1, and salinity of 26. Growth rate and toxin production data suggest that cells produced more toxins during the slowest growth at certain range of treatments. This information gives insight into how changes in environmental conditions may affect toxin production and growth of G. carpenteri.

Breeding new varieties for controlled environments

Agricultural production in controlled environments is increasingly feasible, and may play an important role in providing nutrition and choice to growing urban centres. New technologies in lighting, ventilation, robotics and irrigation are just a few of the innovations that enable production of high-value specialty crops outside of a traditional field setting. However, despite all of the advances in the hardware within the plant factory operation, innovation of the most complex machine has been neglected - the plant itself. Indoor agricultural operations typically rely on legacy varieties, plants selected and bred for field conditions. In the field, phenotypic stability is paramount, as production must be consistent in an unpredictable and changing environment. However, the controlled environment affords focus on different breeding priorities as environmental flux, pests, pathogens and post-harvest quality are less formidable barriers to production. On the contrary, breeding for controlled environments shifts the focus to a completely different set of plant traits, such as rapid growth, performance in low light environments and active manipulation of plant stature. Instead of breeding for phenotypic stability, plants may be bred to maximise genetic plasticity, allowing specific traits to be presented as a function of the quality of the ambient light spectrum. In this scenario plant varieties may be grown with optimal size, supporting a focus on consumer traits like flavour or accumulation of health-related compounds. Gene editing may be a central technology in the production of designer plants for controlled environments. This review considers the opportunity for breeding for controlled environments, with a focus on a revision of priorities for controlled-environment breeders.

Nitrogen-improved photosynthesis quantum yield is driven by increased thylakoid density, enhancing green light absorption

A nitrogen supply is necessary for all plants. The multifaceted reasons why this nutrient stimulates plant dry weight accumulation are assessed herein. We compared tomato plants grown in full sunlight and in low light environments under four N doses and evaluated plant growth, photosynthetic and calorimetric parameters, leaf anatomy, chloroplast transmission electron microscopy (TEM) and a high resolution profile of optical leaf properties. Increases in N supplies allow tomato plants to grow faster in low light environments (91.5% shading), displaying a robust light harvesting machinery and, consequently, improved light harvesting efficiency. Ultrastructurally, high N doses were associated to a high number of grana per chloroplast and greater thylakoid stacking, as well as high electrodensity by TEM. Robust photosynthetic machinery improves green light absorption, but not blue or red. In addition, low construction and dark respiration costs were related to improved total dry weight accumulation in shade conditions. By applying multivariate analyses, we conclude that improved green light absorbance, improved quantum yield and greater palisade parenchyma cell area are the primary components that drive increased plant growth under natural light-limited photosynthesis.

Acclimation responses of macaw palm seedlings to contrasting light environments

The photosynthetic adjustments of macaw palm (Acrocomia aculeata) were evaluated in 30-day-old seedlings exposed to high and low light environments, and sudden transference from low to high light and comparisons were made with the hardening protocol used in nurseries. Furthermore, we evaluated the responses to long-term exposure (265 days) to high and low light environments. Macaw palm seedlings exhibited an efficient mechanism that maximized light capture under scarce conditions, and dissipated excess energy to avoid damaging to the photosystem II under high light. The seedlings showed low saturation irradiance but no photoinhibition when exposed to excess light. When grown under low light intensities, seedlings presented higher photochemical efficiency and minimized the respiratory costs with positive carbon balance at lower irradiance than hardened seedlings did. The hardening procedure did not appear to be an advantageous method during seedling production. Long-term exposure to either low or high light did not cause significant leaf anatomical adjustments. However, the low light seedlings showed higher leaf area and chlorophyll content than those exposed to higher light intensity did, which enabled shaded seedlings to maximize the captured light. Furthermore, the high non-photochemical dissipation allowed rapid acclimation to excessive light exposure. These responses allow macaw palm cultivation and establishment in very different light environments.

Deep Refinement Network for Natural Low-Light Image Enhancement in Symmetric Pathways

Due to the cost limitation of camera sensors, images captured in low-light environments often suffer from low contrast and multiple types of noise. A number of algorithms have been proposed to improve contrast and suppress noise in the input low-light images. In this paper, a deep refinement network, LL-RefineNet, is built to learn from the synthetical dark and noisy training images, and perform image enhancement for natural low-light images in symmetric—forward and backward—pathways. The proposed network utilizes all the useful information from the down-sampling path to produce the high-resolution enhancement result, where global features captured from deeper layers are gradually refined using local features generated by earlier convolutions. We further design the training loss for mixed noise reduction. The experimental results show that the proposed LL-RefineNet outperforms the comparative methods both qualitatively and quantitatively with fast processing speed on both synthetic and natural low-light image datasets.

Responses of foliar carbohydrates and nutrient status of two distinctive cypress species to shading and nitrogen addition

Externally environmental stresses (e.g., low light environment and nutrient shortage) could limit plant growth and recruitment, even causing a species to be rare or endangered. In the present study, we aimed to elucidate whether shading environment formed in the natural forests limited growth of an endangered cypress species, Thuja sutchuenensis, by artificial shading level associated with nitrogen treatment, by compared with a widely distributed species from the same family, Platycladus orientalis. Our results showed that though leaf non-structural carbohydrates decreased with increasing shading level, T.sutchuenensis kept stable or even slight increasing relative growth rate toward lower light environments, which was opposite to the shade-intolerant species, P. orientalis. Our results did not provide evidence for a congruent suite of traits associated with rarity in T.sutchuenensis, but proved it as a shade-tolerant species and had to some extent practical significance for resource management for the targeted species.

Effects of variability in daily light integrals on the photophysiology of the corals Pachyseris speciosa and Acropora millepora.

Phototrophic sessile organisms, such as reef corals, adjust their photosynthetic apparatus to optimize the balance of light capture versus protection in response to variable light availability (photoacclimation). In shallow marine environments, daily light integrals (DLI) can vary several-fold in response to water clarity and clouds. This laboratory study investigated the responses of two coral species to fluctuations in DLI. Corals were exposed to four contrasting DLI treatments: ‘high-light’ (potentially photoinhibiting conditions, 32 mol photons m-2 d-1), ‘low-light’ (potentially light-limiting conditions, 6 mol photons m-2 d-1), and two ‘variable light’ treatments that alternated between high and low conditions every 5 days. In the variable treatments, the shade-tolerant coral Pachyseris speciosa displayed cycles of rapid declines in maximum quantum yield during high-light and subsequent recoveries during low-light, showing photoacclimation at a time scale of 3–5 days. In contrast, the shallow-water coral Acropora millepora showed slow (>20 days) photoacclimation, and minimal changes in photosynthetic yields despite contrasting light exposure. However, growth (change in buoyant weight) in A. millepora was significantly slower under variable light, and even more so under low-light conditions, compared with high-light conditions. The responses of yields in P. speciosa match their preference for low-light environments, but suggest a vulnerability to even short periods of high-light exposure. In contrast, A. millepora had better tolerance of high-light conditions, however its slow photoacclimatory responses limit its growth under low and variable conditions. The study shows contrasting photoacclimatory responses in variable light environments, which is important to identify and understand as many coastal and midshelf reefs are becoming increasingly more turbid, and may experience higher variability in light availability.

Noncontact heart rate measurement using a high-sensitivity camera in a low-light environment

We propose a method for the remote estimation of the heart rate and heart rate variability spectrogram by analyzing the hemoglobin concentration obtained from RGB facial videos taken in a low-light environment. The monitoring of emotion has potential in areas such as market research, safety, and health. In particular, methods of analyzing the heart rate obtained from RGB video are expected to be used practically. However, these studies cannot be applied in dark locations where monitoring is necessary, such as an infant’s bedroom, a crime-prone road, and within a car. The proposed method, therefore, uses a highly sensitivity RGB camera capable of capturing videos at low illuminance. As the result, we could measure the heart rate with accuracy exceeding 99% and estimate the heart rate variability spectrogram with high accuracy for low-light environments of 10 lx, which corresponds to brightness levels of the monitoring environments given above.

Novel Technique for Rapid and Accurate Insertion of a Microguidewire Tail Into Low-Profile Devices During Endovascular Procedures: The Paper Rail Method

Purpose: To report a novel technique (“paper rail”) to facilitate inserting the tail of a microguidewire into the tip of a low-profile device during endovascular procedures. Technique: A sterilized nonwoven fabric tape with a smooth glossy paper backing is used. The tape has several linear folds ideal for a paper rail. Holding each piece of equipment about 5 cm from its respective tip, both the tail of the guidewire and the tip of the catheter are navigated at a 30° angle toward each other in the crease until the guidewire enters the catheter. The paper rail technique was compared with the conventional freehand method under varying luminosities found in an operating room. The paper rail technique was most effective in suboptimal lighting, where the mean time was reduced from 83 seconds with the conventional method to 20 seconds with the paper rail maneuver. The times required to insert the wire with the paper rail method were comparable (~22 seconds) at all light levels. Conclusion: The paper rail method may help improve the speed and accurate insertion of the tail of a microguidewire into the tip of low-profile devices during endovascular procedures. It may be particularly useful for physicians in a low-light environment or trainees.

Mechanical Hedge Pruning Affects Nut Size, Nut Quality, Wind Damage, and Stem Water Potential of Pecan in Humid Conditions

Recent extensive pecan [Carya illinoinensis (Wangenh.) K. Koch] plantings coincided with a shift toward the planting of pecan trees at higher density by Georgia pecan producers in anticipation of maintaining these densities through hedge pruning. Initial studies of mechanical hedge pruning in the low-light environment of the southeastern United States have failed to show significant benefits to pecan production. The objectives of this study were to compare the effects of hedge pruning on pecan nut quality, yield, and midday stem water potential (ψ) of pecan trees in the temperate climate of the southeastern United States and to evaluate the effect of hedge pruning on windstorm damage to pecan trees. Treatments were arranged in a randomized complete block design with three blocks. Two treatments were evaluated; 1) Hedge-pruned; 2) Nonhedge-pruned (control). Midday stem ψ was 8.5%, 17.6%, and 16.6% higher (P ≤ 0.05), indicating less water stress, on hedged trees than on nonhedged trees during 2015, 2016, and 2017, respectively. Nut weight and percent kernel were increased (P ≤ 0.05) by hedge pruning 2 of 3 years of the study. Although no direct positive effect of hedge pruning on in-shell nut yield was observed, hedge pruning was not detrimental to pecan yield in the short term. Hurricane/Tropical Storm Irma brought damaging winds to the entire pecan-producing region of Georgia on 11 Sept. 2017, resulting in blown down trees, broken branches, and immature nuts blown from the trees. Hedged trees had 60% less wind damage in the form of major limb breakage and tree loss than did nonhedged trees.

Canopy openness and topographic habitat drive tree seedling recruitment after snow damage in an old-growth subtropical forest

We investigated the relative and combined effect of topography and light environment on the recruitment of seedlings in a subtropical forest after snow damage to the canopy. The tree seedling community in an old-growth subtropical forest was monitored using 500 2 m × 2 m seedling plots at six-month intervals for 2 years. With a focus on recruitment following canopy damage, we related abiotic and biotic environmental variables to seedling dynamics, and we tested if significant topographic and light habitat associations were present for seedlings recruiting via a torus translation test. Then, we used variance partitioning to examine the relative effects of spatial, topographic and light variables on the temporal assemblages of seedlings. A total of 3047 seedlings from 58 species recruited in the first 2 years following snow damage. At the community level, increases in seedling abundance and richness were positively correlated with canopy openness and negatively correlated with elevation. At the species level, both pioneer and late-successional tree species had more recruits in high light environment than in low light environment. 84.3% of the recruiting seedlings were significantly associated to either light environment (35.7%), topography (26.5%) or both (22.1%). Despite this, at the plot level, spatial variables (PCNM) explained the majority of the variability in seedling composition over time. Our results suggest that snow damage to the canopy increases species richness and abundance via light-facilitated seedling recruitment, and that the composition of recruiting seedlings was largely spatially dependent. Topographic habitat filtering acts as a persistent force in determining the recruitment of seedlings and increases in strength with increased light-facilitated seedling recruitment. Our results highlight that, in this subtropical forest, both light requirement and topographic specialization interact over time to play a key role in promoting coexistence of tree species through selection of individuals at the seedling stage. We also suggest exploring the possibilities of management intervention to speed up the recovery of this forest.

Indoor Person Identification Using a Low-Power FMCW Radar

Contemporary surveillance systems mainly use video cameras as their primary sensor. However, video cameras possess fundamental deficiencies, such as the inability to handle low-light environments, poor weather conditions, and concealing clothing. In contrast, radar devices are able to sense in pitch-dark environments and to see through walls. In this paper, we investigate the use of micro-Doppler (MD) signatures retrieved from a low-power radar device to identify a set of persons based on their gait characteristics. To that end, we propose a robust feature learning approach based on deep convolutional neural networks. Given that we aim at providing a solution for a real-world problem, people are allowed to walk around freely in two different rooms. In this setting, the IDentification with Radar data data set is constructed and published, consisting of 150 min of annotated MD data equally spread over five targets. Through experiments, we investigate the effectiveness of both the Doppler and time dimension, showing that our approach achieves a classification error rate of 24.70% on the validation set and 21.54% on the test set for the five targets used. When experimenting with larger time windows, we are able to further lower the error rate.

Docking at pool and sea by using active marker in turbid and day/night environment

Nowadays, autonomous underwater vehicle (AUV) is playing an important role in human society in different applications such as inspection of underwater structures (dams, bridges). It has been desired to develop AUVs that can work in a sea with a long period of time for the purpose of retrieving methane hydrate, or rare metal, and so on. To achieve such AUVs, the automatic recharging capability of AUVs under the sea is indispensable and it requires AUVs to dock itself to recharging station autonomously. Therefore, we have developed a stereo-vision-based docking methodology for underwater battery recharging to enable the AUV to continue operations without returning surface vehicle for recharging. Since underwater battery recharging units are supposed to be installed in a deep sea, the deep-sea docking experiments cannot avoid turbidity and low-light environment. In this study, the proposed system with a newly designed active—meaning self-lighting—3D marker has been developed to improve the visibility of the marker from an underwater vehicle, especially in turbid water. Experiments to verify the robustness of the proposed docking approach have been conducted in a simulated pool where the lighting conditions change from day to night. Furthermore, sea docking experiment has also been executed to verify the practicality of the active marker. The experimental results have confirmed the effectiveness of the proposed docking system against turbidity and illumination variation.

Altered Expression of the Malate-Permeable Anion Channel OsALMT4 Reduces the Growth of Rice Under Low Radiance.

We examined the function of OsALMT4 in rice (Oryza sativa L.) which is a member of the aluminum-activated malate transporter family. Previous studies showed that OsALMT4 localizes to the plasma membrane and that expression in transgenic rice lines results in a constitutive release of malate from the roots. Here, we show that OsALMT4 is expressed widely in roots, shoots, flowers, and grain but not guard cells. Expression was also affected by ionic and osmotic stress, light and to the hormones ABA, IAA, and salicylic acid. Malate efflux from the transgenic plants over-expressing OsALMT4 was inhibited by niflumate and salicylic acid. Growth of transgenic lines with either increased OsALMT4 expression or reduced expression was measured in different environments. Light intensity caused significant differences in growth between the transgenic lines and controls. When day-time light was reduced from 700 to 300 μmol m-2s-1 independent transgenic lines with either increased or decreased OsALMT4 expression accumulated less biomass compared to their null controls. This response was not associated with differences in photosynthetic capacity, stomatal conductance or sugar concentrations in tissues. We propose that by disrupting malate fluxes across the plasma membrane carbon partitioning and perhaps signaling are affected which compromises growth under low light. We conclude that OsALMT4 is expressed widely in rice and facilitates malate efflux from different cell types. Altering OsALMT4 expression compromises growth in low-light environments.

Shade compromises the photosynthetic efficiency of NADP-ME less than that of PEP-CK and NAD-ME C4 grasses.

The high energy cost and apparently low plasticity of C4 photosynthesis compared with C3 photosynthesis may limit the productivity and distribution of C4 plants in low light (LL) environments. C4 photosynthesis evolved numerous times, but it remains unclear how different biochemical subtypes perform under LL. We grew eight C4 grasses belonging to three biochemical subtypes [NADP-malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME), and phosphoenolpyruvate carboxykinase (PEP-CK)] under shade (16% sunlight) or control (full sunlight) conditions and measured their photosynthetic characteristics at both low and high light. We show for the first time that LL (during measurement or growth) compromised the CO2-concentrating mechanism (CCM) to a greater extent in NAD-ME than in PEP-CK or NADP-ME C4 grasses by virtue of a greater increase in carbon isotope discrimination (∆P) and bundle sheath CO2 leakiness (ϕ), and a greater reduction in photosynthetic quantum yield (Φmax). These responses were partly explained by changes in the ratios of phosphoenolpyruvate carboxylase (PEPC)/initial Rubisco activity and dark respiration/photosynthesis (Rd/A). Shade induced a greater photosynthetic acclimation in NAD-ME than in NADP-ME and PEP-CK species due to a greater Rubisco deactivation. Shade also reduced plant dry mass to a greater extent in NAD-ME and PEP-CK relative to NADP-ME grasses. In conclusion, LL compromised the co-ordination of the C4 and C3 cycles and, hence, the efficiency of the CCM to a greater extent in NAD-ME than in PEP-CK species, while CCM efficiency was less impacted by LL in NADP-ME species. Consequently, NADP-ME species are more efficient at LL, which could explain their agronomic and ecological dominance relative to other C4 grasses.

Ultimate SLAM? Combining Events, Images, and IMU for Robust Visual SLAM in HDR and High-Speed Scenarios

Event cameras are bio-inspired vision sensors that output pixel-level brightness changes instead of standard intensity frames. These cameras do not suffer from motion blur and have a very high dynamic range, which enables them to provide reliable visual information during high speed motions or in scenes characterized by high dynamic range. However, event cameras output only little information when the amount of motion is limited, such as in the case of almost still motion. Conversely, standard cameras provide instant and rich information about the environment most of the time (in low-speed and good lighting scenarios), but they fail severely in case of fast motions, or difficult lighting such as high dynamic range or low light scenes. In this paper, we present the first state estimation pipeline that leverages the complementary advantages of these two sensors by fusing in a tightly-coupled manner events, standard frames, and inertial measurements. We show on the publicly available Event Camera Dataset that our hybrid pipeline leads to an accuracy improvement of 130% over event-only pipelines, and 85% over standard-frames-only visual-inertial systems, while still being computationally tractable. Furthermore, we use our pipeline to demonstrate - to the best of our knowledge - the first autonomous quadrotor flight using an event camera for state estimation, unlocking flight scenarios that were not reachable with traditional visual-inertial odometry, such as low-light environments and high-dynamic range scenes.

Application of a diffusion model to measure ion leakage of resurrection plant leaves undergoing desiccation

Haberlea rhodopensis is a chlorophyll-retaining resurrection plant, which can survive desiccation to air dry state under both low light and sunny environments. Maintaining the integrity of the membrane during dehydration of resurrection plants is extremely important. In the present study, the diffusion model was improved and used for a first time to evaluate the changes in ion leakage through different cellular compartments upon desiccation of H. rhodopensis and to clarify the reasons for significant increase of electrolyte leakage from dry leaves. The applied diffusion approach allowed us to distinguish the performance of plants subjected to dehydration and subsequent rehydration under different light intensities. Well-hydrated (control) shade plants had lower and slower electrolyte leakage compared to control sun plants as revealed by lower values of phase amplitudes, lower rate constants and ion concentration. In well-hydrated and moderately dehydrated plants (50% relative water content, RWC) ion efflux was mainly due to leakage from apoplast. The electrolyte leakage sharply increased in severely desiccated leaves (8% RWC) from both sun and shade plants mainly due to ion efflux from symplast. After 1 day of rehydration the electrolyte leakage was close to control values, indicating fast recovery of plants. We suggest that the enhanced leakage in air-dried leaves should not be considered as damage but rather as a survival mechanism based on a reversible modification in the structure of cell wall, plasma membrane and alterations in vacuolar system of the cells. However, further studies should be conducted to investigate the changes in cell wall/plasma membrane to support this conclusion.

Holobiont Diversity in a Reef-Building Coral over Its Entire Depth Range in the Mesophotic Zone

Mesophotic reef-building coral communities (~30-120 m depth) remain largely unexplored, despite representing roughly three-quarters of the overall depth range at which tropical coral reef ecosystems occur. Although many coral species are restricted to shallow depths, several species occur across large depth ranges, including lower mesophotic depths. Yet, it remains unclear how such species can persist under extreme low-light conditions and how the different symbiotic partners associated with these corals contribute to facilitate such broad depth ranges. We assessed holobiont genetic diversity of the Caribbean coral Agaricia undata over depth in three localities of Colombia: San Andres Island (between 37 and 85 m), Cartagena (between 17 and 45 m) and Parque Nacional Natural Corales de Profundidad (between 77 and 87 m). We used a population genomics approach (NextRAD) for the coral host, and amplicon sequencing for the associated Symbiodinium (non-coding region of the plastid psbA minicircle) and prokaryotic (V4 region of the 16S rRNA gene) symbiont community. For the coral host, genetic structuring was only observed across geographic regions, but not between depths. Bayesian clustering and discriminant analysis of principal components revealed genetic structuring between the three regions, but not between shallow ( 60 m) depths. This pattern was confirmed when evaluating pairwise differentiation (FST) between populations, with much higher values between regions (0.0467-0.1034) compared to between depths [within location; -0.0075-(-0.0007)]. Symbiotic partners, including seven types of zooxanthellae and 325 prokaryotic OTUs, did not exhibit partitioning across depths. All samples hosted Symbiodinium clade C3 and the type C3psbA_e was present in all depths. Alpha microbial diversity was not significantly different between zones (upper vs. lower), which community composition between coral colonies was similar in the two zones (ANOSIM, R = -0.079, P > 0.05). The coral microbiome was dominated by Uncultured Betaproteobacteria in the order EC94 (16%), Unknown-Bacteria (15%), family Cenarchaeaceae (12 %), Burkholderiaceae (10%), and Hahellaceae (10%). The constant coral-holobiont composition along the studied depth range suggests that identity of the symbionts is not responsible for the coral's broad depth range and adaptation to low light environments.

Trade-Offs Associated with Photoprotective Green Fluorescent Protein Expression as Potential Drivers of Balancing Selection for Color Polymorphism in Reef Corals

Photodamage of symbiotic algae exposed to thermal stress is involved in mass coral bleaching, a major cause of reef decline. Photoprotection is therefore a vital part of coral stress physiology. Corals produce a variety of green fluorescent protein (GFP)-like proteins from which some representatives screen the symbiotic algae from excess light. Different tissue concentrations of these GFP-like proteins distinguish colour morphs that are characteristic for many coral species. The question arises whether these pigmentation differences may diversify the niches that can be occupied by corals along the steep light gradient that structures coral reef communities. We assessed the implications of GFP-like protein expression in two color morphs of the symbiotic coral Hydnophora grandis, both associated with the same Symbiodinium sp. (subclade C40). The colour morphs of this species (high fluorescent, HF; and low fluorescent, LF), characterized by markedly different contents of a cyan fluorescent protein, were exposed to different quantities of blue light (470nm) that matched the major absorption band of the host pigment (473nm). High intensities of blue light caused less photodamage to the symbiotic algae of the HF morph and resulted in higher growth rates of these corals compared to representatives of the LF morph. In contrast, under low intensities of blue light, the HF morph showed lower growth rates than the LF morph, indicating that trade-offs are associated with high levels of fluorescent protein expression under this condition. Both morphs showed highest growth rates at medium light intensities with no obvious influence of the tissue pigmentation. Reef coral colour polymorphism caused by photoprotective GFP-like proteins may therefore be a product of balancing selection in which high pigment contents may be beneficial at the upper and detrimental at the lower end of the depth distribution range of symbiotic corals. Conversely, colour morphs with GFP-like proteins that function to optimize symbiont photosynthesis in low light environments could gain an advantage from the benefits offered by high pigment levels in deeper waters.

Exploring the Rhodopsin Dimer Interface in Live Cells

In low-light environments, the G protein-coupled receptor (GPCR), rhodopsin, is responsible for detection of light within the retina. Several studies have shown evidence for dimeric, or oligomeric, conformations of rhodopsin within native disc membranes. However, while many oligomeric packing structures have been proposed, the exact dimerization interface of rhodopsin has yet to be firmly establish. Here we investigate rhodopsin dimerization in live cells with fluorescence correlation spectroscopy (FCS) and pulsed-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). One of the proposed dimerization interfaces is between TM4/5 of rhodopsin. Recently we found evidence for a similar interface in red cone opsin. Based on those previous findings, we have made point mutations along rhodopsin's TM 5, and found significant disruption of the dimerization affinity compared to the wild-type construct. Our results provide new chemical details for the previously proposed packing structure between TM 4 and 5 of rhodopsin. Moreover, our findings further support the idea of conserved dimerization interface motifs shared among class A GPCRs. Such a discovery may provide more general conceptualization of self-association among one of the largest classes of transmembrane receptors, which is essential for properly gauging the potential relationship between dimerization and function.