A Dual Strategy to Cope with High Light in Chlamydomonas reinhardtii

Absorption of light in excess of the capacity for photosynthetic electron transport is damaging to photosynthetic organisms. Several mechanisms exist to avoid photodamage, which are collectively referred to as nonphotochemical quenching. This term comprises at least two major processes. State transitions (qT) represent changes in the relative antenna sizes of photosystems II and I. High energy quenching (qE) is the increased thermal dissipation of light energy triggered by lumen acidification. To investigate the respective roles of qE and qT in photoprotection, a mutant (npq4 stt7-9) was generated in Chlamydomonas reinhardtii by crossing the state transition–deficient mutant (stt7-9) with a strain having a largely reduced qE capacity (npq4). The comparative phenotypic analysis of the wild type, single mutants, and double mutants reveals that both state transitions and qE are induced by high light. Moreover, the double mutant exhibits an increased photosensitivity with respect to the single mutants and the wild type. Therefore, we suggest that besides qE, state transitions also play a photoprotective role during high light acclimation of the cells, most likely by decreasing hydrogen peroxide production. These results are discussed in terms of the relative photoprotective benefit related to thermal dissipation of excess light and/or to the physical displacement of antennas from photosystem II.     

Lysosomal Signaling Licenses Embryonic Stem Cell Differentiation via Inactivation of Tfe3

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Evaluation of the social bond: a new method tested in Mus spicilegus

Innovative and fruitful studies of social bonds have been developed in recent years, although the methods used to establish the existence of a social bond between two individuals have not evolved significantly. Two types of paradigms have been currently used: the separation-reunion paradigm, which evaluates the distress caused by the disruption of the social bond, and choice paradigms, which test the specificity of the bond to a given individual. We have developed a new paradigm based on the idea that the cost an individual was ready to pay in order to gain access to a conspecific depended on the strength of the social bond between the two individuals. To test our paradigm we used mound-building mice, Mus spicilegus that present, in both males and females, a level of tolerance that differs greatly according to the degree of familiarity between the individuals. Our new method for testing social bond revealed unsuspected differences between males and females. Our results suggested that, at least in Mus spicilegus, strong social bonds were not necessary to the development of a high level of tolerance between individuals.     

Cell numbers and leaf development in Arabidopsis: a functional analysis of the STRUWWELPETER gene

The struwwelpeter (swp) mutant in Arabidopsis shows reduced cell numbers in all aerial organs. In certain cases, this defect is partially compensated by an increase in final cell size. Although the mutation does not affect cell cycle duration in the young primordia, it does influence the window of cell proliferation, as cell number is reduced during the very early stages of primordium initiation and a precocious arrest of cell proliferation occurs. In addition, the mutation also perturbs the shoot apical meristem (SAM), which becomes gradually disorganized. SWP encodes a protein with similarities to subunits of the Mediator complex, required for RNA polymerase II recruitment at target promoters in response to specific activators. To gain further insight into its function, we overexpressed the gene under the control of a constitutive promoter. This interfered again with the moment of cell cycle arrest in the young leaf. Our results suggest that the levels of SWP, besides their role in pattern formation at the meristem, play an important role in defining the duration of cell proliferation.     

Proteomics of chloroplast envelope membranes

Proteomics is a very powerful approach to link the information contained in sequenced genomes, like Arabidopsis, to the functional knowledge provided by studies of plant cell compartments, such as chloroplast envelope membranes. This review summarizes the present state of proteomic analyses of highly purified spinach and Arabidopsis envelope membranes. Methods targeted towards the hydrophobic core of the envelope allow identifying new proteins, and especially new transport systems. Common features were identified among the known and newly identified putative envelope inner membrane transporters and were used to mine the complete Arabidopsis genome to establish a virtual plastid envelope integral protein database. Arabidopsis envelope membrane proteins were extracted using different methods, that is, chloroform/methanol extraction, alkaline or saline treatments, in order to retrieve as many proteins as possible, from the most to the less hydrophobic ones. Mass spectrometry analyses lead to the identification of more than 100 proteins. More than 50% of the identified proteins have functions known or very likely to be associated with the chloroplast envelope. These proteins are (a) involved in ion and metabolite transport, (b) components of the protein import machinery and (c) involved in chloroplast lipid metabolism. Some soluble proteins, like proteases, proteins involved in carbon metabolism or in responses to oxidative stress, were associated with envelope membranes. Almost one third of the newly identified proteins have no known function. The present stage of the work demonstrates that a combination of different proteomics approaches together with bioinformatics and the use of different biological models indeed provide a better understanding of chloroplast envelope biochemical machinery at the molecular level.     

Paying attention to neurons with discriminating taste

Traditional theories of attention rely on the idea that when we search for a target in a visual display the brain boosts the activity of neurons optimally tuned for the target features. In this issue of Neuron, Navalpakkam and Itti take a computational approach to show that this strategy is actually very inefficient when the target is surrounded by distractors with similar features. Instead, the optimal strategy is to boost the activity of neurons that best discriminate between target and distractors, while essentially ignoring the neurons that respond best to the target.     

Development of a fluid functionalized lipidic matrix applied to direct in situ polynucleotide detection

This work presents a new approach for direct detection of polyelectrolytes at the air-water interface, based on the investigation of the interfacial properties of an active lipidic matrix especially designed for polynucleotide immobilization. A synthetic lipid with a cationic spermine headgroup, DiOctadecylamidoGlycylSpermine (DOGS), was spread at the interface to form a distortable film able to capture polynucleotides. The control of the organization state of this functionalized monolayer upon compression was achieved by recording surface pressure-area (pi-A) isotherm diagrams, presenting a specific shape with a typical liquid expanded-liquid condensed phase transition on a pure water subphase. In the presence of various dsDNA concentrations in the subphase, the isotherms were markedly modified in a time and concentration-dependent manner. The main modifications, corresponding to a large shift towards higher molecular areas and a clear fading of the phase transition, were corroborated by the fine analysis of the monolayer compressibility profile, thus suggesting a characteristic change in the monolayer fluidity as a function of both time and DNA concentration. Moreover, an ATR-Fourier transform infrared (ATR-FTIR) characterization showed evidences for the adsorption of DNA strands onto the lipidic matrix. The direct observation of the mixed monolayer morphology by Brewster angle microscopy (BAM) strongly suggests that DNA adsorption induces a reorganization of lipids at the interface, as evidenced by the change in the condensed lipidic domains morphology in the presence of DNA in the subphase. The immobilization of various polynucleotidic probes of 4000, 400 and 22 base length, confirmed by fluorescence microscopy, had similar effects on DOGS interfacial properties. Preliminary studies are finally presented to explore the possibility of using this system for the study of hybridization between complementary strands. Hence, this study demonstrates this functionalized matrix behaves as a fluid support where polynucleotide immobilization induces interfacial properties modifications, which could be further exploited through the experimental characterization of Faraday instabilities sensitive to visco-elasticity variations.