Interorganelle Communication: Peroxisomal MALATE DEHYDROGENASE2 Connects Lipid Catabolism to Photosynthesis through Redox Coupling in Chlamydomonas

Plants and algae must tightly coordinate photosynthetic electron transport and metabolic activities given that they often face fluctuating light and nutrient conditions. The exchange of metabolites and signaling molecules between organelles is thought to be central to this regulation but evidence for this is still fragmentary. Here, we show that knocking out the peroxisome-located MALATE DEHYDROGENASE2 (MDH2) of Chlamydomonas reinhardtii results in dramatic alterations not only in peroxisomal fatty acid breakdown but also in chloroplast starch metabolism and photosynthesis. mdh2 mutants accumulated 50% more storage lipid and 2-fold more starch than the wild type during nitrogen deprivation. In parallel, mdh2 showed increased photosystem II yield and photosynthetic CO₂ fixation. Metabolite analyses revealed a >60% reduction in malate, together with increased levels of NADPH and H₂O₂ in mdh2 Similar phenotypes were found upon high light exposure. Furthermore, based on the lack of starch accumulation in a knockout mutant of the H₂O₂-producing peroxisomal ACYL-COA OXIDASE2 and on the effects of H₂O₂ supplementation, we propose that peroxisome-derived H₂O₂ acts as a regulator of chloroplast metabolism. We conclude that peroxisomal MDH2 helps photoautotrophs cope with nitrogen scarcity and high light by transmitting the redox state of the peroxisome to the chloroplast by means of malate shuttle- and H₂O₂-based redox signaling.

Trends in atmospheric particles and their light extinction performance between 1980 and 2015 in Beijing, China

This study explored the interdecadal variations and their horizontal and vertical light extinction performances of atmospheric particulate matter with aerodynamic diameters ≤2.5 μm (PM_2.5), particulate matter with aerodynamic diameters ≤ 10 μm (PM₁₀), and total suspended particulates (TSPs) in Beijing from 1980 to 2015, using data available from historical publications. Prominent declines of PM_2.5, PM₁₀, and TSPs were detected with long-term linear trends of -6.7, -4.3, and -1.9 μg m^-3 yr^-1, respectively. Generally, on the annual scale during the studied period, it was found that PM_2.5 displayed negative correlation (R² = 0.38, p < 0.01) with visibility and positive correlation (R² = 0.41, p < 0.01) with aerosol optical depth (AOD). Comparably, PM₁₀ exhibited robust negative correlation (R² = 0.61, p < 0.01) with visibility and positive correlation (R² = 0.82, p < 0.01) with AOD. The complicated interdecadal variations and light extinction performances of PM_2.5 were found, suggesting the changes on particle composition and vertical distribution of PM_2.5 in the atmosphere.

Lipid catabolism in microalgae

Lipid degradation processes are important in microalgae because survival and growth of microalgal cells under fluctuating environmental conditions require permanent remodeling or turnover of membrane lipids as well as rapid mobilization of storage lipids. Lipid catabolism comprises two major spatially and temporarily separated steps, namely lipolysis, which releases fatty acids and head groups and is catalyzed by lipases at membranes or lipid droplets, and degradation of fatty acids to acetyl-CoA, which occurs in peroxisomes through the β-oxidation pathway in green microalgae, and can sometimes occur in mitochondria in some other algal species. Here we review the current knowledge on the enzymes and regulatory proteins involved in lipolysis and peroxisomal β-oxidation and highlight gaps in our understanding of lipid degradation pathways in microalgae. Metabolic use of acetyl-CoA products via glyoxylate cycle and gluconeogenesis is also reviewed. We then present the implication of various cellular processes such as vesicle trafficking, cell cycle and autophagy on lipid turnover. Finally, physiological roles and the manipulation of lipid catabolism for biotechnological applications in microalgae are discussed.

Identification of Insertion Site by RESDA-PCR in Chlamydomonas Mutants Generated by AphVIII Random Insertional Mutagenesis

Chlamydomonas reinhardtii is frequently used as a model organism to study fundamental processes in photosynthesis, metabolism, and flagellar biology. Versatile tool boxes have been developed for this alga ( Fuhrmann et al., 1999 ; Schroda et al., 2000 ; Schroda, 2006). Among them, forward genetic approach has been intensively used, mostly because of the high efficiency in the generation of hundreds of thousands of mutants by random insertional mutagenesis and the haploid nature therefore phenotypic analysis can be done in the first generation ( Cagnon et al., 2013 ; Tunçay et al., 2013 ). A major bottleneck in the application of high throughput methods in a forward genetic approach is the identification of the genetic lesion(s) responsible for the observed phenotype. In this protocol, we describe in detail an improved version of the restriction enzyme site-directed amplification PCR (RESDA-PCR) originally reported in (González- Ballester et al., 2005 ). The improvement includes optimization of primer combination, the choice of DNA polymerase, optimization of PCR cycle parameters, and application of direct sequencing of the PCR products. These modifications make it easier to get specific PCR products as well as speeding up subcloning steps to obtain sequencing data faster.

Robust Image Segmentation Method for Cotton Leaf Under Natural Conditions Based on Immune Algorithm and PCNN Algorithm

In the actual cotton planting environment, rapid change of light within a day, reflection from different backgrounds and different weather conditions can affect the imaging of cotton. Therefore, the crop object segmentation is difficult. Images which were captured in 12 natural scenes during cotton planting, including three weather conditions, such as sunny, cloudy and rainy and four soil cover conditions, such as white mulch film, black mulch film, straw and bare soil were regarded as the research objects. This paper presents the cotton leaf segmentation method based on Immune algorithm and pulse coupled neural networks (PCNN). First, 17 color components of white mulch film, black mulch film, straw, bare soil and cotton under the conditions of sunny, cloudy and rainy days were analyzed by using statistical method. Three high feasible and anti-light color components were selected by histogram statistical with mean gray value. Second, the optimal parameters of PCNN model and the optimal number of iterations were determined by using immune algorithm optimization theory, and the method in this paper was tested by using 1200 cotton images which were captured under 12 natural scenes. Finally, the test results showed that this method can distinguish cotton target region from soil and other background regions. Meanwhile, for reflection of mulch film, crop shadow, dark light, complex background, noise, etc. which are often appeared in natural scene, four image segmentation methods of Otsu algorithm, [Formula: see text]-Means algorithm, FCM algorithm and PCNN were compared with the proposed method in this paper. The segmentation result showed that the proposed method has good resistance to change of light and complex background. The average [Formula: see text] of the proposed method is 6.5%, significantly lower than that of other four methods and the performance is better than other four methods. This method can segment cotton images in different weather conditions and different backgrounds accurately under complex natural conditions. It will contribute to the subsequent growth status determination and pest diagnosis of cotton.

Covalent Porphyrin Framework-Derived Fe2P@Fe4N-Coupled Nanoparticles Embedded in N-Doped Carbons as Efficient Trifunctional Electrocatalysts

A new porous covalent porphyrin framework (CPF) filled with triphenylphosphine was designed and synthesized using the rigid tetrakis(p-bromophenyl)porphyrin (TBPP) and 1,3,5-benzenetriboronic acid trivalent alcohol ester as building blocks. The carbonization of this special CPF has afforded coupled Fe₂P and Fe₄N nanoparticles embedded in N-doped carbons (Fe₂P/Fe₄N@N-doped carbons). This CPF serves as an "all in one" precursor of Fe, N, P, and C. The porous property and solid skeleton of the CPF endow Fe₂P/Fe₄N@N-doped carbons with porous structure and a high degree of graphitization. As a result, Fe₂P/Fe₄N@N-doped carbons exhibited highly efficient multifunctional electrocatalytic performance for water splitting and oxygen electroreduction. Typically, Fe₂P/Fe₄N@C-800, obtained at a heat-treatment temperature of 800 °C, showed an ORR half-wave potential of 0.80 V in alkaline media and 0.68 V in acidic media, close to that of commercial Pt/C catalysts. Fe₂P/Fe₄N@C-800 also displayed efficient OER and HER activities, comparable to other phosphide and nitride electrocatalysts. The coupled Fe₄N and Fe₂P nanoparticles embedded in carbons exert unique catalytic efficiency for water splitting and fuel cells.

Chlamydomonas carries out fatty acid β-oxidation in ancestral peroxisomes using a bona fide acyl-CoA oxidase

Peroxisomes are thought to have played a key role in the evolution of metabolic networks of photosynthetic organisms by connecting oxidative and biosynthetic routes operating in different compartments. While the various oxidative pathways operating in the peroxisomes of higher plants are fairly well characterized, the reactions present in the primitive peroxisomes (microbodies) of algae are poorly understood. Screening of a Chlamydomonas insertional mutant library identified a strain strongly impaired in oil remobilization and defective in Cre05.g232002 (CrACX2), a gene encoding a member of the acyl-CoA oxidase/dehydrogenase superfamily. The purified recombinant CrACX2 expressed in Escherichia coli catalyzed the oxidation of fatty acyl-CoAs into trans-2-enoyl-CoA and produced H₂ O₂ . This result demonstrated that CrACX2 is a genuine acyl-CoA oxidase, which is responsible for the first step of the peroxisomal fatty acid (FA) β-oxidation spiral. A fluorescent protein-tagging study pointed to a peroxisomal location of CrACX2. The importance of peroxisomal FA β-oxidation in algal physiology was shown by the impact of the mutation on FA turnover during day/night cycles. Moreover, under nitrogen depletion the mutant accumulated 20% more oil than the wild type, illustrating the potential of β-oxidation mutants for algal biotechnology. This study provides experimental evidence that a plant-type FA β-oxidation involving H₂ O₂ -producing acyl-CoA oxidation activity has already evolved in the microbodies of the unicellular green alga Chlamydomonas reinhardtii.

Space-confined synthesis of multilayer Cu–N-doped graphene nanosheets for efficient oxygen electroreduction

Cu–N-doped graphene nanosheets prepared by thermal conversion of montmorillonite-confined Cu(ii) 2,2′-bipyridines exhibit efficient catalytic performance for oxygen electroreduction.

UV-mediated Chlamydomonas mutants with enhanced nuclear transgene expression by disruption of DNA methylation-dependent and independent silencing systems

In this investigation, we succeeded to generate Chlamydomonas mutants that bear dramatically enhanced ability for transgene expression. To yield these mutants, we utilized DNA methyltransferase deficient strain. These mutants must be useful as a plant cell factory. Chlamydomonas reinhardtii (hereafter Chlamydomonas) is a green freshwater microalga. It is a promising cell factory for the production of recombinant proteins because it rapidly grows in simple salt-based media. However, expression of transgenes integrated into the nuclear genome of Chlamydomonas is very poor, probably because of severe transcriptional silencing irrespective of the genomic position. In this study, we generated Chlamydomonas mutants by ultraviolet (UV)-mediated mutagenesis of maintenance-type DNA methyltransferase gene (MET1)-null mutants to overcome this disadvantage. We obtained several mutants with an enhanced ability to overexpress various transgenes irrespective of their integrated genomic positions. In addition, transformation efficiencies were significantly elevated. Our findings indicate that in addition to mechanisms involving MET1, transgene expression is regulated by a DNA methylation-independent transgene silencing system in Chlamydomonas. This is in agreement with the fact that DNA methylation occurs rarely in this organism. The generated mutants may be useful for the low-cost production of therapeutic proteins and eukaryotic enzymes based on their rapid growth in simple salt-based media.