Personal page of Yonghua Li
- Personal page of Yonghua Li
- Personal page of Alexandra DUBINI
Updated in February 2012
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Dr. Yonghua LICEA Research Scientist (sept 2009 - )- Currently serves as the review editorial board for Frontiers in Plant Metabolism and Chemodiversity (Janv 2012 - ) For complete reference on my work, please visit My Google Scholar: http://scholar.google.fr/citations?user=c9FasSUAAAAJ&hl=fr
UMR 6191, University Aix Marseille
Tel : +33 (0) 4 42 25 46 51 Email: yonghua.li@cea.fr |
SHORT BIOGRAPHY |
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Born 1974, China. |
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RESEARCH INTERESTS |
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In view of global warming caused by increased CO2 emission, and soaring fossil fuel prices, there is strong interest in developing alternative and renewable source of energy including wind, hydrogen, and biofuel. Due to its high carbon fixation efficiency, and lack of competition with plants for arable lands, microalgae pose as a competitive platform for bioenergy production (Figure 1). Many microalgae species when subjected to stress such as removal of nitrogen from the media can make significant amount of oil, in some case, it can reach over 70%. This dependence on stress limits biomass therefore overall productivity of the system. Oil is the most reduced form of energy found in nature, and represents twice more energy per gram dry biomass than other storage compounds (starch or protein). Oil (= triacylglycerols), as the name implies, is composed of three often different fatty acids which are esterified to the 3 hydroxyl groups of a glycerol backbone (Figure 2). Function and chemical properties of the oil are conferred largely by the structure of the fatty acids present. Many thousands of fatty acid structures occur in nature. They differ in the total number of carbon, degree of unsaturation, with or without further fatty acid modification (for example, hydroxylation, epoxidation, dicarboxylation etc). Fatty acids and lipids are synthesized by all cell types. Besides their role as a major form of carbon and energy storage, fatty acids are basic building blocks of biological membranes, part of cellular signalling network, form protective outer envelopes. Research on lipids is both fundamental and applied. From a biotechnological point of view, lipids are essential part of our diet, source of chemical feedstock, and a major player as a renewable fuel. My broad interest is to discover genes or proteins involved in lipid metabolism and use this knowledge to increase oil deposition or produce industrially important fatty acids via biotechnological means. To address these questions, we routinely apply a combination of molecular-genetic, biochemical and microscopic approaches in the model plant Arabidopsis thaliana and in the model green algae Chlamydomonas reinhardtii. Specifically, a series of questions we ask:
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CURRENT PROJECTS |
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1. An integrated overview of oil accumulation and degradation in microalgae |
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As a first step toward understanding oil accumulation in Chlamydomonas reinhardtii, we followed the changes in cell morphology, chlorophyll, starch and lipid (membrane and storage) content over time (Figure 3). Upon removal of nitrogen from the media, the appearance of cellular oil droplet and starch granules is coincident with the disappearance of chlorophyll and thylakoid membranes. The stored oil and starch are rapidly degraded upon nitrogen re-availability. The molecular mechanisms of this accumulation and degradation are poorly understood. Results obtained down this line should yield important insights into the lipid homeostasis and therefore the overall fitness of cells. |
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2. Oil bodies |
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Oil bodies are subcellular compartment where neutral lipids are stored. It is ubiquitous in all eukaryotes. Other names for this cellular compartment are lipid droplets or oleosomes. Oil bodies are spherical organelles consisting of a neutral lipid core enclosed by a membrane lipid monolayer coated with proteins. Until fair recently, oil bodies are considered only as an energy and carbon storage site. Modern mass spectrometry has revealed the presence of many proteins in the isolated oil body fraction. Well characterized structural proteins of oil droplet include oleosins found in oilseeds, or perilipin in adipocytes. Using mass spectrometry on purified oil bodies from Chlamydomonas, our lab together with several other labs have identified a novel structural protein of algal oil bodies – named Major Lipid Droplet Protein (MLDP). Besides this protein, numerous metabolic enzymes or lipid trafficking proteins are also present for example acyl activating enzymes, acyltransferases or lipases. The enzymes present span the key steps of the triacylglycerol synthesis pathway and including a glycerol-3-phosphate acyltransferase (GPAT), a lysophosphatidic acid acyltransferase (LPAT) and a putative phospholipid:diacylglycerol acyltransferase (PDAT) (Figure 4). Oil bodies are now believed to be not only the storage compartments (sink) but also are dynamic structures (node) likely to be involved in processes such as oil synthesis, degradation and lipid homeostasis. Detailed characterization of these oil body associated proteins should yield important insight into the compartmentalization and the role of oil bodies in algae. |
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3. Genetic approaches |
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Most of our current knowledge on oil synthesis in algae is deduced from plant pathways based on comparative genomics or sequence homologies. Few proteins of lipid metabolisms have been characterizes so far. To reveal novel players of oil metabolism in algae, we have set up a forward genetic screen (Figure 5). Chlamydomonas is a unicellular microalga and most of its life cycle stays as haploid. Generation of mutants is therefore a very powerful approach because the mutant phenotype can be seen in the first generation. Several methods of high throughput screening have been set up (Nile Red neutral lipid staining coupled with flow cytometer, cell counter and HP-TLC; direct transmethylation and GC-FID). Screening of mutants with one of the following phenotypes is underway:
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PUBLICATIONS |
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PATENTS |
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ORAL PRESENTATIONS in international meetings |
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Figure 1: Chlamydomonas reinhardtii as a model for studying biofuel production. |
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Figure 2: An example of a triacylglycerol molecule. |
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Figure 3: Oil accumulation processes in response to nitrogen starvation. Top panel: Nile red stained cells; bottom panel shows the loss of chlorophyll from the cultures of Chlamydomonas reinhardtii in response to nitrogen depletion with time. |
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Figure 4: Tentative structure of oil bodies of Chlamydomonas reinhardtii and mapping of major oil-body associated proteins to major reactions of lipid synthetic pathways. |
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Figure 5: Generation of mutant banks of Chlamydomonas reinhardtii for genetic screens. |
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