Research Project at UWG
Research Goal: The sensing, response and subsequent acclimation of plants to suboptimal and adverse conditions is of pivotal importance to productivity and crop yield. Natural fluctuations in environmental growth conditions have the potential to strongly alter growth rate and adversely affect fitness of photosynthetic organisms. However, plants and algae have developed elaborate acclimation mechanisms that fine tune the metabolism of the organism to this altered environment, enabling cells to adjust and optimize growth capacity upon establishing a new homeostasis. I am interested broadly in identifying and characterizing novel molecular components essential for photosynthesis and photosynthetic pigment metabolism employing forward and reverse genetics in the model experimental green micro-alga Chlamydomonas reinhardtii.
Research Background: Chlamydomonas reinhardtii is a green micro algae that is haploid, easy to culture in the laboratory, can grow both photosynthetically and heterotrophically (can use acetate as the sole carbon source in the dark like a heterotroph and also can use atmospheric CO2 in the presence of light, like a photosynthetic autotroph), is amenable to both nuclear and chloroplast transformation and its genome has been sequenced. All of these traits make it an ideal, simple and elegant system to study photosynthesis. Additionally, Chlamydomonas has two different pathways to make chlorophyll, unlike angiosperms. It has a strictly light dependent chlorophyll biosynthetic pathway like all photosynthetic higher plants and a light independent chlorophyll biosynthetic pathway that can operate both under dark and light, like in some cyanobacteria and gymnosperms. Photosynthesis is the only O2 generating biochemical reaction on Earth that sustains life. Photosynthesis consists of two sets of reaction, namely the light reaction and the Calvin cycle, that take place in the chloroplast of plant cells. During the light reaction, absorbed light energy is used to extract electrons from water with generation of oxygen as a byproduct. The energy from the electron is converted to chemical energy (ATP and NADPH). This converted chemical energy is then used to assimilate CO2 into carbohydrates. Antenna of a photosystem is comprised of photosynthetic pigment molecules (chlorophyll and carotenoids) bound to LHCs (light harvesting complex proteins) and is present in the thylakoid membrane of chloroplast. It is responsible for trapping solar energy and passing it on to the reaction center for photochemistry. After absorbing the light energy, chlorophyll becomes excited to its singlet excited state and then transfers the absorbed energy to the reaction centers of photosystems, where it drives the initial charge separation reactions of photosynthesis (photochemistry). Besides photochemistry, fluorescence emission, de-excitation by thermal dissipation, and decay through triplet state are some other means by which excited chlorophylls return to ground state. Light is essential for photosynthesis. When plants receive more light than they can utilize, the lifetime of singlet excited chlorophyll extends and the chance of returning to ground state through triplet state chlorophyll is increased. This pathway can dissipate excess energy; however, the generated triplet chlorophyll can transfer its energy to oxygen so that singlet oxygen is produced. Singlet oxygen is a harmful type of reactive oxygen species (ROS) that can cause degradation of membrane and protein structure of photosystems. Plants have several photo-protective mechanisms to protect themselves from excess light. These include chloroplast avoidance movement, reduction of antenna sizes of photosystems, minimizing absorbed energy by non-photochemical quenching (NPQ), reduction of excitation pressure by alternative electron transport directly from PSI to other electron acceptors like oxygen (water-water cycle), photorespiration and PSI cyclic electron flow (under low CO2 conditions), modulation of tetrapyrrole biosynthetic pathways, biosynthesis of antioxidants and efficient repair of damaged photosystems. Any defect in molecular components essential for the light reaction, photo-protection, CO2 assimilation/CCM will be detrimental to photo-autotrophic growth.
Current Research Status: My lab has generated a random nuclear DNA insertional mutant library of C. reinhardtii and has screened it to isolate 21 “interesting” photosynthetic mutants. Out of these 21 mutants, 14 are incapable of photo-autotrophic growth under different light irradiance conditions and seven are pigment deficient but capable of photo-autotrophic growth at a slower rate compared to the parental wild type strain under similar growth conditions. Out of the 14 mutants that are incapable of photo-autotrophic growth, 7 are incapable of photo-autotrophic growth under dim light conditions (15-20 mmol photons m-2s-1), 5 are incapable of photo-autotrophic growth under low-medium light conditions (100-250 mmol photons m-2s-1) and 2 are incapable of photo-autotrophic growth under high light conditions (450-500 mmol photons m-2s-1). Out of the seven pigment deficient mutants that show slow photo-autotrophic growth, 6 are pigment deficient in the light to varying degree and 1 is pigment deficient in the dark but green in the light. We are currently focusing on molecular, biochemical and physiological characterization of these mutants.
Research Significance: The innovative use of plant physiology, biochemistry, genetics, and molecular biology, to dissect complex problems in the eco-physiology of photosynthesis will enable us to assess the relative importance of different processes involved in photosynthetic productivity. Future studies may allow us to manipulate plant productivity and the ability of plants to grow in different, often adverse, environments. As one of the mechanism algae/plants use to cope with high light stress is the regulation of Chl antenna size, my research has the potential of identifying key components in this signal transduction pathway involved in pigment biosynthesis regulation that might be commercial exploited to improve solar conversion efficiency and increase bio mass production (see CV for past research patent).