Wednesday 10 February 2016 at 3:00pm

Title: “Regulation of Cellular Quiescence and Lipid Droplet Formation in Microalgae”

Speaker:
Dr. Christoph Benning, Michigan State University

Abstract:
Increasing the triacylglycerol content of microalgae will enhance the energy density of the biomass and the quality of the feedstock for biofuel production. A key problem hampering maximization of triacylglycerol accumulation is the inverse relationship between cell growth and division on one hand, and triacylglycerol accumulation on the other. N deprivation induces a quiescent state in Chlamydomonas when cell divisions temporarily cease and triacylglycerol biosynthesis is induced. Upon refeeding N, the cells exit quiescence, resume growth and degrade the accumulated triacylglycerol. We have isolated a mutant that does not readily exit quiescence and only very slowly degrades triacylglycerols upon N refeeding. The responsible gene encodes a predicted DNA binding protein. Comparative global transcriptional profiling under different growth conditions suggests that the protein is required for repression of a subset of transcriptional quiescence programs under normal growth conditions and for reinitiating of specific transcriptional programs during quiescence exit following N refeeding. This mutant and the respective protein provide a first insight into how changes of metabolic status of the cell may be linked to the cell division cycle in microalgae. In plants, neutral lipids are frequently synthesized and stored in seed tissues, where the assembly of lipid droplets (LDs) coincides with the accumulation of triacylglycerols (TAGs). In addition, photosynthetic, vegetative cells can form cytosolic LDs and much less is known about the makeup and biogenesis of these LDs. Chlamydomonas is a reference model for LDs in a photosynthetic cell, because in this unicellular green alga LD dynamics can be readily manipulated by N availability. The major lipid droplet protein (MLDP) forms a proteinaceous coat surrounding mature LDs. Reducing the amount of MLDP affects LD size and number, TAG breakdown and timely progression out of cellular quiescence following N resupply.

1. Tsai CH, Warakanont J, Takeuchi T., Sears BB, Moellering ER, Benning C. 2014.The protein COMPROMISED HYDROLYSIS OF TRIACYLGLYCEROLS 7 (CHT7) acts as a repressor of cellular quiescence in Chlamydomonas. Proc Natl Acad Sci U S A, 111: 15833–15838
2. Tsai, CH, Zienkiewicz, K, Amstutz, CL, Brink, BG, Warakanont, J, Roston, R, Benning C. 2015. Dynamics of protein and polar lipid recruitment during lipid droplet assembly in Chlamydomonas reinhardtii. Plant J. 83:650-660

Host: Dr. Yantao Li, Ph.D.