Synergism between Inositol Polyphosphates and TOR Kinase Signaling in Nutrient Sensing, Growth Control, and Lipid Metabolism in Chlamydomonas.
The networks that govern carbon metabolism and control intracellular carbon partitioning in photosynthetic cells are poorly understood. Target of Rapamycin (TOR) kinase is a conserved growth regulator that integrates nutrient signals and modulates cell growth in eukaryotes, though the TOR signaling pathway in plants and algae has yet to be completely elucidated. We screened the unicellular green algaChlamydomonas reinhardtiiusing insertional mutagenesis to find mutants that conferred hypersensitivity to the TOR inhibitor rapamycin. We characterized one mutant,vip1-1, that is predicted to encode a conserved inositol hexakisphosphate kinase from the VIP family that pyrophosphorylates phytic acid (InsP6) to produce the low abundance signaling molecules InsP7and InsP8Unexpectedly, the rapamycin hypersensitive growth arrest ofvip1-1cells was dependent on the presence of external acetate, which normally has a growth-stimulatory effect on Chlamydomonas.vip1-1mutants also constitutively overaccumulated triacylglycerols (TAGs) in a manner that was synergistic with other TAG inducing stimuli such as starvation.vip1-1cells had reduced InsP7and InsP8, both of which are dynamically modulated in wild-type cells by TOR kinase activity and the presence of acetate. Our data uncover an interaction between the TOR kinase and inositol polyphosphate signaling systems that we propose governs carbon metabolism and intracellular pathways that lead to storage lipid accumulation.
Donald Danforth Plant Science Center, St. Louis, Missouri 63132.
Inmaculada
http://orcid.org/0000-0003-2849-675X
I
Couso
Donald Danforth Plant Science Center, St. Louis, Missouri 63132.
Bradley S
BS
Evans
Donald Danforth Plant Science Center, St. Louis, Missouri 63132.
Jia
J
Li
Donald Danforth Plant Science Center, St. Louis, Missouri 63132.
Yu
Y
Liu
Donald Danforth Plant Science Center, St. Louis, Missouri 63132.
Fangfang
http://orcid.org/0000-0001-7220-0675
F
Ma
Earth and Planetary Science, University of California, Berkeley, California 94720.
Spencer
S
Diamond
Donald Danforth Plant Science Center, St. Louis, Missouri 63132.|Agricultural Research Service, U.S. Department of Agriculture, St. Louis, Missouri 63132.
Doug K
http://orcid.org/0000-0001-8599-8946
DK
Allen
Donald Danforth Plant Science Center, St. Louis, Missouri 63132 jumen@danforthcenter.org.
James G
http://orcid.org/0000-0003-4094-9045
JG
Umen