Salinity-induced Palmella Formation Mechanism In Dunaliella Salina Revealed From Physiological And Phosphoproteomics Analysis

Salinity significantly affects metabolic activity in plants and hinder the agricultural productivity throughout the world.Dunaliella salina,which is a halotolerant single-celled green alga,has been recognized as a model photosynthetic organism for studying the salt-tolerant mechanisms in plants.D.salina possesses unique salt tolerance mechanism.Under high salinity conditions,some D.salina choose to form palmelloid to survive under poor growth conditions.Combined morphological,physiological and quantitative phosphoproteomic technology,we tried to clarify the salinity-induced palmella formation mechanism in D.salina.We found salinity-induced palmella formation,decreased cell viability and morphological changes of D.salina.Accumulation of glycerol and increased activity of glycerol metabolic enzyme GPDH can facilitate intracellular concentration osmotically to balance the external salinity,and the production of exopolysaccharides(EPSs)may protect cells from ions toxicity.The decreased chlorophyll content was found and light reaction acitivity and CO2 assimilation were inhibited upon the palmella foramtion of D.salina.In addition,dynamic changes of antioxidant system play important roles in the protection of D.salina response to oxidative stress.Furthermore,using an stable isotope dimethyl labeling and Ti4+-IMAC phosphopeptide enrichment for quantitative phosphoprotemic approach,35 differentially abundant phosphoproteins were identified upon palmella formation of D.salina.These phosphoproteins were mainly involved in photosynthesis,carbohydrate and energy metabolism,metabolism,transcription related,protein synthesis,protein folding and transport,protein degradation,signaling,and cell structure.Taken together,our physiological and phosphoproteomics results may reveal the salinity-induced palmella formation mechanism in D.salina which includes:(1)secrete EPSs by enhancing sugar transport and thereby reduce the penetration of ions through the cell surface;(2)enhance the cyclic electron transport chain by the increaed phosphorylation of LHCb then supply ATP for cell demands,and elevate the ratio of CO2 to O2 by the decreased phosphorylation of pyruvate phosphate dikinase(PPDK)to reduce the energy loss of photorespiration;(3)save metabolic cost by decreasing metabolism activity;(4)enhance the gene expression and protein degration to synthetize novel proteins and degrade unneeded or damaged proteins;(5)minimize the oxidative injury by elevating the activity of superoxide dismutase(SOD)and glutathione peroxidase(GPX)to scavenge reactive oxygen species(ROS);(6)save energy consumption by decreasing several signaling pathways;(7)maintain cell structure and cytoskeleton dynamic by changing the phosphorylation level of some flagellar-realted proteins and a-tubulin.This study provides new clues for studying the salt-tolerant mechanisms in D.salina and lays a solid foundation for further research on salt-tolerance mechanism in plants.