Investigation On The Mechanisms Of Strigolactone In The Regulation Of Lipid Accumulation In Monoraphidium Sp. QLY-1

With the increasing consumption of fossil energy and the emergence of a series of environmental and energy shortages,biodiesel has become a research hotspot in recent years.Biodiesel has become an ideal substitute for fossil energy owing good biodegradability,low sulfur content,sufficient combustion its clean and environmentally friendly traits.As an ideal raw material for the production of biodiesel,microalgae had been widely studied due to its high growth rate,short growth cycle,high lipid content,high photosynthetic efficiency and no occupation of cultivated land.Despite microalgae-based biofuel have many advantages,large-scale production of biodiesel still face many challenges,such as low lipid productivity and high production cost.In the process of microalgae cultivation,exogenous phytohormone application can promote the cell growth,lipid accumulation and shorten the culture period.Strigolactone(SL)is a class of carotenoid-derived phytohormones that have important roles in regulating plant growth and development,stem branching,stomatal closure and abiotic stress.However,the effects and the mechanism of SL in lipid synthesis is not clarified.In this study,taking Monoraphidium sp.QLY-1 for research object,the cultivation system for efficient lipid production in microalgae by using SL induction was conducted.The effects of SL on microalgae growth and lipid accumulation through two-stage comprising heterotrophic cultivation and photo-chemical modulator induction were verified.The changing law of SL regulation of Ca²⁺and NO signals-induced lipid accumulation in microalgae were revealed.The mechanism of SL-mediated jasmonic acid signal to induce lipid accumulation under nitrogen deficiency stress was further explored.This study provide a theoretical foundation for developing of SL or other plant hormones to induce the accumulation of lipid or other high value-added metabolites in microalgae;provide scientific basis and target for improving lipid accumulation by the genetic engineering approach,and it has certain theoretical and practical significance for investigation of high efficient lipid synthesis in microalgae.The results are summarized as follows:(1)BG-11 as the basic media,Monoraphidium sp.QLY-1 was cultivated in two-stage comprising heterotrophic cultivation and photo-chemical modulator induction conditions.The heterotrophic cultivation was applied to achieve high-density cell growth,and addition different concentrations of SL were applied for lipid induction during the second autotrophic phase.The maximum lipid content(48.76%)and lipid productivity(63.65 mg L^-1 d^-1)of this alga was occurred in the condition of 1?M SL,and was 1.28-and 1.31-fold higher those the control group(38.12%,48.61 mg L^-1 d^-1).(2)Induction of SL promoted the contents of lipid and protein but reduced the carbohydrate content,and up-regulated the expression levels of lipid biosynthesis related genes.Moreover,the levels of NO,Ca²⁺and Ca M gene transcription in the alga cells were increased by SL induction.Results showed that SL might regulate the carbon flow and several signal molecules transductions,thereby increasing lipid synthesis.Further experiment revealed that the role of Ca²⁺in SL-induced lipids accumulation in Monoraphidium sp.QLY-1.The results indicated that increased endogenous Ca²⁺could significantly enhance lipid production in the alga compared with the SL induction alone,vice versa.Moreover,the levels of NO and lipid synthesis related genes expression were also upregulated.Collectively,SL induce the lipid synthesis in QLY-1 by mediating Ca²⁺and NO signallings and the expression levels of lipid biosynthetic genes.(3)Nitrogen deficiency is an effective method for lipid production in microalgae.In this study,the lipid content(53.71%)of the alga was obtained in treatment with 1?M SL and nitrogen deficiency conditions,which was 1.16 times compared with the nitrogen conditions alone(46.18%).Meanwhile,JA content was increased from1217.26 pmol L^-1 to 1943.50 pmol L^-1.However,the endogenous GABA content was decreased from 13.18 mg g^-1 to 8.97 mg g^-1.(4)Further studies have found that exogenous JA could further promote the accumulation of lipid and?-aminobutyric acid(GABA)of the alga cells in the presence of SL and nitrogen starvation conditions;JA application also upregulated the levels of lipogenic genes expression and glutathione(GSH),which alleviated the oxidative damage caused by nitrogen deficiency.In addition,when the intracellular JA synthesis was inhibited,the lipid and GABA contents were decreased.These results showed that SL could mediate JA-induced lipid and GABA synthesis in microalgae and increased microalgae tolerance by regulating ROS and GABA signallings under nitrogen deficiency condition.Taken together,these findings further elucidate that the molecular mechanism of SL in regulating lipid synthesis,and provide a theoretical basis for the application of exogenous hormones to regulate the lipid production by microalgae under nitrogen deficiency stress.