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Few information of the function of stearoyl-coenzyme A (CoA) desaturase (SCD) in apicomplaxan parasite has been obtained.In this study,we retrieved a putative fatty acyl-CoA desaturase (TGGT1_238950) by a protein alignment with Plasmodium falciparum SCD in ToxoDB.A typical △9-desaturase domain was revealed in this protein.The putative desaturase was tagged with HA endogenously in Toxoplasma gondii,and the endoplasmic reticulum localization of the putative desaturase was revealed,which was consistent with the fatty acid desaturases in other organisms.Therefore,the TGGT1_238950 was designated T.gondii SCD.Based on CRISPR/Cas9 gene editing technology,SCD conditional knockout mutants in the T.gondii TATi strain were obtained.The growth in vitro and pathogenicity in mice of the mutants suggested that SCD might be dispensable for tachyzoite growth and proliferation.The SCD-overexpressing line was constructed to further explore SCD function.The portion of palmitoleic acid and oleic acid were increased in SCD-overexpressing parasites,compared with the RH parental strain,indicating that T.gondii indeed is competent for unsaturated fatty acid synthesis.The SCD-overexpressing tachyzoites propagated slower than the parental strain,with a decreased invasion capability and weaker pathogenicity in mice.The TglF2α phosphorylation and the expression changes of several genes demonstrated that ER stress was triggered in the SCD-overexpressing parasites,which were more apt toward autophagy and apoptosis.The function of unsaturated fatty acid synthesis of TgSCD was consistent with our hypothesis.On the other hand,SCD might also be involved in tachyzoite autophagy and apoptosis.