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In land plants,the NAD(P)H dehydrogenase(NDH)complex reduces plastoquinones and drives cyclic electron flow(CEF)around photosystem I(PSI).It also produces extra ATP for photosynthesis and improves plant fitness under conditions of abiotic environmental stress.To elucidate the role of CEF in salt tolerance of the photosynthetic apparatus,we monitored Na+concentration,chlorophyll fluorescence,and expression of NDH B and H subunits,as well as of genes related to cellular and vacuolar Na+transport.The salt-tolerant Glycine max(soybean)variety S111-9 exhibited much higher CEF activity and ATP accumulation in light than did the salt-sensitive variety Melrose,but similar leaf Na+concentrations under salt stress.In S111-9 plants,ndhB and ndhH were highly up-regulated under salt stress and their corresponding proteins were maintained at high levels or increased significantly.Under salt stress,S111-9 plants accumulated Na+in the vacuole,but Melrose plants accumulated Na+in the chloroplast.Compared to Melrose,S111-9 plants also showed higher expression of some genes associated with Na+transport into the vacuole and/or cell,such as genes encoding components of the CBL10(Calcineurin B-Like protein 10)-CIPK24(CBL Interacting Protein Kinase 24)-NHX(Na+/H+antiporter)and CBL4(Calcineurin BLike protein 4)-CIPK24-SOS1(Salt Overly Sensitive 1)complexes.Based on our findings,we propose that enhanced NDH-dependent CEF supplies extra ATP used to sequester Na+in the vacuole.This reveals an important mechanism for salt tolerance in soybean and provides new insights into plant resistance to salt stress.