Salinity-responsive key endophytic bacteria in the propagules of Kandelia obovata enhance salt tolerance in rice

Salinity is a major environmental stress affecting crop growth and productivity globally. The application of halo-tolerant plant growth-promoting bacteria (HT-PGPB) has been widely recognized to promote crop growth and reduce the adverse effects of salt stress. Herein, key endophytic bacteria that can respond to salinity changes were identified by analysis of the microbial community in propagules of Kandelia obovate. Delftia tsuruhatensis DYX29, a strain able to grow normally under high salinity conditions with a sodium chloride (NaCl) concentration of 5% (w/v), was obtained by pure culture. DYX29 has the ability to produce siderophores with a siderophore unit value of 87.6% and ACC (1-aminocyclopropane-1-carboxylate) deaminase with 29 U L^-1 and can synthesize intracellular amino acids and auxin induced by high salinity. Inoculation with DYX29 can remarkably promote the salt tolerance of rice. Under salt stress, the addition of DYX29 effectively promoted the growth of rice seedlings through a variety of approaches. It increased the biomass of rice seedlings by 32.9% (dry weight) and promoted the accumulation of soluble sugars by 23.1%. It also increased CAT and POD activities in rice leaves by 37.8 and 88.2%, respectively. Moreover, it maintained ionic homeostasis in roots and leaves. In addition, it upregulated the expression of growth-promoting hormones in roots, such as IAA, BL, ABA and SA, in rice roots by 27.8, 69.5, 123.7 and 28.6%, respectively. This study provides inspiration for screening valuable salt-tolerant pro-biotic bacteria from mangrove ecosystems for crop growth promotion under salt stress. It may also provide useful references for the development of new salt-tolerant and pro-biotic biofertilizers and the investigation of the related mechanism.