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. In this study, key endophytic bacteria that can respond to salinity changes were identified by analyzing the microbial community in propagules of Kandelia obovata. Delftia tsuruhatensis DYX29, a strain that can grow normally under high salinity conditions with a sodium chloride (NaCl) concentration of 5% (w/v), was obtained by pure culture. DYX29 can produce siderophores with a siderophore unit value of 87.6% and 1-aminocyclopropane-1-carboxylate (ACC) deaminase with 29 U L^–1, and its synthesis of intracellular amino acids and auxin can be induced by high salinity. Inoculation with DYX29 can remarkably promote the salt tolerance of rice. Under salt stress, the addition of DYX29 was shown to effectively promote 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 catalase (CAT) and peroxidase (POD) activities in rice leaves by 37.8 and 88.2%, respectively. Moreover, it maintained the ionic homeostasis in rice roots and leaves. In addition, it upregulated the expression of growth-promoting hormones, such as indole-3-acetic acid (IAA), brassinolide (BL), abscisic acid (ABA), and salicylic acid (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 and their use for crop growth promotion under salt stress. It can also provide useful references for the development of new salt-tolerant and pro-biotic biofertilizers, as well as the investigation of the related mechanisms.