Roles of endophytic bacteria in Suaeda salsa grown in coastal wetlands: Plant growth characteristics and salt tolerance mechanisms*

Salinity is a limiting factor in the growth of plants in coastal wetlands. The interaction of halophytes with salttolerant endophytes has been one of the major concerns in this area. However, the mechanism by which endophytes promote halophyte growth remains unclear. The growth and physiological responses of Suaeda salsa inoculated with endophytic bacteria (Sphingomonas prati and Sphingomonas zeicaulis) at 0 %o and 20 %o NaCl were studied. The results showed that Sphingomonas zeicaulis had stronger positive effects on the growth of Suaeda salsa under 0 %o NaCl, and Sphingomonas prati performed better under 20 %o NaCl. Sphingomonas prati inoculation increased the mean height, root length, fresh weight and dry weight by 45.43%, 9.91%, 82.00% and 102.25%, respectively, compared with the uninoculated treatment at 20 %o NaCl. Sphingomonas prati inoculation decreased MDA content by 23.78%, while the soluble sugar and soluble protein contents increased by 15.08% and 12.57%, respectively, compared to the control, at 20 %o NaCl. Increases in SOD and CAT in the Sphingomonas prati inoculation were 1.03 and 1.47-fold greater, respectively, than in the Sphingomonas zeicaulis inoculation, under 20 %o NaCl. Moreover, Sphingomonas prati and Sphingomonas zeicaulis had antagonistic interactions in Suaeda salsa according to the results of the "interaction equation" (most G values were negative). PCA, clustering analysis and the PLS model revealed two mechanisms for regulating plant salt tolerance by which Sphingomonas prati enhanced Suaeda salsa growth: (1) Sphingomonas prati improved intracellular osmotic metabolism and (2) Sphingomonas prati promoted the production of CAT in the antioxidant enzyme system and retained permeability. This study provides new insight into the comprehensive understanding and evaluation of endophytic bacteria as biological inoculants in plants under salt stress.