Endophytic Bacteria Promote the Growth of Suaeda glauca in Saline-Alkali Stress: Regulation of Osmotic Pressure and Antioxidative Defense System

Endophytic bacteria are promising bacterial fertilizers to improve plant growth under adverse environment. For ecological remediation of coastal wetlands, it was necessary to investigate the effect and interaction of endophytes on halophytes under saline-alkali stress. In this study, an endophytic bacterium strain HK1 isolated from halophytes was selected to infect Suaeda glauca under pH (7 and 8) and salinity gradient (150, 300 and 450 mmol L-1). Strain HK1 was identified as Pantoea ananatis and it had ability to fix nitrogen, dissolve inorganic phosphorus and produce indole-3-aceticacid (IAA). The results showed that strain HK1 could promote the growth of S. glauca seedings when the salinity was less than 300 mmol L-1, in view of longer shoot length and heavier fresh weight. The infected plants could produce more proline to decrease the permeability of cells, which content increased by 26.2%-61.1% compared to the non-infected group. Moreover, the oxidative stress of infected plants was relieved with the malondialdehyde (MDA) content decreased by 16.8%-32.9%, and the peroxidase (POD) activity and catalase (CAT) activity increased by 100%-500% and 6.2%-71.4%, respectively. Statistical analysis revealed that increasing proline content and enhancing CAT and POD activities were the main pathways to alleviate saline-alkali stress by strain HK1 infection, and the latter might be more important. This study illustrated that endophytic bacteria could promote the growth of halophytes by regulation of osmotic substances and strengthening antioxidant activities. This finding would be helpful for the bioremediation of coastal soil.