Endophytic bacteria associated with the enhanced cadmium resistance in NHX1-overexpressing tobacco plants

Human industrial activities have left large area of land polluted with cadmium (Cd), which poses tremendous threat to ecosystem. Na+/H+ antiporter 1 (NHX1) was a transporter located on vacuoles, which mediated the compartmentation of Na+ into vacuole and transportation of H+ into the cytoplasm. And the overexpressing of its code gene (NHX1 gene) was previously identified to enhance Cd resistance in tobacco (Nicotiana tabacum) plants possibly by maintaining the change of ion content. However, the mechanisms of Cd resistance in NHX1-overexpressing plants have not been well elucidated yet. In this study, the endophytic bacteria communities, the activities of soil enzymes and the antioxidant enzymes were characterized to explore possible mechanisms of the enhanced Cd resistance in NHX1-overeexpressing tobacco plants. Some differences in the endophytic bacterial community structure in roots of transgenic and WT tobaccos under Cd stress were observed, in which, Sphingomonas, Steroidobacter and Actinoplanes were proposed as vital endophytic bacteria for enhancing Cd resistance of transgenic tobacco plants. In addition, the soil enzymes were increased in the rhizospheric soil around the transgenic tobacco plants compared to WT plants under Cd stress. Under Cd exposure, the SOD and CAT activities were 1.65 times and 1.88 times of that in WT plants, respectively. The content of MDA and H2O2 were also observed to be 35 % and 28 % lower in transgenic tobacco roots compared to WT plants under Cd stress. Moreover, the Cd content of transgenic plants decreased by 38 % under Cd stress compared to WT plants. These findings suggested that the endophytic bacteria communities associated with the NHX1-overexpressing plants might lead to the increased Cd resistance by enhancing the rhizosphere soil enzyme activities, strengthening the antioxidative defense system, and better maintaining of the ion changes. Taken together, this study suggested that the enhanced Cd resistance was observed in transgenic tobacco plants associated with the increase of soil enzyme activities and changes of endophytic bacteria community, and the possible mechanisms involved in this process were also discussed.

Automated summary

The endophytic bacteria communities in roots of NHX1-overexpressing tobacco plants showed differences under Cd stress, with Sphingomonas, Steroidobacter, and Actinoplanes identified as key endophytic bacteria for enhancing Cd resistance. Soil enzyme activities were increased in the rhizospheric soil of transgenic tobacco plants under Cd stress. The activities of SOD and CAT in the transgenic plants were significantly higher, while the content of MDA and H2O2 was lower compared to WT plants under Cd stress. Additionally, the Cd content in transgenic plants decreased and these findings suggest that the enhanced Cd resistance in transgenic tobacco plants may be associated with changes in soil enzyme activities and endophytic bacterial communities.