Study Of Relationship Between Endophyte Bacteria Isolated From Subnival Plants And Chilling Tolerance Of Chorispora Bungeana

Endophytic bacteria can be defined as those bacteria that colonize the internal tissue of the plant showing no external sign of infection or negative effect on their host. Some of them have been shown to enhance plant growth and increase resistance against biotic stress and tolerance to abiotic stress in many plants, including salt injury, chilling stress, water deficit, metal stress, pathogen infection and plant diseases,. The objective of this study was to evaluate the effect of the endophytic bacteria, which were isolated from alpine-subnival plants, on chilling tolerance of host plants and demonstrate the mechanism. We used Chorispora bungeana as research material in this study, which is a representative alpine-subnival plant. Physiological markers for chilling stress were assessed in regenerated plantlets and suspension cultured cells of Chorispora bungeana subjected to chilling stress (0℃or 4℃). The major results are as follows:1. In the present study 7 strains of endophytic bacteria except Sphinqomonas sp. strain Enf42 were capable of colonizing internal tissues of regenerated plantlets of C. bungeana and moved in them under both normal (20℃) and chilling (0℃) conditions. The 7 strains of endophytic bacteria were Sanguibacter sp. strain Enf10, Plantibacter sp. strain Enf69, Rhodococcus sp. strain Enf11, Clavibacter sp. strain Enf12, Pseadomonas sp. strain Enf30, Pseadomonas sp. strain Ens37 and Sphinqomonas sp. strain Enf42. Based on comprehensive analysis we selected endophytic bacteria strains Enf12 and Enf30 for further study.2. Enf12 significantly decreased electrolyte leakage and content of MDA in regenerated plantlets and suspension cultured cells of Chorispora bungeana exposed to chilling stress, indicating that the strain Enf12 may have the potential to enhance stability of biological membrane systems and lower the sensitivity of Chorispora bungeana to chilling injury.3. During chilling treatment, Enf12 inoculation led to further increases in activities of antioxidant enzymes (SOD, CAT, POD and APX) and proline accumulation, and significantly lower levels of ROS (H2O2 and O2-) in the plantlets and suspension cultures. Endophytic bacteria strains Enf12 appears to confer chilling tolerance on Chorispora bungeana by increasing the activities of antioxidant enzymes, which could limit ROS levels as well as lipid peroxidation, thus alleviating chilling-induced oxidative stress and damage.4. Ultrastructure of suspension cultures showed that Enf12 inoculation alleviated the damage in plastid resulted from chilling stress. It may indicate that the endophyte Enf12 improved the tolerance of Chorispora bungeana to low temperature.5. Compared with the non-inoculated plantlets, the aerial part biomass of Enf12-inoculated plantlets was significantly greater under both normal (20℃) and chilling (0℃) conditions, indicating that Enf12 has the ability to enhance the growth of Chorispora bungeana plantlets.6. The study of Enf30-inoculated regenerated plantlets and suspension cultured cells of Chorispora bungeana showed that Enf30 have no significant effect on the growth and tolerance of Chorispora bungeana to chilling stress.Thus, our results demonstrate that the strain Enf12 and Enf30 are capable of colonizing internal tissues of C. bungeana. Enf12 inoculation stimulates the growth of C. bungeana and improves its tolerance to chilling stress through enhancing the antioxidant defense system. However, Enf30 have no significant effect on the growth and tolerance of Chorispora bungeana to chilling stress.