Abstract:

Drought is one of the major constraints on agricultural productivity. Plant growth promoting rhizobacteria（PGPR）could effectively alleviate drought-induced damages to plants. However, the involved mechanisms still need to be explored. This study emphasized the regulatory roles of Burkholderia sp. GD17 in cucumber seedling responses to drought stress. The experiment consisted of GD17-bacterized and non-bacterized plants, with or without drought treatment, then the physiological and biochemical parameters, and gene expression were analyzed at 7 d after withdrawing water, and the mechanisms of GD17 promoting growth and antagonizing were evaluated. As results, on the 5th day of inoculation, the number of GD17 in the roots reached 6.2×10⁶ CFU/g fresh weight, and remained at this level until the 15th day when drought treatment was implemented. Under regular irrigation, fresh and dry weight of aerial part of GD17-bacterized plants were 39% and 36% higher than those of non-bacterized plants, respectively. Following drought stress, the former was 38% and 32% higher than the latter, respectively, and the relative water content was 8.5% higher in the former than the latter. Inoculation of GD17 efficiently alleviated drought-induced oxidation damage as indicated by lower malondialdehyde content（45%）and electrolyte leakage（26%）in GD17-bacterized leaves than in non-bacterized ones following drought stress. The activities of superoxide dismutase, peroxidase and catalase in the GD17-bacterized leaves were significantly lower than those in non-bacterized ones under regular irrigation, while they were higher in the former than the latter under drought stress. Leaf proline content increased significantly under drought stress, with a greater degree in the bacterized plants. The net photosynthetic rate was higher in bacterized plants than in non-bacterized ones under regular irrigation, but the stomatal conductance, intercellular carbon dioxide concentration and transpiration rate did not change significantly. However, the values of these parameters in the bacterized plants were significantly higher than those in non-bacterized plants under drought conditions. Chlorophyll fluorescence imaging further showed that inoculation with GD17 effectively alleviated the drought-induced damage to photosynthetic apparatus and the impairment of photosynthetic efficiency. The expressions of antioxidation-, proline synthesis- and transcription factor-related genes were up-regulated following drought stress, especially in GD17-bacterized plants. In conclusion, inoculation with GD17 efficiently promotes cucumber seedling growth and tolerance to drought. The possible mechanisms might be associated with improving antioxidant defense, alleviating photosynthetic damage, enhancing osmotic substance synthesis and cell water retention, and up-regulating expression of transcription factors. GD17 strain presents potential application in cucumber agricultural production.

Key words: drought stress, Burkholderia, cucumber, oxidative stress, photosynthesis