Plant growth-promoting effects of Hartmannibacter diazotrophicus on summer barley (Hordeum vulgare L.) under salt stress

• Inoculation of barley with strain E19T enhanced plant growth under salt stress.
• Growth promotion is based on production of ACC-deaminase by strain E19T.
• Colonization of strain E19T on barley roots was visualized by a new FISH probe.

Soil salination is a major concern of modern agriculture, specifically regarding irrigation and arid regions. However, plant growth promoting rhizobacteria (PGPR) can increase plant resistance to abiotic stress and represent an environmental friendly approach to alleviate salt stress in crops. The plant growth promoting (PGP) activities of Hartmannibacter diazotrophicus E19T, a new genus recently isolated from the rhizosphere of Plantago winteri from a natural salt meadow, were assessed in pure culture experiments and in vivo assays. ACC-deaminase activity for strain E19T at 1, 2 and 3% NaCl were 0.56 ± 0.20, 1.29 ± 0.82 and 2.60 ± 1.2 μmol α-ketobutyrate mg protein−1 h−1 respectively, and production of IAA was not detected. H. diazotrophicus E19T inoculated summer barley seedlings exposed for 2 h to 200 mM and 400 mM NaCl stress showed reduced ethylene emission in comparison to uninoculated plantlets exposed to same conditions. Inoculation of barley plants (Hordeum vulgare L.) with strain E19T in non-sterile soil under salt stress conditions significantly increased root (308%) and shoot (189%) dry weight. The relative increase of water content in the root system was 378% than the control treatment, and the root-to-shoot ratio was increased more than double compared to control. H. diazotrophicus inoculation showed no effect on both Na+ and K+ concentration in leaf blades or sheaths, but decreased root surface sodium uptake. The capability of strain E19T to colonize barley roots under salt stress conditions was revealed with a specific designed fluorescence in situ hybridization (FISH) probe. H. diazotrophicus strain E19T positively promotes barley growth under salt stress conditions, and indicates that the mode of action is based on ACC-deaminase production.