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Background and Aims Gluconacetobacter diazotrophicus (family Acetobacteraceae), is a N 2 -fixing bacterium with capability of mineral solubilization through organic acid production. The aim of this work was to evaluate its capacity as a phosphate solubilizing bacterium to overcome a phosphorus deficiency in phosphorus-starved strawberry plants. Methods Strawberry plants growing with available and not available phosphorus were inoculated with the wild type G. diazotrophicus PAL5 Pi-solubilizing strain and a Tn5-derivative mutant defective in organic acid production. Growth index, root acidification, root area, MPN, greenness index, and phosphorus content were evaluated. ResultsResults demonstrate that G. diazotrophicus PAL5 can increase the phosphorus content of strawberry plants (root and shoot), and revert phosphorus deficiency. Additionally, bacterial plant growth-promoting effect was represented by higher values of biomass production, root area, and growth index, as compared with the controls. Conclusions G. diazotrophicus can increase the phosphorus content of strawberry plants, promoting the overall plant growth.

The plant growth-promoting strain REC3 of Azospirillum brasilense, isolated from strawberry roots, prompts growth promotion and systemic protection against anthracnose disease in this crop. Hence, we hypothesised that A. brasilense REC3 can induce different physiological, structural and molecular responses in strawberry plants. Therefore, the aim of this work was to study these traits activated in Azospirillum-colonised strawberry plants, which have not been assessed until now. Healthy, in vitro micropropagated plants were root-inoculated with REC3 under hydroponic conditions; root and leaf tissues were sampled at different times, and oxidative burst, phenolic compound content, malondialdehyde (MDA) concentration, callose deposition, cell wall fortification and gene expression were evaluated. Azospirillum inoculation enhanced levels of soluble phenolic compounds after 12 h post-inoculation (hpi), while amounts of cell wall bound phenolics were similar in inoculated and control plants. Other early responses activated by REC3 (at 24 hpi) were a decline of lipid peroxidation and up-regulation of strawberry genes involved in defence (FaPR1), bacterial recognition (FaFLS2) and H₂O₂ depuration (FaCAT and FaAPXc). The last may explain the apparent absence of oxidative burst in leaves after bacterial inoculation. Also, REC3 inoculation induced delayed structural responses such as callose deposition and cell wall fortification (at 72 hpi). Results showed that A. brasilense REC3 is capable of exerting beneficial effects on strawberry plants, reinforcing their physiological and cellular characteristics, which in turns contribute to improve plant performance.