Root-colonizing non-pathogenic bacteria can increase plant resistance to biotic and abiotic stress factors. Bacterial inoculates have been applied as biofertilizers and can increase the effectiveness of phytoremediation. Inoculating plants with non-pathogenic bacteria can provide 'bioprotection' against biotic stresses, and some root-colonizing bacteria increase tolerance against abiotic stresses such as drought, salinity and metal toxicity. Systematic identification of bacterial strains providing cross-protection against multiple stressors would be highly valuable for agricultural production in changing environmental conditions. For bacterial cross-protection to be an effective tool, a better understanding of the underlying morphological, physiological and molecular mechanisms of bacterially mediated stress tolerance, and the phenomenon of cross-protection is critical. Beneficial bacteria-mediated plant gene expression studies under non-stress conditions or during pathogenic rhizobacteria-plant interactions are plentiful, but only few molecular studies on beneficial interactions under abiotic stress situations have been reported. Thus, here we attempt an overview of current knowledge on physiological impacts and modes of action of bacterial mitigation of abiotic stress symptoms in plants. Where available, molecular data will be provided to support physiological or morphological observations. We indicate further research avenues to enable better use of cross-protection capacities of root-colonizing non-pathogenic bacteria in agricultural production systems affected by a changing climate.

Background:The tolerance of plants against abiotic stresses can be greatly influenced by their interaction with microbes. In lowland rice (Oryza sativa) production, the iron toxicity of the soils constitutes a major constraint. Although there are tolerant cultivars, the mechanisms underlying the tolerance against excess iron are not fully understood. Even less is known about the role of microbes in the response to iron toxicity.Aim: In the study presented here, the effects of different Bacillus isolates on the accumulation and distribution of iron within the shoots of different rice cultivars grown under iron toxicity were analyzed.Methods: Three lowland rice cultivars with contrasting tolerance to iron toxicity (IR31785-58-1-2-3-3, Sahel 108, Suakoko 8) were inoculated with three Bacillus isolates (two B. pumilus and one B. megaterium) and, after 1 week, exposed to excess iron (1,000 ppm) for 8 days. Tolerance was evaluated by leaf symptom scoring.Results: Bacterial inoculation mitigated leaf symptoms in the sensitive cultivar IR31785-58-1-2-3-3 despite no significant differences in shoot iron concentration between inoculated and noninoculated plants. In the tolerant excluder cultivar, Suakoko 8, leaf symptoms were exacerbated when inoculated with B. pumilus Ni9MO12. While the total shoot Fe concentration was not affected in this bacteria × cultivar combination, the distribution of iron within the shoot was clearly disturbed. Tolerance to iron toxicity of the tolerant includer cultivar, Sahel 108, was not affected by Bacillus inoculation. Conclusion:In conclusion, our results show that Bacillus inoculation can affect the tolerance of lowland rice to iron toxicity and that the effects strongly depend on the bacteria × cultivar combination.