Influence of a Bacillus sp. on physiological activities of two arbuscular mycorrhizal fungi and on plant responses to PEG-induced drought stress.

The effects of bacterial inoculation (Bacillus sp.) on the development and physiology of the symbiosis between lettuce and the arbuscular mycorrhizal (AM) fungi Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe and Glomus intraradices (Schenck and Smith) were investigated. Plant growth, mineral nutrition and gas-exchange values in response to bacterial inoculation after PEG-induced drought stress were also evaluated. In AM plants, inoculation with Bacillus sp. enhanced fungal development and metabolism, measured as succinate dehydrogenase (SDH) and alkaline phosphatase (ALP) activities, more than plant growth. Under non-stressed conditions, G. intraradices colonization increased all plant physiological values to a higher extent when in dual inoculation with the bacterium. Under stress conditions, the bacterium had an important stimulatory effect on G. intraradices development. Under such conditions, the effects of the bacterium on photosynthetic rate, water use efficiency (WUE) and stomatal conductance of lettuce plants differed with the fungus species. Plant-gas exchange was enhanced in G. intraradices- and reduced in G. mosseae-colonized plants when co-inoculated with Bacillus sp. Thus, the effects of each fungus on plant physiology were modulated by the bacterium. Stress was detrimental, particularly in G. intraradices-colonized plants without the bacterium, reducing intra and extraradical mycelium growth and vitality (SDH), as well as plant-gas exchange. Nevertheless, Bacillus sp. inoculation improved all these plant and fungal parameters to the same level as in non-stressed plants. The highest amount of alive and active AM mycelium for both fungi was obtained after co-inoculation with Bacillus sp. These results suggest that selected free-living bacteria and AM fungi should be co-inoculated to optimize the formation and functioning of the AM symbiosis in both normal and adverse environments.