Interactions between Pseudomonas putida UW4 and Gigaspora rosea BEG9 and their consequences for the growth of cucumber under salt-stress conditions

Aims: After the determination of the toxic but nonlethal concentration of NaCl for cucumber, we examined the interaction between an ACC (1-aminocyclopropane-1-carboxylate) deaminase producing bacterial strain and an arbuscular mycorrhizal fungus (AMF) and their effects on cucumber growth under salinity. Methods and Results: In the first experiment, cucumber seedlings were exposed to 0·1, 50, 100 or 200 mmol l^-1 NaCl, and plant biomass and leaf area were measured. While seeds exposed to 200 mmol l ^-1 NaCl did not germinate, plant growth and leaf size were reduced by 50 or 100 mmol l^-1 salt. The latter salt cancentration caused plant death in 1 month. In the second experiment, seeds were inoculated with the ACC deaminase-producing strain Pseudomonas putida UW4 (AcdS⁺), its mutant unable to produce the enzyme (AcdS^-), or the AMF Gigaspora rosea BEG9, individually or in combination and exposed to 75 mmol l^-1 salt. Plant morphometric and root architectural parameters, mycorrhizal and bacterial colonization and the influence of each micro-organism on the photosynthetic efficiency were evaluated. The AcdS⁺ strain or the AMF, inoculated alone, increased plant growth, affected root architecture and improved photosynthetic activity. Mycorrhizal colonization was inhibited by each bacterial strain. Conclusions: Salinity negatively affects cucumber growth and health, but root colonization by ACC deaminase-producing bacteria or arbuscular mycorrhizal fungi can improve plant tolerance to such stressful condition. Significance and Impact of the Study: Arbuscular mycorrhizal fungus and bacterial ACC deaminase may ameliorate plant growth under stressful conditions. It was previously shown that, under optimal growth conditions, Ps. putida UW4 AcdS⁺ increases root colonization by Gi. rosea resulting in synergistic effects on cucumber growth. These results suggest that while in optimal conditions ACC deaminase is mainly involved in the bacteria/fungus interactions, while under stressful conditions this enzyme plays a role in plant/bacterium interactions. This finding is relevant from an ecological and an applicative point of view.