Synergistic interactions between the ACC deaminase-producing bacterium Pseudomonas putida UW4 and the AM fungus Gigaspora rosea positively affect cucumber plant growth

Bacteria producing 1-aminocyclopropane-1-carboxylate (ACC) deaminase modulate plant ethylene levels. Decreased ethylene levels increase plant tolerance to environmental stresses and promote legume nodulation. On the contrary, the role of ethylene in mycorrhizal symbiosis establishment is still controversial. In this work, the ACC deaminase-producing strain Pseudomonas putida UW4 AcdS⁺ and its mutant AcdS^-, impaired in ACC deaminase synthesis, were inoculated alone or in combination with the AM fungus Gigaspora rosea on cucumber. Mycorrhizal and bacterial colonization as well as plant growth and morphometric parameters were measured. The influence of each microorganism on the photosynthetic efficiency was evaluated on the second and fourth leaf. The strain AcdS⁺, but not the AcdS^- mutant, increased AM colonization and arbuscule abundance. The mycorrhizal fungus, but not the bacterial strains, promoted plant growth. However, the AcdS⁺ strain, inoculated with G. rosea, induced synergistic effects on plant biomass, total root length and total leaf projected area. Finally, the photosynthetic performance index was increased by the strain UW4 AcdS⁺ inoculated in combination with G. rosea BEG9. These results suggest a key role of this enzyme in the establishment and development of AM symbiosis.