ACC deaminase plays a major role in Pseudomonas fluorescens YsS6 ability to promote the nodulation of Alpha- and Betaproteobacteria rhizobial strains.

Ethylene acts as a major regulator of the nodulation process of leguminous plants. Several rhizobial strains possess the ability to modulate plant ethylene levels through the expression of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase; however, rhizobia present low enzymatic activities. One possible alternative to this problem resides on the use of free-living bacteria, such as Pseudomonas, presenting high levels of ACC deaminase activity that may be used as adjuvants in the nodulation process by decreasing inhibitory ethylene levels. Nevertheless, not much is understood about the specific role of ACC deaminase in the possible role of free-living bacteria as nodulation adjuvants. Therefore, this work aims to study the effect of ACC deaminase in the plant growth-promoting bacterium, Pseudomonas fluorescens YsS6, ability to facilitate alpha- and beta-rhizobia nodulation. The ACC deaminase-producing P. fluorescens YsS6 and its ACC deaminase mutant were used in co-inoculation assays to evaluate their impact in the nodulation process of alpha- (Rhizobium tropici CIAT899) and beta-rhizobia (Cupriavidus taiwanensis STM894) representatives, in Phaseolus vulgaris and Mimosa pudica plants, respectively. The results obtained indicate that the wild-type P. fluorescens YsS6, but not its mutant defective in ACC deaminase production, increase the nodulation abilities of both alpha- and beta-rhizobia, resulting in an increased leguminous plant growth. Moreover, this is the first report of the positive effect of free-living bacteria in the nodulation process of beta-rhizobia. The modulation of inhibitory ethylene levels by free-living ACC deaminase-producing bacteria plays an important role in facilitating the nodulation process of alpha- and beta-rhizobia.