Differential Response of Etiolated Pea Seedlings to Inoculation with Rhizobacteria Capable of Utilizing 1-Aminocydopropane-1-Carboxylate or L-Methionine

The majority of soil microorganisms can derive ethylene from L-methionine (L-MET), while some rhizobacteria can hydrolyze 1-aminocyclopropane-1-carboxylate (ACC) due to their ACC-deaminase activity. In this study, three strains having either ACC-deaminase activity (Pseudomonas putida biotype A, $A_7$), or the ability to produce ethylene from L-MET (Acinetobacter calcoaceticus, $M_9$) or both (Pseudomonas fluorescens, $AM_3$) were used for inoculation. The highly ethylene specific bioassay of a classical 'triple' response in pea seedlings was used to investigate the effect of the inoculation with the rhizobacteria in the presence of 10 mM ACC or L-MET. The exogenous application of ACC had a concentration-dependent effect on the etiolated pea seedlings in creating the classical 'triple' response. The inoculation with P. putida diluted the effect of ACC, which was most likely due to its ACC-deaminase activity. Similarly, the application of $Co^{2+}$ reduced the ACC-imposed effect on etiolated pea seedlings. In contrast, the inoculation of A. calcoaceticus or P. fluorescens in the presence of L-MET caused a stronger classical 'triple' response in etiolated pea seedlings; most likely by producing ethylene from L-MET. This is the first study, to our knowledge, reporting on the comparative effect of rhizobacteria capable of utilizing ACC vs L-MET on etiolated pea seedlings.