Effects of rhizobacterial ACC deaminase activity on Arabidopsis indicate that ethylene mediates local root responses to plant growth-promoting rhizobacteria

1-Amino cyclopropane-1-carboxylic acid deaminase (AcdS) is an enzyme that degrades the precursor of plant hormone ethylene. AcdS activity has been identified in many soil bacteria. It has been proposed to play an important role in plant-growth promotion by rhizobacteria. It would lower ethylene level via uncharacterized signaling pathways in the host plant. To further investigate the role of AcdS and the involvement of ethylene signaling pathway in plant development responses to rhizobacteria, we used the model plant Arabidopsis thaliana. We compared the changes in root architecture and root hair length induced by four rhizobacteria (Phyllobacterium brassicacearum STM196, Pseudomonas putida UW4, Rhizobium leguminosarum 6v. viciae 128C53K, Mesorhizobium loti MAFF303099) and by their respective acdS-deficient mutants. All the mutant strains induced similar changes in lateral root development as their WT counterparts. By contrast, root hairs of seedlings inoculated with the acdS mutant strains were significantly longer than those of the plants inoculated with the WT strains. Overall, our results would suggest that rhizobacterial AcdS activity affects local regulatory mechanisms in plant roots, and not lateral root development that is under systemic regulation involving shoot-root dialog. (C) 2008 Elsevier Ireland Ltd. All rights reserved.