The genera Serratia and Enterobacter belong to the Enterobacteriaceae family and several members have been described as plant growth-promoting rhizobacteria (PGPR). However, how these bacteria influence growth and development is unclear. We performed in vitro interaction assays between either Serratia sp. H6 or Enterobacter sp. L7 with Arabidopsis thaliana seedlings to analyze their effects on plant growth. In experiments of co-cultivation distant from the root tip, Enterobacter sp. decreased root length, markedly increased lateral root number, and slightly increased plant biomass by 33%, 230%, and 69%, respectively, and relative to the control. The volatile organic compounds (VOCs) emitted from Serratia sp. H6 but not those from Enterobacter sp. L7 promoted Arabidopsis growth. A blend of volatile compounds from the two bacteria had effects on plant growth that were similar to those observed for volatile compounds from H6 only. At several densities, the direct contact of roots with Serratia sp. H6 had phytostimulant properties but Enterobacter sp. L7 had clear deleterious effects. Together, these results suggest that direct contact and VOCs of Serratia sp. H6 were the main mechanisms to promote plant growth of A. thaliana, while diffusible compounds of Enterobacter sp. L7 were predominant in their PGPR activity.
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