Characterization of Root Stimulating Bacterium and its Root Stimulating Abilities

In this study, the characterization of a novel root stimulating bacterium (RSB) is reported and the root stimulating ability of RSB is compared to Sphingomonas and other related bacteria. A morphological examination of RSB was conducted. Nutritional and biochemical characterization was performed in order to obtain a profile of RSB. Pigment analysis indicated the presence of nostoxanthin, a pigment commonly found in Sphingomonas bacteria. Lipid analysis of RSB revealed the presence of sphingolipids in addition to other fatty acids suggesting similarity with other Sphingomonas bacteria. Pulse-field gel electrophoresis of RSB and Sphingomonas echinoides showed a high level of difference in the DNA of these two organisms suggesting difference at least at the strain level.

The root stimulating ability of RSB, Sphingomonas and other related bacteria were evaluated using mung bean bioassays. Results of these bioassays suggest that the spent medium used for culturing RSB and Sphingopyxis macrogoltabida contain acidic and neutral compounds that contribute to their promotion of adventitious root formation whereas the spent medium used to culture Novosphingobium capsulata, Sphingomonas echinoides and Sphingomonas paucimobilis contains root stimulating components of acidic nature. Thin layer chromatography of acidic fractions obtained from RSB, Sphingomonas echinoides, Sphingomonas paucimobilis, Sphingopyxis macrogoltabida and Novosphingobium capsulata spent growth medium showed Rf values close to those for the standards indolelactic acid (ILA) and indoleacetic acid (IAA).

Based on all these tests, RSB most closely resembles bacteria of the Sphingomonas group and the rooting ability observed for RSB is characteristic of some of the Sphingomonas and related bacteria.