Thin-layer chromatographic analysis of lumichrome, riboflavin and indole acetic acid in cell-free culture filtrate ofPsoralea nodule bacteria grown at different pH, salinity and temperature regimes

Abstract

Using thin-layer chromatography, 16 bacterial isolates from root nodules of 8 differentPsoralea species were quantitatively assessed for their exudation of the metabolites lumichrome, riboflavin and IAA in response to pH, salinity and temperature. Our data showed that the bacterial strains tested differed in their levels of secretion of the three metabolites. For example, strain AS2 produced significantly greater amounts of lumichrome at both pH 5.1 and 8.1, while strains RT1 and PI produced more lumichrome per cell at only pH 8.1. Strains API and RP2 also produced more riboflavin at pH 5.1 than at pH 8.1; conversely strain RTl secreted more riboflavin at pH 8.1 than at pH 5.1. TwoP. repens strains (RP1 and RP2) isolated from very saline environments close to the Indian Ocean produced significant levels of lumichrome and riboflavin at both low and high salinity treatments. However, strains ACI and LI (fromP. aculeata andP. laxa) even produced greater amounts of lumichrome and riboflavin at higher salinity (i.e. 34.2 mM NaCl) and probably originated from naturally saline soils. In this study, high acidity and high temperature induced the synthesis and release of high levels of IAA by bacterial cells. In contrast, there was greater strain secretion of lumichrome at lower temperature (10°C) than at high temperature (30°C). The variations in the secretion of lumichrome, riboflavin and IAA by bacterial strains exposed to different pH, salinity and temperature regimes suggest that genes encoding these metabolites are regulated differently by the imposed environmental factors. The data from this study also suggest that natural changes of pH, salinity and/or temperature in plant rhizospheres could potentially elevate the concentrations of lumichrome, riboflavin and IAA in soils. An accumulation of these molecules in the rhizosphere would have consequences for ecosystem functioning as both lumichrome and riboflavin have been reported to act as developmental signals that affect species in all three plant, animal, and microbial kingdoms.