Influence of Bacterial Sources of Indole-3-acetic Acid on Root Elongation of Sugar Beet

Abstract

Indole-3-acetic acid (IAA) accumulation in culture supernatants of rhizosphere bacterial strains was quantified by high-performance liquid chromatography with two selective detectors. Twelve of 14 rhizobacterial strains produced detectable levels of IAA in culture filtrates. Two strains, 7SR5 and 7SR13, produced large concentrations of IAA (5-10 .mu.g ml-1), reduced root elongation, and increased shoot:root ratios of sugar beet when applied as seed inoculants. A significant linear relationship was observed correlating IAA accumulation of rhizobacterial strains with decreased root elongation and increased shoot:root ratios of sugar beet seedlings. The influence of bacterial sources of IAA on root elongation was also investigated by inoculating sugar beet seeds with IAA-producing strains of Pseudomonas syringae pv. savastanoi. P. s. pv. savastanoi strains 2009 and 2009-6 caused significant decreases in root elongation and increases in shoot:root ratios, whereas two derivative strains, 2009-3 and 2009-561, deficient in IAA production did not.