Production and Metabolism of Indole Acetic Acid in Root Nodules and Symbiont (Rhizobium undicola) Isolated from Root Nodule of Aquatic Medicinal Legume Neptunia oleracea Lour.

Abstract

Indole acetic acid is a phytohormone which plays a vital role in plant growth and development. The purpose of this study was to shed some light on the production of IAA in roots, nodules, and symbionts of an aquatic legume Neptunia oleracea and its possible role in nodular symbiosis. The symbiont (N37) was isolated from nodules of this plant and identified as Rhizobium undicola based on biochemical characteristics, 16S rDNA sequence homology, and DNA-DNA hybridization results. The root nodules were found to contain more IAA and tryptophan than root; however, no detectable amount of IAA was found in root. The IAA metabolizing enzymes IAA oxidase, IAA peroxidase (E.C.1.11.1.7), and polyphenol oxidase (E.C.1.14.18.1) were higher in root than nodule but total phenol and IAA content were reversed. The strain N37 was found to produce copious amount of IAA in YEM broth medium with tryptophan and reached its stationary phase at 20 h. An enrichment of the medium with mannitol, ammonium sulphate, B12, and 4-hydroxybenzaldehyde was found to promote the IAA production. The presence of IAA metabolizing enzymes and IAA production with PGPR traits including ACC deaminase activity of the symbionts was essential for plant microbe interaction and nodule function.