Inoculation of cowpea and wheat with strains of bradyrhizobium sp. That differ in their production of indole acetic acid

Abstract

Sufficient indole acetic acid (IAA) was produced in yeast extract-mannitol (YM) medium by each of two isolates of indigenous Bradyrhizobium sp., to inhibit primary root growth of cowpea. Similar inoculation of cowpea with YM-cultivated strains from a culture collection or with washed cell suspensions of the two IAA-producing isolates had no apparent inhibiting effect on primary root growth. YM-cultivated cultures of each of the two high IAA-producing strains did not prevent nodulation of secondary roots and did not affect plant growth. In tests on agar plates a Bradyrhizobium sp. strain, CB756, frequently used for commercial inoculant production, stimulated root growth of cowpea and wheat seedlings, but when used to inoculate wheat in sand culture, neither strain CB756 nor the IAA-producing isolates measurably affected wheat growth. Production of IAA by the isolates and strains was strongly influenced by growth medium; all required tryptophan to excrete at least 0,5 μg IAA cm−3. Nine 5-methyl-tryptophan-resistant mutants of one of the high IAA-producing isolates were isolated after transposon mutagenesis. Four mutants excreted more IAA than the wild type strain and the nodule mass of plants inoculated with washed cells of the two highest IAA-producing mutants was less than that of plants inoculated with the wild type strain. Primary root growth was unaffected by the mutants. All the mutants were as effective as the wild type strain in their stimulation of cowpea growth. © 1989 Taylor & Francis Group, LLC.