Enhanced suppression of plant growth through production of L-tryptophan-derived compounds by deleterious rhizobacteria

Abstract

Plant-growth-suppressive activity of deleterious rhizobacteria (DRB) may be due to production of metabolites absorbed through roots. Auxins produced in high concentrations in the rhizosphere by DRB contribute to reduced root growth. Selected DRB able to produce excessive amounts of auxin compounds for suppression of weed seedling growth may be effective for biological control of weeds. The objectives to this study were to assess the ability of DRB originating from weed seedlings to synthesize auxins from L-tryptophan (L-TRP), determine effects of DRB with or without L-TRP on seedling root growth, and characterize auxins produced from L-TRP using high performance liquid chromatography (HPLC). Auxins expressed as indole-3-acetic acid (IAA)-equivalents were produced by 22.8% of the DRB tested based on a colorimetric method. Under laboratory conditions, a DRB isolate classified as Enterobacter taylorae with high auxin-producing potential (72 mg L−1 IAA-equivalents) inhibited root growth of field bindweed (Convolvulus arvensis L.) by 90.5% when combined with 10−5M L-TRP compared with non-treated control. Auxin derivatives produced by E. taylorae from L-TRP in broth culture after 24 h incubation identified by HPLC included IAA (102 μg L−1), indole-3-aldehyde (IALD; 0.4 μg L−1), and indole-3-lactic acid (ILA; 7.6 μg L−1). Results suggest that providing L-TRP with selected auxin-producing DRB to increase phytotoxic activity against emerging weed seedlings may be a practical biological control strategy.