ENHANCED SUPPRESSION OF PLANT-GROWTH THROUGH PRODUCTION OF L-TRYPTOPHAN-DERIVED COMPOUNDS BY DELETERIOUS RHIZOBACTERIA

Plant-growth-suppressive activity of deleterious rhizobacteria (DRB) m ay be due to production of metabolites absorbed through roots. Auxins produced in high concentrations in the rhizosphere by DRB contribute t o reduced root growth. Selected DRB able to produce excessive amounts of auxin compounds for suppression of weed seedling growth may be effe ctive for biological control of weeds. The objectives to this study we re to assess the ability of DRB originating from weed seedlings to syn thesize auxins from L-tryptophan (L-TRP), determine effects of DRB wit h or without L-TRP on seedling root growth, and characterize auxins pr oduced from L-TRP using high performance liquid chromatography (HPLC). Auxins expressed as indole-3-acetic acid (IAA)-equivalents were produ ced by 22.8% of the DRB tested based on a colorimetric method. Under l aboratory conditions, a DRB isolate classified as Enterobacter taylora e with high auxin-producing potential (72 mg L(-1) IAA-equivalents) in hibited root growth of field bindweed (Convolvulus arvensis L.) by 90. 5% when combined with 10(-5) M L-TRP compared with non-treated control . Auxin derivatives produced by E. taylorae from L-TRP in broth cultur e after 24 h incubation identified by HPLC included IAA (102 mu g L(-1 )), indole-3-aldehyde (IALD; 0.4 mu g L(-1)), and indole-3-lactic acid (ILA; 7.6 mu g L(-1)). Results suggest that providing L-TRP with sele cted auxin-producing DRB to increase phytotoxic activity against emerg ing weed seedlings may be a practical biological control strategy.