Two Bioassay Methods to Evaluate Root-Accelerating Activity of Streptomyces sp. MBR52 Metabolites

Abstract

Two bioassay methods were established to detect root-growth accelerator(s) among metabolites of Streptomyces sp. strain MBR52. Biological activity of the water-soluble diffusate released from the mycelial mat was tested using broccoli, cucumber, and soybean seedlings. We evaluated root-growth accelerating activity of mat diffusates on adventitious roots emerging from stems of test plants. Among the plants tested, broccoli was most sensitive to the metabolites. The mat diffusate also accelerated growth of its seminal roots. Adventitious and the seminal roots responded differently to auxins. Adventitious root growth increased in response to auxins, whereas seminal root growth did not. The dry weight of adventitious roots increased in response to treatment with an aqueous solution of 0.05 mg/ml freeze-dried metabolites, while elongation of seminal roots responded only to 0.005 mg/ml. The assay methods focusing on both roots reveal different aspects of root-growth accelerating activity of the MBR52 metabolites. Microorganisms produce a variety of secondary metab-olites, including plant hormones, which can affect the physiological properties and growth of plants (Chanway &