A plant growth-promoting bacterium in Atractylodes macrocephala Koidz: isolation, identification, and optimization of culture conditions

Abstract:

[Background] Atractylodes macrocephala Koidz is an important and widely used Chinese medicinal plant, while its cultivation suffers from poor soil fertility which is often dealt with the heavy application of pesticides and fertilizers. [Objective] To provide a material basis for the preparation of a biofertilizer for A. macrocephala with the plant growth-promoting microorganisms isolated from the plant. [Methods] Mo-Sb colorimetry and Salkowski’s method were employed to determine the abilities of the bacterial strain to solubilize inorganic phosphorus and synthesize indole-3-acetic acid (IAA). The effects of the tested strain on plant growth were assessed with the root irrigation method. The strain was identified based on the morphological and physiological characteristics and the 16S rRNA gene sequence. Response surface methodology was employed to optimize the culture conditions of the strain. [Results] The capabilities of the isolate BZ-8 to solubilize inorganic phosphorus and produce IAA reached 4.89 mol/L and 45.52 μg/mL, respectively. Its genome was 1 363 bp long, and phylogenetic analysis identified the strain as Serratia marcescens. The fermentation broth of the strain significantly promoted the growth of A. macrocephala seedlings. The optimized culture conditions of BZ-8 were 5.0 g/L maltose, 3.0 g/L beef extract, 5.0 g/L yeast extract, 10.0 g/L tryptone, 5.0 g/L sodium citrate, initial pH 7.01, and incubation at 30.4 ℃ and 180 r/min. [Conclusion] S. marcescens BZ-8, an endophytic bacterium of A. macrocephala, is capable of solubilizing phosphorus and producing IAA, demonstrating a significant promoting effect on the growth of A. macrocephala. The optimal culture conditions of strain BZ-8 were determined via response surface test.

Key words:plant endophyte;Serratia marcescens;growth-promoting effect;phylogenetic analysis;optimization of culture conditions