Composition and function of endophytic bacteria residing the root tissue of Astragalus mongholicus in Hunyuan

Abstract:

[Objective] This study aimed to isolate and screen bacterial endophytes with potential plant growth-promoting activities from Astragali Radix, and to explore effects of green leaf volatiles (GLVs) on the endophytes.[Methods] Plate culture method was used to isolate bacterial endophytes from the root tissue of 7-year old plants of A. mongholicus in Hunyuan, Shanxi, China and 16S rRNA gene sequencing was used to identify the isolates. The screening for plant growth-promoting endophytes was performed by adding exogenous 1-aminocycline-1-carboxylic acid (ACC), tryptophan, calcium phosphate and feldspar into the medium and by incubation in the medium lacking nitrogen source, further quantification was carried out by inductively coupled plasma mass spectrometry (ICP-MS) and spectrophotometry. Effect of the main GLVs in Astragali Radix on the isolates was investigated by adding synthetic chemicals into the liquid medium and the ten isolates containing ACC deaminase were used as representative strains. Headspace GC-MS was performed to determine the contents of the main GLVs in Astragali Radix.[Results] A total of 85 bacterial strains were obtained and classified into the follow phyla:Proteobacteria, Firmicutes, actinobacteria and Bacteroides. At the genus level, they were categorized into 13 genera and the stains of Pseudomonas, Pantoea and Staphylococcus were more abundant (80.00%) than the ones of the others. Among candidate plant growth-promoting isolates, these that can synthesize indoleacetic acid (IAA) shared the biggest proportion (69.41%), followed by these with ACC deaminase (40.00%) and nitrogen fixation activity (31.76%), and these that can solubilize phosphorus and potassium shared relatively lower proportions (i.e., 14.12% and 7.06%). Among the isolates with double plant growth-promoting activities, the IAA-producing strains containing ACC deaminase shared the biggest proportion (37.65%), followed by the IAA-producing strains with nitrogen fixation activity and the strains with ACC deaminase and nitrogen fixation activity, accounting for 28.24% and 24.71%, respectively, while these that not only contain ACC deaminase but can solubilize potassium bearing minerals shared the smallest part (1.18%). Besides, low concentrations of GLVs (i.e., 2-50 μmol/L of hexanal and Z-3-hexenal, and 5-125 μmol/L hexanol) exerted positive effects on the growth of a few strains with ACC deaminase.[Conclusion] There were bigger proportions of the isolates that can produce IAA and/or contain ACC deaminase and GLV growth-promoting strains in the bacterial community residing the root tissue of A. mongholicus in Hunyuan, which might be resulted from the local adaptation. GLVs might be involved in the shaping of the composition and function of endophytic communities in Astragali Radix in Hunyuan.