Siderophore-producing endophytic bacteria in the root nodules of Lespedeza daurica: identification and examination of growth-promoting effect and stress resistance

Abstract:

[Background] As a special structure of plant-microorganism symbiosis, the root nodules of legumes contain abundant microbial resources. The siderophore-producing endophytic bacteria in the root nodules can promote nodulation and nitrogen fixation. However, it remains unclear whether these bacteria have a direct growth-promoting effect on plants. [Objective] To screen out, identify, and examine the growth-promoting effect of the siderophore-producing endophytic bacteria in the root nodules of Lespedeza daurica, so as to provide a new way for enriching plant growth-promoting bacterial resources. [Methods] The CAS medium was used for the preliminary screening of efficient siderophore-producing endophytic bacteria in the root nodules. The liquid fermentation was carried out to examine the siderophore production. The growth-promoting functional media were used to measure the growth-promoting effects of the strains. The stress resistance of the strains was determined with the YMA liquid medium with different salt concentrations (0.01%–9.00%), pH 3.0–12.0, and culture temperatures (4–60 ℃). The pot experiment was carried out to preliminarily verify the direct growth-promoting effect. [Results] Forty-eight strains possessed siderophore-producing ability, including 9 efficient strains, which were TG4, TG9, TG13, TG15, TG28, TG32, TG38, TG76, and TG78, respectively. The siderophore production of the strains varied within 26%–93%. Except that TG28 was identified as Priestia megaterium, the other 8 strains were identified as Brevibacterium frigoritolerans. TG15, TG28, TG76, and TG78 could secrete indole-3-acetic acid (7.62–64.90 μg/mL) and ACC deaminase, fix nitrogen, and grow at 0.01%–5.00% NaCl, pH 4.0–12.0, and 4–37 ℃. TG15 and TG28 significantly increased the stem diameter, and TG28 significantly increased the plant height of L. daurica.[Conclusion] There were siderophore-producing bacteria with growth-promoting effects and stress resistance in the root nodules of L. davurica. However, the growth-promoting effect on plants was only reflected on the morphological development. The results indicate that the growth-promoting substances such as siderophores may have a positive effect on the nitrogen-fixing bacteria.