Abstract:Background Plant growth-promoting rhizobacteria (PGPR) are among the core microorganisms in the rhizosphere microbiome. They can effectively regulate plant growth and development and improve soil quality, being regarded as potent biofertilizer in modern agriculture. These bacteria are of great significance for the chemical fertilizer reduction, green transformation, and sustainable development of agriculture.Objective To isolate PGPR strains of lavender, identify their taxonomic position, and evaluate their plant growth-promoting effects.Methods Rhizosphere soil samples of the main lavender cultivar ‘French Blue’ were collected from the Ili region of Xinjiang. Rhizosphere bacteria were isolated by the dilution plating method. Bacteria were screened based on plant growth-promoting characteristics such as indole-3-acetic acid (IAA) production, nitrogen fixation, potassium solubilization, and siderophore production. Polyphasic taxonomic methods, including morphological, physiological, and biochemical characterization, phylogenetic analysis, and genomic nucleotide sequence similarity analysis, were used for strain identification. The plant growth-promoting effects were evaluated via maize seed germination and test-tube seedling experiments.Results Three rhizosphere bacterial strains, LR-39, LR-68, and LR-79, exhibiting high IAA production (27.42-31.91 μg/mL), along with capabilities of nitrogen fixation, potassium solubilization, and siderophore production, were isolated from the rhizosphere of ‘French Blue’ lavender. Through polyphasic taxonomic identification, LR-39 was identified as Bacillus velezensis, and LR-68 and LR-79 were identified as Bacillus subtilis. All the three strains exhibited strong adaptability to variations in temperature, pH, and salt concentration. Soaking in medium-concentration LR-39, low-concentration LR-68, and high-concentration LR-79 suspensions had the most significant promoting effects on maize seed germination, increasing the germination rates on the seventh day by 24.0%, 26.7%, and 24.0%, respectively, compared with the LB medium control. Compared with the LB medium control, the medium-concentration LR-68 suspension had the best promoting effects on the total fresh weight and total dry weight of test-tube seedlings of maize, increasing them by 70.3% and 48.6%, respectively. The low-, medium-, and high-concentration suspensions of LR-79 significantly promoted the root growth of maize seedlings, with the highest promoting effect (37.0%) achieved at the low concentration. For the stem length of maize seedlings, the medium-concentration LR-68 suspension had the highest promoting effect, resulting in a significant increase of 40.5%. The COG functional classification of functional genes yielded similar results for strains LR-39 (5.36 Mb, G+C content 46.4%), LR-68 (4.06 Mb, G+C content 43.4%), and LR-79 (4.34 Mb, G+C content 43.6%), which were concentrated in amino acid transport and metabolism and conformed to the characteristics of Bacillus. The prediction via antiSMASH revealed 14, 10, and 10 synthetic gene clusters of secondary metabolites for LR-39, LR-68, and LR-79, respectively, most of which were homologous to the synthetic gene clusters of antimicrobial substances or siderophores.Conclusion B. velezensis LR-39, B. subtilis LR-68, and B. subtilis LR-79 isolated from the lavender rhizosphere produce high levels of IAA and possess good capabilities of nitrogen fixation, potassium solubilization, and siderophore production. They exert significant promoting effects on maize seed germination and seedling growth and serve as promising candidates to be developed as microbial fertilizers for application in Xinjiang to reduce chemical fertilizer inputs.
