Identification of endophytic bacterium BN7 in Boju and study on its biocontrol and growth promoting functions
Plant endophytic bacteria refer to a group of microorganisms that live in healthy plant tissues or cells during a certain stage or the entire stage of their life history and do not cause obvious disease symptoms in the host. The entire growth process of plants is the life process of the host plant and endophytic community. Endophytes form complex ecological relationships with host plants and their surrounding living environments. Endophytic bacteria can produce metabolites similar to those of the host and endow plants with the ability to resist biotic and abiotic stresses. During the long-term interaction between endophytic bacteria and plants, endophytic bacteria exhibit the characteristic of enhancing the host plant’s decomposition and absorption of mineral elements such as phosphorus and potassium. Screening multifunctional bacterial strains that have antagonistic effects on plant pathogens, as well as the ability to hydrolyze phosphorus, potassium, and produce elongation, has great potential for application in agriculture.

Chrysanthemums have the effects of clearing heat, relieving heat, and antioxidation, as well as antibacterial, anti-inflammatory, weight loss, and relieving diseases related to high uric acid blood. As one of the four major medicinal chrysanthemums, our research group has conducted preliminary studies on its endophytic fungi. This experiment studied the antibacterial, phosphate solubilizing, potassium solubilizing, growth promoting, cellulose degrading, and antioxidant activities of endophytic bacterium BN7 in Chrysanthemum morifolium, and preliminarily identified it, laying the foundation for enriching biological control resources and developing bio fertilizers.

In the late 1970s, Dobereiner et al. discovered that nitrogen fixing snails could promote the growth of non leguminous plants; In the late 1980s, Philom Bios, a Canadian company, began producing and selling phosphate solubilizing bacteria, nitrogen fixing bacteria, and mixed phosphate and nitrogen fixing bacteria that could increase crop yields by 10% to 15% for crops such as wheat, rapeseed, and peas. Subsequently, endophytic bacteria in plants gradually became a hot research topic both domestically and internationally. In 2009, Cheng et al. screened an endophytic bacterium PCE45 from Huazhong Building, which produced an antibacterial peptide PCP-1 that had inhibitory effects on various pathogens such as corn curveback Boed and rice blast fungus (Pyricularia oryzae) Cav. In 2010, Bressan et al. isolated six endophytic Bacillus species capable of producing chitinase from maize plant tissues, all of which exhibited inhibitory effects on Fusarium graminearum. In 2015, Zhao et al. screened 36 phosphorus solubilizing endophytic bacteria from soybean nodules, among which strain DD291 had a soluble phosphorus content of up to 452mg/L in the ethanol solution, and some phosphorus solubilizing strains had a promoting effect on soybean growth. In 2015, Kang et al. found that endophytic bacteria ASR16 and ALR33 in alfalfa can produce plant growth hormone and siderophore, with phosphorus solubilizing activity, and have a significant promoting effect on alfalfa growth. In 2018, Gao et al. isolated 57 endophytic bacteria from wild rice in Niwala, among which 44 endophytic nitrogen fixing bacteria had the ability to produce lignin, and 25 strains had varying degrees of potassium solubilization ability. In 2018, Bai et al. isolated an endophytic Bacillus HJ-1 from wild Polygonatum sibiricum roots, which exhibited significant antibacterial activity against Salmonella typhimurium, Pseudomonas aeruginosa, and Bacillus thuringiensis.

At present, there is relatively little research on the endophytic bacteria of Chrysanthemum morifolium, and there are few reports on the promotion of growth by endophytic bacteria of Chrysanthemum morifolium. In this study, the endophytic bacterium BN7 of Chrysanthemum morifolium exhibited antagonistic effects against six plant pathogens, including corn curveball fungus CY1, with the highest inhibition rate of 68% against wheat Fusarium graminearum FM1. This strain also has certain abilities to hydrolyze phosphorus, potassium, and produce IAA, with concentrations of 108.38, 32.03, and 164.39 mg/L, respectively. The ability of strain BN7 to hydrolyze inorganic phosphorus (68.71mg/L) is about three times that of endophytic Bacillus subtilis ZS-3 (20.16mg/L) in camphor trees, which may be related to the shorter cultivation time of the latter (24h); In addition, the ability of strain BN7 to produce long hormones is four times that of strain N8 (41.66mg/L), which was selected by Gao Chen et al. as the strain with the strongest ability to produce long hormones. Strain BN7 also has the ability to degrade cellulose, with CMCase and FPase enzyme activities of 168.78U/mL and 79.87U/mL, respectively, on the third day. In addition, it has a DPPH free radical scavenging activity of up to 82.13%, which is much higher than the DPPH scavenging rate of the previously screened endophytic fungus BJF10 (67.7%). Based on morphological characteristics, physiological and biochemical experiments, and analysis of the 16S rDNA sequence, strain BN7 was identified as Bacillus subtilis. Therefore, Bacillus subtilis BN7 is an endophytic bacterium with the ability to antagonize plant pathogens, hydrolyze phosphorus, potassium, and produce IAA, and has the potential to be developed as a biocontrol and growth promoting agent.