Mechanism of oxalate decarboxylase Oxd_S12 from Bacillus velezensis BvZ45-1 in defense against cotton verticillium wilt

Verticillium wilt, a soilborne vascular disease caused by Verticillium dahliae, strongly affects cotton yield and quality. In this study, an isolated rhizosphere bacterium, designated Bacillus velezensis BvZ45-1, exhibited greater than 46% biocontrol efficacy against cotton verticillium wilt under greenhouse and field conditions. Moreover, through crude protein extraction and mass spectrometry analysis, we found many antifungal compounds present in the crude protein extract of BvZ45-1. The purified oxalate decarboxylase Odx_S12 from BvZ45-1 inhibited the growth of V. dahliae Vd080 by reducing the spore yield, causing mycelia to rupture, and causing spore morphology changes, cell membrane rupture and cell death. Subsequently, overexpression of Odx_S12 in Arabidopsis significantly improved plant resistance to V. dahliae. Through studies of the resistance mechanism of Odx_S12, V. dahliae was shown to produce oxalic acid (OA), which has a toxic effect on Arabidopsis leaves. Odx_S12 overexpression reduced Arabidopsis OA content, enhanced tolerance to OA, and improved resistance to verticillium wilt. Transcriptome analysis and quantitative real-time PCR (qRT-PCR) analysis revealed that Odx_S12 promoted the outbreak of reactive oxygen species (ROS) and a salicylic acid (SA) and abscisic acid (ABA) mediated defense response in Arabidopsis. In summary, this study not only identified B. velezensis BvZ45-1 as an efficient biological control agent, but also identified the resistance gene Odx_S12 as a candidate for cotton breeding against verticillium wilt.

Beneficial rhizobacterium Bacillus velezensis SQR9 regulates plant nitrogen uptake via endogenous signaling pathway

Nitrogen fertilizer is widely used in agriculture to boost crop yields, plant growth-promoting rhizobacteria (PGPRs) can increase plant nitrogen use efficiency through nitrogen fixation and organic nitrogen mineralization. However, it is not known if they can activate the plant uptake of nitrogen. In this study, we investigated the effects of a PGPR strain Bacillus velezensis SQR9-emitted volatile compounds (VCs) on plant nitrogen uptake. Strain SQR9 VCs promoted nitrogen accumulation in both rice and Arabidopsis. In addition, isotope labeling experiments showed that strain SQR9 VCs promoted the absorption of nitrate and ammonium. Several key nitrogen uptake genes were up-regulated by strain SQR9 VCs, such as AtNRT2.1 in Arabidopsis and OsNAR2.1, OsNRT2.3a and OsAMT1 family members in rice, and the deletion of these genes compromised the promoting effect of SQR9 VCs on plant nitrogen absorption. Furthermore, the calcium (Ca2+) and transcription factor NIN-LIKE PROTEIN 7 play an important role in strain SQR9 VCs-promoted nitrate uptake. Taken together, our results suggest that PGPRs can promote nitrogen uptake through regulating the plant's endogenous signaling and nitrogen transport pathways.

Isolation, identification, and evaluation of the biocontrol potential of a Bacillus velezensis strain against tobacco root rot caused by Fusarium oxysporum

AIMS

Tobacco (Nicotiana tabacum) is an economically important crop. Root rot caused by Fusarium oxysporum has become a damaging disease in N. tabacum crops grown in Henan province of China. Therefore, the objectives of this study were to screen bacterial isolates against F. oxysporum from rhizosphere soils of tobacco growing areas and to evaluate their antifungal activities, biocontrol effects, and effects on plant growth.

METHODS AND RESULTS

Nineteen strains with antifungal inhibition effects of >60% against F. oxysporum were obtained using the method of flat confrontation; the strain Ba-0321 was the strongest, with an antifungal effect of 75%. Moreover, this strain had broad spectrum antimicrobial activity to eight additional tobacco pathogens. The strain was identified as Bacillus velezensis by morphology and the 16S rDNA sequence. The B. velezensis strain Ba-0321 had strong UV resistance as well as tolerance to high temperatures and low nutrition. The bacteria inhibited spore germination and mycelial growth of F. oxysporum under in vitro co-culture conditions. In vivo assays demonstrated that the Ba-0321 strain significantly reduced the pathogenicity of F. oxysporum, resulting in a control effect on tobacco root rot of 81.00%. Simultaneously, the bacteria significantly promoted root development and the growth of tobacco plants.

CONCLUSION

Our results confirmed that the B. velezensis strain Ba-0321 has a strong antifungal effect and stress resistance that enable it to be used as a biological control agent for tobacco root rot caused by F. oxysporum.

SIGNIFICANCE AND IMPACT OF THE STUDY

Tobacco root rot caused by F. oxysporum has become a damaging disease in China. The B. velezensis strain Ba-0321 has promising application value for controlling tobacco root rot diseases, and it could provide a new biocontrol agent against root rot caused by F. oxysporum in other plant species.

Bacillus velezensis (strains A6 & P42) as a potential biocontrol agent against Klebsiella variicola, a new causal agent of soft rot disease in carrot

Bacterial soft rot is one of the most devastating diseases and a major constraint encountered during carrot farming. Biological agents are the best eco-friendly alternatives to agrochemicals to manage soft rot disease to ensure environmental sustainability. In this study, about eight isolates of bacterial pathogen causing soft rot in carrots were collected from Karnataka, India. Based on the 16S rRNA sequencing the pathogen isolates causing soft rot of carrot were identified as Klebsiella variicola. The morphological characteristics of K. variicola was investigated under scanning electron microscopy. The pathogenicity assay showed that all eight isolates were pathogenic to the carrot. An in vitro and in planta assay of two novel strains of Bacillus velezensis (A6 and P42) against K. variicola indicated that both strains had strong antagonistic activity against all the pathogen strains. Furthermore, the volatile bioactive compounds produced by A6 and P42 strains were analyzed in GC-MS, which revealed the presence of 10 and 6 bioactive compounds in their culture filtrate, respectively, with antibacterial and antifungal properties. The present study suggests that both A6 and P42 strains of B. velezensis were antagonistic to K. variicola and can be used as biocontrol agents to manage soft rot diseases of carrot under field conditions.

Extracellular lactonase mediated quorum quenching by a novel Bacillus velezensis

Many Gram-negative bacteria coordinate their gene expression via quorum sensing (QS) mediated by small diffusible autoinducer molecules such as Acyl Homoseriene Lactones (AHL). The degradation of AHLs or quorum quenching (QQ) imparts less selection pressure on the target organisms leading to a possible alternative to antimicrobial agents and traditional biofilm control methods. Here, a novel strain of Bacillus velezensis, strain PM7, exhibiting extracellular quorum quenching (QQ) activity against Gram-negative bacteria has been isolated. Analytical studies of the AHL degradation using LC-MS as well as bioreporters revealed the mechanism of QQ as homoserine lactone ring hydrolysis. Using molecular techniques, the presence of an aiiA homologues gene, specific for acyl homoserine lactonase enzyme, was confirmed in the bacterium. A biofilm inhibitory activity in the range of 36.9 to 77.4% was achieved by the PM7 against three different Gram-negative bacteria. Moreover, a significant reduction (p < 0.05) in the quorum-sensing controlled traits such as violacein production in Chromobacterium violaceum (73%), swarming motility and prodigiosin production (67.2%) in Serratia marcescens, exopolysaccharide production (97.9%) in Pseudomonas aeruginosa, was also observed. The extracellular and broad-spectrum lactonase of this novel strain can be beneficial for its use in QQ applications.

Isolation of Bacillus spp. with High Fibrinolytic Activity and Performance Evaluation in Fermented Douchi

ABSTRACT

Fibrinolytic enzymes are effective and highly safe in treating cardiovascular and cerebrovascular diseases. Therefore, screening fibrinolytic enzyme-producing microbial strains with excellent fermentation performance is of great value to industrial applications. The fibrin plate method was used in screening strains with high yields of fibrinolytic enzymes from different fermented food products, and the screened strains were preliminarily identified using molecular biology. Then, the strains were used for solid-state fermentation of soybeans. Moreover, the fermentation product douchi was subjected to fibrinolytic activity measurement, sensory evaluation, and biogenic amine content determination. The fermentation performance of each strain was comprehensively evaluated through principal component analysis. Finally, the target strain was identified based on strain morphology, physiological and biochemical characteristics, 16S rDNA sequence, and phylogenetic analysis results. A total of 15 Bacillus species with high fibrinolysin activity were selected. Their fibrinolytic enzyme-producing activity levels were higher than 5,500 IU/g. Through molecular biology analysis, we found 4 strains of Bacillus subtilis, 10 strains of Bacillus amyloliquefaciens, and 1 strain of Bacillus velezensis. The principal component analysis results showed that SN-14 had the best fermentation performance and reduced the accumulation of histamine and total amine, the fibrinolytic activity of fermented douchi reached 5,920.5 ± 107.7 IU/g, and the sensory score was 4.6 ± 0.3 (out of 5 points). Finally, the combined results of physiological and biochemical analyses showed SN-14 was Bacillus velezensis. The high-yield fibrinolytic and excellent fermentation performance strain Bacillus velezensis SN-14 has potential industrial application.

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