Banana wilt disease is an important disease in banana production, causing significant losses. Herein, we investigated the mechanism by which Bacillus velezensis isolate X5 enhances the resistance of different resistant banana cultivars to Fusarium oxysporum f. sp. cubense race 4 (Foc4). From the perspectives of metabolism, transcriptome, and key genes in important pathways, this study analyzed the composition and content changes of other types of signaling molecules, such as free amino acids and soluble sugars, in resistant/susceptible varieties. The results indicate that under pathogen stress, the contents of root-secreted metabolite components in both resistant and susceptible varieties increase significantly overall, and the increase in susceptible varieties is generally higher. For example, the free amino acid components in susceptible varieties are significantly more than those in resistant varieties. However, the addition of biocontrol bacteria can inhibit this increase. Exogenous addition experiments prove that differential metabolites can either promote or inhibit Foc4 and X5 at certain concentrations. The results of KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment and GO (Gene Ontology) annotation show that resistant varieties have more defense pathways compared to susceptible varieties. Under X5 treatment, more defense genes in resistant varieties are activated or their expression is enhanced, promoting the plant roots to secrete more substances related to plant resistance, such as phenylpropanoids and lignin. This research revealed the effects of Bacillus velezensis on bananas and pathogens, allowing valuable conclusions to be drawn. The results have good application potential to understand the relationships among the three species, defining the biocontrol effect and mechanism of Bacillus velezensis, and providing a theoretical basis for the biological control of soil-borne diseases such as banana wilt disease.

Trichoderma is a widely used bio-control agent, and has excellent ability to antagonize plant pathogens and promoting plant growth. It has been successfully used to control various plant diseases. The premise of using Trichoderma is accurate identification of it. In this study, four sets of species/genus-specific primers were designed based on the translation elongation factor 1-alpha (tef1) gene and internal transcribed spacer (ITS) region, in order to identify Trichoderma harzianum, T. koningiopsis, and T.virens, and detect the genus of Trichoderma. Here, the rapid, simple, and reliable PCR methods were development using the species-specific primers EHarF2/EHarR2, EKoisF/EKoisR, and EVireF/EVireR to produce 253 bp (tef1 gene), 255 bp (tef1 gene), and 263 bp (tef1 gene) DNA bands to identify T. harzianum, T. koningiopsis, and T. virens, respectively. The genus-specific primers ITricF/ITricR produces a single DNA band in the range of 103–113 bp (ITS region) to distinguish Trichoderma from other genera fungi, and the size of the band is related to the species of Trichoderma. In addition, ITricF/ITricR could use for real-time PCR amplification for detection the quantity of Trichoderma spp..