Background
Verticillium dahliae is one of the most destructive pathogens in many cotton-growing countries around the globe. There is still the lack of high-resistant upland cotton cultivars against this pathogen, and the usage of fungicides is threatening the environment. Biocontrol is an environment-friendly and a green technique in plant protection. In this regard, the present work was aimed to investigate antagonistic bacterial strains and to introduce them as new bioinoculants against V. dahliae in cotton.

Main body of the abstract
Numerous bacteria were isolated from cotton roots, and their antagonism was examined against V. dahliae. Strain SZAD2 which was identified as Bacillus velezensis on the bases of 16S rRNA gene analysis had significantly high biocontrol potential both in plate assay and under greenhouse conditions. The biocontrol efficacy in seed treatment was 60.31%, while in the soil drench method, it was 79.19%. Transmission electron microscopy (TEM) analysis revealed that the strain systemically colonized the roots and lived within the intercellular space. Hydrogen peroxide staining exposed that the strain increased plant basal defense response by the accumulation of hydrogen peroxide (H2O2) in both roots and leaves. Furthermore, the bacterization of cotton roots triggered induced systemic resistance (ISR) in the roots and caused the activation of the antioxidant enzymes such as phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), peroxidase (POD), and phenol contents. Moreover, the antifungal extract produced by the strain contained the activity of both cellulase and chitinase, which were able to suppress fungal mycelia both in the presence and absence of cotton.

Short conclusions
Strain SZAD2 systemically colonized the plant, showed multiple modes of action against the pathogen which played a vital role in V. dahliae control, and is recommended for commercial exploration as a safe alternative bioinoculant instead of hazardous chemical fungicides.