Antagonistic Streptomyces enhances defense-related responses in cotton for biocontrol of wilt caused by phytotoxin of Verticillium dahliae

Verticillium wilt of cotton is a severe soilborne disease caused by Verticillium dahliae worldwide. Phytotoxin of V. dahliae (VD-toxin) plays a crucial role in inducing the wilt syndrome in cotton. In this study, spores of four antagonistic isolates were used in the form of powdered biocontrol agent (BCA) to protect cotton from the toxic and wilting effect of VD-toxin. The application of powdered Streptomyces BCA to soil at the time of sowing considerably enhanced defense-related enzyme activities and increased ortho-dihydric phenol and lignin contents in cotton plants after 6 week cultivation in a greenhouse. Compared to the control treatment, Streptomyces BCA treatment severely enhanced defense-related responses in cotton plants within 24 h post-VD-toxin treatment, which further decreased the accumulation of malondialdehyde and mitigated the losses of root activity, leaf total chlorophyll content, and leaf water content. Ultimately, the Streptomyces BCA treatment weakened the pathogenic effect of VD-toxin on cotton and reduced Verticillium wilt by 20.5% to 68.2% at 72 h. These results indicate that the induction of systemic disease resistance and the reduction of VD-toxin's pathogenic effect are two important mechanisms of antagonistic Streptomyces BCA for biocontrol of Verticillium wilt of cotton.