PPtase-activated production of borrelidin from extremophilic actinobacteria against multidrug-resistant cotton pathogen Verticillium dahliae.

As the most devastating disease in cotton crops, Verticillium wilt caused by Verticillium dahliae led to fiber quality reduction and extensive yield loss. In recent years, Verticillium wilt has been increasingly serious in Xinjiang, China, the world's largest cotton production area. In this study, 52 actinobacterial strains were selected for the discovery of NPs with anti-V. dahliae activity, from our strain collection isolated from three types of extreme habitats (high salinity, high temperature or plant endophytes) in Xinjiang. In the culture broth extracts of phosphopantetheinyl transferase (PPtase)-activated actinobacterial strains, four showed good anti-V. dahliae activities. By isolation, purification and spectral analysis of the antifungal metabolite, its structure was elucidated as borrelidin from Streptomyces rochei TRM46813. This is the first report that borrelidin shows anti-V. dahliae activity. Notably, borrelidin could effectively inhibit different commercial fungicides-resistant V. dahliae pathogens with MICs of 0.125 to 2 μg/mL. The growth-inhibitory effect was antagonized by L-threonine in a dose-dependent manner, suggesting that threonyl-tRNA synthetase (ThrRS) may be the target of borrelidin in V. dahliae. Interestingly, although borrelidin was previously reported to bind to ThrRS and thus inhibit protein translation in Gram-positive pathogens, our mode-of-action analysis indicates that borrelidin led to the accumulation of cell wall precursor, which may be due to the inhibited effects on protein translation. Our findings suggest that borrelidin is a promising candidate for the development of novel antifungal agents to overcome the growing problem of Verticillium wilt.